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A DCS node is equipped with a switch fabric so that it can switch virtual tributaries from one input interface to an output interface. Specifically, the switch fabric can switch the data carried in one or more time slots of each incoming frame from any input interface to the same number of time slots, but not necessarily in the same position, of the outgoing frame of any output interface. It serves all of the input interfaces simultaneously.
SONET/SDH rings are typically deployed in a metropolitan area, either as metro edge rings or as metro core rings. A metro edge ring is used to transport traffic between customers and a hub, which is a SONET/SDH node that is attached to a metro core ring. Typical customers include: ADSL-based access networks, cable-based access networks, small telephone switches (private branch exchange, or PBX), storage access networks (SAN), and enterprise networks. A metro core ring interconnects metro edge rings, large telephone switches, and ISP points of presence (POP). It also sends and receives traffic to and from larger regional and long-haul networks. Traffic demands on a metro core ring are dynamic, unlike a metro edge ring, which has fairly static traffic patterns. Metro core rings are interconnected using DCS nodes to form a mesh network.
2.6 SELF-HEALING SONET/SDH RINGS
SONET/SDH rings have been specially architected so that they are highly reliable. Specifically, they are available 99.999% of the time, which translates to an average downtime for the network of only six minutes per year! One of the main causes for a ring to go down is failure of a fiber link. This can happen when the fiber is accidentally cutoff (backhoe fade), or when the transmission or receiver equipment on the fiber link fail. Also, link failure can occur when a SONET/SDH device fails, although this happens very rarely since these devices have a high degree of redundancy. Fiber cuts due to digging in an area where fiber cables pass through, however, are quite common. SONET/SDH rings are self-healing, so that the ring’s services can be automatically restored following a link failure or a degradation in the network signal. This is done using the automatic protection switching (APS) protocol. The time to restore the services has to be less than 50 msec.
In this section, we first describe protection schemes for point-to-point SONET/SDH links, and then we describe several self-healing SONET/SDH ring architectures.
The simplest SONET/SDH network is a point-to-point fiber link that connects two SONET/SDH devices. Link protection can be done in a dedicated 1 + 1 manner, or in a shared 1:1 or a 1 : N manner. In the 1 + 1 scheme, the two devices are connected with two different fibers (see Figure 2.15). One is designated as a working fiber, and the other as a protection fiber. The SONET/SDH signal is split and then transmitted simultaneously over both fibers. The destination selects the best of the two signals based on their quality. If one fiber fails, the destination continues to receive the signal from the other fiber. The working and protection fibers have to be diversely routed. That is, the two fibers use separate conduits and different physical routes. Often, for economic reasons, the two
SONET/SDH AND THE GENERIC FRAME PROCEDURE (GFP)
Figure 2.15 The 1 + 1 protection scheme.
(a) Two-fiber ring (b) Four-fiber ring
Figure 2.16 SONET/SDH 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 SONET/SDH ring architectures are distinguished by the following three features:
• Number of fibers: A SONET/SDH 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 SONET/SDH ring, there are two working rings and two protection rings (one per working ring).