THE SONET STS-3 FRAME STRUCTURE

36 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP ADM Working Protection ADM Figure 2.15 The 1 + 1 protection scheme. a Two-fiber ring b Four-fiber ring 1 2 3 4 5 6 7 8 ADM ADM ADM ADM ADM ADM ADM ADM Figure 2.16 SONETSDH rings. fibers use different conduits, but they use the same physical path. In this case, we say that they are structurally diverse. In the 1:1 scheme, there are still two diversely routed fibers: a working fiber and a protection fiber. The signal is transmitted over the working fiber. If this fiber fails, then the source and destination both switch to the protection fiber. The 1:N scheme is a generalization of the 1:1 scheme, whereby N working fibers are protected by a single protection fiber. Since there is one protection fiber, only one working fiber can be protected at any time. Once a working fiber has been repaired, the signal is switched back, either automatically or manually, from the protection fiber to the working fiber. Self-healing SONETSDH ring architectures are distinguished by the following three features: • Number of fibers : A SONETSDH ring can consist of either two or four fibers see Figure 2.16. In the two-fiber ring, fibers 1, 2, 3, and 4 are used to form the working ring , and fibers 5, 6, 7, and 8 are used to form the protection ring. Transmission on the working ring is clockwise; on the protection ring, it is counter-clockwise as indicated by the arrows in Figure 2.16. In another variation of the two-fiber ring, each set of fibers i.e. fibers 1, 2, 3, 4 and fibers 5, 6, 7, 8 form a ring that can function as both a working ring and a protection ring. In this case, the capacity of each fiber is divided into two equal parts: one for working traffic and the other for protection traffic. In a four-fiber SONETSDH ring, there are two working rings and two protection rings one per working ring. As in the case of a point-to-point SONETSDH fiber link, the working and protection rings are route diverse. That is, the fibers between two adjacent SONETSDH devices use different conduits and different physical routes. The working and protection rings can also be structurally diverse, which is typically more economical. In this case, the fibers between two adjacent SONETSDH devices use different conduits, but they follow the same physical path. SELF-HEALING SONETSDH RINGS 37 • Direction of transmission : A SONETSDH ring can be unidirectional or bidirectional. In a unidirectional ring, signals are only transmitted in one direction of the ring; in a bidirectional ring, signals are transmitted in both directions. • Line or path switching : Protection on a SONETSDH ring can be at the level of a line or a path. Recall from Section 2.3.1 that a line is a link between two SONETSDH devices and might include regenerators. A path is an end-to-end connection between the point where the SPE originates and the point where it terminates. Note that Section 9.2 refers to line protection as link protection. Line switching restores all of the traffic that pass through a failed link, and path switching restores some of the connections that are affected by a link failure. Based on these three features, we have the following two-fiber or four-fiber ring archi- tectures: unidirectional line switched ring ULSR, bidirectional line switched ring BLSR, unidirectional path switched ring UPSR , and bidirectional path switched ring BPSR. Of these rings, the following three are currently used: two-fiber unidirectional path switched ring 2F-UPSR, two-fiber bidirectional line switched ring 2F-BLSR , and four-fiber bidi- rectional line switched ring 4F-BLSR These three ring architectures are discussed below in detail.

2.6.1 Two-fiber Unidirectional Path Switched Ring 2F-UPSR

This ring architecture, as its name implies, consists of two fibers with unidirectional transmission and path switching. Figure 2.17 shows an example of this ring architecture type. The working ring consists of fibers 1, 2, 3, and 4; the protection ring consists of fibers 5, 6, 7, and 8. The ring is unidirectional, meaning that traffic is transmitted in the same direction. That is, A transmits to B over fiber 1 of the working ring, and B transmits over fibers 2, 3, and 4 of the working ring. Protection is provided at the path level using a scheme similar to the 1 + 1 described above. That is, the signal transmitted by A is split into two; one copy is transmitted over the working fiber fiber 1, and the other copy is transmitted over the protection fibers fibers 8, 7, and 6. During normal operation, B receives two identical signals from A and selects the one with the best quality. If fiber 1 fails, then B will continue to receive A’s signal over the protection path. The same applies if there is a node failure. This is a simple ring architecture; it is used as a metro edge ring to interconnect PBXs and access networks to a metro core ring. Typical transmission speeds are OC-3STM- 1 and OC-12STM-4. The disadvantage of this ring architecture is that the maximum amount of traffic it can carry is equal to the traffic it can carry over a single fiber. ADM 1 ADM 2 ADM 3 ADM 4 5 2 6 4 8 3 7 A Protection ring Working ring 1 B Figure 2.17 An example of a 2F-UPSR.