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.

Figure 6-27. Two-Fiber Unidirectional Ring

[View full size image]

A two-fiber STM-N bidirectional system with two nodes is illustrated in Figure 6-28.

On each fiber, a maximum of half the bandwidth or number of channels are defined

as working channels, and the other half are defined as protection channels. Fiber

span 1 carries (N/2) working channels and (N/2) protection channels eastbound, and

fiber span 5 carries (N/2) working channels and (N/2) protection channels

westbound. For example, an STM-16 system would carry eight working VC-4s and

eight protection VC-4s eastbound from NE1 to NE2, while carrying eight working VC-

4s and eight protection VC-4s westbound from NE2 to NE1. Each fiber has a set of

SDH transport and POH bytes for the working and protection channels.

Figure 6-28. Two-Fiber Bidirectional Ring

A four-fiber STM-N bidirectional system with two nodes is shown in Figure 6-29.

Fiber pair span 1A and 5A carry N working channels full duplex east and westbound,

while fiber pair span 1B and 5B carry N protection channels full duplex east and

westbound. For example, an STM-16 system would carry 16 working VC-4s

eastbound from NE1 to NE2 as well as 16 working VC-4s westbound from NE2 to

NE1. The same system would also carry 16 protection VC-4s eastbound from NE1 to

NE2 as well as 16 protection VC-4s westbound from NE2 to NE1. A set of SDH

transport and POH bytes is dedicated either to working or protection channels for the four-fiber ring.

Figure 6-29. Four-Fiber Bidirectional Ring

[View full size image]

As can be seen for an STM-N fiber system, two-fiber SNCP provides N * VC-4s,

whereas two-fiber MS-SPRing provides (N/2) * VC-4s in either direction. Four-fiber

MS-SPRing, on the other hand, provides N * VC-4s in either direction. Usually two

segment failures will cause a network failure or outage on a two-fiber ring of either

type. However, four-fiber systems with diverse routing can suffer multiple failures

and still function. Four-fiber systems are widely used for rings spanning large

geographical areas or when the traffic being carried on the network is mission critical.

SDH rings are limited to 16 nodes per ring because the K1/K2 bytes that define the

source and destination node were defined with only 4 bits each. However, vendors

have implemented proprietary mechanisms that use unused bytes from other fields

in the SDH header to extend the limit on the number of nodes. For example, the

Cisco ONS 15454 SDH uses 4 bits from the K1/K2 fields and 2 additional bits of the

K3 byte in MS-SPRing configurations. The K3 byte also carries information on the

K1/K2 bytes. Out of the 2 K3 bits, 1 bit is used to define the source and the other bit

is used to define the destination node. Use of the 2 additional K3 bits increases the

node count to 32 NEs per MS-SPRing ring. In such a case, however, if the span has

to pass through third-party equipment, the K3 byte needs to be remapped to an unused SDH overhead byte, such as the E2 or F1 byte.

Path and Multiplex Section Switching

Path switching works by restoring working channels at a level below the entire STM-

N capacity in a single protection operation. This means that levels lower than an

STM-N, such as VC-3s, VC-12, or VC-11s, can be restored in the event of a failure.

Path switching is shown in Figure 6-30. Live protected user traffic is always sent on

the working fiber. However, a copy of the protected traffic is also transmitted over

the protection fiber. The receiver constantly senses the signal level of both the

working and protection fibers. In the event of a fiber cut or signal degradation on the

working fiber, the receiver switches to the incoming signal available on the

protection fiber. All unprotected traffic is dropped for the duration of the outage. Path switching is mostly implemented on two-fiber SNCP rings.

Figure 6-30. Path Switching

[View full size image]

MS switching works by restoring all working channels of the entire STM-N capacity in

a single protection operation. The protection channels or fiber are idle while the ring

operates normally. MS switching is shown in Figure 6-31. Live protected user traffic

is always sent on the working channels or fiber. In the event of a fiber or node

failure, the protected traffic is switched to the protection channels or fiber at both

ends of the span. Channels within the MS are switched this way, which is why it is

called line switching. In the event of a failure, all unprotected traffic being

transmitted on the protection link or protected channels is dropped. This is called

protection channel access (PCA), and the traffic carried this way is called extra

traffic. Carriers typically discount unprotected PCA bandwidth, thereby enabling

customers to maintain a more cost-effective network without having to pay for a

five-nines service level agreement (SLA). Line-switching systems are able to restore

service within 50 ms. Line switching is mostly implemented on two-fiber and four-fiber bidirectional rings.

Figure 6-31. Multiplex Section Switching

[View full size image]

Dual-Ring Interconnect

The dual-ring interconnect (DRI) architecture allows subtending rings sharing traffic

to be resilient from a matching node failure perspective. As shown in Figure 6-32, a

DRI topology uses two interconnecting matching nodes, DRI node 3 and DRI node 4,

to connect the two STM-N rings. If one of the interconnected nodes fails, traffic is

routed through the surviving DRI node. The benefit to the service provider is that

continuous network operation is maintained even though a node failure has occurred.

The DRI topology provides an extra level of path protection between rings. In a DRI

configuration, traffic is dropped and continued at the interconnecting nodes to

eliminate single points of failure. Each ring protects against failures within itself using

path-switched and/or MS-switched protection mechanisms, whereas DRI provides

protection against failures at the interconnections. DRI cannot provide protection if

both DRI node 3 and DRI node 4 experience simultaneous failure.

Figure 6-32. Dual-Ring Interconnect

[View full size image]

As shown in Figure 6-32, a signal input at node 1 destined for node 7 is bridged east

and west. The downstream primary eastbound signal passes through node 2 and

arrives at the DRI node 3. At DRI node 3, a duplicate copy of the signal is dropped

and transmitted to DRI node 4. Similarly, the downstream secondary westbound

signal passes through node 5 and arrives at the DRI node 4. At DRI node 4, a

duplicate copy of the signal is dropped and transmitted to DRI node 3. The

downstream path selector at node 3 always selects the primary downstream signal

during steady-state normal operation. However, the downstream path selector at

node 4 always selects the secondary downstream signal during steady-state normal

operation. The primary downstream signal at node 3 is then continued and

transmitted to node 6 on ring 2 that acts as a pass-through node and transmits the

signal to node 7. Similarly, the secondary downstream signal at node 4 is then

continued and transmitted to node 8 on ring 2 that acts as a pass-through node and transmits the signal to node 7.

Node 7 receives two copies of the downstream signal (primary and secondary).

However, the path selector in node 7 always selects the primary downstream signal

during steady-state normal operation. A similar process takes place with the primary

and secondary upstream signal. Suppose that DRI node 3 fails. In such an event, the

path selector at node 7 will switch to the secondary downstream signal. The

upstream traffic is not affected, because the primary upstream path is through the surviving node 4.

Subnetwork Connection Protection Rings

An SNCP ring is a survivable, closed-loop, transport architecture that protects

against fiber cuts and node failures by providing duplicate, geographically diverse

paths for each circuit. SNCP provides dual fiber paths around the ring. Working traffic

flows clockwise in one direction, and protection traffic flows counterclockwise in the

opposite direction. If fiber or node failure occurs in the working traffic path, the

receiving node switches to the path coming from the opposite direction. Because

each traffic path is transported around the entire ring, SNCPs are best suited for

networks where traffic concentrates at one or two locations and is not widely

distributed. SNCP capacity is equal to its bit rate. This means that an STM-N SNCP

ring will always provide N * VC-4s of capacity. Services can originate and terminate

on the same core SNCP ring, or can be passed across a matching node to an access

ring for transport to the service-terminating location. Figure 6-33 shows a basic

SNCP configuration. This drawing can also be used to explain basic two-fiber SNCP

operation with its various subtleties. The schematic illustrates the operation of a two-

fiber STM-16 ring using SNCP as its protection mechanism. The outer Fiber 1 is the

working fiber that carries traffic in a clockwise direction. The inner Fiber 2 is the

protection fiber that carries a copy of the working traffic in a counterclockwise direction.

Figure 6-33. Two-Fiber SNCP

If node A sends a signal S1 to node B, the working signal travels on the working

traffic path to node B. The same signal is also sent on the protect traffic path from

node A to node B, via nodes D and C. For node B to reply to node A, the signal uses

the working VC-4 path around the ring via nodes C and D. Note that signal S1 or VC-

4(1)A is the first VC-4 of the STM-16. Signal S1 consumes the entire VC-4 around

the ring. Therefore, it is not possible for another signal, such as S1 VC-4(1)B, to be

transmitted between node C and node D using SNCP protection. However, VC-4(1)B

could be transmitted unprotected between nodes C and D. This circuit would be dropped in the event SNCP protection is invoked.

Signal S2 is added at node A and dropped at node C. Signal S2 contains VC-4(2–5)

or [VC-4, number 2 to number 5 of the STM-16]. This also means that VC-4(2–5)

cannot be used for adds or drops at other nodes on the ring. For node C to reply to

node A, the signal uses the working VC-4 path around the ring via node D. Signal S3

VC-4(6) is added at node B and dropped at node D, effectively blocking any other

adds or drops for VC-4(6) at any of the other nodes. For node D to reply to node B,

the signal uses the working VC-4 path around the ring via node A. Finally, signal S4

is added at node A and dropped at node C. Signal S4 contains VC-4(7–16); that

means VC-4(7–16) cannot be used for adds or drops at other nodes on the ring. For

node C to reply to node A, the signal uses the working VC-4 path around the ring via node D.

As shown in Figure 6-34, if a fiber break occurs on the working Fiber 1 between node

A and node B, node B switches its active receiver to the protect signal coming

through node C. Signals S1 and S3 would be received on the protection fiber for the

duration of the outage. The switchover would happen within the 50-ms SDH

restoration time. Signals S2 and S4 would be received on node C via node D on the

protection fiber. If there were a fiber cut on the protection Fiber 2, however, the

system would continue operating without any disruption. The element management

system would detect the fiber cut and report the LOS on the protection fiber. Repairs could be performed on the Fiber 2 without service interruption.

Figure 6-34. Two-Fiber SNCP Protection

Asymmetrical Delay

As shown in Figure 6-33, any signal from node A to node B traverses a single span.

When node B has to reply to node A, however, the signal has to traverse multiple

spans via node C and node D. In the case of small metropolitan rings, this does not

create any issues. However, for large transcontinental rings, a finite delay could

affect voice or data applications. In the case of voice applications, the cumulative

delay should not exceed 100 ms. So long as the asymmetric delay does not exceed

100 ms, the human user would not perceive any delay. In the case of data, transport

layer windowing comes into play. With asymmetric delay, two end hosts might

experience a 40-ms round-trip delay. One host might perceive a 5-ms inbound delay

with a 35-ms outbound delay. It would be the exact opposite for the other host.

Issues occur when one data application tries to adjust its window size for a 20/20-ms

split in delay, while data keeps arriving early (5 ms). The host on the other end

experiences exactly the opposite effect, when adjusting its window size for a 20/20-

ms split, with data arriving late (25 ms).

Multiplex Section-Shared Protection Rings

MS-SPRing uses bidirectional multiplex section–switched protection mechanisms.

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.

Two-Fiber MS-SPRing

In a two-fiber MS-SPRing ring, each fiber carries working and protection VC-3s. In an

STM-16 MS-SPRing, as shown in Figure 6-35, for example, VC-4s 1 through 8 carry

the working traffic, and VC-4s 9 through 16 are reserved for protection. Working

traffic travels clockwise in one direction on one fiber and counterclockwise in the opposite direction on the second fiber.

Figure 6-35. Two-Fiber MS-SPRing

In Figure 6-35, signal S1 VC-4(1)A added at node A, destined for a drop at node B,

typically will travel on Fiber 1, unless that fiber is full (in which case, circuits will be

routed on Fiber 2 through nodes C and D). Traffic from node A to node C (or node B

to node D) can be routed on either fiber, depending on circuit-provisioning

requirements and traffic loads. For node B to reply to node A, the signal uses the

working VC-4(1) path on Fiber 2.

Signal S2 VC-4(2–5) added at node A, destined for a drop at node C, typically will

travel on Fiber 1 via node B, unless that fiber is full (in which case, the circuit will be

routed on Fiber 2 via node D). For node C to reply to node A, the signal uses the

working VC-4(2–5) path on Fiber 2 via node B. Signal S3 VC-4(6) added at node B,

destined for a drop at node D, typically will travel on Fiber 1 via node C, unless that

fiber is full (in which case, the circuit will be routed on Fiber 2 via node A). For node

D to reply to node B, the signal uses the working VC-4(6) path on Fiber 2 via node C.

It is quite apparent that only VC-4 * 8 worth of bandwidth can be configured on a

two-fiber STM-16 MS-SPRing. This is not entirely true. Unlike SNCP, the provisioning

of VC-4(1) does not consume the entire first VC-4 of the STM-16 around the ring.

Bandwidth is reusable, as shown by S1 VC-4(1)B in Figure 6-35, and can be

provisioned between nodes C and D. With careful bandwidth-capacity planning, MS-SPRing could be quite efficient.

NOTE

The bidirectional bandwidth capacities of two-fiber MS-SPRings is the STM-N rate

divided by two, multiplied by the number of nodes in the ring, minus the number of pass-through VC-4 circuits.

The SDH K1 and K2 bytes carry the information that governs MS-SPRing protection

switching. Each MS-SPRing node monitors the K bytes to determine when to switch

the SDH signal to an alternate physical path. The K bytes communicate failure

conditions and actions taken between nodes in the ring. If a break occurs on one

fiber, working traffic targeted for a node beyond the break switches to the protect

bandwidth on the second fiber. The traffic travels in reverse direction on the protect

bandwidth until it reaches its destination node. At that point, traffic is switched back to the working bandwidth.

As shown in Figure 6-36, if a break occurs in Fiber 1 between node A and node B,

signal S1 VC-4(1)A that would normally travel between node A and B using VC-4(1)

of Fiber 1 would MS switch to VC-4(9) of Fiber 2 and reach node B via nodes D and C

for the duration of the outage. The switchover would happen within the 50-ms SDH

restoration time. Signal S2 VC-4(2–5) added at node A and destined for node C

would also be affected. S2 would be MS switched to VC-4(10–13) of Fiber 2 and

would reach node C via node D. Signal S3 VC-4(6) would not be affected. Now

consider the case where Fiber 1 is intact and there is a break in Fiber 2 between

nodes A and B. In such a case, the return path for signal S1 VC-4(1)A between node

B and node A is lost. An MS switch would occur and signal VC-4(1)A would switch to

VC-4(9) of Fiber 1 and reach node A via nodes C and D. The return path for signal S2

VC-4(2–5) between node C, destined for node A, would also be affected. Node C

would transmit signal S2 VC-4(2–5) back to node A over Fiber 2. However, the fiber

cut on Fiber 2 (between nodes A and B), detected by node B, would cause all return

traffic to node A to be MS switched to VC-4(10–13) of Fiber 1 and retransmitted to node A via nodes C and D.

Figure 6-36. Two-Fiber MS-SPRing Protection

Finally, consider a case of a dual fiber cut of both Fiber 1 and Fiber 2 between nodes

A and B. In such a case, signal S1 VC-4(1)A added at node A and destined for node

B would be MS switched to VC-4(9) of Fiber 2 and would reach node B via nodes D

and C. The return path for signal S1 VC-4(1)A between node B and node A would MS

switch to VC-4(9) of Fiber 1 and reach node A via nodes C and D. Signal S2 VC-4(2-

5) added at node A and destined for node C would be MS switched to VC-4(10–13) of

Fiber 2 and would reach node C via node D. Node C would transmit the return signal

S2 VC-4(2–5) back to node A over Fiber 2. However, the fiber cut on Fiber 2

(between node A and B), detected by node B, would cause all return traffic to node A

to be MS switched to VC-4(10–13) of Fiber 1 and retransmitted to node A via nodes

C and D. All unprotected traffic carried over the protection VC-4s is dropped in the event of an MS switch.

MS-SPRing Node Failure

MS-SPRing restoration gets complex in the event of a node failure. MS-SPRing uses a

protection scheme called shared protection. Shared protection is required because of

the construction of the MS-SPRing ring and the reuse of VC-4s around the ring. This

creates a situation in which the VC-4s on a protection fiber cannot be guaranteed to

protect traffic from a specific working VC-4. Shared protection, which provides MS-

SPRing its capability to reuse bandwidth, brings with it additional problems when a

node failure occurs in a MS-SPRing ring. Consider the MS-SPRing schematic in Figure

6-37. This schematic shows a complete failure of node D. Trace the path of signal S3

as it gets added on to node B with a destination node D. Signal S3 VC-4(6) gets sent

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)

switching occurs when a working span fails. Traffic switches to the protect fibers

between the nodes and then returns to the working fibers. Multiple span switches

can occur at the same time. MS (ring) switching occurs when a span switch cannot

recover traffic, such as when both the working and protect fibers fail on the same

span. In an MS (ring) switch, traffic is routed to the protect fibers throughout the full ring.

As shown in Figure 6-39, if the working fiber pair between node A and B fails, all

working traffic between these nodes is shunted onto the protection fiber pair. Any

unprotected traffic mapped between other nodes on the ring is unaffected by this outage.

Figure 6-39. Four-Fiber MS-SPRing Span Switch

[View full size image]

Signal S1 from node A to node B would use VC-4(1) of protection Fiber 2, and the

return path from node B to node A would use VC-4(1) of protection Fiber-4. Signal

S2, added at node A and destined for node C would use VC-4(2–5) of protection

Fiber 2 between node A and B, after which it would revert to VC-4(2–5) of the

working Fiber 1 between nodes B and C. Signal S2 would use VC-4(2–5) of the

working Fiber-3 for its return path from node C to node B, after which it would use

VC-4(2–5) of protection Fiber-4 between nodes B and A. Signal S3 would be

unaffected. However, signal S4, added at node A and destined for node C, would use

VC-4(7–12) of protection Fiber 2 between node A and node B, after which it would

revert to VC-4(7–12) of the working Fiber 1 between nodes B and C. Signal S4 would

use VC-4(7–12) of the working Fiber-3 for its return path from node C to node B,

after which it would use VC-4(7–12) of protection Fiber-4 between nodes B and A.

Four-fiber MS-SPRing ring switching is shown in Figure 6-40. If both fiber pairs

between node A and B fail, all working traffic between these nodes is wrapped onto

the protection fiber pairs. Any unprotected traffic mapped between other nodes on

the ring is preempted and dropped because all the protection pairs are used during the outage.

Figure 6-40. Four-Fiber MS-SPRing Ring Switch

[View full size image]

Signal S1 from node A to node B would use VC-4(1) of protection Fiber-4 via nodes

D and C, and the return path from node B to node A would use VC-4(1) of protection

Fiber 2 via nodes C and D. Signal S2, added at node A and destined for node C,

would use VC-4(2–5) of protection Fiber-4 via node D. On its return path from node

C to node A, signal S2 would use VC-4(2–5) of the working Fiber-3 between node C

to node B. Node B would cause a wrap and switch the traffic to VC-4(2–5) of

protection Fiber 2 for a drop at node A via nodes C and D. Signal S3 would be

unaffected. Signal S4, added at node A and destined for node C, would use VC-4(7–

12) of protection Fiber-4 via node D. On its return path from node C to node A,

signal S4 would use VC-4(7–12) of the working Fiber-3 between node C to node B.

Node B would cause a wrap and switch the traffic to VC-4(7–12) of protection Fiber 2 for a drop at node A via nodes C and D.

SDH Network Management

SDH NEs need OAM&P support to be managed by carriers and service providers. The

OAM&P of an NE is the task of its EM. EMs are device-specific and vary by vendor. In

a typical service provider environment, there could be multiple EMs. The integration

of the various EMs along with fault management (FM), performance management

(PM), accounting management (AM), security management (SM), configuration

management (CM), and trouble ticketing and billing applications is the function of the

OSS. Multiple OSS systems that manage the data communications network (DCN)

constitute the TMN. The TMN has been standardized by the ITU-T under

Recommendation M.3010. SDH devices can be remotely managed through the use of

in-band management channels in the RSOH and MSOH, known as DCCs. Figure 6-41

shows an intercarrier TMN model. OSS-1 is operated by Carrier 1 and OSS-N is

operated by Carrier N. The OSS accesses the DCN via a gateway network element (GNE).

Figure 6-41. OSS and TMN Schematic

[View full size image]

The DCC channels can transport operations and management messages that let OSS

systems comply with the TMN specification. However, many SDH equipment vendors

have established proprietary element management schemes, and there is little

interoperability between vendors in the use of these bytes. The DCC bytes, D1

through D3 in the RSOH, are known as DCCR. The 3 DCCR bytes provide a 192-kbps

communications channel. The DCC bytes, D4 through D12 in the MSOH, are known

as DCCM. The 9 DCCM bytes provide a 576-kbps communications channel. Most SDH

systems use DCCR bytes for management purposes and don't use the DCCM bytes by

themselves. The Cisco Transport Manager (CTM) enables service providers to

manage their Cisco SDH and optical transport devices collectively under one

management system. Cisco also uses a craft tool and element management system

(EMS) for comprehensive SDH and optical transport management called Cisco

Transport Controller (CTC). Cisco has developed its management application based

on an IP stack coupled with an Open Shortest Path First (OSPF)-based topology

discovery mechanism. Furthermore, Cisco ONS devices can tunnel their DCCR bytes

through an ONS network. The bytes are tunneled by copying the DCCR bytes into 3

of the DCCM bytes. Because there are 3 DCCR bytes and 9 DCCM bytes, the ONS

devices have the capability to transport traffic from 3 different SDH networks

simultaneously across any given span. The DCCR bytes are restored as they leave the

ONS network, thus permitting interoperability with non-ONS networks. The Cisco

EMS supports Transaction Language 1 (TL-1), Common Object Request Broker

Architecture (CORBA), and SNMP for OAM&P purposes.

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