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OFC '97 Technical Digest
Thursday Afternoon
successfuldeployment. Without this, there is the risk that time and effort
(i.e., dollars) will be spent mairiaining and operating separate networks.
Once all of this is considered, we must also investigate the physical
viability for technology deployment. High-capacity optical networks
have two potential avenues for growth: time-division multiplexing
(TDM) and wavelength-division multiplexing (WDM).Each technology
has its own physical limitations for deployment. We must determine if
the technology meets the requirements for the deployment area. If the
technology does not match the requirements of the physical environment, than other alternatives niust be considered.
A new technology technically proves in if the above criteria are met.
The final consideration is the economics of the technology; is there any
financial gain in deploying it? A new technology might meet all of our
technical criteria for deployment, however if it is not cost effective, then
we will not deploy it. If the gains do not outweigh the costs, then a sound
business case for deploying the new technology cannot be supported.
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WDM four-fiber ring with add/drop acousto-optic
tunable filter and 4 x 4 optical switch
S. Hamada, K. Asahi, T. tlosoi,* K. Nakaya,** D. Uehara,
C. Konishi, S. Fujita, 1 s t 'Transmission Division, 1753,
Shimonumabe, Nakahara, Kawasaki, 211, Japan; E-mail:
[email protected]. nec. co.jp
The wavelength-division multiplexing (WDM) ring has been considered
a cost-effective,reliable network architecture due to its features of bandwidth sharing and improved survivability.'
In a previously reported WDM ring network using transit tunable
filters, a tunable filter was employed in an optical wavelength add/drop
multiplexer because it is an effective component for reconfiguration in a
WDM ring.' However, the multiplexer, consisting of optical star couplers and convenient transit tunable filters, was very complex and generated excess signal loss. As the result, electrical regenerators were required to restore the signal-to-noise ratio. A four-port add/drop
acousto-optic tunable filter (AOTF) is desirable to realize an optical
add/drop multiplexer with simple configuration and low insertion loss.
A four-fiber ring using 2 X 2 optical switches has been also reported.3 In this system, the protection lines carry only the signals rerouted by
the 2 X 2 optical switch in the :vent of a failure on one of the working
paths. On the other hand, 4 X 4 optical switches offer double the
transmission capacity of the 2 X 2 optical switches in the ring, because a
low-priority signal transmission is performed through the protection
line under normal conditions.
We propose a four-fiber WDM ring employing both add/drop
AOTFs and 4 X 4 optical switchcs.The structure and performance of this
ring are detailed below.
Figure 1 shows the configuration of the WDM four-fiber ring with
four stations. There are two working fibers and two protection fibers.
Each station employs four-port LiNbO, AOTFs for optical add-anddrop multiplexing and LiNbO, 4 X 4 optical matrix switches for selfhealing on the WDM ring. The through port and drop port insertion loss,
and the cross talk in the AOTF were 8 dB, 8.3 dB and -15.2 dB,
respectively.The average insertion loss and cross talk in the 4 X 4 optical
switch were 7.2 dB and 29.8 dI!, respectively. The per-channel output
LXI
4x4 Optical Switch
'111
I OG-L~
0
313
IIOG-LT~
IIOG-LTIIIOG-LTI
T h o 2 Fig. 1. Configuration of WDM four-fiber ring. AOTF: acousto-optic
tunable filter; O A optical amplifier; 4 X 4 S W 4 X 4 optical matrix switch; 10
GLT: 10 Gbit/s line terminal; Rx: optical receiver; Tx: optical transmitter.
power from the booster erbium-doped fiber amplifier (EDFA) in each
station was set at +5 dBm. Standard single-mode fibers of 50 km and
dispersion-compensation fibers were utilized in all transmission lines.
A specific wavelength (1549.6 nm, 1550.8 nm, 1552.0 nm, 1553.2
nm) was assigned to each station and transmitted to another station. A
wavelength may be dropped from the optical transmission path and be
connected to a line terminal.
Figure 2 shows an example of signal flow between station 1 and
station 3. When a transmission failure occurs on working transmission
lines, traffic through the protection lines is rerouted by the 4 X 4 optical
switches [Fig. 2(b)].Even during a critical failure, such as the cut of a pair
of optical fibers, the ring resumes communication through protection
lines provided [Fig. 2(c)].
The nonblocking configuration in the 4 X 4 optical switchesand the
tunableness of the AOTFs also enable low-priority traffic between station
3 and station 4 to be transmitted through the protection lines under the
normal conditions. With this function, capability can be doubled.
Transmission experiments at 10 Gbitls were carried out on the
different paths shown in Figs. 2(a), (b), and (c). Figure 3 shows the
bit-error-rate (BER) characteristics when the input power per channel at
station 2 was set at -18 dBm and the input power at station 3 was
changed. The receiver sensitivity at IO-" BER was around -29 dBm/ch
for all channels. Receiver sensitivity of -29 dBm/ch was also obtained for
(a)Normal
14 13
condition
(c)All fiber cut
T h o 2 Fig. 2. Signal flow on the WDM ring. (a) normal condition; (b) working fiber cut; (c) all fiber cut.
314
0
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-38 -36 -34 -32 -30 -28 -26 -24 -22 -20
Average Received Power : Pr(dBm)
Tho2 Fig. 3. Bit-error-ratecharacteristics.
the transmission path between station 1 and station 4 through station 2
and station 3 with each input power of - 18 dBm/ch. Allowable transmission loss of 23 dB was confirmed for all transmission paths.
*Opto-Electronics Research Laboratories, NEC corporation, 4-1 -1,
Miyazaki, Miyamae-ku, Kasawaski, 21 6, Japan
““2nd Transmission Division, NEC Corporation
1. R.E. Wagner et al., IEEE J. Lightwave Technol. 14, 1349-1355
(1996).
2. P.A. Perrier et al., in Optical Fiber Communication Conference, Vol.
2, 1996 OSA Technical Digest Series (Optical Society of America,
Washington, D.C., 1996), paper ThD3.
3. R.E. Wagner etal., presented at OEC’94,1994, Japan, paper 14C3-1.
4. T. Hosoi etal., in Optical Fiber Communication Conference, Vol. 2 of
1996 OSA Technical Digest Series (Optical Society of America,
Washington, D.C., 1996), paper ThL2.
5. W. Nakabayashi et al., presented at ECOC’96,Oslo, Norway, paper
Thd2-4.
Automatic protection switching for multicasting in
optical mesh networks
OFC ’97 Technical Digest
tection) (see Fig. 1). Restoration times are roughly 50 ms (e.g.,
SONET) for low-speed rotary switches and a few ps’ for high-speed
switches. APS requires a priori knowledge of the full network to
preplan routes and sufficient spare capacity to accommodate rerouted traffic. Another common approach, based on digital crossconnect switches (DCS), performs dynamic local rerouting around a
link and generally leads to better bandwidth utilization.2 However,
the restoration times are of the order of seconds.
The most common approach to APS relies on self-healing rings
(SHRs), as in SONET, interconnected with diversity protection (DP),
where a logical link may be routed along more than one physical link.
Limiting ourselves to such building blocks, however, affects the cost of
the network. For instance, if the cost of laying fiber to be proportional to
length, then for certain node configurations a physical ring connection
may not be the cheapest redundant option.
In order to extend path protection to arbitrary redundant networks,
schemes have been developed for finding a pair of link or node-disjoint
paths for every pair of nodes.3 In a network with multicasting, such an
approach may be very inefficient in bandwidth. Trees are desirable for
multicasting, particularly in optical networks, where multicasting may be
performed by signal splitting.
Schemes for finding spanning trees which are edge-di~joint~
impose
more topology constraints than simple redundancy (see Fig. 2), because
an edge cannot be used in both the primary and the secondary tree.
Schemes that look at arcs (directed edges)5 do not require stricter requirements than redundancy, but fail to take into account that failure of
communications in one direction usually implies failure in the reverse
direction also (see Fig. 2, where failure of the circled arcs disconnects the
destination).
Our novel approach finds redundant directed trees in arbitrary
redundant mesh networks where failure of one edge entails failure of
communications in both directions on that edge. Thus, the problem of
finding the minimum cost redundant topology and the problem of the
APS scheme can be handled separately. For every source node s, we find
two spanning trees, which we shall denote by Blue and Red, rooted at s.
Blue is the tree used when there are no failures. The trees are selected so
that, if a failure of a link or a node occurs, the traffic upstream of that
failure on Blue need not be re-routed, while the traffic downstream of the
failure will use Red. Figure 3 gives an example of our construction. We
see, in contrast with Fig. 2, that several edges are used by both the primary
and the secondary tree but that failure of two arcs on the same edge does
not disconnect any node from the source. We can view our method as a
Muriel Medard, Steve Finn, Rick Barry, MIT Lincoln Laboratoiy,
244 Wood Street, Lexington, Massachusetts 021 73; E-mail:
medard@l\.miL edu
Reliability in optical networks is a growing area of concern as highbandwidth networks are expanded and interconnected. Even if robust
operating conditions are established, redundancy is necessary to preclude catastrophic loss of data in case of failure of a link or of a node in a
network. We shall give a brief overview of the common approaches to
failure recovery, discuss the related topological issues and present a new
approach to multicasting, which reduces topological constraints to a
provable minimum.
In very high bandwidth networks, service restoration time is
important because many bits may be lost while service is disrupted.
Therefore, we look at automatic protection switching (APS) schemes,
which rely on pre-planned alternative end-to-end routes (path pro-
Tho3 Fig. 1. Example of path rerouting.