SDH Ring Protection Mechanisms

The ring protection types are as follows;

1. In Europe, 2f-MS-SPRing, 4f-MS-SPRing or 2f-SNCP

2. In USA, 2f-BLSR, 4f-BLSR or 2f-UPSR

Where, BLSR is Bi-Directional Line-Switched Rings, UPSR is Uni-Directional Path-

Switched Rings and MS-SPRing is the Multiplex Section-Shared Protected Rings, SNCP

ADM

DXC

ADM

DXC

ADM

is Sub-Network Connection Protection. Table 2.3 illustrates this logical equivalence of

terminology.

SONET in USA SDH in Europe

2 fiber Uni-directional Path-Switched

Ring (2f UPSR)

2 fiber Sub-Network Connection Protection Ring

(2f SNCP Ring)

2 fiber Bi-directional Line-Switched

Ring (2f BLSR)

2 fiber Multiplex Section Shared Protected Rings

(2f MS-SPRing)

4 fiber Bi-directional Line-Switched

Ring (4f BLSR)

4 fiber Multiplex Section Shared Protected Rings

(4f MS-SPRing)

Table 2.3 The Ring Protection Architectures

SDH rings are also called self-healing rings since they incorporate protection mechanisms

that detect failures and reroute traffic onto reserved channels rapidly.

In a dedicated protection ring, every normal path has a corresponding protection path and

in a shared protection ring, several normal paths may use a single protection path.

Although SDH meshed architectures offer more advanced functionalities and ring

networks require high capacity and have limited flexibility, ring networks have dominant

advantages as fast restoration, being economical, practical and easy management.

The main advantage of ring protection schemes is fast restoration time since the network

switching time is a major factor for the transmission networks. The switch completion

time in a SDH ring for a failure on a single span is defined as less than 50 msec by ITU

Telecommunication Standardization Sector (ITU-T) Recommendation G-841. Ring

networks are also preferred due to economical reasons since their lower nodal costs in

Metropolitan Area Networks [8].

MS-SPRing architecture implements traffic routing function electrically in the SDH

equipment of nodes. The nodes adjacent to a section or node failure are responsible for

the protection switching action. Protection is shared at the Multiplex Section level by

dividing the capacity of the SDH frames in half for service and protection channels [9].

The communication protocol restricts the number of nodes to 16 in a ring in the MSSPRing

mechanisms.

There are two types of MS-SPRing; 2-fiber and 4-fiber rings that are shown in Fig. 2.13,

Fig. 2.14 respectively.

Figure 2.13 Two-fiber MS-SPRing

Figure 2.14 Four-fiber MS-SPRing

: Working Channels

: Protection Channels

: Working Channels

: Protection Channels

ADM 4 ADM 3

ADM 2 ADM 1

: Protection Channels

: Working Channels

2-fiber MS-SPRing protection architecture uses half of the capacity of the ring for

working traffic, and reserves the other half of the ring capacity for protection. This

protection mechanism uses both of the fibers to carry working traffic, but half of the

capacity on each fiber is reserved for protection purpose. 2-fiber rings with shared

protection use ring switching for protection purposes.

4-fiber MS-SPRing uses two fibers for working traffic and reserves the other two fibers

for hot stand-by in case of a failure. A 4-fiber MS-SPRing has two types of protection

mechanisms, span switching and ring switching. 4-fiber rings enable the protection

mechanisms of either ring or span switching, but not both of them simultaneously.

Figure 2.15 Two-fiber MS-SPRing Protection

under link failure condition

An example of MS-SPRing protection with two fibers against a single link failure is

given in Figure 2.15. In normal conditions, both channels of the demand 2-4 travel

through 2-1-4. When a failure occurs on the fiber between node 1 and node 2, the

adjacent ADMs to the failure node 1 and node 2 are responsible for protection switching.

In the case of the failure condition, the traffic is looped back to the channels dedicated for

protection as illustrated in the Fig.2.15. Since MS-SPRing Protection architecture is used,

the half of the capacity is reserved to handle failures.

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