SDH Ring Architectures This section examines SDH unidirectional and bidirectional ring architectures and examines the differences between two-fiber and four-fiber SDH rings. A comparison is also made between multiplex section (ring) switching versus path (span) switching. SDH provides for three attributes with two choices each, as illustrated in Table 6-13 . Table 6-13. SDH Ring Types SDH Attribute Value Fibers per link 2-fiber 4-fiber Signal direction Unidirectional Bidirectional Protection switching Multiplex section switching Path switching Table 6-13 shows various SDH ring configurations that differ in at least one major attribute. The commonly used ring types and topologies are as follows: Two-fiber subnetwork connection protection ring (two-fiber SNCP) Two-fiber multiplex section-shared protection ring (two-fiber MS-SPRing) Four-fiber multiplex section-shared protection ring (four-fiber MS-SPRing) Unidirectional Versus Bidirectional Rings In a unidirectional ring, the working traffic is routed over the clockwise spans around the ring, and the counterclockwise spans are protection spans used to carry traffic when the working spans fail. Consider the two-fiber ring schematic presented in Figure 6-26 . Traffic from NE1 to NE2 traverses span 1 in a clockwise flow, and traffic from NE2 to NE1 traverses span 2, span 3, and span 4 in a clockwise flow as well. Spans 5, 6, 7 are used as protection spans and carry production traffic when one of the working clockwise spans fail. Figure 6-26. Unidirectional Versus Bidirectional Rings
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SDH Ring Architectures
This section examines SDH unidirectional and bidirectional ring architectures and
examines the differences between two-fiber and four-fiber SDH rings. A comparison
is also made between multiplex section (ring) switching versus path (span)
switching. SDH provides for three attributes with two choices each, as illustrated in Table 6-13.
Table 6-13. SDH Ring Types
SDH Attribute Value
Fibers per link 2-fiber
4-fiber
Signal direction Unidirectional
Bidirectional
Protection switching Multiplex section switching
Path switching
Table 6-13 shows various SDH ring configurations that differ in at least one major
attribute. The commonly used ring types and topologies are as follows:
Two-fiber subnetwork connection protection ring (two-fiber SNCP)
Two-fiber multiplex section-shared protection ring (two-fiber MS-SPRing) Four-fiber multiplex section-shared protection ring (four-fiber MS-SPRing)
Unidirectional Versus Bidirectional Rings
In a unidirectional ring, the working traffic is routed over the clockwise spans around
the ring, and the counterclockwise spans are protection spans used to carry traffic
when the working spans fail. Consider the two-fiber ring schematic presented in
Figure 6-26. Traffic from NE1 to NE2 traverses span 1 in a clockwise flow, and traffic
from NE2 to NE1 traverses span 2, span 3, and span 4 in a clockwise flow as well.
Spans 5, 6, 7 are used as protection spans and carry production traffic when one of the working clockwise spans fail.
Figure 6-26. Unidirectional Versus Bidirectional Rings
Bidirectional traffic flows can also be illustrated using the schematic of Figure 6-26.
In a bidirectional ring, traffic from NE1 to NE2 would traverse span 1 in a clockwise
flow. However, traffic from NE2 to NE1 would traverse span 5 in a counterclockwise
fashion. If the links between NE1 and NE2 were to fail, traffic between NE1 and NE2 would use the spans between NE2-NE3, NE3-NE4, and NE4-NE1.
Two-Fiber Versus Four-Fiber Rings
Unidirectional and bidirectional systems both implement two-fiber and four-fiber
systems. Most commercial unidirectional systems, such as SNCP, are two-fiber
systems, whereas bidirectional systems, such as MS-SPRing, implement both two-
fiber and four-fiber infrastructures. A two-fiber STM-N unidirectional system with two
nodes is illustrated in Figure 6-27. Fiber span 1 carries N working channels
eastbound, and fiber span 5 carries N protection channels westbound. For example,
an STM-16 system would carry 16 working VC-4s eastbound from NE1 to NE2, while
carrying 16 separate protection VC-4s westbound from NE2 to NE1. The SDH transport and POH bytes are carried on both working and protection fiber spans.
MS-SPRing is commonly implemented on two-fiber as well as four-fiber systems. MS-
SPRing nodes can terminate traffic that is fed from either side of the ring and are
suited for distributed node-to-node traffic applications, such as interoffice networks
and access networks. MS-SPRings allow bandwidth to be reused around the ring and
can carry more traffic than a network with traffic flowing through one central hub. MS-SPRing supports nonrevertive and revertive protection mechanisms.
out to node D on Fiber 1 and proceeds to node C. Node C has sensed an LOS from
failed node D and reroutes S3 on to Fiber 2 as signal S3 VC-4(12). Signal S3 passes
via node B and arrives at node A. However, because node A cannot deliver this traffic
to node D, it places S3 on Fiber 1 as S3 VC-4(6). This signal gets dropped off at
node B, because VC-4(6) already has a connection from node A to node B (signal
S4). This event results in the traffic being delivered to the wrong node and is called a
misconnection. In some situations, it is possible that bridging traffic after a node failure could also lead to a misconnection.
Figure 6-37. MS-SPRing Node Failure
MS-SPRing misconnections can be avoided by using the squelching mechanism. The
squelching feature uses automatically generated squelch maps that require no
manual record-keeping to maintain. Each node maintains squelch tables to know
which connections need to be squelched in the event of a node failure. The squelch
table contains a list of inaccessible nodes. Any traffic received by a node for the
inaccessible node is never placed on the fiber and is removed if discovered.
Squelching involves sending the AIS in all channels that normally terminated in the
failed node rather than real traffic. The misconnection is avoided by the insertion of
an AIS path by nodes A and C into channel VC-4(6). In an AIS path, all the bits
belonging to that path are set to 1 so that the information carried in that channel is
invalidated. This way, node B is informed about the error condition of the ring, and a
misconnection is prevented. Misconnection can occur only in MS-SPRing when a node
is cut off and traffic happens to be terminated on that node from both directions on
the same channel (VC-4). In some implementations, the path trace might also be
used to avoid this problem. If node B monitors the path trace byte, it will recognize
that it has changed after the misconnection. This change should be a sufficient indication that a fault has occurred, and that traffic should not be terminated.
Four-Fiber MS-SPRing
Four-fiber MS-SPRings double the bandwidth of two-fiber MS-SPRings. As shown in
Figure 6-38, two fibers are allocated for working traffic and two fibers are allocated
for protection. Signal S1 from node A to node B would use VC-4(1) of the working
Fiber 1, and the return path from node B to node A would use VC-4(1) of the
working Fiber-3. Signal S2, added at node A and destined for node C, would use VC-
4(2–5) of the working Fiber 1, and would use VC-4(2–5) of the working Fiber-3 for
its return path from node C to node A.
Figure 6-38. Four-Fiber MS-SPRing
[View full size image]
Signal S3, added at node B and destined for node D, would use VC-4(6) of the
working Fiber 1 via node C, and would use VC-4(6) of the working Fiber-3 for its
return path from node D to node B, via node C. Signal S4, added at node A and
destined for node C, would use VC-4(7–12) of the working Fiber 1, and would use VC-4(7–12) of the working Fiber-3 for its return path from node C to node A.
Four-fiber MS-SPRing allows path (span) switching as well as MS (ring) switching,
thereby increasing the reliability and flexibility of traffic protection. Path (span)