U.S. patent application number 12/146579 was filed with the patent office on 2009-03-12 for communication system and its device.
Invention is credited to Yoshihiro Ashi, Masahiko MIZUTANI.
Application Number | 20090067850 12/146579 |
Document ID | / |
Family ID | 39720743 |
Filed Date | 2009-03-12 |
United States Patent
Application |
20090067850 |
Kind Code |
A1 |
MIZUTANI; Masahiko ; et
al. |
March 12, 2009 |
Communication System and Its Device
Abstract
A transmission device uses transmission delay time of
reciprocation with the terminal and time information of the
transmission device to create a correction value of time
information of the terminal and transmits it to the terminal. The
terminal includes expected arrival time information based on the
time information of the terminal and the correction value received
from the transmission device in a frame transmitted to the
transmission device. The transmission device compares reception
time of the frame with the expected arrival time information in the
frame. If they match, the time information of the terminal
synchronizes with the time information of the transmission device.
If they do not match, the transmission device transmits a new
correction value to the terminal. The terminal transmits a frame
including expected arrival time information using the new
correction value to the transmission device.
Inventors: |
MIZUTANI; Masahiko;
(Yokohama, JP) ; Ashi; Yoshihiro; (Yokohama,
JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
39720743 |
Appl. No.: |
12/146579 |
Filed: |
June 26, 2008 |
Current U.S.
Class: |
398/154 |
Current CPC
Class: |
H04J 3/0605 20130101;
H04Q 2011/0088 20130101; H04L 7/0016 20130101; H04L 43/0864
20130101; H04Q 2011/0064 20130101; H04Q 11/0067 20130101; H04J
3/1694 20130101; H04Q 2011/0079 20130101 |
Class at
Publication: |
398/154 |
International
Class: |
H04B 10/00 20060101
H04B010/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 2007 |
JP |
2007-231379 |
Claims
1. A communication system comprising: a terminal including: a first
storage device that stores first time information; a receiving
device that receives a correction value to correct the first time
information; and a transmitter that transmits a frame including
expected arrival time information based on the first time
information and the correctness value; and a transmission device
including: a receiving device that is connected with the terminal
through a communication line, and receives the frame from the
terminal through the communication line; a second storage device
that stores second time information and transmission delay time
(RTD) of reciprocation with the terminal; and a transmitter that
transmits the correction value based on the second time information
and the transmission delay time (RTD) of the reciprocation, and the
correction value modified based on the difference between reception
time relative to the second time information of the frame and the
expected arrival time information in the received frame to the
terminal through the communication line.
2. The communication system according to claim 1, wherein the
terminal corrects the first time information within the first
storage device in response to the reception of the modified
correction value, and the transmitter of the terminal transmits a
frame including new expected arrival time information based on the
corrected first time information and the modified correction
value.
3. The communication system according to claim 1, wherein when the
difference between reception time relative to the second time
information of the frame and the expected arrival time information
in the received frame exceeds a specific value determined
correspondingly to time accuracy demanded from the terminal, the
transmitter of the transmission device transmits the modified
correction value to the terminal through the communication
line.
4. The communication system according to claim 1, wherein the
second time information stored in the second storage device is
distributed from GPS and the like that satisfy time accuracy
demanded from the terminal, and updated by a clock within the
transmission device, and the first time information stored in the
first storage device is updated by a clock within the terminal.
5. The communication system according to claim 1, wherein the
terminal is an ONU (Optical Network Unit), and the transmission
device is a PON (Passive Optical Network) system that is configured
with an OLT (Optical Line Terminal) that is connected with the ONU
through an optical network as the communication line and has
ranging means that determines transmission delay time (RTD) of the
reciprocation.
6. A transmission device comprising: a receiving device that is
connected with a terminal through a communication line, and
receives a frame including expected arrival time information of the
frame from terminal through the communication line; a storage
device that stores time information and transmission delay time
(RTD) of reciprocation with the terminal; and a transmitter that
transmits a time correction value of the terminal based on a
difference between reception time relative to the time information
of the frame and the expected arrival time information in the
received frame to the terminal through the communication line.
7. The transmission device according to claim 6, wherein when the
difference between reception time relative to the time information
of the frame and the expected arrival time information in the
received frame exceeds a specific value determined correspondingly
to time accuracy demanded from the terminal, the transmitter of the
transmission device transmits the time correction value to the
terminal through the communication line.
8. The transmission device according to claim 6, wherein the time
information is distributed from a GPS and the like satisfying time
accuracy demanded from the terminal, and updated by a clock within
the transmission device.
9. The transmission device according to claim 6, wherein the
terminal is an ONU (Optical Network Unit) in a PON (Passive Optical
Network) system, and the transmission device is an OLT (Optical
Line Terminal) in the PON (Passive Optical Network) system that is
connected with the ONU through an optical network as the
communication line and has ranging means that determines the
transmission delay time (RTD) of the reciprocation.
10. A terminal comprising: a storage device that stores time
information; a transmitter that is connected with a transmission
device through a communication line, and transmits a frame
including expected arrival time information of the frame to the
transmission device; a receiving device that is used along with the
time information to create expected arrival time information of the
frame, and receives a time correction value created by the
transmission device based on a difference between the expected
arrival time information and time information owned by the
transmission device from the transmission device through the
communication line; and correction means that correct time
information stored in the storage device based on the received time
correction value.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to technology for time
synchronization between devices in a communication system.
Specifically, it relates to technology for a transmission device
connected with a terminal to convey time information to the
terminal.
[0002] Time distribution services having been conventionally
offered to general users include time distribution by use of radio
waves, notification services by use of time, and services by use of
the Internet. A typical example of the application of time
distribution technology by use of radio waves is a wave clock. As
time distribution on the Internet, for example, a time
synchronization method by use of NTP (network time protocol) is
available.
[0003] Major applications of traditional standard time distribution
have been synchronization between transmission devices, the
management of fault occurrence time, and the use (keeping the
consistency of time) of time stamp in the exchange of mail, data,
and the like. Time accuracy in standard time distribution by use of
NTP and existing phone lines is no more than several milliseconds.
This is a sufficient accuracy in time adjustment of individual PCs
and servers. This is because, in the range of traditional
applications, an object has been to independently confirm each of
the times of devices existing on a network. For example, although
mail arrival time is highly different from transmission time due to
the influence of transmission delay and the like on the network,
the difference is no problem in terms of use. In downloading of Web
data, inherently, it has not been necessary at all to take PC
setting time of individual users into account.
[0004] The construction of an optical access network has been
promoted for general users. One of its representative system is a
G-PON (Gigabit Capable Passive Optical Network)(ITU-T
Recommendation G.984.1 (2003), Gigabit-capable Passive Optical
Networks (G-PON): General Characteristics, ITU-T Recommendation
G.984.3 (2004), Gigabit-capable Passive Optical Networks (G-PON):
Transmission convergence layer specification, ITU-T Recommendation
G.984.3 Amendment 1(2005), Gigabit-capable Passive Optical Networks
(G-PON): Transmission convergence layer specification) standardized
in ITU-T. Since G-PON uses basic cycle frames of 125 microseconds
for data transmission control, it can accommodate information
distribution at a fixed rate and at a fixed cycle such as E1 and T1
lines (specifications of high speed digital line) having been
conventionally used in private line services, and best-effort type
communications of variable bands as typified by Ethernet
(registered trademark) having been used in data communications. A
PON is a system that is being promoted for its introduction as
users of the Internet increase. The PON is in the form of PDS
(Passive Double Star) using a passive optical element, and
accommodates plural user terminals at the same time for one base
station device (transmission device). Multiplexing communications
of each user on an identical optical fiber has the advantages of
facilitating user management (increase or decrease in facilities,
fault monitoring, etc.) and reducing optical fiber installation
costs.
SUMMARY OF THE INVENTION
[0005] With the emergence of high speed networks and the
establishment of IT environments in homes and enterprises, there is
a growing situation in which not only users download information
but also plural users use common applications through the Internet.
Not only terminals (clients) are distributedly disposed and
connected through a network, but also there are increasing systems
in which the relationship between terminals is functionally dense,
and the operation of the system is increasing. For example, there
are commercial transactions on the Internet. In the online
reservation of tickets and the online trading of stocks, the
recording of access time is very important. In the case where one
application is used at the same time by a large number of users,
since information and profits obtained by individual users may
differ depending on mutual timing among the users, high time
accuracy is desired.
[0006] Furthermore, also in communication network services such as
distributed databases and sensor networks, higher accurate time
synchronization than traditional time accuracy (msec order) is
demanded.
[0007] One of important things in distributed databases is the
holding of consistency among distributed data. When other data in
other sites is updated in relation to the updating of certain data,
a database management system takes countermeasures on the
assumption that times of different sites do not synchronize (there
is data transmission delay time) with high accuracy. In other
words, if times of different sites synchronize with high accuracy,
it is possible to reduce the burden of taking countermeasures in
the database management system.
[0008] A sensor network requires correctly grasping the occurrence
time of events detected by a large number of sensors for each of
the sensors.
[0009] In contrast to this, time synchronization accuracy in
traditional time distribution services has been not necessarily
enough. The accuracy of time distribution by JJY using phone lines
has been several milliseconds so far. Time distribution services
over radio waves used for wave clocks cannot be used when terminals
and transponders are installed in places where radio waves do not
reach. Even when NTP is used, time synchronization accuracy is
several milliseconds, and becomes worse as relaying servers
increase (as the number of pops increases).
[0010] Accordingly, an object of the present invention is to
achieve more accurate time synchronization among terminals, and
guarantee the correctness of the time.
[0011] Another object of the present invention is to achieve highly
accurate time synchronization for transmission devices in a network
such as PON.
[0012] One aspect of the present invention is a communication
system that synchronizes time information of a terminal connected
to a transmission device through a communication line to time
information of the transmission device with high accuracy. The
transmission device uses transmission delay time (RTD) of
reciprocation with the terminal and time information of the
transmission device to create a correction value of time
information of the terminal, and transmits it to the terminal
through the communication line. The terminal creates a frame to be
transmitted to the transmission device. The frame includes expected
arrival time information based on time information of the terminal
and the correction value received from the transmission device. The
transmission device that has received the frame compares reception
time of the frame based on time information of the transmission
device with the expected arrival time information in the received
frame. As a result of the comparison, if they match or a difference
between them is equal to or less than a specific value, time
information of the terminal is considered to synchronize with time
information of the transmission device. If the difference is equal
to or greater than the specific value, the transmission device
transmits the difference to the terminal as a new correction value.
The terminal transmits a frame including expected arrival time
information based on the new correction value to the transmission
device. This processing is repeated until the difference between
reception time and expected arrival time information becomes equal
to or less than the specific value to synchronize time of the
terminal to time of the transmission device.
[0013] Another aspect of the present invention is in each of the
transmission device and the terminal that constitute the
communication system described previously. The transmission device
achieves feedback by synchronizing time of the terminal and
prompting the terminal to transmit a frame including expected
arrival time information to check synchronization accuracy.
[0014] On the other hand, the terminal transmits a frame including
expected arrival time information to the transmission device in
response to the reception of a correction value.
[0015] It will be apparent to those skilled in the art that
transmission delay time (RTD) of reciprocation in these disclosures
of the invention includes not only delay time due to a transmission
path but also time involved in transmission and reception.
[0016] Although the present invention is represented as the
invention of each of a communication system constituted by a
transmission device and a terminal, and the transmission device and
the terminal, as is apparent from the disclosure of the
specifications, the present invention is an invention of the
relationship between a server having highly accurate time
information and a client time-synchronized with time information of
the server with high accuracy. As an important point in this case,
without being bound to the functions of the server and the client,
one has highly accurate time information, and the other performs
feedback control to time-synchronize to the former with high
accuracy.
[0017] The present invention achieves highly accurate time
synchronization of a terminal with a transmission device, and
guarantees the correctness of the time. When it is found that, by
comparing reception time of a frame including expected arrival time
information from the terminal with expected arrival time
information included in the frame, synchronization accuracy is not
obtained, highly accurate time synchronization can be achieved by
repeating the transmission of a time correction value from the
transmission device to the terminal and the comparison described
previously.
[0018] By prompting the terminal to transmit a frame including
expected arrival time information, the accuracy of time
synchronization of the terminal is checked, and the correctness of
time of the terminal is guaranteed.
[0019] It will be apparent that the above-described effects become
conspicuous by realizing the present invention in a transmission
device and a terminal in a PON system having a ranging
function.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a block diagram showing the configuration of a
subscriber terminating network configured using a GPON system, and
a time distribution system using it;
[0021] FIG. 2 shows a basic sequence of a time distribution method
performed of a PON section;
[0022] FIG. 3 shows an example of a frame structure for indicating
time information in a first stage;
[0023] FIG. 4 shows an example of a frame structure for time
notification in a second stage (PON section);
[0024] FIG. 5 shows the device configuration of OLT and GPS
receivers;
[0025] FIG. 6 is a functional block diagram of an ONU;
[0026] FIG. 7 is a sequence showing the relationship between
processing from the calculation of transmission time to time
notification in a basic sequence of time distribution, and ranging
processing;
[0027] FIG. 8 is a sequence showing the flow of processing of
modifying time information in an operation state;
[0028] FIG. 9 is a flowchart showing the flow of processing from
the acquisition of time information in an OLT to notification to an
ONU;
[0029] FIG. 10 is a flowchart showing the flow of processing from
the acquisition of time information from an OLT in an ONU to the
registration to a device;
[0030] FIG. 11 shows an example of the structure of a table that
manages RTD information and EqD information;
[0031] FIG. 12 shows an example of a table structure for time
management held in an OLT;
[0032] FIG. 13 shows an example of the structure of an ONU-based
time information management table held in an OLT;
[0033] FIG. 14 shows an example of a table structure for time
management held in an ONU;
[0034] FIG. 15 shows the relationship between clock and time
information within a device (OLT) and time information supplied
from a GPS receiver;
[0035] FIG. 16 is a schematic diagram showing time required for
reciprocation of a PON section;
[0036] FIG. 17 shows a sequence of basic time adjustment;
[0037] FIG. 18 is a sequence diagram showing time elapse during
time adjustment;
[0038] FIG. 19 is a flowchart showing the flow of processing within
an ONU during time notification from the ONU to an OLT;
[0039] FIG. 20 is a flowchart for time adjustment processing within
an OLT;
[0040] FIG. 21 shows an example of the structure of a time
notification frame from an OLT to an ONU;
[0041] FIG. 22 shows an example of the structure of an upstream
frame for transferring time information from an ONU to an OLT;
[0042] FIGS. 23A and 23B are conceptual diagrams showing the
operation of time distribution;
[0043] FIG. 24 is a time chart showing changes in time setting
situation within an ONU; and
[0044] FIG. 25 shows the effects of an embodiment of use of a
distributed database.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0045] FIG. 1 is a block diagram showing the configuration of a
subscriber terminating network configured using a GPON (Gigabit
Capable PON) system in this embodiment, and a time distribution
system using it.
[0046] This network includes OLTs (Optical Line Terminal) 1-1 to
1-3, ONUs (Optical Network Unit) 2-1 to 2-3, optical splitters 3-1
to 3-3, optical fibers 10-1 to 10-3, and 11-1-1 to 11-3-3. Plural
OLTs 1-1 to 1-3 are provided in the edge of a user accommodation
network, and the individual OLTs each accommodate plural ONUs.
[0047] The OLTs 1-1 to 1-3 each include plural PON-IFs (described
later in the description of FIG. 2). For example, OLT 1-2 is
connected with ONU 2-1 through an optical fiber 10-1, splitter 3-1,
and optical fiber 11-1-2, with the ONU 2-2 through an optical fiber
10-2, the splitter 3-2, and optical fiber 11-2-2, with the ONU 2-3
through an optical fiber 10-3, the splitter 3-3, and optical fiber
11-3-2. Splitters 10-1 to 10-3 branch (copy) a signal transmitted
through the optical fibers 10-1 to 10-3 from the OLT 1-2 to the
optical fibers 11-1-1 to 11-1-3, 11-2-1 to 11-2-3, and 11-3-1 to
11-3-3 branched to the ONU side, respectively. Moreover, the
splitters transmit a signal (upstream signal) from an ONU to the
OLT 1-2 sent through the optical fibers 11-1-1 to 11-1-3, 11-2-1 to
11-2-3, and 11-3-1 to 11-3-3 to the OLT 1-2 through the common
optical fibers 10-1, 10-2, and 10-3, respectively.
[0048] For example, with respect to the optical fiber 10-2,
upstream signals sent through the optical fibers 11-2-1 to 11-2-3
from plural ONUs including ONU 2-2 are transmitted to the common
optical fiber 10-2 each time they are received. To prevent upstream
signals from different ONUs from overlapping, a multiplex system by
TDMA (Time Division Multiple Access) is used. The OLT 1-2, in
communications with ONUs connected to the optical fibers 10-1 to
10-3, respectively, notify individual ONUs of transmission timing
used for upstream communications and the amount of transmittable
data, that is, performs transmission timing control by use of the
TDMA system that assigns a communication band to the individual
ONUs.
[0049] The ONU 2-1 to 2-n accommodate subscriber data communication
terminals 20-1 to 20-n and TDM (Time Division Multiplexing)
terminals 30-1 to 30-n, respectively. The former is used for
services requiring data transmission efficiency including browsing
of WWW (World Wide Web) information and downloading of data such as
PC and mobile communication terminals. A primary connection service
is the use of Ethernet. The TDM terminals are connected to TDM
interfaces (described later) of ONUs. By the interfaces, the ONU
2-1 to 2-3 accommodate synchronous multiplexed frame communication
(TDM communication) by T1 or E1 lines. This service is a
communication system multiplexing information by synchronizing
communication control between an information transmitting device
and information receiving device. Its representative example is
telephone service that uses a line exchange system. Plural data
communication terminals and TDM terminals can be accommodated in
each ONU.
[0050] One of network services requiring highly accurate time
synchronization is a distributed database system that uses a
distributed hash table. In FIG. 1, in a local network or private
network 50-3 and 50-n, servers 60-3 and 60-n, and storages 70-3 and
70-n being components of the database system are located. When
these databases are located in geographically near locations, they
may be located respectively under control of different ONUs under
the management of an identical OLT. Of course, plural databases may
exist under control of an identical ONU. When databases are built
using a wide area network, plural databases linked may be located
under control of different OLTs, respectively.
[0051] When plural nodes targeted for time synchronization exist
under control of a single OLT, time synchronization is required
between PON sections. In the case where time synchronization is
performed for plural nodes over a wide area network, when databases
are mutually consulted, usually, communications with devices (OLT)
different in operation clock must be performed, in most cases,
through plural routers and switches passing between them. At this
time, maintaining time information with high accuracy between the
communication devices enables authentication and correct data
processing, and makes it possible to prevent data tampering and
illegal access from users.
[0052] The case of a sensor network also requires time
synchronization like the case of databases. It is different from
databases in that more nodes are distributedly located in local
networks and private networks under control of individual ONUs, and
a sensor network is formed there. Information obtained from sensor
nodes is conceivably processed in, for example, one (e.g., network
50-1) of sites connected to an access line and a server located in
an upper network 1000. Therefore, requirements for the system are
expected to be higher accuracy because of the large number of
nodes, but are basically the same as those for the distributed
database. Here, the processing of cooperation among distributed
databases is representatively described.
[0053] FIG. 2 shows a basic sequence of a time distribution method
performed of a PON section in the system of FIG. 1. An ONU under
control of OLT 1 is representatively shown by the ONU 2, and a
terminal located in a local network or private network (hereinafter
referred to as a local network) connected to the ONU, or a
management server of a distributed database is representatively
shown by a terminal/distributed DB 60.
[0054] The process of time distribution can be broadly divided to
three stages: first, from a time information server to the OLT;
second, time information distribution of a PON section; and third,
time distribution from ONU to the local network 50. Here, in a
first process, a GPS-based standard time receiving device is
assumed as a time distribution server. The GPS is a system that can
tell time having today's highest possible accuracy (several
microseconds). A standard time receiver 100 may be mounted in a
cabinet separate from the OLT 1 or, for example, in a board form in
a single cabinet. The adoption of any of the methods will not
impair the contents of the present invention.
[0055] The standard time receiver 100 includes a GPS receiver (see
a drawing shown later), and receives standard time information
(201). The standard time information is conveyed to a PON interface
board of the OLT 1 by using an internal communication frame within
the device or an existing communication protocol in between the
devices (202).
[0056] The OLT 1 determines transmission time (correction value of
time information) conveyed to individual ONUs, based on received
time information, or by referring to the distances between the ONUs
managed by its own device and the PON (203). At this time, the OLT
1 uses a ranging result (RTD; Round Trip Delay measurement result)
for the ONUs. To convey time determined here to each ONU, the OLT 1
inserts necessary information in a PON section transmission frame
(see a drawing shown later) to generate a downstream frame (204).
The downstream frame, after delay time required to process a time
information frame to the OLU 2 is calculated, is transmitted
inserting time for which necessary corrections are performed
(205).
[0057] On receiving the standard time, the ONU 2 registers time
information within its own device (the information is mapped to an
operation clock within the device) (206). In the stage of initial
setting, the OLT 1 cannot always grasp correct transmission time to
the ONU 2. To correctly adjust time, the time set temporarily in
the ONU 2 must be compared with the standard time in the OLT 1 for
re-notification. In feedback processing 207, a correct PON section
transmission delay is determined based on an EqD computation model
(see ITU-T Recommendation G.984.3 (2004), Gigabit-capable Passive
Optical Network (G-PON): Transmission convergence layer
specification) used for ranging processing. By using feedback from
the ONU 2, time can be more correctly than during transmission from
the OLT 1 to the ONU 2. The feedback processing will be described
in more detail in and after FIG. 17.
[0058] The ONU 2 conveys time having been correctly adjusted as a
result of the feedback processing 207 to a terminal 60 under
control of the ONU 2. Time distribution to a device connected to
the local network 50 is performed by an existing protocol or
specific frame format (208). On receiving the information, the
terminal 60 registers the information in its own device (209).
[0059] FIG. 3 shows an example of a frame structure for indicating
time information in the first stage. Representative examples of
formats used here are, for example, a frame for internal processing
having a header (internal header) for processing within the device,
Ethernet frame, and SDH frame. Differences of the formats exert no
influence on the nature of this embodiment. The OLT 1 has only to
be able to acquire standard time. Here, only basic conditions are
described.
[0060] A header part (time information header) 310 includes
destination information 311, source information 312, other
information 313. In the destination information 311, a destination
address in the case of Ethernet, and channel information in the
case of SDH (Synchronous Digital Hierarchy) frame are inserted. In
the case of an internal header, an identifier that allows a
functional block receiving the frame to determine whether frame
processing is required is inserted. Also for the source, an
identifier for identifying a source is inserted. Usually, since
time information does not naturally involve frame retransmission,
source information may be inserted as required. Since a delay in
the notification of time information for some cause significantly
impairs the accuracy of time, other information 313 effectively
contains information (numeric indication of priority or an
identifier such as tag indicating a frame set with high priority)
indicating processing priority during transmission of the
frame.
[0061] A data part 320 (time data) contains standard time
information to be conveyed. Time (hour) 321, time (minute) 322,
total time 323 from reference time, year 324, and other information
325 are contained. For example, for time notification of push type
from the standard receiver 100 and the OLT 1, instead of
transmitting the above-described information necessary to set
standard time all the time, the amount of band use of a PON section
can be curbed by minimizing the notification of these pieces of
basic information, and conveying only a time correction amount
necessary to maintain time information during normal operation of
the PON system.
[0062] FIG. 4 shows an example of a frame structure for time
notification in a PON section in the second stage of time
notification. The frame structure is described using an example of
using a GEM (G-PON Encapsulation Method) frame. A PON downstream
frame is formed of a continuous concatenation of cycle frames. The
separation of each cycle frame is set every 125 microseconds in the
case of GPON. In the unit of the cycle, a downstream frame
including a header 410 and a payload 420 is transmitted. The header
410 of a downstream frame includes Psync header for frame
synchronization and other frame information.
[0063] A portion other than the above of the downstream frame is
used as the payload 420. The payload 420 is stored with a normal
data frame 450 (a format called a GEM frame is used in the case of
GPON) and a frame for time notification 440. To transmit time
information to individual ONUs by specifying a destination, the
frame header 410 includes destination information and other header.
The destination information contains an identifier for determining
whether to receive the time information in the ONU. For example, if
the GEM frame structure is used, an identifier called Port-ID442 is
inserted in the destination information. Other header information
(GEM header) 413 includes a PLI (Payload length indicator) 441
indicating the length of a time information frame (GEM frame in
FIG. 4), a PTI (Payload type indicator) 443 indicating the type of
in-frame information such as maintenance management information or
normal data, and a HEC (Header Error Control) field 444 added for
error detection and modification in the GEM header.
[0064] Furthermore, in FIG. 4, as information included in a data
part 414 (GEM payload), the time information header 310 and the
time data 320 shown in FIG. 3 are shown. Thus, a time information
frame received by the OLT 1 in the format shown in FIG. 3 may be
capsuled in the PON section transmission frame shown in FIG. 4
without modifications. As another method, when information
contained in the time information header 310 is unnecessary, the
filed may be deleted and only payload (time data) 320 may be
transferred. This is because the header information of the PON
section transmission frame (GEM frame in GPON) is usually
considered sufficient to identify a destination. If there is a
field necessary to confirm time information in information within
the header 310, in the OLT 1, a frame with only the relevant
information left may be capsuled for transfer to a PON section.
[0065] FIG. 5 shows a device configuration of the OLTs 1-1 to 1-3
(hereinafter, OLT is representatively described as OLT 1) and GPS
receivers 100-1 to 100-3 (hereinafter, representatively described
as a GPS receiver 100) that constitute the network of FIG. 1. Since
an Ethernet interface is recently general, subsequent descriptions,
for simplicity of description, assume an Ethernet interface as an
SNI (Service Network Interface) interface. Of course, descriptions
of Ethernet may also apply to TDM.
[0066] The OLT 1 includes a GPS receiver 100, L2SW 560, connection
unit 570, and PON interfaces 200-a and 200-b.
[0067] The GPS receiver 100 receives standard waves from a GPS
satellite and conveys them to the OLT 1. A general GPS receiver
includes an antenna 501, a signal amplifier 502, a down converter
503, a signal processing unit 510, a transmission processing unit
520, a CPU 530, time processing unit 540, and an output control
unit 550. The down converter 503 frequency-converts GPS radio waves
of 1.5 GHz band into intermediate frequencies. As intermediate
frequencies, about 4 MHz or 1 MHz is often used.
[0068] A signal detection unit 511 inversely diffuses a spread
spectrum signal to extract an original carrier signal. A
synchronization tracing unit 512 corrects phase deviations of a
spread spectrum signal to enable continuous signal detection. A GPS
carrier signal is modulated by a data signal of about 50 bps as a
navigation message, and a message decoding 513 decodes the
navigation message. Time deviations between a satellite clock and a
receiver clock are detected from the navigation message to
determine the position of the receiver. The obtained message or
information about position and distance is sent to the time
processing unit 540. If signals from four or more satellites are
obtained, a correct time (accuracy of several microseconds) is
obtained. A time determining unit 541 determines a correct time
from satellite messages, and stores it in a synchronization
information database 543. A reception information database 542 is
used to record received information (message) used for time
determination.
[0069] The transmission processing unit 520 generates a frame for
conveying the time information obtained here to the OLT. The format
of the frame to be used does not matter. To convey a correct time
to the OLT, the output control unit 550 manages frame output
timing. Specifically, it previously grasps a transmission delay
time taken to capture time information by the OLT and conveys time
corrected by it to the OLT. Correction based on delay may be
performed at the time of reception of the information in the OLT.
Moreover, a frame generating unit 521 previously performs time
correction with a delay in mind, and may insert the corrected time
in a frame. On receiving the time information, the OLT 1 performs
the above-described correction if necessary before storing it in a
time information database 264 in a memory.
[0070] The L2SW 560 multiplexes signals from an Ethernet interface
(not shown) included in its upper network and sends the multiplexed
signals to the PON interface 200. Moreover, it sends an upstream
signal received from the PON interface to the Ethernet interface of
SNI.
[0071] The PON interface unit 200 of the OLT 1 includes a frame
processing unit 210 that queues frames transmitted and received in
an access network side (SNI; Service Network Interface side) and
performs header processing; a PON terminating unit 230 that
performs frame processing for mutual connection between a
communication system in a PON section and a communication method in
an external network such as Ethernet; an optical module 240 that
converts an electrical signal into an optical signal after
generating a downstream in the PON terminating unit 230 and
converts an optical signal into an electrical signal for
transmission to the PON terminating unit 230 when receiving an
upstream signal through an optical fiber; a CPU 250 that performs
various operations; and a memory 260 that performs data management
for communication control and holds programs. Information held in
the memory 260 includes information about frame analyzing results
and the setting of band control.
[0072] Data inputs by the SNI circuit 201 is temporarily stored in
a data queue 212 included in the Ethernet frame processing unit
210. The stored data is read out by a command from a queue control
unit 211, and is reorganized to a downstream frame for PON section
communication in the PON terminating unit 230. The queue control
unit 211 reads a frame held in the data queue 212 according to a
command from the PON terminating unit 230. In the case of TDM data,
because of a transmission system by synchronous multiframe,
permissible conditions of transmission delay are severer than those
of Ethernet, and data received in a fixed cycle over a TDM line 202
is transmitted to the PON terminating unit 230 at the same rate as
it. A data transmission/reception cycle over the TDM line is 125
microseconds in the case of SDH, and also in the standardization
recommendations of G-PON, cycle transmission/reception control in a
PON section is advised to be performed in units of 125 microseconds
(see ITU-T Recommendation G.984.3 (2004), Gigabit-capable Passive
Optical Network (G-PON): Transmission convergence layer
specification).
[0073] The PON terminating unit 230 generates a downstream frame
transmitted to a PON section (NNI; Network Node Interface) from the
received data stored in the Ethernet frame processing unit 210 and
the TDM processing unit 220. Since the PON section follows
communication control using TDMA, frame generation is periodically
performed. In G-PON, downstream and upstream frames are transmitted
or received at a cycle of 125 microseconds. Hereinafter, the frames
are called basic cycle frames, and the frame cycle is called a
basic cycle. The data stored in the Ethernet frame processing unit
210 is reorganized to the basic cycle frame format. The basic cycle
frame is multiplexed with data directed to plural ONUs 2
(subscribers or subscriber terminals), and an identifier indicating
a destination ONU 2 is inserted along with the data. The format of
a basic cycle frame will be described later.
[0074] The PON terminating unit 230, in an upstream frame analyzing
unit 231, determines an SNI output port (Ethernet port) of a
transfer destination of a frame sent in an upstream direction.
Moreover, it extracts traffic reservation information included in
the upstream frame, that is, transmission-waiting data storage
information in an upstream frame transmission queue provided within
the ONU 2. The information is held in the memory 260 as band
request information 265. The band request is used for communication
band allocation from the OLT 1 to the ONU 2. Upstream data
transmission timing in the PON section follows data transmission
approval given to the ONU 2 by the OLT 1. The ONU 2 transmits a
specified amount of data in a specified timing according to a
transmission schedule set so that transmission data does not
overlap after multiplexing in a splitter. Thereby, the OLT 1 can
identify the sources of individual frames.
[0075] The memory 260 includes a delay computing unit 261 a time
adjusting unit 262. The delay computing unit 261 measures the
difference between frame transmission time from the OLT 1 and the
arrival time of an upstream frame from the ONU 2 responding to it
in the ranging process of measuring a transmission distance of a
PON section. The ranging process, which is performed for each of
ONUs, is used for startup processing when an ONU is newly
connected, and when starting an ONU again when communication
synchronization with the ONU is lost for some cause. Usually, in
the course of operation, the delay computing unit 261 manages the
arrival timing of upstream frames from the ONU by RTD (Round Trip
Delay) information and DBA (Dynamic Bandwidth Assignment) (see
ITU-T Recommendation G.984.3 (2004), Gigabit-capable Passive
Optical Network (G-PON): Transmission convergence layer
specification) that are obtained as a result of ranging. When it is
found that arrival time deviates more greatly than expected from
the result of initial ranging, the delay computing unit 261
commands the ONU to change response timing of the ONU for a
downstream frame, that is, a transmission timing command from the
OLT.
[0076] The response timing differs depending on the ONU. When all
ONUs respond to an identical transmission command from the OLT 1,
the delay computing unit 261 makes adjustment so that upstream
frames from all ONUs arrive in the OLT at the same time. That is,
response time is set for each ONU so as to reduce the distance
difference of the PON section (between the OLT and the ONU). After
thus grasping a distance (communication time) from each ONU, the
delay computing unit 261 conveys transmission start timing and
communication time from the OLT 1 individually to ONUs so that
upstream frames from the individual ONUs do not overlap in terms of
time in the course of operation. Response time adjusted here and
set in each ONU is called EqD (Equalization-Delay). To zero the
distance difference between ONUs, response timing from the farthest
ONU is adjusted as one reference. The EqD information is held in
EqD information 263 within the memory 260 along with an ONU
identifier. When EqD is readjusted in the course of operation or at
the time of communication failure, the EqD information 263 is also
rewritten as required.
[0077] An ONU control unit 266 performs ONU startup processing as
described above and manages communication statuses. It commands the
transmission of a downstream signal necessary for the ONU in each
process at the startup of the ONU. The downstream signal includes a
response request (ranging and serial number investigation) for
requesting a reply to the ONU, and a message for setting ID, EqD,
and the like for the ONU. Since it must the status of ONU (message
transmission/reception status and communication states) to manage
startup processes, it includes an ONU state management table
265.
[0078] The memory 260 includes a time information table 264. The
table is used to hold standard time information received via the
connection unit 570 from the GPS receiver 100. The time is used for
not only ONU control but also to correctly maintain OLT time by
being mapped with an operation clock within the OLT.
[0079] The time information processing unit 262 computes time for
notification to each ONU, based on time stored in the time
information table 264, that is, time set in the OLT 1 itself. A
distance from the OLT 1 depends on ONUs. This is because the
distance of a branch optical fiber depends on the installation
locations of ONUs. After referring to response time obtained as a
result of the above ranging and calculating the propagation time of
a signal from the OLT to the ONU, it conveys time to individual
ONUs. At this time, the OLT 1 determines a correction value of time
information, based on response time measured for each of ONUs. When
corrected time inserted in a downstream frame arrives in the ONU,
the time adjusting unit 262 makes corrections so as to indicate
correct time within the ONU.
[0080] Although EqD adjustment for DBA is sufficiently made with
knowledge of the difference of relative response delay times
between ONUs, and all response times until completion of signal
reciprocation observable in the OLT, some device is required to
increase accuracy for the case of setting time for each of ONUs.
The algorithm will be described later (see the description of FIG.
18).
[0081] Part of functions of the GPS receiving unit 100 may be
installed as an external device independently of the OLT 1.
[0082] FIG. 6 is a functional block diagram of the ONU 2 shown in
the system of FIG. 1. Here, a PON interface 300 within the ONU is
described. The ONU 2 includes an optical module 340 terminating an
optical fiber, a PON terminating unit 330, a memory 350, an
Ethernet line terminating unit 310 accommodating an Ethernet line
301. Like the case of OLT, a TDM line can include an interface
directly accommodated, or can be accommodated indirectly via L2SW.
Here, for simplicity of description, the type of an interface is
standardized to Ethernet.
[0083] The Ethernet line terminating unit 310 reads a signal
inputted via an Ethernet line 301 from a transmission/reception
buffer 360 of a UNI (User Network Interface) and transfers it to
the PON terminating unit 330. An Ethernet frame extracted by the
Ethernet line terminating unit 310 is stored in a data queue 352 of
the memory 350. The data queue 352 is managed by a queue control
unit 351, and is read out according to a command sent from an
upstream frame generating unit 332 of the PON terminating unit 330
to the memory 350. An Ethernet frame produced by performing
processing for transfer such as header conversion for a downstream
frame received in the optical module 340 is stored in a data queue
(transmission queue) for downstream Ethernet frames within the data
queue 352 of the memory 350. A downstream queue control unit within
the queue control unit 351, according to a read command from the
Ethernet line terminating unit 310, successively transfers frames
to the Ethernet line terminating unit 310 from the data queue.
[0084] In the ONU 2, the data queue 352 may be used in common
between the transmission/reception buffer 360 of the following
stage of the line terminating unit 310 and the NNI
transmission/reception buffer 370 of the preceding stage of the
optical module 340. PON-IF is a set of a series of functional
blocks formed on ASIC, and may adopt any configuration if the
above-described processing can be performed.
[0085] A downstream frame analyzing unit 331 of the PON terminating
unit 330 extracts information of a relevant frame, based on header
information and payload information (if necessary) of a downstream
PON section communication frame stored in the downstream frame
buffer 370 or 352. For example, for user data, header processing is
performed for transmission from a UNI interface, and for a control
frame of a PON section, the reception of the frame is conveyed to
firmware to extract its content for specified processing. Band
allocation information from the OLT 1 is stored in a band
allocation register 333, and for EqD setting information, extracted
EqD is stored in an EqD register 334. An ONU control unit 335
manages state transition at the startup of the ONU, and manages
failures within the ONU device. The ONU control unit 335 primarily
manages device states, and transmits and receives messages for PON
section control.
[0086] The downstream frame analyzing unit 331 extracts device
control information and time information sent from the OLT 1. Time
information is held in a time information management unit 337
included in the PON terminating unit 330. The time information
management unit 337 maps an operation clock within its own device
and the received time information. By managing changes in mutual
correspondences (clock phase), it maintains time information within
its own device. A time control unit 336 controls such time
information. The time information management unit 337 is referred
to by the upstream frame generating unit 332 when time information
of its own device is conveyed to the OLT 1 according to a command
from the OLT 1 (described later).
[0087] FIG. 7 is a sequence showing the relationship between
processing from the calculation 203 of transmission time to time
notification 205 in a basic sequence of time distribution shown in
FIG. 2, and ranging processing. Although only processes necessary
for time notification are described in FIG. 2, execution of these
processes requires parameters held in the OLT in the course of ONU
state management at and after ONU startup. Here, processing in the
stage of initial setting of time is described along the sequence of
FIG. 2. Feedback processing 207 of FIG. 2 will be described in
FIGS. 17 to 20.
[0088] The OLT 1, in the ONU startup sequence, assigns identifiers
to individual ONUs to control the ONUs (701). On normally receiving
the identifiers, an ONU proceeds to a distance measurement wait
state (ranging state 702). The distance measurement measures the
distance (distance of PON section) between the ONU and the OLT 1.
The OLT 1 issues a response request for distance measurement to the
ONU 2 in a ranging state (703). The time of return to the OLT 1 of
a reply (ranging transmission) transmitted from the ONU 2 in
response to the response request (ranging request) (704) is
recorded in the OLT 1 as a reciprocation communication time
(705).
[0089] In actual operation, RTD added with EqD is response time of
each ONU. To assign an upstream band to each ONU in the OLT 1, the
response time must be standardized among all ONUs. After totaling
RTDs, the OLT 1 calculates EqD to be assigned to each ONU so that
total delays are equal (706). EqD determined here is stored in a
database within the OLT 1 along with RTD, and at the same time is
conveyed to the ONU by a downstream communication from the OLT 1
(707). On receiving EqD, the ONU 2 registers it in an EqD
information database within its own memory (708). When EqD has been
normally registered, the ONU 2 enters a normal operation state and
starts data communications with the OLT 1 (709).
[0090] The above process is a standard startup sequence of the ONU
2. Actually, request 703, transmission 704, EqD notification 707,
and the like of the ranging processing are transmitted plural times
in terms of resistance to failure, and when messages have been
received plural times (twice or more in the case of GPON), it is
determined that the information has been correctly transmitted or
received. These operations and design details are not limited in
this embodiment. In this embodiment, RTD for each ONU obtained in
the ranging process is used during time distribution.
[0091] Time information is placed in a position superior to the OLT
1 or acquired from the GPS receiver 100 attached to the OLT 1. The
acquired information is conveyed to the OLT 1 (710). The OLT 1
compares the received time with a device time controlled by an
operation clock of its own device, and makes corrections if
necessary. After that, the OLT 1 calculates time to be conveyed to
each ONU under control of the device (711). This is described using
the ONU 2 as a representative example. When time arrives from the
OLT 1 to the ONU 2, the time must match standard time. That is, a
communication delay occurring during transmission in the PON
section is taken into account previously, time produced by
correcting time held in the OLT 1 by the delay is conveyed to
individual ONUs (712). Therefore, as many times as there are ONUs
must be managed. On receiving time information, the ONU 2 stores it
in the time information database within the device memory. Also in
this case, time notification 712 may be transmitted plural times in
terms of resistance to failure to determine that the information
has been correctly transmitted or received, from plural (twice or
more in the case of GPON) message receptions.
[0092] FIG. 8 shows a sequence showing the flow of the processing
of correcting time information, based on the result of phase
confirmation processing of an upstream signal in an operation
state. Here, for simplicity of description, the situation in which
time correction need not be performed is described. Processing for
accurately adjusting the setting time of the ONU 2 will be
described in FIGS. 17 to 20. A phase change in an upstream frame
described here is a method for coping with problems caused by the
outside of the system that are not intended in the management side
(OLT 1), such as expansion or contraction of optical fibers. On the
other hand, time adjustment described later is a procedure
necessary to increase time synchronization accuracy to a maximum
extent, using the mechanism of PON in a certain natural
requirement. These processes occur independently of each other,
depending on phase variation and ONU startup timing. EqD
information notification 707 from the OLT 1, EqD registration 708
in the ONU 2, and transition to operation state 709 are the same
processes as those in FIG. 7.
[0093] To perform upstream communications in an operation state,
the ONU 2 applies for transmission approval to the OLT 1 in a
downstream frame (band notification is not always made by the first
preceding frame because response speed in the OLT 1 changes
depending on the setting of DBA cycle) transmitted earlier than a
relevant frame. This application is made by conveying the amount of
data stored in a frame queue included in the ONU 2 for upstream
communications, for example, in the case of G-PON. The OLT 1
compares the amounts of transmission-waiting data sent from ONUs,
and determines an upstream band to be assigned to each ONU.
[0094] The ONU 2, according to a command from the OLT 1, transmits
an upstream frame so as not to overlap with upstream frames
transmitted by other ONUs. Data transmitted by each ONU constitutes
a frame formed of a header and payload. The OLT 1 refers to a
signal pattern called preamble and delimiter contained in the start
of a header of each frame to compensate for the frame. At this
time, since the OLT 1 is expected to receive a signal from an ONU
in timing specified previously by it, the signal may be checked
within a specific range including the expected timing.
[0095] For the frame synchronization processing, the OLT 1 checks
transmission timing from the ONU 2 (802). When the timing is close
to the expected timing, EqD need not be corrected. However, when
the timing is more than a given value off, as described previously,
an EqD value set in the ONU 2 must be corrected (803). Possible
causes for the change are expansion or contraction of optical
fibers due to temperature, measurement errors during ranging
processing, and some processing error occurring during setting.
When corrections have been made, correction values are conveyed to
the ONU 2 (804). For each checking, notification may be made.
However, to effectively use the band of the PON section, it is
desirable to avoid unnecessary communications.
[0096] The GPS receiver 100 continues to receive radio waves from a
satellite (or standard radio wave source), and continues to supply
time information to the OLT 1 at a fixed interval (806). This
interval, which depends on the implementation, need not always be
conveyed to the OLT 1 each time information is received, and may be
designed to be optimum with necessary accuracy. On receiving time,
the OLT 1 compares it with time of its own device, and make
corrections if a difference exists between the times. At the same
time, the OLT 1 refers to RTD information to calculate a correct
time to be conveyed to the ONU 2 (807). When communication timing
from the ONU 2 is not intended one as described above, since RTDs
are different, EqD must be corrected, and of course, correction
time for notification from the OLT 1 to the ONU 2 must also be
corrected. Correction time notification (808) and time information
storage within the ONU 2 (809) are the same as those in FIG. 7.
[0097] FIG. 9 is a flowchart showing the operation of the OLT 1 in
an initial stage of time setting to the ONU shown in FIGS. 7 and 8.
The flowchart shows processing from the acquisition of time
information from the GPS receiver 100 in FIG. 2 to time
notification 205 to the ONU 2 via the calculation of transmission
time 203.
[0098] On receiving a signal from an upper (SNI side, that is,
network 1000) device, the OLT 1 determines whether it is time
information (S101). To check the signal, ID indicating that time
information is included, the source address of a received frame, a
port number that received the frame that are included in the frame
(e.g., Ethernet frame) can be used. As ID, a dedicated
identification field may be defined within a header, or
identification can be made by inserting a specific value in an
existing field. As a specific field, for example, a Type field,
VLAN ID, and MPLS label information of an Ethernet frame can be
used.
[0099] When it is determined that the signal is time information,
the OLT 1 compares the received time information with time
information within the device (OLT 1), and determines whether the
device is operating with a correct time (S102). If time adjustment
is not necessary in determining whether the received time is normal
(S103), the processing proceeds to the next step. Otherwise, it
corrects the time information of its own device (S104), then
proceeds to the next step.
[0100] When the OLT 1 is operating at a correct time, it calculates
time to be conveyed to the ONU 2, based on time information of its
own device (S105). In this step, it corrects the time of its own
device by transmission time to an ONU 2 and time required for frame
processing and determines the corrected time for each of ONUs.
Therefore, for time required for communications with individual
ONUs, RTD information measured in the ranging processing is
referred to. Calculated time is put in a PON downstream frame for
notification to each ONU (S106, S107). Times (or time correction
amounts for time of the OLT 1 itself) to be conveyed to all ONUs
are held within the OLT 1, and conveyed to the individual ONUs one
after another (S108).
[0101] FIG. 10 is a flowchart showing the flow of processing until
the registration of time information by ONU 2 after the processing
in the OLT 1 of FIG. 9.
[0102] On receiving a signal from the OLT 1, the ONU 2 determines
whether it is time information (S201). For the determination of the
signal, ID indicating that time information is included, the source
address of a received frame (e.g., Ethernet frame), a port number
that received the frame, and the like that are included in the
frame can be used. As the ID, a dedicated identification field may
be defined within the header, or identification may be made by
inserting a specific value in an existing field. As specific
fields, for example, Type field, VLAN ID, and MPLS label
information of an Ethernet frame can be used.
[0103] When it is determined that the signal is time information,
the ONU 2 compares the received time information and time
information within the device (ONU 2), and determines whether the
device is operating with a correct time (S202). If time adjustment
is not necessary in determining whether the received time is normal
(S203), the processing terminates immediately. Otherwise, it
corrects the time information of its own device (S204). For other
than a frame of time information, the ONU 2 performs normal data
processing (205).
[0104] FIG. 11 shows an example of the structure of a table that
manages RTD information and EqD information calculated based on it
for each of ONUs that are acquired in the ranging processing. This
table includes an ONU identifier 1101, RTD 1102, and EqD 1103. If
necessary, it may include an option field 1104 for holding a flag
and other information. The table is stored in the EqD information
database of FIG. 5.
[0105] An ONU identifier is assigned by the OLT 1 in the startup
stage of each ONU. For each ONU, the arrival time of ranging
transmission sent for a ranging request sent from the OLT 1 is
measured, and recorded as RTD 1102. Since the RTDs vary depending
on the distance of an optical fiber to each ONU and processing
speed, EqD 1103 is calculated to standardize response time among
all ONUs, and stored in the table. The value of EqD 1103 is
represented as the amount of data (the number of bytes) transmitted
at a specific transmission speed as time for standardizing response
time. Another field 1104 can be used as an auxiliary field for ONU
state management and time management to indicate whether time
notification is completed for each of ONUs, whether a relevant
entry is valid, that is, the ONU can be used, and as elapsed time
(timer) from the transmission of time information. If necessary,
the other field 1104 may be divided into plural fields.
[0106] FIG. 12 shows an example of a table structure for time
management held in OLT. Time information is used to calculate a
relative error from the comparison between received time and time
within the device, and change the mapping relationship between time
and clock registered in the memory (register) to correct the error.
This processing, when consistency with highly accurate standard
time is required, is superior in efficiency in terms of rewriting
time information registered in the register at each reception of
time information to reading and writing a table held in firmware or
RAM. Therefore, this table may be placed in any form of firmware
and hardware. An object to present this table is to show the
principle of time adjustment, and its form need not be strictly
limited.
[0107] This table shows the correspondence between time 1201
recognized within the OLT and OLT internal clocks. To adjust time,
the boundary of a periodic signal synchronizing with a clock is
adjusted to match time information. At this time, if there is an
in-device periodic signal as reference (fixedly afforded), the
correspondence position 1202 of a periodic signal with the boundary
of a reference cycle counted as a reference point that is used for
time count is determined. As a reference periodic signal, instead
of a signal existing fixedly, a periodic clock control signal that
exists as the timing of time afforded previously at initial setting
may be used. When the latter method is used, the fields 1202 and
1204 that hold clock counts are not required as described
later.
[0108] On receiving standard time from the GPS receiver 100, the
OLT compares the received in-OLT 1 time 1201 with reception time
1203. If time is strictly controlled, time information received
from the GPS receiver 100 in this stage must also allow for time
from electrical processing after the reception of radio waves in
the GPS receiver 100 to the arrival of time in the OLT 1. This
point, which is a function in the GPS receiver, is excluded from a
description here.
[0109] The timing in which time 1203 was received is compared with
the boundary position of the above-described periodic signal
synchronizing with an in-device clock of the OLT 1, and a clock
number relative from the reference point is recorded. This is time
reception timing information 1204. Synchronous correction amount
1205 is the difference between the time reception timing 1204 and
in-device time timing 1202, and time information within the OLT 1
is corrected based on it.
[0110] As another method, each time information is received,
in-device time 1201 is compared with received time 1203 to find a
difference between them, and it is stored in an error field 1205.
At this time, an internal reference clock 1202 need not be held as
a field. Likewise, time reception timing 1204 need not be grasped
by a clock number. Although a calculated error is represented here
by the number of in-device clocks, it may be represented by time
(e.g., microsecond unit) (the field is replaced in this case), or
both of these representations may be held in different fields at
the same time, respectively. Anyway, a time error must be converted
into the difference between the numbers of clocks to correct time
within its own device, and these pieces of information are
necessary regardless of whether they are held on the table.
[0111] FIG. 13 shows an example of the structure of an ONU 2-based
time information management table held in the OLT 1. The table
includes ONU identifier 1301, time information 1302 calculated
based on RTD for each of ONUs, and another field 1303.
[0112] The time information 1302 represents a correction amount for
time within the OLT 1. When a frame is generated, time information
to be conveyed to each ONU is determined by totaling the
information and time information of the OLT 1. As another table
structure example, the time correction amount field 1302 can be
used as a field to store time itself to be conveyed. At this time,
for each ONU, notification time directed to each ONU that is
calculated based on RTD and time information of the OLT 1 is stored
in the table, and read when a frame is generated.
[0113] The another field is used as a field to store information
indicating whether an ONU (entry) is valid, information indicating
whether a frame for conveying time information has already been
transmitted, or elapsed time (timer) from the previous notification
of time information. These fields may exist plurally, in which case
the number of fields within the table may be added.
[0114] FIG. 14 shows an example of a table structure for time
management held in ONU. Time information is used to calculate a
relative error from the comparison between received time and time
within the device, and change the mapping relationship between time
and clock registered in the memory (register) to correct the error.
This processing, when consistency with highly accurate standard
time is required, is superior in efficiency in terms of rewriting
time information registered in the register at each reception of
time information to reading and writing a table held in firmware or
RAM. Therefore, this table may be placed in any form of firmware
and hardware. An object to present this table is to show the
principle of time adjustment, and its form need not be strictly
limited. Since the table structure, and the meanings and use method
of individual fields are the same as those of FIG. 12, a
description of them is omitted.
[0115] FIG. 15 shows the relationship between clock and time
information within the device (OLT) having been described so far
and time information supplied from the GPS receiver.
[0116] Normal communication devices have a specific operation clock
1501. Time information held in the OLT 1 in this state has
boundaries mapped periodically in units of, for example, minute and
second (or finer units of millisecond and the like). The boundaries
are boundary positions 1511 to 1513 of an in-device time 1502. On
the other hand, assume that time information 1503 received from the
GSP receiver 100 has time separations in positions indicated by
1521 to 1523 of the drawing. Then, the OLT 1, to correct time,
shifts the boundary positions 1511 to 1513 held in its own device
so as to align them with 1521 to 1523. By corrections 1531 to 1533
of time boundary positions, time within the device can be matched
to the standard time.
[0117] When there is a periodic signal fixedly afforded within the
device, it can be used as an index for managing the mapping between
in-device clock and time information as a reference periodic signal
1504. At this time, the boundary position of the in-device clock is
represented as a difference 1510 from the boundary position, and
for a reception clock, reception timing is represented as a
difference 1520 from the reference clock. Clock number count fields
1202, 1204, 1402, and 1404 of FIGS. 12 and 14 are managed with the
difference clock count. At this time, time corrections 1531 to 1533
are obtained as the difference between 1520 and 1510. By adding the
difference 1533 to 1510, the OLT 1 adjusts a time cycle within the
device to a correct position 1520 (six clocks from the reference
boundary in the example).
[0118] FIGS. 12 and 14 show tables for achieving the latter method.
By using this method, if a reference clock is known, for example,
at startup, rough time information can be determined. Therefore,
this method is effective to hold time information. With the latter,
since some timing must be initially afforded as the boundary
position of a clock signal at the startup of the device, an initial
error become large. However, in comparison with the tables of FIGS.
12 and 14, there are expected effects that there are fewer fields,
and that a calculation procedure can be simplified because a
reference clock does not intervene.
[0119] FIG. 16 is a schematic diagram showing time required for
reciprocation of a PON section. Points called phase specification
points are provided in signal processing units of the OLT 1 and the
ONU 2 (1612 of the OLT 1 and 1622 of the ONU 2), respectively. The
points are a measurement start point and a delay detection point in
the OLT 1, and a signal loopback point (ONU phase determination
position) in the ONU 2. A frame transmitted from the OLT 1 causes
TiS1 delay until being created as an optical signal in the OLT 1
(point S in the OLT 1). This is time required to convert an
electric signal into an optical signal. Next, the signal arrives in
a signal processing unit 1622 of the ONU 2 via time Tpd required to
pass through an optical fiber, further time TiO1 required for
conversion into an electric signal within the ONU 2. Likewise,
communication from the ONU 2 to the OLT 1 also requires E/O
conversion time, optical fiber transmission time, and O/E
conversion time. RTD measured in ranging includes signal processing
time Ts within the ONU 2 in addition to these values. The result of
totaling these is compared with other ONUs, and EqD is determined
so that total RTDs added with EqD are equal among all ONUs.
[0120] Since time required for generation and transmission of a
downstream frame in the OLT 1 and time (clock number) required for
the reception and analysis of an upstream frame can be grasped
depending on the design of a signal processing unit of the OLT 1,
they need not be considered here. Processing time Ts in the ONU 2
is the summation of times required for all from frame analysis to
frame generation and transmission.
[0121] As time conveyed from the OLT 1 to the ONU 2, for example,
time in the portion of acquiring a frame by the ONU 2, that is, at
the stage of the completion of O/E conversion through an optical
fiber after transmission from the OLT 1 is conceivable. By
conveying this time, since a clock number of the signal processing
unit can be grasped within the ONU 2, reception time can be
determined from reception time and a clock number of processing.
Therefore, external time and time within its own device can be
correctly compared.
[0122] However, the OLT 1 can actually know only RTD as a
parameter. Even though a communication distance to the ONU 2 can be
approximated at a half value of RTD, it is difficult to afford
correct time at the point of entry to the signal processing unit of
the ONU 2. Since the recommendation (G.984.3) stipulates that frame
processing time within an ONU is 35 microseconds (accuracy of one
microsecond) (ITU-T Recommendation G.984.3 Amendment 1 (2005),
Gigabit-capable Passive Optical Network (G-PON): Transmission
convergence layer specification), an error of initially set time
from the standard time can be suppressed by employing the half of a
value produced by subtracting 35 microseconds from RTD. However,
this method also inevitably produces some errors. This is solved by
employing the following method.
[0123] The ONU 2 conveys time information held in the ONU 2 to the
OLT 1. The OLT 1 grasps correct time through the GPS receiver.
Since an upstream frame from the ONU 2 follows an upstream band
(transmission timing) command from the OLT 1, the OLT 1 can expect
the arrival timing and time of an upstream frame to be sent from
the ONU 2.
[0124] Accordingly, time set in the ONU 2 can be adjusted using the
ranging information (RTD and EqD). When the ONU 2 is commanded to
report time information of the ONU 2 to the OLT 1, the ONU 2
captures device time at the generation of a frame and transmits an
upstream frame. The OLT 1 can adjust internal time of the ONU 2
with accuracy (in the case of GPON, ranging accuracy is one bit,
that is, about 1 ns in the case of 1.25 Gbps) of the same level as
ranging accuracy by comparing upstream frame reception time (the
time matches an expected upstream arrival time provided that EqD is
correct) from the ONU 2 with time inserted in the frame by the ONU
2, recognizing the difference, and correcting a correction value
1302 of notification time for notification to the ONU 2.
[0125] Times may be compared at any stage if time expected by the
OLT 1 and time when the ONU 2 inserts time may match. Although
several methods are conceivable to do this, one example is
described here.
[0126] A correction value of time information calculated for each
ONU is inserted in a downstream frame including a time report
command from the OLT 1.
[0127] The ONU 2 records time when a downstream frame of the OLT 1
arrives. The time can be calculated, when it is determined that
time is requested after the analysis of the received downstream
frame, from a clock number required for the processing and time at
that point. Moreover, signal processing time within the ONU 2 is
calculated from a clock number required for signal processing after
the generation of an upstream frame until the frame can be
transmitted. On receiving a time report request from the OLT 1, the
ONU 2 calculates (time added with signal processing time to the
arrival time of the downstream frame) transmission time of an
upstream frame for time notification, based on time within its own
device, and from the result, adds a correction value included in
the downstream to obtain expected time when the relevant frame
arrives in the OLT 1. The time is inserted in the frame, which is
transmitted as an upstream frame.
[0128] Comparison and verification of time is performed in the OLT
that grasps the standard time. The OLT 1 commands the ONU 2 to
include expected time of the arrival of an upstream frame in the
OLT 1 within the upstream frame. For replay (upstream frame) from
the ONU 2, arrival time is compared with "expected arrival time"
within the frame. When the times are different, the cause of the
difference lies in a correction value of time for individual ONUs.
For example, when the half of RTD is used as an initial correction
value, actual arrival time of a downstream frame for time
notification in the ONU 2 will be a little earlier than it. This is
because signal processing time within the ONU 2 is included in the
initial correction value. Accordingly, when the arrival time
expected in the ONU 2 of the upstream frame lags behind the
standard time, it indicates that a time correction value held in
the OLT 1 is too large and time set in the ONU 2 is delayed.
Conversely, when time expected in the ONU 2 is ahead of the actual
standard time, the time of the ONU 2 is ahead. Accordingly, the
difference between the arrival times is calculated, and by
reflecting the difference in the correction value, the internal
time of the ONU 2 can be approximated to a correct time.
[0129] This embodiment assumes that signal processing delay (clock
number) within the ONU 2 is fixed, and transmission delay Tpd of
the optical fiber is equal in upstream and downstream directions.
Thereby, it assumes that a time notification frame is free of
transmission delay due to firmware processing and transmission
queuing within the ONU 2. This can be achieved by realizing
transmission/reception of time information by hardware, or always
putting the time information in the start of a transmission wait
queue.
[0130] Also when EqD is included in processing delay within the ONU
2, likewise, the ONU grasps processing time (clock number)
including EqD, calculates an expected arrival time in the OLT 1,
and uses the half of a difference from the standard time as an
(initial) correction value. Furthermore, usually, as a band
allocated by DBA, transmission start time is defined. Ultimately,
the ONU 2 must internally include, as processing time of an
upstream frame for reply, transmission start time following
necessary processing time (clocks) based on its own circuit design,
EqD, and DBA.
[0131] As a correction method, a method of making corrections
little by little (by one to several bits) until times match is
conceivable. Although the latter is resistant to measurement error
or the like, much time is required until completion of time
adjustment. At least during initial setting, determining directly a
correction amount from a correction value is more efficient.
[0132] The above content is described using a drawing. FIG. 17
shows a processing sequence in basic time adjustment stage of this
embodiment. The OLT 1 command the ONU 2 to report time within the
ONU 2 by a time information request 1701. In the downstream frame,
the period during which the relevant ONU 2 is to start and continue
transmission is indicated by DBA. At the same time, the frame
includes a correction value 1302 concerning the relevant ONU 2 held
by the OLT 1.
[0133] On receiving the command, the ONU 2 transmits an upstream
frame containing time information. Time inserted here is time when
the upstream frame 1702 is expected to arrive in the OLT 1.
[0134] The OLT 1 compares arrival time of the upstream frame 1702
with expected arrival time stored in the upstream frame 1702
(1703). As a result, when the times do not match, it corrects the
time correction value 1302 and reregisters the corrected value. For
match, no special action is necessary (1704).
[0135] The OLT 1 transmits time information directed to the ONU 2
calculated with a new correction value by a downstream frame
(1705). The ONU 2 registers received time in a time information
database in its own memory (1706).
[0136] FIG. 18 is a sequence diagram showing time elapse during
time adjustment described in FIGS. 16 and 17. The OLT 1 notifies
the ONU 2 of time matching the timing in which the ONU 2 receives
downstream frames containing time information 707, 712 (FIG. 7),
804, and 808 (FIG. 9).
[0137] On receiving a time request 1701 from the OLT 1, the ONU 2,
as a reply to it, calculates time 1804 when an upstream frame from
the ONU 2 arrives in the OLT 1, and puts it in the upstream frame
for transmission. The ONU 2 can grasp arrival time 1802 of the time
request frame 1701 from set in its own device. Since signal
processing delay Ts can be obtained based on device specifications,
time 1802 and the transmission time of a upstream frame 1702 as
reply to it can also be grasped.
[0138] The time request 1701 received by the ONU 2 includes a time
correction value 1302 used by the OLT 1 to set time in a relevant
ONU 2, and in this embodiment, the value is the sum of E/O
conversion delay TiS1, transmission delay Tpd, and O/E conversion
delay TiO1. On the assumption that these signal conversion delays
and transmission delay over the optical fiber are equal in upstream
and downstream directions, a correction value conveyed by the OLT 1
can be considered as time required for the transmission of an
upstream frame, that is, delay time from times from 1803 to 1804.
Therefore, the ONU 2 stores a value produced by adding the
correction value 1302 to the transmission time 1803 of the upstream
frame 1702 calculated above in the frame as expected arrival time
for transmission to the OLT 1.
[0139] The OLT 1 compares the reception time 1804 recorded with the
standard time and expected arrival time included in the upstream
frame 1702, and determines whether correction is necessary. If
necessary, it again conveys a correction value to the ONU 2
according to FIG. 17.
[0140] FIG. 19 is a flowchart showing the flow of processing within
the ONU 2 during time notification from the ONU 2 to the OLT 1.
[0141] On receiving a downstream frame, the ONU 2 determines
whether the frame is directed to its own device (S301). This
determination is made based on frame header information used for
PON section transmission. For example, for GPON, Port-ID needs to
be referred to. When not directed to its own device, the processing
terminates immediately. When directed to its own device, the ONU 2
records time when the frame is received (S302). Then, it determines
whether the frame is a time request (S303). If not so, the ONU 2
processes it as a normal frame and terminates (S304). In the case
of a time request, it calculates transmission time an upstream
frame for reply (S305). In this processing, as described
previously, expected time of frame transmission is obtained based
on frame processing time (clock number). The ONU 2 stores a value
produced by adding a correction value within the received time
request frame to the obtained expected transmission time in an
upstream frame as expected arrival time (S306), transmits it in
timing specified by the OLT 1 (transmission time calculated in
S305) (S307), and completes a series of processing.
[0142] FIG. 20 is a flowchart for time adjustment processing within
the OLT 1. On receiving an upstream frame, the OLT 1 records the
reception time of the frame (S401). It determines whether the frame
is time notification for a time report request 1701 of the OLT 1
(S402). When it is time information, the OLT 1 extracts expected
arrival time stored in the frame (S403), and compares it with the
time recorded in Step 401, that is, time when the frame was
actually received (S404). If the times match, the processing
terminates. In the determination here, the times are considered to
match if the comparison result is within permissible values
determined from required time accuracy. If they do not match, the
OLT 1 finds the difference between the times (S405), and rewrites
the time correction value 1302 having been held for the ONU (S406).
Finally, the OLT 1 puts standard time corrected by the modified
correction value in a downstream frame for notification to the ONU
2 (S407), and completes the processing.
[0143] FIGS. 19 and 20 show examples of recording the reception
time of a received frame. It is first determined whether a relevant
frame is a frame related to time information, then if necessary,
reception time and expected transmission time in the case of an
upstream frame may be calculated based on in-device processing time
(clock number). Since S404 and S405 that determine whether times
match perform the same processing in terms of finding the
difference between the times, they may not be separated.
[0144] FIG. 21 shows an example of the structure of a time
notification frame from the OLT 1 to the ONU 2. An object to define
the frame is to issue a time request individually to each ONU.
Therefore, it has only to include an ONU identifier 4131 and a flag
4132 indicating a time report request. In the case of GPON, this
can be achieved by defining a new message by using, for example, a
PLOAM (Physical Layer OAM) (ITU-T Recommendation G.984.3 (2004),
Gigabit-capable Passive Optical Network (G-PON): Transmission
convergence layer specification) message frame. Specifically, for
example, by setting the first bit of a message ID field of PLOAM to
1, this message can be distinguished from existing messages.
Therefore, a message ID of 10000001 may be allocated. Thereby, this
embodiment can be realized without greatly changing an existing
message system.
[0145] FIG. 22 shows an example of the structure of an upstream
frame for transferring time information from the ONU 2 to the OLT
1. Here, an example of using the PLOAM field to transmit time
information is shown, corresponding to FIGS. 23A and 23B.
[0146] In upstream communications, frames issued from plural ONUs
exist in 125 microseconds. The header 2210 of an upstream header
includes PLOu (Physical Layer Overhead Upstream) 2211, PLOAMu 2212,
PLSU (Power leveling Sequence Upstream) 2213, and DBRu (Dynamic
Bandwidth Report Upstream) 2214. PLOAMu, which corresponds to
PLOAMd of a downstream frame, performs control for the operation of
the ONU 2. Information stored in the PLOu 2211 includes preamble
for frame synchronization, a signal pattern for delimiter, and
ONU-ID identifying a source ONU 2. PLSu 2213 is used to monitor the
transmission power of the ONU 2 and determine whether adjustment is
required. DBRu 2214 is used when the ONU 2 notifies the OLT 1 of an
upstream transmission request.
[0147] PLOAMu 2212 includes ONU-ID 22121, message identifier MSG-ID
22122, message body 22123, and CRC 22124 for error detection and
correction. Since time information is conveyed during notification
in the form of response to a downstream shown in FIG. 21, the
message identifier MSG-ID 22122 can use 10000010, for example, as a
vender-specific message. Time (expected value) when the upstream
frame arrives in the OLT 1 is inserted in the message field. The
OLT 1 can identify a source ONU 2 and time 22123 by the format, and
can compare the reception time of the frame with respect to
standard time with these for time adjustment.
[0148] FIGS. 23A and 23B are conceptual diagrams showing the
operation of time distribution. Reference numerals in the drawing
are the same as those having been used in the above descriptions.
FIG. 23A shows the state of initial time setting at the startup of
the ONU 2. The OLT 1 receives standard time from the GPS receiver
100 connected to the SNI side, and sets the standard time 2110 in
its own device. It determines time (correction value 1302) conveyed
to ONU from the RTD 1102 database (see FIG. 11) obtained from
ranging processing for each ONU and in-device standard time 2110.
In this case, since the correction value 1302 is calculated based
on RTD, accuracy may somewhat decrease.
[0149] FIG. 23B shows the state of correcting time set in the ONU 2
by using ranging information (correctly, a model for ranging has a
meaning. See FIG. 16). On receiving a time information transmission
request 1701 from the OLT 1, the ONU 2 calculates expected arrival
time 2120 of an upstream 1702 in the OLT 1 based on time setting
within the ONU 2 in timing specified in the frame 1701, and
transmits the upstream frame 1702. The OLT checks upstream frame
arrival time 2140 from the ONU 2, based on the in-device standard
time 2110. The OLT 1 calculates a correction value 2-Ir (2130) from
the expected arrival time 2120 within the upstream frame and
arrival time 2140 by the method described previously, and based on
it, determines again time 2-Ir (2121) conveyed to the ONU 2 (2121).
The OLT 1 re-conveys the time to the ONU 2 (1705), and terminates
adjustment.
[0150] FIG. 24 is a time chart showing changes in time setting
situation within the ONU 2. ONU time 2402 immediately after startup
generally lags (2450) behind OLT time 2401 by a lag 2450. For the
cycle boundaries 2411 to 2413 of OLT time at minute or second
intervals, times of the ONU 2 are, for example, in positions
indicated by 2421 and 2422 (in FIG. 24, two ONUs are shown as 2421a
and 2421b). When the OLT 1 sets time based on RTD measurement
values during ranging, the half of RTD is most simply set as a
reference value, as described previously. In that case, however,
since internal processing time of the ONU 2 cannot be correctly
measured, there are errors 2460 between modified times ONU times
2431 to 2433 and the OLT times 2411 to 2413. Accordingly, next, the
OLT 1 commands the ONU 2 to convey time information, compares the
arrival time of an upstream frame to the OLT 1 with the internal
time of the ONU 2 recorded in the frame, and further modifies an
error. If necessary, this processing is repeated to match ONU times
2441 to 2443 to OLT times 2411 to 2413.
[0151] FIG. 24 shows an example of time adjustment when two ONUs
exist. Assume that setting time at startup stage of ONU(b) deviates
reversely from that of ONU(a) with respect to OLT time(2421b,
2422b). Next, when an time initial setting sequence is observed
between the OLT and the OLU, it is appreciated that ONU(b) is
closer to the standard time than ONU(a). This is an error occurring
due to a relative ratio between the transmission delay of an
optical fiber and processing delay time within the ONU 2. For
example, when ONU(b) is larger in optical fiber delay (more
distant) than ONU(a), the state in FIG. 24 could occur. Finally,
through time readjustment based on feedback from the ONU 2, both
ONU(a) and ONU(b) are set to standard time.
[0152] Time of each ONU is calculated from a time correction value
obtained by the method having been described so far. After the
processing has been performed within the OLT as in the above
embodiment, correct time is conveyed to ONU. As another method, a
correction value itself is conveyed in advance to ONUs, and the OLT
spreads standard time to all ONUs. In this case, the ONU uses the
correction value to calculate time for its own device. The standard
time is afforded from the OLT as in the above-described embodiment.
That is, the OLT may afford it if necessary when distance
modification is necessary, while monitoring communication timing
from ONU, or may afford it continuously at a certain cycle. Time
adjustment may be started by issuing a time checking request from
the ONU. When standard time is conveyed periodically from the OLT,
the OLT stores standard time in a cycle (every 125 microseconds)
boundary of a downstream in a frame for notification, while the ONU
applies a correction value afforded previously, based on time when
the frame boundary is received.
[0153] FIG. 25 shows the effects of this embodiment when a
distribute database and a distributed application are used based on
the system diagram of FIG. 1. FIG. 25 shows an example of the
disposition of two types of distributed databases. One is the case
where OLT is the same and ONUs are different. Databases 70-1-2 and
70-1-3 correspond to this. The other is the case where OLTs are
different. For example, the relationship between databases 70-1-2
and 70-n correspond to it.
[0154] A distributed database is one designed so that databases
placed distributedly to plural servers can be handled in each site
or server as if they were a single database. Objects to introduce a
distributed database are various such as an expansion of database
capacity, on-demanding of information utilization, an increase in
response time, and the attainment of fault tolerance.
[0155] An expansion of database capacity requires exponential
increase in costs and is limited in terms of CPU processing
capacity. A problem of processing performance has become a driving
force of distribution architecture development, and a distributed
system has been created which includes inexpensive and advanced
small and middle-sized computer systems having easy-to-use network
functions. Users must analyze various data according to their
objects and quickly make decisions. That is, an environment has
been desired which allows the users to have at hand and process and
analyze by themselves data in which they are most interested. A
tendency toward authority transfer of information management based
on reexamination of the roles and responsibility sharing of an
enterprise organization based on the idea of reengineering is also
becoming a driving force to promote distribution. Communication
costs and response speed are problematic. If there is a local
database in a remote place and information frequently accessed is
placed there, response is improved and communication costs can be
reduced. In a system with overconcentration of access, if it is
stopped, business operations may be stopped. To avoid such a
situation, it is necessary to distribute data against disasters and
hold duplication of data in different locations.
[0156] What is important in a distributed database as well as data
management and countermeasures against faults is the holding of
consistency. In a distributed environment, since, in some cases,
one record is referred to in plural sites, data cannot be updated
at sole discretion, and updating must be performed with
coordination among the sites. Since data is placed distributedly to
plural sites, in relation operations (join, etc.) of a relational
database, to which site data is sent and in which site data
processing is performed exert a great influence on performance,
depending on the amount of data transferred. Therefore, a
distributed database system need be equipped with a mechanism of
optimization for this. Although communication traffic is referred
to as costs in optimization, there are also an approach of
minimizing overall costs or the policy to minimize response time.
Respective companies attempt to increase the performance of
products by putting thought into algorithms such as the use of
statistical information about the distribution of data. In such
information management, time synchronization among databases or
sites is indispensable.
[0157] FIG. 25 shows the state in which a user works in site
50-1-1, and collects information from databases distributed to
plural sites 50-1-2, 50-1-3, and 50-n. In a Web site and the like,
a system that allows users to view collectively information from
plural sites by using a cache server is established. In this case,
somewhat static (there is not change in information, or update
interval is relatively long) information is targeted. In a
distributed database, in a situation in which information changing
as required is distributed to plural databases and shared among
plural users, for example, as in the sharing of product development
information, there is a demand to reflect the situation in more
real-time. In the drawing, information A from a database 70-1-3,
information B from a database 70-1-2, and information C from a
database 70-n are collected to be one piece of information for
users to use. Since time of all devices is standardized, users can
handle the information as if it existed in their own terminals.
[0158] Time synchronization is necessary not only for the sharing
of information but also for the case where servers placed in
different sites use applications distributedly placed to operate in
coordination with each other. Specifically, by plural applications
transferring mutual processing results parallelly in coordination
to execute a given task, information necessary for users can be
created, or user-required working environments can be offered as
required with users' situations in mind.
[0159] Sensor networks require more accurate time synchronization
than distributed databases to perform time synchronization between
sensors. One possible method is, as shown in FIG. 25, to integrate
sensor information to understand targeted events. Sensors may be
provided with some function (issuing a signal, switching conditions
(modes) of information collection, etc.) to collect information
more effectively by communications between the sensors under some
conditions.
[0160] This embodiment relates to time synchronization (time
distribution) between terminals (ONU) and transmission devices
(OLT), and it does not matter whether optical signals or electric
signals are used as transmission media in the physical layer of the
OSI reference model. Moreover, the type of transmission protocols
need not be specially limited. Here, however, as a preferred
application example, a time distribution system and a device that
use ITU-T GPON have been described.
[0161] The GPS receiver 100 in this embodiment may include a
standard time generating device having an oscillator. However,
since time synchronization processing is performed for each of
optical access lines, in terms of device development, installation,
and maintenance costs, it is more effective to use a system that
receives standard time distributed by GPS and conveys time
information to a PON system, based on the information. From the
standpoint of accuracy, the use of conventional standard radio
waves capable of time synchronization with an accuracy of several
milliseconds is conceivable for use for traditional services.
[0162] When the ONU 2 conveys time to a subordinate terminal (208
of FIG. 2), the ONU may passively afford information for a request
from the terminal 60 as if it were one NTP server, or it may
continuously distribute time information by time transmission of
push type.
* * * * *