U.S. patent application number 12/292337 was filed with the patent office on 2009-07-23 for optical access network system.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Yasuhiko Aoki, Yutaka Kai.
Application Number | 20090185804 12/292337 |
Document ID | / |
Family ID | 38778171 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090185804 |
Kind Code |
A1 |
Kai; Yutaka ; et
al. |
July 23, 2009 |
Optical access network system
Abstract
In an optical access network system in which a relay station
receives a wavelength multiplexed and time-division multiplexed
optical signal (WDM optical signal) from an optical network, and
the relay station transmits optical signals having specified
wavelengths to subscriber units, the relay station demultiplexes
the WDM optical signal into a plurality of groups with one group
having a plurality of optical signals having a fixed wavelength
interval between signals, then divides the optical signals of each
group into k branches (k is the number of subscriber units) and
inputs the signals to optical switches, and by turning ON/OFF the
optical switches such that they are spaced in time, inputs the
branched optical signals to the respective specified subscribers
units.
Inventors: |
Kai; Yutaka; (Kawasaki,
JP) ; Aoki; Yasuhiko; (Kawasaki, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700, 1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
38778171 |
Appl. No.: |
12/292337 |
Filed: |
November 17, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP2006/310442 |
May 25, 2006 |
|
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12292337 |
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Current U.S.
Class: |
398/48 |
Current CPC
Class: |
H04J 14/0282 20130101;
H04J 14/0227 20130101; H04J 14/0257 20130101; H04J 14/0226
20130101; H04J 14/0238 20130101; H04J 14/0247 20130101; H04J
14/0232 20130101; H04J 14/0246 20130101; H04J 14/0252 20130101;
H04J 14/025 20130101 |
Class at
Publication: |
398/48 |
International
Class: |
H04J 14/00 20060101
H04J014/00 |
Claims
1. An optical access network system comprising a relay station that
receives a wavelength multiplexed optical signal (WDM optical
signal) from an optical network, and transmits optical signals
having specified wavelengths to subscriber units, wherein said
relay station comprises: optical demultiplexing means for
demultiplexing said WDM optical signal into a plurality of optical
signals having fixed wavelength intervals between them as one
group; optical branching means for dividing the demultiplexed group
of optical signals into a plurality of branches; and switching
means for transmitting the divided group of optical signals to
specified subscriber units.
2. The optical access network system of claim 1, wherein said
optical demultiplexing means is a cyclic
multiplexing/demultiplexing filter that demultiplexes each of the
optical signals of a plurality of groups from said WDM optical
signal, and multiplexes the optical signals of each group, then
outputs the signal from a specified port; and said relay station
comprises a said optical branching means and optical switching
means for each group.
3. The optical access network system of claim 1 wherein said
optical switching means comprises a plurality of optical
semiconductor amplifiers having a switching function for turning
ON/OFF a connection between input ports and output ports.
4. The optical access network system of claim 3, wherein said group
of optical signals is created by multiplexing optical signals
transmitted to subscriber units that have been shifted in time, and
wherein by shifting the times when the said plurality of optical
semiconductor amplifiers are turned ON/OFF, specified optical
signals are input to specified subscriber units.
5. The optical access network system of claim 3 wherein by driving
said plurality of optical semiconductor amplifiers of said group
optical switching means so they go ON at the same time, the same
plurality of optical signals that have been branched off are
transmitted to a plurality of subscriber units at the same
time.
6. The optical access network system of claim 1 wherein said
wavelength interval between optical signals of a group is an
interval for which 4-wave optical mixing does not occur.
7. The optical access network system of claim 1 wherein said
wavelength multiplexed group of optical signals comprise optical
signals that transmit subscriber data, and a control optical signal
having a different wavelength from the wavelengths of the data
optical signals, and for each group, said relay station comprises:
demultiplexing means for demultiplexing said control optical
signal; and a control circuit for performing switching of
high-speed switching elements of said optical switching means using
the demultiplexed control optical signal.
8. The optical access network system of claim 7 wherein said
control circuit performs switching by separating said group of
optical signals into 100 .mu.s or less optical burst signals.
9. The optical access network system of claim 7 wherein when said
control optical signal is phase modulated, said relay station
comprises: an optical modulation unit that performs intensity
modulation of the control optical signal using data that is
transmitted from a said subscriber unit and outputs the result; and
multiplexing means for multiplexing the optical signals that are
output from the optical modulation units of each group and
transmits the result to the transmission side via a transmission
path that differs from a DOWN transmission path.
10. The optical access network system of claim 9 wherein said relay
station comprises a transmission data multiplexing unit that
demodulates the transmission data input by optical signals from a
plurality of subscriber units, and multiplexes that plurality of
transmission data, and said optical modulation unit performs the
intensity modulation of said control optical signal using the
multiplexed transmission data.
11. The optical access network system of claim 9 wherein said relay
station comprises an optical amplifier that amplifies in the
saturation region said control optical signal that was
demultiplexed by said demultiplexing means and inputs the result to
said optical modulation means.
12. The optical access network system of claim 2 wherein a first
portion of the relay station that comprises said cyclic
multiplexing/demultiplexing filter, a second portion of the relay
station that comprises said optical branching means and optical
switching means, and the subscriber units are physically connected
in a tree shape.
13. The optical access network system of claim 1 wherein said
subscriber unit has a wavelength selection filter that separates
optical signals of each wavelength from the input optical signals
and inputs each of the optical signals to respective subscriber
terminals.
14. The optical access network system of claim 1 wherein said
subscriber unit has a wavelength selection filter that separates
out optical signals of each wavelength from the input wavelength
multiplexed optical signal, and a decoding circuit that decodes
signals that were encoded on the transmission side using the
optical signals that were separated out, and inputs the result to
subscriber terminals.
15. The optical access network system of claim 7 wherein each
subscriber unit comprises an optical transmitter that modulates an
optical signal having a wavelength that is different from the
wavelength of a DOWN optical signal with transmission data and
outputs the result, and said relay station comprises: a combination
means for shifting in time and combining the optical signals that
were input from said transmitters of each of the subscriber units
to output an optical signal having one wavelength; a multiplexing
unit that multiplexes said optical signals that were input from
said combination means of a plurality of groups, and an optical
coupler that transmits an optical signal that was output from the
multiplexing unit to the transmission side by the same transmission
path as the DOWN transmission path; and by separating the optical
signals that were transmitted from the subscriber side along said
transmission path on the transmission side, bi-directional
transmission is performed using one fiber optic transmission
path.
16. The optical access network system of claim 15 wherein said
optical coupler is an optical wavelength band
multiplexing/demultiplexing filter, or optical circulator.
17. The optical access network system of claim 15 wherein said
combination means comprises: switching means for shifting in time
and switching optical signals that are input from each subscriber
unit; and a multiplexing unit that multiplexes the optical signals
that are output from the switching means and outputs the result as
an optical signal having one wavelength; wherein said control
optical signal includes a control signal that controls the
switching by the switching means.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an optical access network
system that transmits high-speed optical signals to subscribers,
and more particularly to an optical access network system that
receives a multiplexed optical signal of a plurality of optical
signals having different wavelengths from a broadband optical
network, and transmits optical signals having specified wavelengths
to subscribers.
[0002] Various methods have been proposed as a next-generation
optical access network system that transmits optical signals to a
subscriber as is without performing electrical conversion. The work
of global standardization is advancing, and as examples of such an
access system, there is (1) E-PON (Ethernet PON Ethernet frames
that is obtained by modifying a B-PON (Broadband Passive Optical
Network) for ATM frames, (2) GE-PON (Giga Ethernet PON) for Giga
Bit Ethernet, and (3) Unique-standard G-PON. These PON systems do
not use active elements that perform electrical-to-optical
conversion in the network, such as in the case of ADS (Active
Double Star), but rather comprise only passive optical parts, which
make it possible for a low-cost End-to-End optical network.
Moreover, even though equipment cost is high, adoption of a WDM-PON
that uses a Wavelength Division Multiplex (WDM) signal for larger
capacity is being investigated because it lowers the unit cost per
bit. Ethernet is a registered trademark.
[0003] FIG. 16 is a drawing of a conventional GE-PON, in which a
center station 2 that is connected to the Internet or Broadband
Ethernet 1 is connected to users 3 by optical paths 5, 6.sub.1,
6.sub.2, 6.sub.3, . . . via an optical coupler 4. An Optical Line
Terminal (OLT), which is the center station 2, has a 100/1000
BASE-T or 1000 BASE-SX/LK interface. A 10/100 BASE interface
subscriber unit (Optical Network Unit: ONU) 3a and a personal
computer 3b, which is the user terminal that is connected to the
ONU, are located at subscribers' homes 3.sub.1, 3.sub.2, 3.sub.3, .
. .
[0004] The maximum distance between the center station 2 of the
communication carrier and the users 3.sub.1, 3.sub.2, 3.sub.3
(distance between access intervals) is approximately 20 km, and by
using a wavelength of 1490 nm for DOWN optical signals, and a
wavelength of 1310 nm for UP optical signals, 1 Gbps communication
is performed in both directions simultaneously.
[0005] The center station 2 transmits Ethernet frames that were
received from the Internet or broadband Ethernet 1 and destined for
users 3.sub.1, 3.sub.2, 3.sub.3 to an optical transmission path 5.
The optical coupler 4 splits the DOWN Ethernet frames to a maximum
of 32 branches, and transmits them to optical transmission paths
6.sub.1, 6.sub.2, 6.sub.3, . . . , and the ONU unit 3a of each user
3.sub.1, 3.sub.2, 3.sub.3, . . . extracts out its own respective
packet, and inputs it to the personal computer 3b. Furthermore, the
ONU 3a converts the transmission data from the personal computer 3b
of each user 3.sub.1, 3.sub.2, 3.sub.3, . . . to an optical signal
and transmits the signals to the optical transmission paths
6.sub.1, 6.sub.2, 6.sub.3, . . . , and the optical coupler 4
combines the signals and sends them to the center station 2.
[0006] FIG. 17 is a drawing of a conventional WDM-PON, where the
maximum distance between the communication carrier and user is
approximately 20 km, and communication is performed by multiplexing
32 optical signals having different wavelengths. The wavelengths of
the optical signals that are used for UP and DOWN communication are
the same, .lamda.1 to .lamda.32, and DOWN and UP communication are
performed alternately (ping-pong transmission). A transmission unit
(Tx) at a relay station of the communication carrier multiplexes 32
optical signals .lamda.1 to .lamda.32 having different wavelengths
that were modulated with the data destined for each subscriber at
an optical multiplexer (MUX) 3a, and outputs the multiplexed signal
to a transmission path 7 via an optical circulator 3b. An optical
multiplexing/demultiplexing filter (MUX/DMUX) 8 receives a
Wavelength-Division Multiplexed optical signal (WDM optical
signal), and divides that signal into optical signals .lamda.1 to
.lamda.32 for each wavelength, and inputs the signals to the ONU
units 9.sub.1 to 9.sub.32, which are the subscriber units. The ONU
units 9.sub.1 to 9.sub.32 convert the signals to electrical signals
and input the signals to the personal computers (not shown in the
figure). Moreover, the personal computers of the subscribers input
transmission data to the ONU units 9.sub.1 to 9.sub.32, then those
ONU units 9.sub.1 to 9.sub.32 modulate and transmit the signals of
the wavelengths .lamda.1 to .lamda.32, and the optical
multiplexing/demultiplexing filter 8 combines the signals having
different wavelengths that were output from the ONU units 9.sub.1
to 9.sub.32, and outputs the result to the transmission path 7. The
optical circulator 3b located at the relay station of the
communication carrier inputs the received optical signal to an
optical demultiplexing filter (DMUX) 3c of a receiving unit (Rx),
and that demultiplexing filter 3c separates and outputs optical
signals .lamda.1 to .lamda.32 for each wavelength.
[0007] Moreover, as related art, there is a communication network
in which PONs are connected in a ring shape such that transmission
is possible in both directions, with a plurality of terminals
connected to each PON such that communication can be performed
among the terminals (Refer to WIPO No. WO95/19689 (Japanese Patent
Publication No. H9-510053)). In the prior optical communication
network, communication channels between terminals are multiplexed
using various wavelengths, and use wavelengths that are assigned by
a wavelength assignment unit to perform communication between
terminals.
[0008] In the case of a PON system, a network can be created using
inexpensive passive optical elements, however, there is a problem
in that it is possible to use only a bandwidth of approximately 30
to 100 Mbps per subscriber, and high-speed communication exceeding
100 Mbps is not possible. Moreover, in a PON system, the network is
constructed such that branching is performed by an optical coupler,
so the same signal is distributed to all of the subscriber units
that are connected to that optical coupler. Therefore, the danger
of communication contents being decoded increases, making it
impossible to maintain physical secrecy, and thus in a certain
sense, the system can be said to be the same as a public wireless
LAN. As can be seen from the above explanation, a conventional PON
system has the disadvantage in that there is insufficient security
for financial or business uses.
[0009] On the other hand, in a WDM-PON, it is possible to assign
one wavelength for each user, and thus it is possible to maintain
physical secrecy on the ONU side. However, since WDM optical
signals are used, the number of users that can use a system is
limited to a few tens of users due to restrictions of the number of
multiplexed wavelengths, and to build a WDM-PON system, an enormous
amount of fiber optic cables which connect between the
communication carrier and users is necessary, which is reflected in
the cost of the fiber optic equipment. On the ONU side as well,
when WDM optical signals are used, signal wavelength management of
each individual ONU is necessary, which increases the cost of the
ONU terminal. In addition, in a WDM-PON, it is possible to maintain
sufficient bandwidth, however, wasted bandwidth occurs due to
ping-pong transmission.
SUMMARY OF THE INVENTION
[0010] Taking the aforementioned problems into consideration, the
object of present invention is to provide an optical access network
system that is capable of expanding bandwidth and increasing number
of subscribers.
[0011] Another object of the present invention is to provide an
optical access network system for which long-distance transmission
and tree-shaped or star-shaped network topology can be applied such
that it is widely used from access to a metro area.
[0012] Moreover, another object of the present invention is to
provide an optical access network system that takes into
consideration physical security such that even enterprise users can
feel at ease in using the system.
[0013] Furthermore, another object of the present invention is to
provide an optical access network system that can lower cost by
increasing the number of users per system.
[0014] A first form of the present invention is an optical access
network system comprising a relay station that receives wavelength
multiplexed optical signals (WDM optical signal) from an optical
network, and transmits optical signals having specified wavelengths
to subscriber units, wherein the relay station comprises: optical
demultiplexing means for demultiplexing a WDM optical signal into a
plurality of optical signals having fixed wavelength intervals
between them as one group; optical branching means for dividing the
demultiplexed group of optical signals into a plurality of
branches; and switching means for transmitting the divided group of
optical signals to specified subscriber units. The optical
switching means comprises a plurality of optical semiconductor
amplifiers having a switching function for turning ON/OFF a
connection between input ports and output ports.
[0015] The group of optical signals is created by multiplexing
optical signals transmitted to subscriber units that have been
spaced in time and by shifting the times when the plurality of
optical semiconductor amplifiers are to be turned ON/OFF, specified
optical signals are input to specified subscriber units.
[0016] The group of wavelength multiplexed optical signals
comprises optical signals that transmit subscriber data, and a
control optical signal having a different wavelength from the
wavelengths of the data optical signals, and for each group, the
relay station comprises a demultiplexing filter for demultiplexing
the control optical signal, and a control circuit for performing
switching of high-speed switching elements of the optical switching
means using the demultiplexed control optical signal.
[0017] A subscriber unit has a wavelength selection filter that
separates optical signals of each wavelength from the input optical
signals and inputs each of the optical signals to respective
subscriber terminals, or a subscriber unit has a wavelength
selection filter that separates out optical signals of each
wavelength from the input wavelength multiplexed optical signal,
and a decoding circuit that decodes signals that were encoded on
the transmission side using the optical signals that were separated
out, and inputs the result to subscriber terminals.
[0018] A second form of the present invention is an optical access
network system in which the DOWN transmission path is separate from
the UP transmission path. When the control optical signal is phase
modulated, the relay station comprises: an optical modulation unit
that performs intensity modulation of the control optical signal
using data that is transmitted from a subscriber unit and outputs
the result, and multiplexing means that multiplexes the optical
signals that are output from the optical modulation units of each
group and transmits the result to the transmission side via a
transmission path that differs from the DOWN transmission path.
Moreover, the relay station comprises a transmission data
multiplexing unit that demodulates the transmission data input by
optical signals from a plurality of subscriber units, and
multiplexes that plurality of transmission data, and the optical
modulation unit performs the intensity modulation of the control
optical signal using the multiplexed transmission data.
[0019] A third form of the invention is an optical access network
system that performs bi-directional transmission over one fiber
optic transmission path, where each subscriber unit comprises an
optical transmitter that modulates an optical signal having a
wavelength that is different from the wavelength of a DOWN optical
signal with transmission data and outputs the result, and the relay
station comprises: a combination means for shifting in time and
combining the optical signals that were input from said
transmitters of each of the subscriber units to output an optical
signal having one wavelength; a multiplexing unit that multiplexes
said optical signals that were input from the combination means of
a plurality of groups, and an optical coupler that transmits an
optical signal that was output from the multiplexing unit to the
transmission side by the same transmission path as the DOWN
transmission path; and by separating the optical signals that were
transmitted from the subscriber side along the transmission path on
the transmission side, bi-directional transmission is performed
using one fiber optic transmission path.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a drawing showing the overall communication
network system of the present invention.
[0021] FIG. 2 is a drawing that explains the wavelengths of the
optical data signals and control signals that are assigned to
groups.
[0022] FIG. 3 is a drawing for explaining the wavelength
multiplexed and time-division multiplexed status for each
group.
[0023] FIG. 4 is a drawing showing the overall construction of an
optical communication network system that comprises a first
embodiment of the optical access network system of the present
invention.
[0024] FIG. 5 is a first drawing showing the construction of a
receiver in a subscriber unit.
[0025] FIG. 6 is a second drawing showing the construction of a
receiver in a subscriber unit.
[0026] FIG. 7 is a drawing explaining the optical path of a control
signal having a wavelength .lamda.a.
[0027] FIG. 8 is a drawing showing the construction of a
receiving/multiplexing unit.
[0028] FIG. 9 is a drawing showing the construction of an Optical
Line Terminal (OLT), which is a unit on the transmission side.
[0029] FIG. 10 is a drawing showing the construction of a signal
generation unit for the case in which encoding is performed.
[0030] FIG. 11 is a drawing explaining the signal-switching
format.
[0031] FIG. 12 is a drawing explaining the signal-switching
timing.
[0032] FIG. 13 is a drawing explaining the signal-switching timing
for different groups.
[0033] FIG. 14 is a drawing showing the overall construction of an
optical communication network system that comprises a second
embodiment of the optical access network system of the present
invention.
[0034] FIG. 15 is a drawing explaining the signal-switching
format.
[0035] FIG. 16 is a drawing showing the construction of a
conventional GE-PON.
[0036] FIG. 17 is a drawing showing the construction of a
convention WDM-PON
DESCRIPTION OF THE PREFERRED EMBODIMENTS
(A) Overview of the Invention
[0037] FIG. 1 is a drawing showing the overall construction of a
communication network system of the present invention, where the
transmission side SDS, which includes providers, video distribution
centers, telephone exchange stations, mobile communication
companies or the like, is connected to an optical access network
system PACN via a broadband communication network BPN, and on the
optical access network side PACN, relay stations RST are connected
with the user side USS by optical transmission paths. The relay
stations RST are divided into first and second relay stations RST1,
RST2, and the optical access network system PACN is built in a tree
shape. The number of users that are allowed for one relay station
RST is kN, which are divided into a maximum of N number of groups
of k users each. There are second relay stations RST2-1 to RST2-8
for each group, and expansion is possible in group units. In the
figure, the maximum number of groups is N=8, where one group
comprises k=16 users U1-1 to U1-16.
[0038] In a case where a relay station RST is able to handle up to
a maximum of 40 waves of WDM optical signals (wavelength
multiplexed signals), the transmission side SDS comprises four wave
data transmitters 11.sub.1 to 11.sub.4 and one wave control
transmitter 11.sub.5 per group, so by combining the four data waves
and one control wave, there is a total of five waves assigned to
one group. The wavelengths of the five waves that are assigned to
each group are spaced such that there is no Four Wave Mixing (FWM).
That is, of the 40 waves, 32 continuous waves are for data, and 8
continuous waves are for control, where four waves (wavelengths:
.lamda.1, .lamda.9, .lamda.17, .lamda.25) having a fixed wavelength
interval are selected from the optical signals of these 32
continuous waves for the data of group 1, and one wave (wavelength:
.lamda.a) is selected from the 8 continuous waves for control of
group 1. Similarly, optical signals are assigned for data and
control of other groups. As a result, the relationship between
groups 1 to 8 and the wavelengths of the optical signals for data
and control for the groups is as shown in FIG. 2.
[0039] The transmitters 11.sub.1 to 11.sub.4 of group 1 shift in
time and modulate the four optical signals (wavelengths: .lamda.1,
.lamda.9, .lamda.17, .lamda.25) with the user data D1-1 to D1-16 of
group 1, and an optical multiplexer 12 performs wavelength
multiplexing of the modulated optical signals, then sends the
result to an optical transmission path 21. The transmitters of the
other groups i (i=1 to 8) similarly shift in time and modulate the
four optical signals with the user data Di-1 to Di-16 of the
respective group i, and the optical multiplexer 12 performs
wavelength multiplexing of all the modulated optical signals and
sends the result to the optical transmission path 21. As a result,
as shown in FIG. 3, wavelength multiplexing and time division
multiplexing is performed for a total of 32 waves, and the optical
signals are sent to the relay station RST via the optical
transmission path 21. Wavelength multiplexing and time division
multiplexing are also performed for the control optical signals of
each group and sent to the relay station RST via the optical
transmission path 21 in the same way as the data optical
signals.
[0040] In the description of the present invention, the term `time
division multiplexing` means to shift the signals in time and
perform multiplexing, where the destination of the user data
assigned to each timeslot is not fixed, but dynamically changes.
The control data specifies the user destination for the data of
each timeslot.
[0041] A cyclic multiplexing/demultiplexing filter 31 divides out
the wavelength multiplexed optical signals into data optical
signals and control optical signals for each group, and combines
the optical signals of each group, then inputs the results to the
second relay stations RST2-1 to RST2-8 from ports corresponding to
each respective group. The second relay stations RST2-1 to RST2-8
comprise a branch coupler 41 and high-speed optical switch unit 42.
The branch coupler of group GI branches four wave WDM optical
signals (wavelengths: .lamda.1, .lamda.9, .lamda.17, .lamda.25)
that were input for group 1, and inputs the results to the 16 input
ports of the high-speed switch unit 42. The high-speed switch unit
42 comprises 16 optical switches that turn ON/OFF the connections
between the 16 input ports and 16 output ports, and each optical
switch shifts the optical switching time based on the control data
that is sent by the control optical signals, or in other words,
performs ON/OFF control in order at the time divided timeslot
periods. By doing this, the WDM optical signals (D1-1 to D1-16) for
each user that are assigned to each of the timeslots are input to
the subscriber units 61.sub.1 to 61.sub.16 of the users by way of
the optical transmission paths 51.sub.1 to 51.sub.16.
[0042] From the operation described above, the WDM optical signals
are shifted in time and distributed to the users, so only the
pertinent WDM optical signal is distributed to the subscriber units
61.sub.1 to 61.sub.16 of the target users. As a result, it becomes
possible to maintain physical secrecy. Moreover, when secrecy is
not necessary, or when it is preferred that low-cost service be
provided, it is possible to further increase the user capacity by
further allocating each of the four waves of WDM optical signals
that are distributed to the subscriber units 61.sub.1 to 61.sub.16
to one user.
[0043] When phase modulation is performed for the control optical
signals that are received from the transmission side SDS,
transmission of UP data from the subscriber units is performed by
performing intensity modulation of the control optical signals with
the transmission data (UP data). In other words, the subscriber
units 61.sub.1 to 61.sub.16 use a 1.3 .mu.m band signal to transmit
transmission data to the second relay station RST2 bi-directionally
over one cable, and the second relay station RST2 performs time
division multiplexing of the UP signals from the subscriber units
61.sub.1 to 61.sub.16 to generate one high-speed data signal, then
an optical modulator (not shown in the figure) performs intensity
modulation of the control optical signals using that data signal,
and a multiplexing unit combines the optical modulated signals of
each group and transmits the result to the transmission side via a
transmission path (not shown in the figure) that is different from
the DOWN transmission path. With this construction, it is possible
to create subscriber units inexpensively.
[0044] When it is desired to reduce cost by performing
bi-directional transmission using one optical transmission path, an
optical modulation unit, which uses transmission data to modulate
an optical signal having a different wavelength from that of the
transmission side, is mounted in the subscriber unit, and that
subscriber unit transmits an optical modulated signal to the second
relay station RST2, after which that second relay station RST2
performs time division multiplexing of the optical modulated
signals that were sent from the 16 subscriber units for each group,
then performs wavelength multiplexing of the time division
multiplexed optical signals for each group and returns the result
as a WDM optical signal.
(B) First Embodiment
[0045] FIG. 4 is a drawing showing the overall construction of an
optical communication network system that comprises the optical
access network system of a first embodiment of the invention, where
the transmission side SDS is connected to the optical access
network system PACN via a broadband optical communication network
BPN, and where in the optical access network system PACN, relay
stations RST are connected to the user side by optical transmission
paths.
[0046] The maximum distance between the transmission side SDS and a
relay station RST is 300 km, the maximum distance between a first
and second relay station RST1, 2 is 20 km, and the distance between
a relay station RST and the user side USS is several km or less.
The relay stations RST are divided into first and second relay
stations RST1, RST2, and the optical access network PACN is built
in a tree shape from the first relay station to a subscriber unit.
The allowable number of users for one relay station RST is kN,
where kN users are divided in to a maximum of N number of groups k
by k. Second relay stations RST2-1 to RST2-8 are provided for each
group, and construction is such that expansion is possible in group
units. In the figure, the maximum number of groups is N=8, with a
group comprising k=16 users U1-1 to U1-16. The optical access
network system PACN is not limited to a tree shape, and could also
be built in a star shape.
(a) DOWN Transmission
[0047] In the case where a relay station RST can handle WDM optical
signals (wavelength multiplexed signals) for up to a maximum of 40
waves, the transmission side SDS comprises data transmitters
11.sub.1 to 11.sub.4, . . . , 11.sub.36 to 1139 for four waves per
group, and control transmitters 11.sub.5, . . . , 11.sub.40 for one
wave per group, so there is a total of four waves for data and one
wave for control, and five waves are assigned for one group. The
interval between the five wavelengths assigned to each group is an
interval in which four-wave mixing does not occur, for example, an
interval of 400 GHz, where the relationships between the groups 1
to 8, and the wavelengths of the data signals and control signals
assigned to each group are as shown in FIG. 2. The DOWN optical
signals use 40 waves in C-band which is a 1.5 .mu.m band.
[0048] The transmitters 11.sub.1 to 11.sub.4 for group 1 shift four
optical signals (wavelengths: .lamda.1, .lamda.9, .lamda.17,
.lamda.25) in time in conformity with the time slot of the user
data D1-1 to D1-16 for group 1, or in other words, perform
time-division modulation, and transmitter 11.sub.5 shifts the
optical signals (wavelength: .lamda.a) in time in conformity with
the time slot of the control data for each user of the group 1, or
in other words, performs time-division modulation, then a
multiplexer 12.sub.1 performs wavelength multiplexing of the
time-division modulated optical signals. The modulation speed is 10
Gbps.
[0049] Other groups operate similarly. For example, The
transmitters 11.sub.36 to 11.sub.39 for group 8 perform
time-division modulation of four optical signals for data
(wavelengths: .lamda.8, .lamda.16, .lamda.24, .lamda.32) based upon
the user data D8-1 to D8-16 for group 8, and transmitter 11.sub.40
performs time-division modulation of the optical signals for
control (wavelength: .lamda.h) based upon the control data for each
user of group 8, or in other words, performs time-division
modulation, then a multiplexer 12.sub.8 performs wavelength
multiplexing of the time-division modulated optical signals.
[0050] The multiplexers 12.sub.1 to 12.sub.8 could also be
commercially sold MUX filters having dielectric multi layer film
that is overlapped in multiple stages, or they could also be AWG
(Arrayed Waveguide Grating) or Fiber Bragg Grating (FBG). Arrayed
Waveguide Grating (AWG) has the function as a
multiplexer/demultiplexer, and the function of the demultiplexer
separates the WDM optical signal into signals for each wavelength,
and outputs the signals to a plurality of waveguide paths, and the
function of the multiplexer performs optical multiplexing of the
lights having different wavelengths that are input from the
plurality of waveguide paths, and outputs the result as wavelength
multiplexed light.
[0051] Wavelength groups can be arbitrarily configured. For
example, groups can be divided according to services such that
wavelengths .lamda.1 to .lamda.8 are assigned for providers,
wavelengths .lamda.9 to .lamda.16 are assigned for audio
(telephone), wavelengths .lamda.17 to .lamda.25 are assigned for
video, and wavelengths .lamda.26 to .lamda.32 are assigned for
other mobile uses. By doing so, it is possible to flexibly upgrade
from a conventional FTTH system.
[0052] Multiplexer 13 multiplexes the WDM optical signals that are
output from each of the multiplexers 12.sub.1 to 12.sub.8 (see FIG.
1 and FIG. 3), and an optical amplifier 14 amplifies the signal and
sends the result to one optical transmission path 21. The
modulation speed for each optical signal is 10 Gbps, and there are
a total of 40 wavelengths, so the DOWN transmission speed is 10
Gbps.times.40. A cyclic multiplexing/demultiplexing filter, for
example, a cyclic AWG can be used as the multiplexer 13. By
designing the waveguide path, a cyclic AWG multiplexes the WDM
optical signals for a plurality of groups that are input from a
plurality of input ports P1 to P8 for each specified wavelength,
and outputs the result to one output port. When a cyclic
multiplexing/demultiplexing filter is used, it is possible to
reduce the number of ports and thus it is possible to lower the
cost, as well as expansion in group units becomes possible.
[0053] The optical amplifier 32 of the first relay station of the
optical access network system PACN amplifies the WDM optical signal
that is input from the optical transmission path 21, and the cyclic
multiplexing/demultiplexing filter 31 demultiplexes the
data/control optical signals for each group from the WDM optical
signal and sends those signals from ports P1 to P8 that correspond
to the respective groups to optical transmission paths 23.sub.1 to
23.sub.8. The cyclic multiplexing/demultiplexing filter 31
comprises a cyclic AWG (Arrayed Waveguide Grating), and
demultiplexes the WDM optical signal that is input from one port to
optical signals for each specified wavelength, the multiplexes
these for each group and transmits the multiplexed optical signals
for each group from specified port P1 to P8 to the optical
transmission paths 23.sub.1-23.sub.8. In other words, the cyclic
multiplexing/demultiplexing filter 31 transmits the WDM optical
signal (.lamda.1, .lamda.9, .lamda.17, .lamda.25, .lamda.a) for
group 1 to the optical transmission path 23.sub.1 from port P1, and
similarly outputs WDM optical signals for groups 2 to 8 to optical
transmission paths 23.sub.2 to 23.sub.8 from ports P2 to P8.
[0054] From the above, data and control optical signals for each
group are input to the second relay stations RST2-1 to RST2-8 for
each group via the optical transmission paths 23.sub.1 to
23.sub.8.
[0055] At the second relay RST2-1, a fixed demultiplexing filter 43
for group 1 demultiplexes the WDM optical signal that was input
from the optical transmission path 23a, into optical signals for
data having wavelengths .lamda.1, .lamda.9, .lamda.17, .lamda.25,
and an optical signal for control having a wavelength .lamda.a,
then inputs the former signals to a k-branch coupler 41, and inputs
the latter signal to a 2-branch coupler 44. The k-branch coupler
branches the input four wave WDM optical signals for group 1
(wavelengths: .lamda.1, .lamda.9, .lamda.17, .lamda.25) and inputs
the branched four wave WDM optical signals to the 16 input ports of
the high-speed switch unit 42. The 2-branch coupler branches the
one wave control optical signal and inputs one to a switch control
circuit 45, and the other to an optical amplifier circuit 46.
[0056] The high-speed switch unit 42 comprises 16 switches SW1 to
SW16 that turn ON/OFF a connection between 16 input ports and 16
output ports, and based on control data that is sent in the control
optical signal, the switch control circuit 45 shifts the switching
timings of switches SW1 to SW16 one timeslot by one timeslot) and
performs ON/OFF control. It is required that each of the 16
switches SW1 to SW16 has Tr and Tf characteristics (rise and fall
characteristics) of a few ns or less. Therefore, as these switches,
a Semiconductor Optical Amplifier (SOA) can be used for turning
ON/OFF the conduction of optical signals at high speed by turning
ON/OFF a bias power supply. From the above, by performing ON/OFF
control of each semiconductor optical amplifier SOA based on the
control data, the switch control circuit 45 is able to transmit
four wave WDM optical signals that are assigned to specified
timeslots to the subscriber units 61.sub.1 to 61.sub.16 of
specified users via an optical coupler 47 and optical transmission
paths 51.sub.1 to 51.sub.16.
[0057] For example, at a certain timeslot time T1, the switch
element SW1 to which the subscriber unit 61.sub.1 is connected is
turned ON, and all other switch elements are OFF, then at the next
timeslot time T2, the switch element SW2 to which the subscriber
unit 61.sub.2 is connected is turned ON, and all other switch
elements are turned OFF, and after that similar ON/OFF control is
performed in order to input the four wave WDM optical signals
assigned to specified timeslots to the subscriber units 61.sub.1 to
61.sub.16 of specified users. Moreover, when a plurality of switch
elements are turned ON at the same time, it is possible to perform
a multicast (simultaneous broadcast).
[0058] The subscriber units, which are Optical Network Units (ONU)
61.sub.1 to 61.sub.16 comprise an optical coupler 62, receiver 63
and transmitter 64. The receiver 63 decodes packets from the
wavelength multiplexed four wave WDM optical signal, and inputs
them to the user's personal computer (not shown in the figure).
[0059] By turning ON/OFF the optical switching elements in a time
sequence as described above, and inputting the four wave WDM
optical signals of the specified timeslot to specified subscriber
unit, it is possible for the optical switching elements to block
the received signals for a subscriber unit from being distributed
to other subscriber units, and thus it is possible to dramatically
improve secrecy. In addition, by using optical switches as the
switches, it is possible to reduce cost. That is, when switching is
performed by electrical signals without using optical switches, a
total of 16 branches.times.2=32 converters for performing O/E and
E/O conversion are necessary for the switch unit 42 so cost is
greatly increased.
[0060] Examples of candidates for use as high-speed switching
elements SW1 to SW16 are types that use a semiconductor optical
amplifier (SOA), switches that use a compound semiconductor such as
Gallium Arsenide (GaAs) that is used in laser diodes or the like,
and field-effect optical switches that use PLZT thin film. In the
case of a switch that uses SOA, the SOA also functions as an
optical amplifier, so it is possible to compensate for optical
distribution loss in the k-branch coupler 41, and is also effective
in further increasing the transmission distance from the second
relay station RST2 to the subscriber unit.
[0061] Moreover, in the construction shown in FIG. 4, a cyclic
multiplexing/demultiplexing filter 31 is used, and this is
effective for the case in which semiconductor optical amplifiers
(SOA) are used as the switches SW1 to SW16. When a semiconductor
optical amplifier (SOA) amplifies the four wave WDM optical signals
all together, which are the WDM optical signals having a 100 GHz
interval, for example, third harmonic wave distortion occurs due to
a non-linear optical effect called Four Wave Mixing (FWM), so
transmission characteristics greatly deteriorate. A cyclic
multiplexing/demultiplexing filter 31 is also effective in solving
this problem, so the wavelengths for each group shown in FIG. 2 can
be sufficiently separated by an interval of 800 GHz, which has the
effect of suppressing FWM.
--Construction of a Subscriber Unit Receiver
[0062] FIG. 5 is a drawing showing first construction of a receiver
for a subscriber unit of group 1, and shows an example in which
desired signals are decoded from four wave WDM optical signals and
input to the user's personal computer. The construction of the
receivers for other groups is the same as shown in FIG. 5.
[0063] Assuming that an ciphering process is performed on the
transmission side, and that four wave optical signals are modulated
and transmitted according to the obtained ciphered data, a fixed
wavelength selection filter 63a.sub.1 selects a signal light having
wavelength .lamda.1 from the WDM optical signals (wavelengths:
.lamda.1, .lamda.9, .lamda.17, .lamda.25) of group 1, and inputs
that signal light to the receiving unit 63b.sub.1, after which the
receiving unit 63b.sub.1 demodulates the data that is sent in the
signal light having wavelength .lamda.1 and inputs the result to a
decoding circuit 63c.
[0064] Next, a fixed wavelength selection filter 63a2 selects a
signal light having wavelength .lamda.9 from the WDM optical
signals (wavelengths: .lamda.9, .lamda.17, .lamda.25) that are
output from the fixed, wavelength selection filter 63a.sub.1, and
inputs that signal light to the receiving unit 63b.sub.2, after
which that receiving unit 63b.sub.2 demodulates the data that is
sent in the signal lights having wavelength .lamda.9, and inputs
the result to a decoding circuit 63c. Similarly, a fixed wavelength
selection filter 63a.sub.3 selects signal light having wavelengths
.lamda.17 and .lamda.25 from the WDM optical signals (wavelengths:
.lamda.17, .lamda.25) that are output from the fixed wavelength
selection filters 63a.sub.2, and inputs the respective signal light
to the receiving units 63b.sub.3 and 63b.sub.4, after which the
receiving unit 63b.sub.3 demodulates the data that is sent in the
signal light having wavelength .lamda.17 and inputs the result to a
decoding circuit 63c, and receiving unit 63b.sub.4 demodulates the
data that is sent in the signal light having wavelength .lamda.25
and inputs the result to a decoding circuit 63c.
[0065] The decoding circuit 63c performs a decoding process using
the four wave data that was demodulated by each demodulation
circuit to decode the data to the original data, and inputs that
data to the personal computer 71. In other words, the decoding
circuit 63c generates packet signals for each wavelength and stores
the packets in an internal buffer, then when all of the four wave
packets have been generated, uses those four packets to decode one
desired packet, and inputs the result to the personal computer 71.
When data is not ciphered, the four wave packets that are sent are
input in order as is to the personal computer 71. As was described
above, the data transmission speed for one wave is 10 Gbps, and by
performing wavelength multiplexing for the four waves, the data
transmission speed becomes 40 Gbps. Time-division multiplexing is
performed for the user data for 16 users at this data transmission
speed of 40 Gbps, so the average data transmission speed per user
is 40/16=2.5 Gbps. This data transmission speed of 2.5 Gbps is
remarkably faster than the conventional transmission speed of 30 to
100 Mbps, making high-speed transmission possible with this
invention. Moreover, the allowable number of users that can be
handled is 16 users per group, and transmission is possible for 8
groups at the same time, so a maximum of 16.times.8=128 users can
be handled by one second relay station RST2-1 to RST2-8.
[0066] FIG. 6 is a drawing showing a second example of construction
of a receiver for a subscriber unit of group 1, and is an example
of construction in which four wave WDM optical signals are allotted
to users with one more wave allotted per user. This example of
construction corresponds to the cases in which the user does not
have such a great need for a high-speed line, or in which a normal
level of security is allowed, so it is possible to quadruple the
number of users that can be handled.
[0067] A fixed wavelength filter 63a.sub.1 selects a signal light
having wavelength .lamda.1 from the WDM optical signals
(wavelengths: .lamda.1, .lamda.9, .lamda.17, .lamda.25) of group 1,
and inputs that signal light to the receiving unit 63b.sub.1, after
which that receiving unit 63b.sub.1 demodulates the packet that is
sent in that signal light having wavelength .lamda.1, and inputs
that packet to the first user terminal 71.sub.1.
[0068] Moreover, a fixed wavelength filter 63a.sub.2 selects a
signal light having wavelength .lamda.9 from the WDM optical
signals (wavelengths: .lamda.9, .lamda.17, .lamda.25) that are
output from the fixed wavelength selection filter 63a.sub.1, and
inputs that signal light to the receiving unit 63b.sub.2, after
which that receiving unit 63b.sub.2 demodulates the packet that is
sent in that signal light having wavelength .lamda.9, and inputs
that packet to the second user terminal 71.sub.2. Similarly, a
fixed wavelength selection filter 63a.sub.3 selects signal light
having wavelengths .lamda.17 and .lamda.25 from the WDM optical
signals (wavelengths: .lamda.17, .lamda.25) that are output from
the fixed wavelength selection filter 63a.sub.2, and inputs the
respective signal light to the receiving units 63b.sub.3 and
63b.sub.4, after which receiving unit 63b.sub.3 demodulates the
packets that is sent in the signal light having wavelength
.lamda.17 and inputs that packet to the third user terminal
71.sub.3, and the receiving unit 63b.sub.4 demodulates the packet
that is sent in the signal light having wavelength .lamda.25 and
inputs that packet to the fourth user terminal 71.sub.4. The
process described above is for DOWN transmission of group 1,
however the second relay stations RST2-2 to RST2-8 operate in the
same way for other groups as well.
[0069] As mentioned above, the data transmission speed for one wave
is 10 Gbps, and by performing wavelength multiplexing for four
waves, the data transmission speed becomes 40 Gbps. In the
construction shown in FIG. 6, user data for 16.times.4 users is
transmitted using time-division multiplexing at a data transmission
speed of 40 Gbps, so the average data transmission speed per user
is 40/(16.times.4) Gbps=625 Mbps, which is 1/4 that of the case
shown in FIG. 5. However, when compared with the conventional
transmission speed of 30 to 100 Mbps, high-speed transmission is
possible. Also, when increasing the number of users that can be
handled, since the optical signal already comes close to houses, it
is possible to increase users by using a normal electric signal
switching device (router or switching hub).
[0070] The case in which N=8 and k=16 was explained above, however,
generally, it is possible to effectively use a bandwidth with a
maximum of N.times.k user per relay station. At the same time it is
possible to create a flexible system that has both expandability
and secrecy.
(b) UP Transmission
[0071] In UP transmission, a control optical signal that is
branched by a 2-branch coupler 44 is used. In other words, part of
a control signal light is input to a switch control circuit 45,
however, the remaining light is used as is and is reused as the
return signal wavelength for data that is transmitted from a
subscriber unit. Therefore, when the transmission side SDS uses
control data to modulate an optical signal having a wavelength of
.lamda.a, instead of performing intensity modulation, phase
modulation is performed in which the optical power does not change
even after modulation. The purpose of the control data that is
superimposed on that control optical signal is for specifying the
switching destination and the timing at which switching is
performed, and since it is sufficient to have at least several ten
Mbps, there is no problem even though phase modulation is
performed.
[0072] The transmitters 64 of the subscriber units 61.sub.1 to
61.sub.16 all modulate 1.3 .mu.m band optical signals using 1 Gbps
UP transmission data, and transmit the optical modulation signals
to the second relay station RST2-1 via optical transmission paths
51.sub.1 to 51.sub.16 bi-directionally over one cable. The reason
for bi-directional transmission over one cable is to lower the cost
of the subscriber units, and to lower the transmission cost from
the subscriber units to the second relay station RST2. Therefore,
1.3 .mu.m band optical signals are used for transmission as used in
a GE-PON system.
[0073] The optical coupler 47 of the second relay station RST2-1
separates the 1.3 .mu.m band optical signals that are input from
the subscriber units 61.sub.1 to 61.sub.16, and inputs the signals
to a receiving/multiplexing unit 48. The receiving/multiplexing
unit 48 demodulates the transmission data of the subscriber units
61.sub.1 to 61.sub.16 from the 1.3 .mu.m band optical signals, and
shifts the demodulated transmission data of the subscriber units
61.sub.1 to 61.sub.16 in time, then multiplexes the data and
generates a 10 Gbps high-speed data signal.
[0074] An optical amplifier 46 amplifies in the saturation range
the control optical signal having a wavelength of .lamda.a that was
branched by the 2-branch coupler 44, to make the power of the
control optical signal constant and inputs the amplified signal to
an optical modulation unit 49. The optical modulation unit 49
performs intensity modulation of the control optical signal using
the 10 Gbps high-speed data that was input from the
receiving/multiplexing unit 48, and sends that signal to the
optical transmission path 24.sub.1. The optical modulation units
for the other groups also perform intensity modulation of the
control optical signals having wavelengths .lamda.b to .lamda.h
using 10 Gbps high-speed data, and send the signals to the optical
transmission paths 24.sub.2 to 24.sub.8. A multiplexer 33 of the
first relay station RST1 multiplexes the optical signals that are
input from the optical transmission paths 24.sub.1 to 24.sub.8, and
an optical amplifier 34 performs optical amplification of the
multiplexed WDM optical signal, then sends the signal to an UP
transmission path 22 that is different from the DOWN transmission
path 21, and transmits the signal to the transmission side SDS. On
the transmission side SDS, an optical amplifier 15 amplifies the
WDM optical signal, and a demultiplexing filter 16 demultiplexes
the signal into optical signals for each wave length .lamda.a to
.lamda.h, then inputs the signals to receivers 17.sub.1 to
17.sub.8. The reason for keeping the UP transmission path different
from the DOWN transmission path 21 is that due to the effect of
interference, bi-directional transmission over one cable is not
possible.
[0075] Using a dotted line, FIG. 7 shows the path of the control
optical signal having a wavelength of .lamda.a for the optical
communication network system shown in FIG. 4.
[0076] FIG. 8 is a drawing showing the construction of the
receiving/multiplexing unit 48, where the same reference numbers
are used for parts that are identical to those shown in FIG. 4 and
FIG. 7. Optical signal receiving units 48a.sub.1 to 48a.sub.16
receive 1.3 .mu.m band optical signals from the subscriber units
61.sub.1 to 61.sub.16 that have been separated by optical couplers
47.sub.1 to 47.sub.16, and demodulates the 1 Gbps UP transmission
data and inputs the respective transmission data to a serializer
48b. By shifting the timing of the 1 Gbps UP transmission data and
performing multiplexing, the serializer 48b generates 10 Gbps
time-division multiplexed data. An amplifier 48c amplifies that
time-division multiplexed data and inputs the result to an optical
modulation unit 49. The optical modulation unit 49 uses the
time-division multiplexed data to perform intensity modulation of
the control optical signal.
(c) Construction of the Transmission Side
[0077] FIG. 9 is a drawing showing the construction of an Optical
Line Terminal (OLT), which is a unit on the transmission side,
where the same reference numbers are used for parts that are the
same as those shown in FIG. 4. The construction of the transmission
unit 11 for group 1 is shown in detail, and the other groups 2 to 8
have the same construction.
[0078] In transmission unit 11, optical signal generation units
11a.sub.1 to 11a.sub.5 generate data transmission signal light and
control signal light for group 1. That is, the optical signal
generation units 11a.sub.1 to 11a.sub.4 generate data signal light
having wavelengths .lamda.1, .lamda.9, .lamda.17, .lamda.25, and
optical signal generation unit 11a.sub.5 generates a control signal
light having a wavelength of .lamda.a. Optical modulation units
11b.sub.1 to 11b.sub.4 perform intensity modulation of the data
signal light using the transmission data that is output from the
transmission signal generation unit 11c, and inputs the optical
modulation signal to a multiplexer 12.sub.1. The optical modulation
unit 11b.sub.5 performs modulation of the control signal light
using the control data (switching control data) that is output from
the transmission signal generation unit 11c, and inputs modulated
optical signal to a multiplexer 12.sub.1. As shown in FIG. 5, at
the second relay station, if intensity modulation of the control
signal light is performed using the UP transmission data and
returned to the transmission side, the optical modulation unit
11b.sub.5 performs phase modulation and does not perform intensity
modulation. The multiplexer 12.sub.1 multiplexes the input 5 wave
optical signal, and the cyclic multiplexing/demultiplexing filter
13 multiplexes the optical signal of each group and sends the
result to the transmission path 21.
[0079] The transmission signal generation unit 11c shifts
respectively the data and control data to be transmitted to the 16
users of group 1 one timeslot by one timeslot and performs
multiplexing (time-division multiplexing), then inputs the data to
the optical modulation units 11b.sub.1 to 11b.sub.5 via amplifiers
11d.sub.1 to 11d.sub.5. The data that is sent to each of 1b users
is input to the transmission signal generation unit 11c from a
provider center 81, video distribution center 82, telephone
exchange station 83, mobile communications company 84, or the like.
The transmission source for data that is sent to each user can be
the same or different.
[0080] In the case where encoding is not performed and four waves
are used for one user, the signal generation unit 11c divides the
packet to be sent to the first user into 4-bit units, and inputs
the four bits to optical modulation units 11b.sub.1 to 11b.sub.4
during a specified timeslot (for example, the first timeslot
period). Next, the signal generation unit 11c divides the packet to
be sent to the second user into 4-bit units, and sends the four
bits to the optical modulation units 11b.sub.1 to 11b.sub.4 during
the second timeslot period, then similarly inputs the data for each
of the users to the optical modulation units 11b.sub.1 to
11b.sub.4. In addition, the signal generation unit 11c generates
control data that specifies the transmission destination of the
transmission data for each timeslot, and inputs that data to the
optical modulation unit 11b.sub.5. By the processing described
above, packets and control data for each user undergo time-division
multiplexing and wavelength multiplexing and are sent to the user
side.
[0081] On the other hand, in the case where ciphering is performed
and data (packets) is transmitted, the signal generation unit 11c
performs ciphering and creates four packets using the packet to be
sent to the first user, inputs one bit for each packet to the
optical modulation units 11b.sub.1 to 11b.sub.4 respectively during
the first timeslot period. Next, the signal generation unit 11c
performs ciphering and creates four packets using the packet to be
sent to the second user, then inputs one bit for each packet to the
optical modulation units 11b.sub.1 to 11b.sub.4 during the second
timeslot period, after which the signal generation unit 11c
similarly ciphers the packets for each user and inputs the packets
to the optical modulation units 11b.sub.1 to 11b.sub.4. In
addition, the signal generation unit 11c generates control data
that specifies the transmission destination of transmission data
for each timeslot, and inputs that data to the optical modulation
unit 11b.sub.5. From the process described above, the ciphered
packets and control data for each user undergoes time-division
multiplexing and wavelength multiplexing, and are transmitted to
the user side.
[0082] FIG. 10 is a drawing showing the construction of the signal
generation unit 11c in the case when ciphering is performed, where
a selector 11c.sub.1 selects for each timeslot the packets for the
users that are input from a provider center 81, video distribution
center 82, telephone exchange station 83, mobile communications
company 84, and together with inputting the packets to an ciphering
circuit 11c.sub.2, inputs the packets to a control data generation
unit 11c.sub.3. The ciphering circuit 11c.sub.2 uses the input
packets to perform ciphering and create four packets, then inputs
one bit for each packet to the optical modulation units 11b.sub.1
to 11b.sub.4 at a specified speed during the timeslot periods.
Moreover, a control data generation unit 11c.sub.3 identifies the
destinations from the input packets and generates control data
(switching control signal) and inputs that data to the operation
modulation unit 11b.sub.5.
[0083] When one wave is used for one user, the signal generation
unit 11c inputs the packets to be sent to the first to fourth user
one bit at a time to the optical modulation units 11b.sub.1 to
11b.sub.4 during the first timeslot.
[0084] Next, the signal generation unit 11c inputs the packets to
be sent to the fifth to eighth users one bit at a time to the
optical modulation units 11b.sub.1 to 11b.sub.4 during the second
timeslot, then similarly inputs in order the packets for four users
one bit at a time to the optical modulation units 11b.sub.1 to
11b.sub.4. Moreover, the signal generation unit 11c generates
control data that specifies the destination (subscriber unit) of
the transmission data for each timeslot and inputs that data to the
optical modulation unit 11b.sub.5. From the process described
above, the packets and control data for each user undergo
time-division multiplexing and wavelength multiplexing, and are
transmitted to the user side. The transmission source of the
packets to be sent to each user can be the same or different.
[0085] Returning to FIG. 9, a demultiplexing filter 16
demultiplexes the optical signals from the WDM optical signal that
was input from the UP transmission path 22 for the wavelengths
.lamda.a to .lamda.h, and inputs the signal to the receivers
17.sub.1 to 17.sub.8, after which the receivers 17.sub.1 to
17.sub.8 demodulate the data sent from the subscriber units of each
of the groups 1 to 8, and input the results to the deserializers
18.sub.1 to 18.sub.8 of the corresponding group. Each deserializer
18.sub.1 to 18.sub.8 converts the input serial data (time-division
multiplexed data for 16 users) to parallel data, inputs the
transmission data for each user to the transmitters 19.sub.1 to
19.sub.4 that correspond to the respective destination centers, and
transmits the data to each center 81 to 84.
(d) Signal Switching
[0086] FIG. 11 is a drawing explaining the signal switching format,
and shows the case for group 1 when there is 16 users per group and
4 wavelength is used for each user. In the timeslot T1 on the right
side, data for the first user is transmitted, in the timeslot T2,
the data for the second user is transmitted, in the timeslot T3,
the data for the third user is transmitted, . . . , in the timeslot
T15, the data for the fifteenth user is transmitted, and in the
16th timeslot, the user data for the sixteenth user is transmitted.
In this case, the switching control signal is transmitted before
the data, for example, one timeslot before, and the switching
signal specifies the destination for the data after the next
timeslot.
[0087] In the case of transmitting one packet in one wave for each
timeslot, the data time width for one wave in each timeslot is 1 to
several 10 microseconds (.mu.sec). This is because a packet length
is 1 kbyte to several 10 kbytes, and the data transmission speed
for one wave is 10 Gbps. As a result, the maximum transmission
interval for each user is approximately 16 to 1000 microseconds
(.mu.sec). In other words, the time width of each timeslot and the
transmission interval T are variable.
[0088] FIG. 12 is a drawing that explains the signal switching
timing, and shows the case for group 1. In timeslot T1, the data
for the first user 1-1 is transmitted, in timeslot T2, the data for
the eighth user 1-8 is transmitted, in timeslot T3, the date for
the fifteenth user 1-15 is transmitted, and assuming that the
transmission is performed in the same way after that, the switching
signal is transmitted before the data, for example, one timeslot
before, and this switching signal specifies the destination of the
data for the next timeslot. In other words, based on the switching
signal that was received one timeslot before, the switching control
circuit 45 (see FIG. 4) sets the optical switch that will be turned
ON/OFF for the current timeslot, then performs ON/OFF control and
transmits an optical signal to a specified user.
[0089] FIG. 13 is a drawing explaining the signal switching timing
for each group, and is an example in which in timeslot T1, the data
for the first user 1-1 in group 1 and the data for the eighth user
2-8 in group 2 are transmitted, and in timeslot T2, data for the
second user 1-2 in group 1, the data for the second user 2-2 in
group 2 and the data for the thirteenth user 8-13 in group 8 are
transmitted, after which transmission is performed in a similar
way.
(C) Second Embodiment
[0090] FIG. 14 shows a second embodiment, where the same reference
numbers are given to parts that are the same as those of the first
embodiment shown in FIG. 4. The construction of this embodiment
differs in that: (1) C-L couplers 20, 35 are provided on the
transmission side and relay station side of a common transmission
path 25; (2) the transmitter 65 of a subscriber unit performs
intensity modulation of L-band signal light of a 1.5 .mu.m band
using transmission data, and transmits that signal light to the
second relay station RST2-1; and (3) a serializer 50 is provided
for each group in the second relay stations RST2-1 to RST2-8, and
by shifting the optical signals that are input from each subscriber
unit in time and combining them, outputs the signal as an optical
signal having one wavelength. The C-L couplers 20, 35 are both
optical wavelength multiplexing/demultiplexing filters, or optical
circulators, and they select and transmit C-band optical signals in
the DOWN direction, and select and transmit L-band optical signals
in the UP direction.
[0091] During DOWN transmission, the C-L coupler 20 transmits a
C-band 40 wave WDM optical signal that was multiplexed by a cyclic
multiplexing/demultiplexing filter 13 to the transmission path 25,
and the C-L coupler 35 selects the C-band 40 wave WDM optical
signal that was input from that transmission path 25, and inputs
the signal to a cyclic multiplexing/demultiplexing filter 31 via an
optical amplifier 32. After that, by performing the same control
that was performed in the first embodiment shown in FIG. 4, signal
light having specified wavelengths is transmitted to the subscriber
units of each group.
[0092] During UP transmission, the transmitters 65 of the
subscriber units 61.sub.1 to 61.sub.16 of group 1 perform intensity
modulation of L-band signal light of a 1.5 .mu.m band having a
specified wavelength .lamda.L1 using transmission data, and
transmits that light to the second relay station RST2-1. The
optical coupler 47 of the second relay station RST2-1 separates the
optical signals having wavelength .lamda.L1 that were input from
the subscriber units 61.sub.1 to 61.sub.16, and inputs the signals
to the serializer 50.
[0093] The serializer 50 comprises a high-speed switch unit 50a
that shifts each of the optical signals that were input from the
subscriber units 61.sub.1 to 61.sub.16 in time and outputs the
signals, and a multiplexing unit 50b that combines the optical
signals that were output from the high-speed switch 50a to output a
serial signal. The high-speed switch unit 50a comprises the same
construction as the high-speed switch unit 42, and comprises 16
optical switches SW1' to SW16' that perform
connection/disconnection between an input port and output port; On
timing of the optical switches SW1' to SW16' are controlled by
switch signals that are input from the switch control circuit 45.
From this, the optical signals that are input from the subscriber
units 61.sub.1 to 61.sub.16 are shifted in time and input to the
multiplexing unit 50b. The multiplexing unit 50b combines the
optical signals that are inputted from the optical switches SW1' to
SW16' of the high-speed switch unit 50a, and transmits the result
to the transmission path 24.sub.1 as a time-division multiplexed
optical signal having a transmission speed of 10 Gbps and
wavelength of .lamda.L1.
[0094] Similarly, for other groups as well, the multiplexing unit
50b transmits optical signals having a transmission speed of 10
Gbps and wavelengths of .lamda.L2 to .lamda.L8 to transmission
paths 24.sub.2 to 24.sub.8.
[0095] The multiplexer 33 combines the optical signals having
wavelengths .lamda.L1 to .lamda.L8 that are input from the optical
transmission paths 24.sub.1 to 24.sub.8, and an optical amplifier
34 performs optical amplification of the combined WDM optical
signal then inputs the result to the C-L coupler 35. The C-L
coupler 35 selects the L-band 8 wave WDM signal and transmits the
signal to the transmission path 25 in the DOWN direction. The C-L
coupler 20 on the transmission side selects the L-band 8 wave WDM
optical signal in the DOWN direction and inputs the signal to a
demultiplexing filter 16 via an optical amplifier 15, and that
demultiplexing filter 16 demultiplexes the optical signal into each
of the wavelengths .lamda.L1 to .lamda.L8 and inputs the results to
the receivers 17.sub.1 to 17.sub.8.
[0096] The switch control circuit 45 generates a switch signal
based on control data that is included in the DOWN control optical
signal. FIG. 15 is a drawing explaining the signal switching
format, and is for the case of group 1 when there is 16 users per
group and 4 wavelengths is used for each user.
[0097] In the case where UP transmission data is transmitted 14
timeslots after a subscriber unit receives data, the transmitting
side knows the timing at which the subscriber unit receives data,
so the transmitting side already knows the timeslot that a
subscriber unit transmits data. Therefore, the transmitting side
specifies the timing (timeslot) that a subscriber unit transmits
data.
[0098] In FIG. 15, the transmission side specifies the destination
user for receiving a packet by the control data C1, and specifies
the source user for transmitting a packet by the control data C2.
In other words, in FIG. 15, according to control data C1, the
transmitting side transmits first user data at timeslot T1,
transmits second user data at timeslot T2, transmits third user
data at timeslot T3, . . . , transmits fifteenth user data at
timeslot T15, and transmits sixteenth user data at timeslot T16.
Moreover, according to control data C2, data is transmitted from
the first user at timeslot T15, data is transmitted from the second
user at timeslot T16, and data is transmitted from the third user
at timeslot T1.
[0099] Based on control data C1 that is received one timeslot
before, the switch control circuit 45 sets the optical switch of
the switch unit 42 to turn ON/OFF in the current timeslot, and
performs ON/OFF control to transmit an optical signal to a
specified user. Moreover, based on a control signal C2 that is
received one timeslot before, the switch control circuit 45 sets
the optical switch of the switch unit 50 to turn ON/OFF in the
current timeslot, and performs ON/OFF control to output an optical
signal to the multiplexer 50b.
[0100] With this second embodiment, bi-directional transmission can
be performed over one fiber optic transmission path.
[0101] Effect
[0102] With the present invention a plurality of users (for example
16 users) are formed into a group and the data for the users in
that group undergo wavelength multiplexing and time-division
multiplexing before being transmitted, so it is possible to build
an End-to-End optical access network system that is capable of
keeping down the cost per bit.
[0103] Particularly, in a subscriber device, by separating a
received optical signal into each wavelength, and inputting the
light of each wavelength to each of the subscriber terminals, it is
possible to further increase the number of users that can be
handled, and suppress the cost per bit.
[0104] With the present invention, active optical elements
(high-speed switching elements) are used in a relay station, and by
shifting the ON/OFF timing of the switching elements, the
time-division multiplexed data for each user is transmitted only to
the subscriber unit of the respective users, so it is possible to
build an optical access system that is capable of maintaining
physical secrecy.
[0105] Moreover, with the present invention, a subscriber unit
optical separates out signals having a plurality of wavelengths
from an input optical signal, and uses the separated plurality of
optical signals to decode a signal that was ciphered on the
transmission side, then inputs that signal to a subscriber
terminal, so it is possible to build an optical access system that
is capable of further maintaining secrecy.
[0106] With the present invention, Semiconductor Optical Amplifiers
(SOA) that are capable of being turned ON/OFF are used as switching
elements, so there is no need to provide special optical
amplifiers, as well as multicasting is possible, thus a low cost
and compact relay station is possible.
[0107] With the present invention, the relay station is divided
into two or more, with switching being performed among stations, so
it is possible to build an optical access network system using tree
shape or star shape network topology construction.
[0108] With the present invention, by using a cyclic
multiplexing/demultiplexing filter, it is possible to reduce the
number of ports of a multiplexer/demultiplexer, and to expand the
number of users that can be handled in group units, thus it is
possible to reduce the initial cost for employing the system.
[0109] With the present invention, it is possible to switch
Wavelength Division Multiplexed (WDM) packet signals altogether, so
it becomes easy to increase or decrease bandwidth according to need
on the user side.
[0110] With the present invention, the switching elements are
turned ON/OFF using control optical signals that are transmitted
together with the data optical signals from the transmission side,
so it is possible to transmit time-division multiplexed data for
each user to just the respective subscriber units of the users.
Moreover, with this invention, a control optical signal is used as
a return optical signal, and that optical signal is modulated with
the UP data, so a low-cost ONU is possible.
[0111] With the present invention, an optical transmitter that uses
transmission data to modulate and output an optical signal having a
different wavelength than a DOWN optical signal is provided in a
subscriber unit, where optically modulated signals that are input
from optical transmitters of a plurality of subscriber units in
each group undergo time-division multiplexing to form an optical
signal having one wavelength, and the optical signals from each
group are multiplexed and transmitted to the transmission side via
the same transmission path as the DOWN transmission path so
bi-directional transmission is possible over one fiber optic
transmission path.
* * * * *