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As in the case of a point-to-point SONET/SDH fiber link, the working and protection rings are route diverse. That is, the fibers between two adjacent SONET/SDH 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 SONET/SDH devices use different conduits, but they follow the same physical path.
SELF-HEALING SONET/SDH RINGS
• Direction of transmission: A SONET/SDH 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 SONET/SDH 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 SONET/SDH 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 architectures: 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 bidirectional 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-3/STM-1 and OC-12/STM-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.
Figure 2.17 An example of a 2F-UPSR.
38 SONET/SDH AND THE GENERIC FRAME PROCEDURE (GFP)
2.6.2 Two-fiber Bidirectional Line Switched Ring (2F-BLSR)
This is a two-fiber ring with bidirectional transmission and line switching. It is used in metro core rings. As shown in Figure 2.18, fibers 1, 2, 3, 4, 5 and 6 form a ring (Ring 1), on which transmission is clockwise. Fibers 7, 8, 9, 10, 11, and 12 meanwhile form another ring (Ring 2), on which transmission is counter-clockwise. Unlike the 2F-UPSR, both Rings 1 and 2 carry working and protection traffic. This is done by dividing the capacity of each fiber on Rings 1 and 2 into two parts. One part is used to carry working traffic, and the other part to carry protection traffic. For instance, let us assume that the transmission speed of each fiber is OC-12/STM-4. Then, two OC-3/STM-1s are allocated to working traffic and the other two to protection traffic. Since only two OC-3/STM-1s can be used for working traffic, the maximum capacity that the 2F-BLSR can carry over both Rings 1 and 2 is OC-12/STM-4. The capacity allocated to protection traffic on either Rings 1 and 2 can be used to carry low priority traffic. This traffic can be preempted in case of failure of a fiber.
The ring is bidirectional, which means that a user can transmit in either direction. That is, it can transmit on either Ring 1 or Ring 2, depending on the route of the shortest path to the destination. In view of this, under normal operation, A transmits to B over the working part of fibers 1 and 2 of Ring 1, and B transmits to A over the working part of fibers 8 and 7 of Ring 2.
Assume that fiber 2 fails. Since the ring provides line switching, all of the traffic that goes over fiber 2 will be automatically switched to the protection part of Ring 2. That is, all of the traffic will be rerouted to ADM 3 over the protection part of Ring 2 using fibers 7, 12, 11, 10, and 9. From there, the traffic for each connection will continue on following the original path of the connection. For instance, let us consider a connection from A to C, as indicated in Figure 2.18 by the solid line. When fiber 2 fails, the traffic from A will be rerouted, as shown in Figure 2.18 by the dotted line. At ADM 3, the traffic will continue along the same path as the original connection. That is, it will be routed to back to ADM 4 over fiber 3.