U.S. patent application number 11/270645 was filed with the patent office on 2006-03-23 for dynamic bandwidth assignment system and dynamic bandwidth assignment method capable of reducing cell transmission delay.
This patent application is currently assigned to MITSUBISHI DENKI KABUSHIKI KAISHA. Invention is credited to Minoru Akita, Yoshihiro Asashiba, Seiji Kozaki, Takamasa Suzuki, Tetsuya Yokotani.
Application Number | 20060062169 11/270645 |
Document ID | / |
Family ID | 18682926 |
Filed Date | 2006-03-23 |
United States Patent
Application |
20060062169 |
Kind Code |
A1 |
Suzuki; Takamasa ; et
al. |
March 23, 2006 |
Dynamic bandwidth assignment system and dynamic bandwidth
assignment method capable of reducing cell transmission delay
Abstract
A dynamic bandwidth assignment system includes a network unit
for carrying out cell slot assignment, and a network termination
for transmitting cells to the network unit by means of cell slots
assigned by the network unit. When the number of consecutive valid
cells the network unit receives from the network terminations
exceeds a predetermined threshold value, the network unit increases
the number of the cell slots to be assigned to the network
termination. The system can solve a problem of a conventional
system in that it is unavoidable that a cell transfer delay and a
bursting tendency of cells are statistically increased.
Inventors: |
Suzuki; Takamasa; (Tokyo,
JP) ; Akita; Minoru; (Tokyo, JP) ; Asashiba;
Yoshihiro; (Tokyo, JP) ; Kozaki; Seiji;
(Tokyo, JP) ; Yokotani; Tetsuya; (Tokyo,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
MITSUBISHI DENKI KABUSHIKI
KAISHA
Tokyo
JP
|
Family ID: |
18682926 |
Appl. No.: |
11/270645 |
Filed: |
November 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09880115 |
Jun 14, 2001 |
|
|
|
11270645 |
Nov 10, 2005 |
|
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Current U.S.
Class: |
370/314 |
Current CPC
Class: |
H04Q 2011/0064 20130101;
H04Q 11/0067 20130101; H04Q 2011/0086 20130101; H04L 2012/5649
20130101; H04L 2012/5632 20130101; H04Q 11/0066 20130101; H04Q
11/0478 20130101 |
Class at
Publication: |
370/314 |
International
Class: |
H04Q 7/00 20060101
H04Q007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 16, 2000 |
JP |
2000-182090 |
Claims
1-14. (canceled)
15. A dynamic bandwidth assignment system including a network unit
configured to control cell slot assignment, and a network
termination unit configured to transmit cells to the network unit
using cell slots assigned by the network unit, the transmitted
cells including valid cells having a first cell header and idle
cells having a second cell header, said network unit comprising: a
detection unit configured to detect valid cells and idle cells
received from said network termination unit based on cell headers
in the received cells; a decision unit configured to output a
decision result on a cell slot assignment to the network
termination unit based on the valid cells and idle cells detected
by said detection unit; and a cell slot assignment unit configured
to control the cell slot assignment to the network termination unit
in response to the decision result of said decision unit.
16. The dynamic bandwidth assignment system according to claim 15,
wherein said decision unit is configured to supply the decision
result to said cell slot assignment unit when a number of
consecutive valid cells said network unit receives from said
network termination unit exceeds a first predetermined threshold
value, and wherein said cell slot assignment unit is configured to
increase the number of the cell slots to be assigned to said
network termination unit in response to the decision result.
17. The dynamic bandwidth assignment system according to claim 15,
wherein said decision unit is configured to supply the decision
result to said cell slot assignment unit when a number of valid
cells said network unit receives from said network termination unit
in a decision period becomes less than a second predetermined
threshold value, and wherein said cell slot assignment unit is
configured to reduce a number of the cell slots to be assigned to
said network termination unit in response to the decision
result.
18. The dynamic bandwidth assignment system according to claim 15,
wherein said decision unit is configured to supply the decision
result to said cell slot assignment unit when a number of valid
cells said network unit receives from said network termination unit
in a decision period exceeds a first predetermined threshold value,
and wherein said cell slot assignment unit is configured to
increase a number of the cell slots to be assigned to said network
termination unit in response to the decision result.
19. The dynamic bandwidth assignment system according to claim 16,
wherein said decision unit is configured to determine the first
threshold value in accordance with the total number of cells said
network unit receives from said network termination unit in a
decision period.
20. The dynamic bandwidth assignment system according to claim 17,
wherein said decision unit is configured to determine the second
threshold value in accordance with the total number of cells said
network unit receives from said network termination unit in the
decision period.
21. The dynamic bandwidth assignment system according to claim 18,
wherein said decision unit is configured to determine the first
threshold value in accordance with the total number of cells said
network unit receives from said network termination unit in the
decision period.
22. A dynamic bandwidth assignment method in a network unit
comprising: receiving, from a network termination, cells including
valid cells having a first cell header and idle cells having a
second cell header; detecting valid cells and idle cells based on
cell headers in the received cells; producing a decision result on
a cell slot assignment to the network termination unit based on the
detected valid cells and idle cells; and controlling cell slot
assignment to the network termination unit in response to the
decision result.
23. The dynamic bandwidth assignment method according to claim 22,
wherein when a number of consecutive valid cells said network unit
receives from said network termination unit exceeds a first
predetermined threshold value, the step of controlling cell slot
assignment increases the number of the cell slots to be assigned to
said network termination unit in response to the decision
result.
24. The dynamic bandwidth assignment method according to claim 22,
wherein when a number of the valid cells said network unit receives
from said network termination unit becomes less than a second
predetermined threshold value, the step of controlling cell slot
assignment decreases the number of the cell slots to be assigned to
said network termination unit in response to the decision
result.
25. The dynamic bandwidth assignment method according to claim 22,
wherein when a number of the valid cells said network unit receives
from said network termination unit exceeds a first predetermined
threshold value, the step of controlling cell slot assignment
increases the number of the cell slots to be assigned to said
network termination unit in response to the decision result.
26. The dynamic bandwidth assignment method according to claim 23,
wherein the first threshold value is determined in accordance with
the total number of cells said network unit receives from said
network termination unit in the decision period.
27. The dynamic bandwidth assignment method according to claim 24,
wherein the second threshold value is determined in accordance with
the total number of cells said network unit receives from said
network termination unit in the decision period.
28. The dynamic bandwidth assignment method according to claim 25,
wherein the first threshold value is determined in accordance with
the total number of cells said network unit receives from said
network termination unit in the decision period.
29. A dynamic bandwidth assignment system including a network unit
configured to control cell slot assignment, and a network
termination unit configured to transmit cells to the network unit
via an Asynchronous Transfer Mode Passive Optical Network using
cell slots assigned by the network unit, said network unit
comprising: a detection unit configured to detect valid cells and
idle cells said network unit receives from said network termination
unit; a decision unit configured to output a decision result on a
cell slot assignment to the network termination unit based on the
valid cells and idle cells detected by said detection unit; and a
cell slot assignment unit configured to control the cell slot
assignment to the network termination unit based on the decision
result of said decision unit.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an FSAN (Full Service
Access Network) and an ATM-PON (Asynchronous Transfer Mode-Passive
Optical Network) which is under international standardization by
ITU-T (International Telecommunication Union-Telecommunication
Standardization Sector), and particularly to a dynamic bandwidth
assignment system and a dynamic bandwidth assignment method for
carrying out dynamic bandwidth assignment of the uplink band from a
network termination to a network unit in these networks.
[0003] 2. Description of Related Art
[0004] FIGS. 8A and 8B are block diagrams showing a configuration
of a conventional dynamic bandwidth assignment system disclosed in
Japanese patent application laid-open No. 7-135502, for example. In
these figures, reference numerals 12a, 12b and 12c each designate
user equipment; 14a, 14b and 14c each designate a transmission
line; 15 designates a distributor; 16 designates a transmission
line; and 17 designates a network unit. The user equipment 12a, 12b
and 12c is connected to the distributor 15 via the transmission
lines 14a, 14b, 14c, and the distributor 15 is connected to the
network unit 17 via the transmission line 16. FIG. 8A shows the
flow of downlink signals from the network unit 17 to the user
equipment 12a, 12b and 12c, whereas FIG. 8B shows the flow of
uplink signals from the user equipment 12a, 12b and 12c to the
network unit 17.
[0005] Next, the operation of the conventional system will be
described.
[0006] In FIG. 8A, the network unit 17 transmits a frame identifier
F, followed by supplying the transmission line 16 with information
a to the user equipment 12a, information b to the user equipment
12b, and information to the user equipment 12c. The information a,
band c consists of information items the network unit 17 receives
from a communication network. The network unit 17 provides the
information items a, band c with user equipment information about
the destinations.
[0007] The distributor 15 supplies all the transmission lines 14a,
14b and 14c with the same signal. The user equipment 12a, 12b and
12c each receives the signal from the transmission lines 14a, 14b
and 14c, and extracts the information addressed to itself. The
frame identifier F in FIG. 8A has information indicating the time
slots to be used in the next uplink frame by the user equipment
12a, 12b and 12c as shown in FIG. 8B.
[0008] In FIG. 8B, the user equipment 12a, 12b and 12c each
recognizes its own time slots from the frame identifier F in FIG.
8A, and supplies the time slots with the information items a, b and
c, respectively.
[0009] Incidentally, in order to increase the number of time slots
to be transmitted, the user equipment 12a, 12b and 12c supplies the
information to all the time slots assigned. As for the frame
identifier F, it can be always transmitted by the user equipment
12a, for example.
[0010] The distributor 15 supplies the transmission line 16 with
the information items a, b and c in this order. When the
transmission lines 14a, 14b and 14c and the transmission line 16
consist of an optical fiber, an optical star coupler can be used as
the distributor 15. The network unit 17 receives the frame
identifier F and information items a, b and c from the transmission
line 16, and transfers them to the communication network.
[0011] Receiving the entire information of a frame, the network
unit 17 makes a decision as to whether all the time slots assigned
to the equipment 12a, 12b or 12c of each user carry the
information, and when the decision result is positive, it increases
the number of the time slots assigned to the user equipment,
whereas when the decision result is negative, it decreases or holds
the number of the time slots, and the information about the
increase or decrease is added to the uplink frame identifier F of
FIG. 8A.
[0012] Thus, the network unit 17 can increase the number of the
time slots to be assigned to the user equipment that requires the
increase of the uplink time slots.
[0013] With the foregoing configuration, the conventional dynamic
bandwidth assignment system has a problem of statistically
increasing a cell transfer delay and a bursting tendency of a
cell.
[0014] Here, the problem will be described in more detail taking an
example of the user equipment 12a, which problem also holds true
for the user equipment 12b or 12c.
[0015] FIG. 9 is a diagram illustrating relationships between
assignment positions of time slots and output information in
reference to the necessary bandwidth of the user equipment 12a. In
FIG. 9, frames 1 and 2 are named so for the convenience of
explanation, and the frame identifier F is omitted from the output
information of the user equipment 12a.
[0016] FIG. 9 illustrates an example where the user equipment 12a
makes a request for increasing its necessary bandwidth at a
midpoint in the frame 1. As typical services that request an
increase or decrease of the necessary bandwidth, there are such
services as data transmission of videos, data transmission by
computers, etc. In the example of FIG. 9, the user equipment 12a
halts the output of the information at the assigned time slot
before the request for increasing the necessary bandwidth because
it does not need to increase the number of the time slots before
the request, but after that it supplies the information to all the
assigned time slots because of the increase of the necessary
bandwidth.
[0017] However, it is not until the network unit 17 receives the
final information in frame 2 from the user equipment 12a that it
can recognize the request to increase the number of time slots from
the user equipment 12a, because the user equipment 12a does not
transmit its information in the time slot in frame 1. Waiting for
the arrival of the frame 2 in its entirety, the network unit 17
also recognizes the request to increase the number of time slots
from other user equipment, and starts the reassignment of the time
slots. When completing the reassignment, the network unit 17
provides the downlink frame identifier F with time slot
information. Receiving the time slot information, the user
equipment 12a can output its information to the reassigned time
slots from the next uplink frame.
[0018] As described above, it takes one to two frame periods for
the network unit 17 to start the reassignment of the time slots in
response to the request to increase the necessary bandwidth from
the user equipment. Generally, when the user equipment is not
assigned necessary time slots, it stores the information to be
transmitted in its buffer memory. Thus, the delay of the
reassignment of the time slots will increase the capacity of the
buffer memory, along with the amount of information to be stored in
the buffer, resulting in an increase in the transfer delay.
Besides, the increase in the number of cells stored in the buffer
increases the bursting tendency of the cells. Since a larger
capacity buffer memory is usually required to transmit the
information with high bursting tendency through the communication
network, it is preferable for the equipment of the communication
network to transmit information with low bursting tendency.
[0019] Next, FIG. 10 is a diagram illustrating relationships
between the number of time slots assigned to the user equipment 12a
and the amount of the output information of the user equipment 12a
when the necessary bandwidth of the user equipment 12a is increased
and held thereafter. In FIG. 10, there are shown two delay times: a
delay time X between the time when the user equipment 12a makes a
request to increase the number of the time slots and the time when
it actually outputs its information using the time slots with their
number being increased, and a delay time Y between the time when
the user equipment 12a halts the request to increase the number of
the time slots and the time when it actually transmits its
information using the time slots with their number being reduced.
Thus, when the necessary bandwidth of the user equipment 12a is
increased, the number of the time slots assigned to the user
equipment 12a is increased after the time X, and hence the amount
of the output information of the user equipment 12a is
increased.
[0020] For convenience of explanation, it is further assumed that
the bandwidth of the time slots whose number is increased satisfies
the necessary bandwidth of the user equipment 12a, and that when
all the time slots assigned are not supplied with the information,
the number of the time slots assigned is decreased without
exception.
[0021] Afterward, when the user equipment 12a transmits the entire
information stored in the buffer memory, it does not supply all the
assigned time slots with its information. In response to this, the
network unit 17 decides that it can reduce the number of time slots
to be assigned to the user equipment 12a, and actually reduces the
number of the time slots after the time period Y. Thus, the amount
of the output information of the user equipment 12a is further
decreased, and the information is stored in the buffer memory,
again. The operation is repeated until the necessary bandwidth of
the user equipment 12a is reduced.
[0022] The delay time increases with an increase of the amount of
information in the buffer memory. The foregoing operation brings
about the states alternately where a large amount and a small
amount of information is transmitted, thereby increasing the
bursting tendency of the cells.
SUMMARY OF THE INVENTION
[0023] The present invention is implemented to solve the foregoing
problem. It is therefore an object of the present invention to
provide a dynamic bandwidth assignment system and a dynamic
bandwidth assignment method capable of statistically reducing the
cell transfer delay and the bursting tendency of the cells.
[0024] According to a first aspect of the present invention, there
is provided a dynamic bandwidth assignment system including a
network unit for carrying out cell slot assignment, and a network
termination for transmitting cells to the network unit by means of
cell slots assigned by the network unit, the network unit
comprising: a detecting section for detecting a number of valid
cells the network unit receives from the network termination; a
decision section for outputting a decision result in accordance
with the number of valid cells; and a cell slot assignment section
for controlling the cell slot assignment to the network termination
in response to the decision result of the decision section.
[0025] Here, the decision section may supply its decision result to
the cell slot assignment section when a number of consecutive valid
cells the network unit receives from the network termination
exceeds a first predetermined threshold value, and the cell slot
assignment section may increase the number of the cell slots to be
assigned to the network termination in response to the decision
result.
[0026] The decision section may supply its decision result to the
cell slot assignment section when a number of valid cells the
network unit receives from the network termination in a decision
period becomes less than a second predetermined threshold value,
and the cell slot assignment section may reduce a number of the
cell slots to be assigned to the network termination in response to
the decision result.
[0027] The decision section may supply its decision result to the
cell slot assignment section when a number of valid cells the
network unit receives from the network termination in a decision
period exceeds a first predetermined threshold value, and the cell
slot assignment section may increase a number of the cell slots to
be assigned to the network termination in response to the decision
result.
[0028] The decision section may determine the first threshold value
and the second threshold value in accordance with the total number
of cells the network unit receives from the network termination in
the decision period.
[0029] According to a second aspect of the present invention, there
is provided a dynamic bandwidth assignment method in a network unit
comprising the steps of: producing a decision result in accordance
with a number of valid cells the network unit receives from a
network termination; and controlling cell slot assignment to the
network termination in response to the decision result.
[0030] Here, when a number of consecutive valid cells the network
unit receives from the network termination exceeds a first
predetermined threshold value, the step of controlling cell slot
assignment may increase the number of the cell slots to be assigned
to the network termination in response to the decision result.
[0031] When a number of the valid cells the network unit receives
from the network termination becomes less than a second
predetermined threshold value, the step of controlling cell slot
assignment may decrease the number of the cell slots to be assigned
to the network termination in response to the decision result.
[0032] When a number of the valid cells the network unit receives
from the network termination exceeds a first predetermined
threshold value, the step of controlling cell slot assignment may
increase the number of the cell slots to be assigned to the network
termination in response to the decision result.
[0033] The first threshold value and the second threshold value may
be determined in accordance with the total number of cells the
network unit receives from the network termination in the decision
period.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIGS. 1A and 1B are block diagrams showing a configuration
of an embodiment 1 of a dynamic bandwidth assignment system in
accordance with the present invention;
[0035] FIG. 2 is a diagram illustrating relationships between the
necessary bandwidth of a network termination, cell slot positions
assigned to the network termination, and the output information of
the network termination;
[0036] FIG. 3 is a diagram illustrating relationships between the
necessary bandwidth of a network termination, the number of
assigned cell slots per frame of the network termination, the total
number of cells per frame received by the network unit from the
network termination, the number of valid output cells per frame of
the network termination and the number of valid cells per frame
received by the network unit from the network termination;
[0037] FIG. 4 is a diagram illustrating relationships between the
necessary bandwidth of a network termination, the number of
assigned cell slots per frame of the network termination, the total
number of cells per frame received by the network unit from the
network termination, the number of valid output cells per frame of
the network termination and the number of valid cells per frame
received by the network unit from the network termination;
[0038] FIG. 5 is a flowchart illustrating a dynamic bandwidth
assignment method in the embodiment 1;
[0039] FIG. 6 is a diagram illustrating relationships between the
necessary bandwidth of a network termination, the number of
assigned cell slots per frame of the network termination, the total
number of cells per frame received by the network unit from the
network termination, the number of valid output cells per frame of
the network termination and the number of valid cells per frame
received by the network unit from the network termination;
[0040] FIG. 7 is a flowchart illustrating a dynamic bandwidth
assignment method in an embodiment 2 in accordance with the present
invention;
[0041] FIGS. 8A and 8B are block diagrams showing a configuration
of a conventional dynamic bandwidth assignment system;
[0042] FIG. 9 is a diagram illustrating relationships between
assignment positions of the time slots and output information with
respect to a necessary bandwidth of the user equipment; and
[0043] FIG. 10 is a diagram illustrating relationships between the
number of time slots assigned to the user equipment and the amount
of output information of the user equipment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] The invention will now be described with reference to the
accompanying drawings.
Embodiment 1
[0045] FIGS. 1A and 1B are block diagrams showing a configuration
of an embodiment 1 of a dynamic bandwidth assignment system in
accordance with the present invention. In these figures, reference
numerals 11a, 11b and 11c each designate a network termination for
transmitting cells to a network unit 18 using cell slots assigned
by the network unit 18; 12a, 12b and 12c each designate user
equipment; 13a, 13b and 13c each designate a transmission line;
14a, 14b and 14c each designate transmission line; 15 designates a
distributor; 16 designates a transmission line; and 18 designates
the network unit. The network unit 18 comprises a detecting section
21, a decision section 23 and a cell slot assignment section 25.
The detecting section includes a total cell counter 21a for
counting the number of the cells received from the network
terminations 11a, 11b or 11c in a predetermined decision period,
and a valid cell counter 21b for counting the number of valid
cells. The decision section 23 makes a decision as to whether the
number of consecutive valid cells exceeds a first predetermined
threshold value, or the number of valid cells received in the
decision period drops below a second predetermined threshold value.
The cell slot assignment section 25 assigns an increasing number of
cell slots to the network termination 11a, 11b or 11c when the
number of the consecutive valid cells received from it exceeds the
first predetermined threshold value, and assigns a decreasing
number of cell slots to it when the number of valid cells received
from it in the decision period reduces below the second
predetermined threshold value.
[0046] The decision section 23 can make the decision about the
increase or decrease in the number of cell slots at every single
frame or several frame interval. In the following description, it
is assumed that it makes the decision frame by frame.
[0047] The user equipment 12a, 12b and 12c, which is described in
the conventional technique in connection with FIGS. 8A and 8B, is
individually connected to the network termination as shown in FIGS.
1A and 1B in the present embodiment 1: The network termination 11a
is connected to the user equipment 12a; the network termination 11b
to the user equipment 12b; and the network termination 11c to the
user equipment 12c because such configuration is practically used
in the current ATM-PON. The network terminations 11a, 11b and 11c
currently provide user-network interface for the user equipment
12a, 12b and 12c as equipment on a carrier network side.
[0048] The user equipment 12a, 12b and 12c is each connected to the
network terminations 11a, 11b and 11c via the transmission lines
13a, 13b and 13c. The network terminations 11a, 11b and 11c are
connected to the distributor 15 via the transmission lines 14a, 14b
and 14c. The distributor 15 is connected to the network unit 18 via
the transmission line 16.
[0049] FIG. 1A shows a flow of downlink signals from the network
unit 18 to the user equipment 12a, 12b and 12c, whereas FIG. 1B
shows a flow of uplink signals from the user equipment 12a, 12b and
12c to the network unit 18. In the present embodiment 1, it is
assumed that symbols a, b and c in a frame in FIGS. 1A and 1B
designate a cell because information is transmitted using a fixed
length packets called cells in the ATM-PON, though these symbols
are assumed to designate information items in the conventional
technique.
[0050] Next, the operation of the present embodiment 1 will be
described.
[0051] In FIG. 1A, the network unit 18 transmits a PL-OAM (Physical
Layer-Operation and Maintenance) cell P, followed by supplying the
transmission line 16 with a cell a to the network termination 11a,
a cell b to the network termination 11b, and a cell c to the
network termination 11c, successively. The cells a, b and c are
those the network unit 18 receives from the communication network,
and include user equipment information about their
destinations.
[0052] The distributor 15 supplies the transmission lines 14a, 14b
and 14c with the same signal as it receives via the transmission
line 16. The network terminations 11a, 11b and 11c each receive the
signal from the transmission lines 14a, 14b and 14c, and extracts
from it the information directed to themselves, thereby
transmitting the cells a, b and c to the user equipment 12a, 12b
and 12c, respectively.
[0053] The PL-OAM cell P in FIG. 1A includes information indicating
cell slot positions the network terminations 11a, 11b and 11c
utilize from the next frames in the uplink transmission in FIG. 1B.
Although a cell slot is assumed to be a time slot in the
conventional technique, it is handled as the cell slot itself
because the ATM-PON uses cells. In addition, although the PL-OAM
cells are output once in a frame in FIGS. 1A and 1B, they are
output multiple times in a single frame, and the PL-OAM cell at the
initial position of the frame can be identified from the
information inserted into the cell.
[0054] In FIG. 1B, the user equipment 12a, 12b and 12c supplies
cells a, b and c to the network terminations 11a, 11b and 11c. The
network terminations 11a, 11b and 11c, recognizing from the PL-OAM
cell shown in FIG. 1A the cell slots assigned to them to transmit
their own information, and supplies the cell slots with the cells
a, b and c. Here, the bandwidths of the cells the network
terminations 11a, 11b and 11c receive from the user equipment 12a,
12b and 12c differ from those of the cells they transmit to the
distributor 15 in the short term, although they become equal in the
long term.
[0055] The network terminations 11a, 11b and 11c each include a
buffer memory for storing the cells to absorb the difference in the
bandwidth in the short term between the cells received from the
user equipment 12a, 12b and 12c and the cells transmitted to the
distributor 15. As long as the buffer memories store cells, the
network terminations 11a, 11b and 11c transmit valid cells using
all the cell slots assigned to them. In contrast, while the buffer
memories store no cell, they output idle cells to the cell slots
assigned. Although the idle cells take a form of a cell, they do
not carry any information.
[0056] The distributor 15 supplies the transmission line 16 with
the cells a, b and c in this order. When the transmission lines
14a, 14b and 14c and the transmission line 16 are an optical fiber,
an optical star coupler is used as the distributor 15. The network
unit 18 receives the cells a, b and c from the transmission line
16, and transfers them to the communication network. Although the
PL-OAM cell is not provided in the uplink transmission, this frame
identifier is not necessary because the network unit 18 measures
the delay of the uplink frame with respect to the downlink frame in
advance in the ATM-PON, and handles the cell that arrives at the
network unit 18 when the delay time has elapsed after the
transmission of the PL-OAM cell at the initial position of the
downlink frame, as the initial cell of the uplink frame.
[0057] Subsequently, receiving the entire information of the frame,
the network unit 18 supervises the received cells for each of the
network terminations 11a, 11b and 11c, carries out the assignment
of cell slots, and inserts the information about the assignment
into the downlink PM-OAM cells P as shown in FIG. 1A.
[0058] Next, the cell slot assignment will be described in detail.
Although the operation of the user equipment 12a in connection with
the network termination 11a will be described in the following, the
operation of the user equipment 12b and network termination 11b and
that of the user equipment 12c and network termination 11c are the
same.
(1) First, the Case Will be Described Where the Bandwidth of the
Cells Output by the User Equipment 12a is Increased.
[0059] An increasing bandwidth of the cells output from the user
equipment 12a will increase the uplink bandwidth required by the
network termination 11a. FIG. 2 is a diagram illustrating
relationships between the bandwidth required by the network
termination 11a, cell slot positions assigned to the network
termination 11a, and the output information of the network
termination 11a.
[0060] In FIG. 2, before increasing its necessary bandwidth, the
network termination 11a has completed the transmission of all the
cells in its buffer, and outputs an idle cell. Subsequently, the
bandwidth of the cells output from the user equipment 12a is
increased, and hence the valid cells are output successively.
[0061] It is assumed here that S (cells) is the threshold value of
the number of consecutive valid cells determined in accordance with
the total number of the cells the network termination 11a receives
from the user equipment 12a by the point of time at which a
decision is made about whether to increase the number of cell slots
to be assigned to the network termination 11a. In the case of FIG.
2, S+1 valid cells continue, which exceeds the threshold value S.
Therefore, the network unit 18 starts the increase in the number of
cell slots to be assigned to the network termination 11a. Here, the
threshold value is referred to as an increasing threshold value
because it is used to increase the number of the cell slots.
[0062] The increase of the other network terminations is decided in
the same manner.
[0063] Since the cell slots assigned to the network termination 11a
are provided with either the valid cell or idle cell of the network
termination 11a without fail, the number of the cell slots assigned
to the network termination 11a in a single frame equals the total
number of cells per frame that the network unit 18 receives from
the network termination 11a after the fixed delay time.
[0064] Since the decision section 23 determines the increasing
threshold value of the number of consecutive valid cells in
accordance with the total number of cells per frame, the increasing
threshold value varies in response to the total number of the cells
received.
[0065] The total number of received cells and the number of valid
cells are counted as follows.
[0066] The detecting section 21 of the network unit 18 recognizes
the cell that arrives at the point of time when the delay time
measured in advance has elapsed after transmitting the initial
PL-OAM of the downlink frame, as the initial cell of the uplink
frame. Since the network unit 18 knows the positions of the cell
slots assigned to the network termination 11a in the frame, the
detecting section 21 can recognize the cell time at which it
receives the cells from the network termination 11a.
[0067] At the time it receives the cells from the network
termination 11a, the detecting section 21 counts the total number
of the cells and the number of valid cells as follows. First, it
causes the total cell counter 21a to count the total number of the
cells of the network termination 11a. Then, detecting the valid
cells by referring to the ATM cell header, it causes the valid cell
counter 21b to count the number of the valid cells of the network
termination 11a.
[0068] It is necessary for the network unit 18 to assign the cell
slots as even as possible to reduce the bursting tendency of the
cells. In this case, when the necessary bandwidth of the network
termination is increased, the possibility that the network unit 18
can decide the increase in the number of the cell slots at the end
of the frame is about L/K, where K is the number of the cell slots
assigned to the network termination, and L is the increasing
threshold value of the number of consecutive valid cells of the
network termination at the time when the decision is made as to the
increase of the cell slots.
[0069] Thus, the present embodiment 1 can statistically reduce the
time taken from the increase in the necessary bandwidth of the
network termination to the decision as to the increase by the
network unit as compared with the conventional technique that
decides the increase in the number of the cell slots at the end of
the next frame. For example, when K=100 and L=50, the possibility
is about 1/2 that the network unit 18 can make a decision on the
increase in the number of cell slots at the end of the frame after
the network termination increases its necessary bandwidth. When the
network unit 18 cannot decide the increase in the number of the
cell slots at the end of the frame after increasing the necessary
bandwidth of the network termination, it makes the decision on the
cell slots at the end of the next frame. Accordingly, the average
time from the increase in the necessary bandwidth of the network
termination to the decision of the increase in the number of cell
slots by the network unit 18 is 1.0 frame time when K=100 and
L=50.
[0070] Since the conventional technique makes the decision of the
increase from one frame to two frame afterward, the mean time for
making the decision is 1.5 frame time. Thus, the present embodiment
1 can reduce the time by 0.5 frame time as compared with the
conventional technique. This effect can be improved by reducing
K/L.
[0071] However, because of the fluctuations of the cells the user
equipment outputs, it may output the valid cells consecutively, and
hence K/L cannot be reduced extremely. Thus, the increasing
threshold value is determined considering parameters such as the
cell characteristics of the user equipment, the increase in the
number of cell slots to be assigned to the network termination,
etc.
[0072] As described above, the present embodiment 1 can
statistically reduce the time taken from the increase in the
necessary bandwidth by the network termination 11a, 11b or 11c to
the decision about the increase in the number of the cell slots by
the network unit 18. As a result, it offers an advantage of being
able to statistically reduce the number of cells stored in the
buffer memory in the network terminations 11a, 11b and 11c, and
statistically reduce the cell transfer delay and the bursting
tendency of the cells.
(2) Second, the Case Will be Described Where the Bandwidth of the
Cells Output from the User Equipment 12a is Decreased.
[0073] FIG. 3 is a diagram illustrating relationships between the
necessary bandwidth of the network termination 11a, the number of
assigned cell slots per frame of the network termination 11a, the
total number of cells per frame the network unit 18 receives from
the network termination 11a, the number of valid cells per frame
output from the network termination 11a and the number of valid
cells per frame the network unit 18 receives from the network
termination 11a.
[0074] It is assumed in FIG. 3, that the initial value of the
number of the assigned cell slots per frame of the network
termination 11a is "100", the initial value of the total number of
cells per frame the network unit 18 receives from network
termination 11a is "100", the initial value of the number of valid
cells per frame output from the network termination 11a is "80",
and the initial value of the number of valid cells per frame the
network unit 18 receives from network termination 11a is "80". For
the convenience of explanation, it is further assumed that the
number of cell slots assigned to the network termination 11a is
either 100 or 50 cells per frame. In practice, however, more than
two values can be defined as the number of the assigned cell
slots.
[0075] As for the cell slots assigned to the network termination
11a, either a valid cell or idle cell of the network termination
11a are inserted into them without fail. Therefore, the number of
the assigned cell slots per frame of the network termination 11a
equals the total number of cells per frame the network unit 18
receives from the network termination 11a after time Z, where z is
the delay time from the transmission of the cell by the network
termination 11a to the reception of the cell by the network unit
18. Likewise, the number of the valid cells per frame of the
network termination 11a equals the number of valid cells per frame
the network unit 18 receives from the network termination 11a after
the time Z. In addition, the graph of the number of valid cells per
frame the network unit 18 receives from the network termination 11a
illustrates a decreasing threshold value used for decreasing the
number of cell slots to be assigned. The decreasing threshold value
varies in accordance with the total number of cells per frame the
network unit 18 receives from the network termination 11a.
[0076] As the bandwidth of the cells output from the user equipment
12a decreases, the bandwidth required by the network termination
11a reduces, and the number of valid cells per frame output from
the network termination 11a reduces. In FIG. 3, it is assumed that
the number of valid output cells per frame is reduced from "80" to
"45" (FIG. 3(d)). In connection with this, the number of valid
cells per frame the network unit 18 receives from the network
termination 11a is also reduced from "80" to "45" after the time Z
(FIG. 3(e)).
[0077] When the total number of cells per frame the network unit 18
receives from the network termination 11a is "100" (FIG. 3(c)), the
decreasing threshold value is set at "70" (FIG. 3(e)). Let us
assume in FIG. 3 that the number of valid cells per frame output
from the network termination 11a reduces (FIG. 3(d)), and hence the
number of valid cells per frame the network unit 18 receives from
the network termination 11a becomes equal to or less than the
decreasing threshold value (FIG. 3(e)), and the number of cell
slots to be assigned is reduced (FIG. 3(b)), and that the delay
time is Y from the time when the network termination 11a reduces
the number of valid cells per frame to the time when it outputs the
cells to the cell slots whose number is actually reduced (FIGS.
3(a) and 3(b)), then the number of the assigned cell slots per
frame of the network termination 11a is reduced (FIG. 3(b)) when
the time Y has elapsed after the number of valid cells per frame
output from the network termination 11a is reduced to "45" (FIG.
3(d)). In FIG. 3, it is reduced to "50" which is held thereafter
(FIG. 3(b)).
[0078] When the total number of cells the network unit 18 receives
from the network termination 11a is "50", the decreasing threshold
value is set at "35". However, even if the number of valid cells
received becomes less than "35", since the number of the assigned
cell slots per frame is assumed to take one of the two values "50"
and "100", the number of the assigned cell slots per frame of the
network termination 11a is not reduced. The decreasing threshold
value is determined considering parameters such as the cell
characteristics of the user equipment, the number of an increase in
the cell slots for the network termination, etc.
(3) Next, the Case Will be Described Where the Bandwidth of the
Cells Output from the User Equipment 12a is Increased, and the
Increased State Continues.
[0079] FIG. 4 is a diagram illustrating relationships between the
necessary bandwidth of the network termination 11a, the number of
assigned cell slots per frame of the network termination 11a, the
total number of cells per frame the network unit 18 receives from
the network termination 11a, the number of valid cells per frame
output from the network termination 11a and the number of valid
cells per frame the network unit 18 receives from the network
termination 11a. The relationships are the same as those of FIG.
3.
[0080] It is assumed in FIG. 4 that the initial value of the number
of the assigned cell slots per frame of the network termination 11a
is "50", the initial value of the total number of cells per frame
the network unit 18 receives from network termination 11a is "50",
the initial value of the number of valid cells per frame output
from the network termination 11a is "40", and the initial value of
the number of valid cells per frame the network unit 18 receives
from network termination 11a is "40".
[0081] When the bandwidth of the cells the user equipment 12a
outputs increases, and continues the increased state, the bandwidth
required by the network termination 11a increases, and continues
the increased state (FIG. 4(a)).
[0082] When the bandwidth required by the network termination 11a
increases, the number of the valid output cells per frame increases
to the number of the cell slots assigned. In FIG. 4, the number of
valid output cells per frame becomes "50" (FIG. 4(d)).
[0083] When the number of valid cells per frame output from the
network termination 11a increases up to the number of the assigned
cell slots (FIG. 4(d)), the number of consecutive valid cells the
network unit 18 receives from the network termination 11a at the
time when the decision section 23 makes its decision, exceeds the
increasing threshold value. Thus, the network termination 11a is
assigned an increasing number of the cell slots per frame in
accordance with the principle as described above. In this case, the
delay time is X from the increase in the number of valid cells per
frame output from the network termination 11a to the actual output
of the cells using the increased number of cell slots (FIGS. 4(d)
and 4(b)). Thus, when the time X has elapsed after the number of
valid cells per frame output from the network termination 11a
increases, the number of the assigned cell slots per frame of the
network termination 11a increases (FIG. 4(d) and 4(b)), and in
conjunction with this, the number of valid cells per frame output
from the network termination 11a also increases (FIG. 4(d)). After
the additional time Z, the total number of cells per frame the
network unit 18 receives from the network termination 11a increases
(FIG. 4(c)) together with the number of valid cells in the frame
(FIG. 4(e)).
[0084] In FIG. 4, it is assumed that the number of the assigned
cell slots and the number of valid output cells per frame of the
network termination 11a both become "100" (FIGS. 4(b) and 4(d)),
and hence the total number of cells and the number of valid cells
per frame the network unit 18 receives from the network termination
11a also become "100" after the time Z (FIGS. 4(c) and 4(d)). It is
further assumed that when the total number of cells per frame the
network unit 18 receives from the network termination 11a is 100,
the decreasing threshold value is set at 80 cells (FIG. 4(e)).
[0085] Subsequently, when all the cells stored in the buffer memory
of the network termination 11a have been transmitted, the number of
valid cells per frame output from the network termination 11a is
reduced (FIG. 4(d)). In this case, when the number of valid cells
per frame the network unit 18 receives from the network termination
11a is greater than the decreasing threshold value (FIG. 4(e)), the
number of the assigned cell slots is not reduced, holding this
state thereafter. In FIG. 4, the number of valid cells per frame
output from the network termination 11a becomes 90 (FIG. 4(d)), and
hence the number of valid cells per frame the network unit 18
receives from the network termination 11a also becomes 90 after the
time Z (FIG. 4(e)). Since it is greater than the decreasing
threshold value 80, this state is maintained from then on.
[0086] As described above, the present embodiment 1 is configured
such that as long as the number of cells per frame output from the
network termination 11a is greater than the decreasing threshold
value, the number of valid cells output from the network
termination 11a does not repeat the increase and decrease. As a
result, the present embodiment 1 can reduce the number of cells
stored in the buffer memory, and statistically reduce the cell
transfer delay and the bursting tendency of the cells.
[0087] FIG. 5 is a flowchart illustrating a dynamic bandwidth
assignment method of the present embodiment 1. It is assumed in
this flowchart that N network terminations are connected to the
network unit 18 via the distributor 15.
[0088] First, waiting for the completion of a frame, the network
unit 18 starts updating the assignment of the cell slots (step
ST1).
[0089] Subsequently, the decision section 23 of the network unit 18
makes a decision as to whether the number of consecutive valid
cells it receives from a first network termination at the decision
time exceeds the increasing threshold value set in accordance with
the total number of the cells per frame the network unit 18
receives from the first network termination (steps ST2 and
ST3).
[0090] When the decision result is positive, the decision section
23 requests the cell slot assignment section 25 of the network unit
18 to increase the number of cell slots to be assigned to the first
network termination (step ST4). In contrast, when the decision
result is negative, the decision section 23 makes a decision as to
whether the number of valid cells per frame the network unit 18
receives from the network termination is less than or equal to the
decreasing threshold value set in accordance with the total number
of the cells (step ST5).
[0091] When the decision result is positive, the decision section
23 requests the cell slot assignment section 25 to reduce the
number of cell slots to be assigned to the first network
termination (step ST6). When the decision result is negative, the
decision section 23 requests the cell slot assignment section 25 to
maintain the number of the cell slots to be assigned to the first
network termination (step ST7). The foregoing operation is
performed for the N network terminations (step ST8). After
completing the process, the cell slot assignment section 25 updates
the number of the cell slots to be assigned to each of the network
terminations in response to the request from the network
terminations (step ST9).
[0092] As for each of the network terminations, it has an internal
buffer memory, writes the cells it receives from the user equipment
into the buffer memory, reads the cells and supply them to the
assigned cell slots to be output, and outputs an idle cell to an
assigned cell slot when no cell is present in the buffer
memory.
Embodiment 2
[0093] In the present embodiment 2, the network unit 18 operates
such that it increases the number of the cell slots to be assigned
to the network termination from which the network unit receives in
a decision period a greater number of valid cells than a first
threshold value that is set in accordance with the total number of
cells the network unit receives from the network termination in the
decision period, and that it decreases the number of the cell slots
to be assigned to the network termination from which the network
unit receives in a decision period a smaller number of valid cells
than a second threshold value that is set in accordance with the
total number of cells the network unit receives from the network
termination in the decision period.
[0094] The threshold values that are set in accordance with the
total number of cells the network unit receives from the network
termination in the decision period include two types: an increasing
threshold value for increasing the number of the cell slots; and a
decreasing threshold value for decreasing the number of the cell
slots. The increase or decrease in the number of the cell slots can
be made either at every one frame interval or at every several
frame interval. In the following description, it is assumed that
the decision as to the increase or decrease in the number of the
cell slots is made frame by frame. FIGS. 1A and 1B are applied to
the present embodiment 2. Since the operation of the present
embodiment 2 is the same as that of the embodiment 1 except when
the bandwidth of the cells the user equipment 12a outputs
increases, only the description thereof is made without describing
the remaining operation.
[0095] The case will be described where the bandwidth of the cells
the user equipment 12a outputs increases. Although the operation of
the user equipment 12a in connection with the network termination
11a will be described in the following, the operation of the user
equipment 12b and the network termination 11b and that of the user
equipment 12c and the network termination 11c are the same.
[0096] FIG. 6 is a diagram illustrating relationships between the
necessary bandwidth of the network termination 11a, the number of
assigned cell slots per frame of the network termination 11a, the
total number of cells per frame the network unit 18 receives from
the network termination 11a, the number of valid cells per frame
output from the network termination 11a and the number of valid
cells per frame the network unit 18 receives from the network
termination 11a. Their relationships are the same as those of FIG.
3. For convenience of explanation, it is further assumed that the
number of cell slots assigned to the network termination 11a is
either 100 or 50 cells per frame. In practice, however, more than
two values can be defined as the number of cell slots to be
assigned.
[0097] It is assumed in FIG. 6 that the initial value of the number
of the assigned cell slots per frame of the network termination 11a
is "50", the initial value of the total number of cells per frame
the network unit 18 receives from network termination 11a is "50",
the initial value of the number of valid cells per frame output
from the network termination 11a is "40", and the initial value of
the number of valid cells per frame the network unit 18 receives
from network termination 11a is "40". In addition, the increasing
threshold value is placed at "45" and "95", when the total number
of cells per frame the network unit 18 receives from network
termination 11a is "50" and "100", respectively.
[0098] When the bandwidth of the cells the user equipment 12a
outputs is increased, and continues the increased state, the
bandwidth required by the network termination 11a is increased, and
continues the increased state.
[0099] When the bandwidth required by the network termination 11a
is increased, the number of the valid output cells per frame is
increases up to the number of the cell slots assigned (FIG. 6(d)).
In FIG. 6, the number of valid output cells per frame becomes "50"
(FIG. 6(d)). When the time Z has elapsed, the number of valid cells
per frame the network unit 18 receives from the network termination
11a increases to "50", exceeding the increasing threshold value
(FIG. 6(e)). Accordingly, the cell slot assignment section 25 of
the network unit 18 increases the number of the cell slots to be
assigned to the network termination 11a, so that the number of the
assigned cell slots per frame of the network termination 11a is
increased to "100" when the time X has elapsed (FIG. 6(b)).
[0100] When the number of the assigned cell slots per frame of the
network termination 11a increases to "100", the number of valid
cells per frame the network termination 11a outputs is increased up
to "100" (FIG. 6(d)), and the total number of cells per frame the
network unit 18 receives from the network termination 11a also
increases to "100" when the time Z has elapsed (FIG. 6(c)). Thus,
the number of valid cells per frame the network unit 18 receives
from the network termination 11a also becomes "100" (FIG.
6(e)).
[0101] Subsequently, when all the cells stored in the buffer memory
of the network termination 11a have been transmitted, the number of
valid cells per frame output from the network termination 11a is
assumed to be reduced to 90 cells, for example (FIG. 6(d)). Since
the increasing threshold value is "95" when the total number of
cells the network unit 18 receives from the network termination 11a
is "100" (FIG. 6(e)), the number of the assigned cell slots per
frame of the network termination 11a is not increased from the
viewpoint of the threshold value. Besides, since the number of the
assigned cell slots per frame is assumed to take one of the two
values "50" and "100", the number of the assigned cell slots per
frame of the network termination 11a is not increased even when the
total number of the cells received exceeds "95". The increasing
threshold value is determined considering parameters such as the
cell characteristics of the user equipment, the number of an
increase in the cell slots for the network termination, etc.
[0102] Since the operation when the bandwidth of the cells the user
equipment 12a outputs is reduced is the same as that of the
foregoing embodiment 1, the description thereof is omitted here. In
addition, in the case where the bandwidth of the cells the user
equipment 12a outputs is increased, and the increased state
continues, since the operation is the same as that of the foregoing
embodiment 1 except that the method of deciding the increase in the
number of the assigned cell slots per frame of the network
termination 11a is replaced by the decision method described in the
present embodiment 2, the same effect as the foregoing embodiment 1
is achieved.
[0103] Thus, when the bandwidth of the cells the user equipment 12a
outputs is increased, and the increased state continues, and when
the number of valid cells per frame output from the network
termination 11a is greater than the decreasing threshold value, the
number of cells output from the network termination 11a does not
repeat the increase and decrease. As a result, the present
embodiment 2 can reduce the number of cells stored in the buffer
memory, and statistically reduce the cell transfer delay and the
bursting tendency of the cells.
[0104] Furthermore, since the network unit 18 sets both the
increasing threshold value and decreasing threshold value in
accordance with the total number of cells per frame the network
unit 18 receives from the network termination in the present
embodiment 2, the circuit can be used in common, enabling the
circuit to be reduced in its size.
[0105] FIG. 7 is a flowchart illustrating a dynamic bandwidth
assignment method of the present embodiment 2. It is assumed in
this flowchart that N network terminations are connected to the
network unit 18 via the distributor 15.
[0106] First, waiting for the completion of a frame, the network
unit 18 starts updating the assignment of the cell slots (step
ST11). Subsequently, the decision section 23 of the network unit 18
makes a decision as to whether the number of valid cells per frame
the network unit 18 receives from a first network termination
exceeds the increasing threshold value set in accordance with the
total number of the cells per frame the network unit 18 receives
from the first network termination (steps ST12 and ST13).
[0107] When the decision result is positive, the decision section
23 requests the cell slot assignment section 25 of the network unit
18 to increase the number of cell slots to be assigned to the first
network termination (step ST14). In contrast, when the decision
result is negative, the decision section 23 makes a decision as to
whether the number of valid cells per frame the network unit 18
receives from the network termination is less than or equal to the
decreasing threshold value set in accordance with the total number
of the cells (step ST15).
[0108] When the decision result is positive, the decision section
23 requests the cell slot assignment section 25 to reduce the
number of cell slots to be assigned to the first network
termination (step ST16). When the decision result is negative, the
decision section 23 requests the cell slot assignment section 25 to
maintain the number of the cell slots to be assigned to the first
network termination (step ST17). The foregoing operation is
performed for the N network terminations (step ST18). After
completing the process, the cell slot assignment section 25 updates
the number of the cell slots to be assigned to each of the network
terminations in response to the request from the network
terminations (step ST19).
[0109] As for each of the network terminations, it has an internal
buffer memory, writes the cells it receives from the user equipment
into the buffer memory, reads the cells and supply them to the
assigned cell slots to be output, and outputs an idle cell to an
assigned cell slot when no cell is present in the buffer
memory.
* * * * *