Optical Networks: A Practical Perspective - Part 58 pdf

540 NETWORK SURVIVABILITY SONET/SDH. A ring is the simplest topology offering an alternate route around a failure. In the optical layer, many protection schemes have been designed to operate over true mesh topologies. Protection may be dedicated or shared. In dedicated protection, each working connection is assigned its own dedicated bandwidth in the network over which it can be rerouted in case of a failure. In shared protection, we make use of the fact that not all working connections in the network fail simultaneously (for example, if they are in different parts of the network). Therefore, by careful design, we can make multiple working connections share protection bandwidth among themselves. This helps reduce the amount of bandwidth needed in the network for protection. Another advantage of shared protection is that the protection bandwidth is available to carry low-priority traffic under normal conditions. This low-priority traffic is discarded in the event of a failure when the bandwidth is needed to protect a connection. Protection schemes can either be revertive or nonrevertive. In both schemes, if a failure occurs, traffic is switched from the working path to the protect path. In a nonrevertive scheme, the traffic remains on the protect path until it is manually switched back onto the original working path, usually by a user through the network management system. In a revertive scheme, once the working path is repaired, the traffic is automatically switched back from the protect path onto the working path. Reversion allows the network to return to its original state once the failure is restored. Dedicated protection schemes may be revertive or nonrevertive; however, shared protection schemes are usually revertive. Since multiple working connections share a common protection bandwidth, the protection bandwidth must be freed up as soon as possible after the original failure has been repaired, so that it can be used to protect other connections in the event of another failure occurring. To confuse terminology further, the protection switching can be unidirectional or bidirectional. This is not to be confused with unidirectional transmission or bidi- rectional transmission over a fiber. Figure 10.1 illustrates the two schemes for the case where two fiber pairs are used on the point-to-point link, with each fiber carry- ing traffic in one direction (unidirectional transmission). In unidirectional protection switching, each direction of traffic is handled independent of the other. Thus in the event of a single fiber cut, only one direction of traffic is switched over to the protection fiber and the other direction remains on the original working fiber. In bidirectional switching, both directions are switched over to the protection fibers. For the case where bidirectional transmission is used, the switching mostly becomes bidirectional by default because both directions of traffic are lost when a fiber is cut (both directions may not be lost if there is an equipment failure, rather than a fiber cut). Unidirectional protection switching is used in conjuction with dedicated protec- tion schemes since it can be implemented very easily by switching the traffic at the 10.1 Basic Concepts 541 Figure 10.1 Unidirectional and bidirectional protection switching. (a) The link is shown under normal operation. (b) Unidirectional protection switching. After a unidirectional fiber cut, only the affected direction of traffic is switched over to the protection fiber. (c) Bidirectional protection switching. After a undirectional fiber cut, both directions of traffic are switched over to the protection fibers. receiving end from the working to the protect path, without requiring a signaling protocol between the receiver and the transmitter. For example, in Figure 10.1, if a fiber carrying traffic from left to right is cut, without affecting the fiber carrying traffic from right to left, the transmitter on the left is not aware that there has been a failure. In the case of unidirectional dedicated protection, if traffic is transmitted simultaneously on the working and protect paths, the receiver at the end of the paths simply selects the better of the two arriving signals. However, if bidirectional switch- ing is required, the receiver needs to inform the transmitter that there has been a cut. This requires a signaling protocol, called an automatic protection-switching (APS) protocol. 542 NETWORK SURVIVABILITY A simple APS protocol works as follows: if a receiver in a node detects a fiber cut, it turns off its transmitter on the working fiber and then switches over to the protection fiber to transmit traffic. The receiver at the other node then also detects the loss of signal on the working fiber and then switches its traffic over to the protection fiber. Actual APS protocols used in SONET and optical networks are quite a bit more complicated because they have to deal with many different possible scenarios than the one described here. In a bidirectional communication system, where traffic is transmitted in both directions over a single fiber, a fiber cut will be detected by both the source and the destination. While no APS protocol is required to deal with fiber cuts, an APS protocol will still be needed to deal with unidirectional equipment failures and to support other maintenance functions. In the case of shared protection schemes, an APS protocol is required to coordi- nate access to the shared protection bandwidth. Therefore most shared protection schemes use bidirectional protection switching because it is easier to control and manage in a more complex network than unidirectional switching. There is also the question of how and where the traffic is rerouted in the event of a failure. Here we distinguish between path switching, span switching, and ring switching. Figure 10.2 illustrates these concepts. In path switching (Figure 10.2(b)), the connection is rerouted end to end from its source to its destination along an alter- nate path. In span switching (Figure 10.2(c)), the connection is rerouted on a spare link between the nodes adjacent to the failure. In ring switching (Figure 10.2(d)), the connection is rerouted on a ring between the nodes adjacent to the failure. Finally, different protection schemes operate at different layers in the network (for example, SONET/SDH, ATM, MPLS, IP) and at different sublayers within a layer. For example, there are schemes that protect one connection at a time, as well as schemes that protect all connections on a failed fiber together. In SONET/SDH networks, the former schemes operate at the path layer, and the latter schemes operate at the line (multiplex section in SDH) layer. In many cases, path layer schemes operate end to end, rerouting traffic along an alternate path all the way from the source to the destination. In contrast, line layer schemes are almost all localized~that is, they reroute traffic around the failed link. Similarly, in the optical layer, we have schemes operating either at the optical channel layer or the optical multiplex section layer. 10.2 Protection in SONET/SDH A major accomplishment of SONET and SDH network deployment was to provide a significant improvement in the availability and reliability of the overall network. This was done through the use of an extensive set of protection techniques. Similar 10.2 Protection in SONET/SDH 543 Figure 10.2 Path, span, and ring switching. (a) Working path for the connection under normal operation. (b) Path switching, where the connection is rerouted end to end on an alternate path. (c) Span switching, where the connection is rerouted on a spare link between the nodes adjacent to the failure. (d) Ring switching, where the connection is rerouted on a ring between the nodes adjacent to the failure. schemes are used in both SONET and SDH, but their nomenclature is different. We will specify both nomenclatures but use the SONET nomenclature for the most part. A taxonomy of the different protection schemes is given in Table 10.1. We will start by describing the different types of protection mechanisms that are used for simple point-to-point links, and then discuss how these can be applied for networks. Each protection scheme can be associated with a specific layer in the network. As we saw in Chapter 6, the SONET layer includes a path layer and a line layer. Both path layer and line layer protection schemes are used in practice. Equivalently, SDH networks use both channel layer and multiplex section (MS) layer protection schemes. A path layer protection scheme operates on individual paths or connections in the network. For example, in an OC-48 (2.5 Gb/s) ring supporting STS-1 (51 Mb/s) connections, a path layer scheme would treat each STS-1 connection independently 544 NETWORK SURVIVABILITY Table 10.1 A summary of protection schemes in SONET and SDH. N denotes the number of working interfaces that share a single protection interface. The schemes operate either in the path layer or in the SONET line layer/SDH multiplex section (MS) layer. Path layer ring schemes include unidirectional path-switched ring (UPSR) or 1 + 1 subnetwork connection protection (SNCP). Line layer ring schemes include bidirectional line-switched ring (BLSR) or, equivalently, multiplexed section-shared protection ring (MS-SPRing). Protection Scheme SONET Term 1 + 1 I:N UPSR BLSR SDH Term 1 + 1 I:N SNCP MS-SPRing Type Dedicated Shared Dedicated Dedicated Shared Topology Point-point Point-point Ring Ring/mesh Ring Layer Line/MS Line/MS Path/- -/path Line/MS and switch them independently of each other. A line layer scheme on the other hand, operates on the entire set of connections at once and generally does not distinguish between the different connections that are part of the aggregate signal. In the former example, a line layer protection scheme in an OC-48 ring would switch all the connections within the OC-48 together. (There are some exceptions to this statement. The bidirectional line-switched rings (BLSRs) that we will study later do allow bits to be set for each connection. In the event of a failure, only those connections that are specified are switched. This is needed to ensure that some connections can be left unprotected if so desired, and also to handle node failures, as we will see in Section 10.2.4.) 10.2.1 Point-to-Point Links Two fundamental types of protection mechanisms are used in point-to-point links: 1 + 1 protection and 1:1 or, more generally, 1 :N protection, as shown in Figure 10.3. Both operate in the line or multiplex section layer. In 1 + 1 protection, traffic is transmitted simultaneously on two separate fibers (usually over disjoint routes) from the source to the destination. Assuming unidirec- tional protection switching, the destination simply selects one of the two fibers for reception. If that fiber is cut, the destination simply switches over to the other fiber and continues to receive data. This form of protection is very fast and requires no signaling protocol between the two ends. Note that since connections are usually full duplex, there is actually a pair of fibers between the two nodes, say, node A and 10.2 Protection in SONET/SDH 545 Figure 10.3 Different types of protection techniques for point-to-point links: (a) 1 + 1 protection, where the signal is simultaneously transmitted over two paths; (b) 1:1 pro- tection, where the signal is transmitted over a working path under normal conditions but switched to a protect path after a failure; and (c) I:N protection, which is a more generalized form of 1:1 protection, where N working paths share a single protection path. 546 NETWORK SURVIVABILITY node B for the working traffic. One fiber carries traffic from A to B, and the other carries traffic from B to A. Likewise there is another pair of fibers for protection traffic. Node A's receiver and node B's receiver can make the switching decisions independently. In 1:1 protection, there are still two fibers from the source to the destination. However, traffic is transmitted over only one fiber at a time, say, the working fiber. If that fiber is cut, the source and destination both switch over to the other protection fiber. As we discussed earlier, an APS protocol is required for signaling between the source and destination. For this reason, 1:1 protection is not as quick as unidi- rectional 1 + 1 protection in restoring traffic because of the added communication overhead involved. However, it offers two main advantages over 1 + 1 protection. The first is that under normal operation, the protection fiber is unused. Therefore, it can be used to transmit lower-priority traffic. This lower-priority traffic must be discarded if the working fiber is cut. SONET and SDH equipment in the field does provide support for this lower-priority or extra traffic. This capability is not widely used today, but carriers in the past have used this capability on occasion to carry "lower-priority" data traffic or even voice traffic, when their networks are temporar- ily over capacity. This is likely to change in the future with the advent of data services, as we shall see in Section 10.4. Best-effort data services, in particular, can use this capability. Another advantage is that the 1:1 protection can be extended so as to share a single protection fiber among many working fibers. In a more general 1 :N protection scheme, N working fibers share a single protection fiber. This arrangement can handle the failure of any single working fiber. Note that in the event of multiple failures, the APS protocol must ensure that only traffic on one of the failed fibers is switched over to the protection fiber. In the previous discussion we talked about how the protection is done, but skimmed over what the triggers are for initiating protection switching. In SONET/SDH, the incoming signal is continously monitored. Protection switching is initiated if a signal fail or a signal degrade condition is detected on the line. A signal fail represents a hard failure and is detected typically as a loss of signal or as a loss of the SONET/SDH frame. Out of the 60 ms allowed for restoration, detecting the failure and initiating protection switching must be performed within 10 ms. 10.2.2 Self-Healing Rings Ring networks have become very popular in the carrier world as well as in enterprise networks. A ring is the simplest topology that is 2-connected, that is, provides two 10.2 Protection in SONET/SDH 547 separate paths between any pair of nodes that do not have any nodes or links in common except the source and destination nodes. This allows a ring network to be resilient to failures. Rings are also efficient from a fiber layout perspectivemmultiple sites can be interconnected with a single physical ring. In contrast, a hubbed ap- proach would require fibers to be laid between each site and a hub node, and would require two disjoint routes between each site and the hub, which is a more expensive proposition. Much of the carrier infrastructure today uses SONET/SDH rings. These rings are called sel~-healing since they incorporate protection mechanisms that automati- cally detect failures and reroute traffic away from failed links and nodes onto other routes rapidly. The rings are implemented using SONET/SDH add/drop multiplex- ers (ADMs), which we studied in Section 6.1. These ADMs selectively drop and add traffic from/to the ring as well as protect the traffic against failures. The different types of ring architectures differ in two aspects: in the direction- ality of traffic and in the protection mechanisms used. A unidirectional ring carries working traffic in only one direction of the ring (say, clockwise), as shown in Fig- ure 10.4. Working traffic from node A to node B is carried clockwise along the ring, and working traffic from B to A is also carried clockwise, on a different set of links in the ring. A bidirectional ring carries working traffic in both directions. Figure 10.5 shows a four-fiber bidirectional ring. Working traffic from A to B is carried clockwise, and working traffic from B to A is carried counterclockwise along the ring. Note that in both unidirectional and bidirectional SONETISDH rings, all connections are bidirectional and use up the same amount of bandwidth in both directions. The two directions of a connection are routed differently based on the type of ring, as we discussed earlier. The SONET/SDH standards dictate that in SONET/SDH rings, service must be restored within 60 ms after a failure. This time includes several components: the time needed to detect the failure, for which 10 ms is allocated; the time needed to signal to other nodes in the network (if needed), including the propagation delays; the actual switching time; and the time to reacquire the frame synchronization after the switch-over has occurred. Three ring architectures have been widely deployed: two-fiber unidirectional path-switched rings (UPSR), four-fiber bidirectional line-switched rings (BLSR/4), and two-fiber bidirectional line-switched rings (BLSR/2). In SDH, the 1 + 1 path protection has been defined to operate in a more general mesh topology and is called subnetwork connection protection (SNCP). SDH multiplex section shared protection ring/4 (MS-SPRing/4) and MS-SPRing/2 are similar to BLSR/4 and BLSR/2, respec- tively. Table 10.2 summarizes the features of the different architectures, which we will discuss in detail in the following sections. 548 NETWORK SURVIVABILITY Figure 10.4 A unidirectional path-switched ring (UPSR). One of the fibers is considered the working fiber and the other the protection fiber. Traffic is transmitted simultaneously on the working fiber in the clockwise direction and on the protection fiber in the coun- terclockwise direction. Protection is done at the path layer. Table 10.2 Comparison of different types of self-healing rings. Parameter UPSR B LSR/4 B LSR/2 SNCP MS-SPRing/4 MS-SPRing/2 Fiber pairs 1 2 1 TX/RX pairs/node 2 4 2 Protection type Dedicated Shared Shared Protection capacity = Working - Working = Working capacity capacity capacity Link failure Path Span/ring Ring switch switch switch Node failure Path Ring Ring switch switch switch Restoration speed Faster Slower Slower Implementation Simple Complex Complex 10.2 Protection in SONET/SDH 549 Figure 10.5 A four-fiber bidirectional line-switched ring (BLSR/4). The ring has two working fibers and two protection fibers. Traffic between two nodes is transmitted nor- mally on the shortest path between them, and either span or ring switching is used to restore service after a failure. 10.2.3 Unidirectional Path-Switched Rings Figure 10.4 shows a UPSR. One fiber is used as the working fiber and the other as the protection fiber. Traffic from node A to node B is sent simultaneously on the working fiber in the clockwise direction and on the protection fiber in the counterclockwise direction. The protection is performed at the path layer for each connection as follows. Node B continuously monitors both the working and protection fiber and selects the better signal between the two for each SONET connection. Under normal operation, suppose node B receives traffic from the working fiber. If there is a link failure, say, of link AB, then B will switch over to the protection fiber and continue to receive the data. Note that the switch-over is done on a connection-by-connection basis (see Problem 10.8). Observe that this is essentially like the 1 + 1 scheme that we studied earlier, except that it is operating at the path layer in a ring rather than at the line layer in a point-to-point configuration. . Each protection scheme can be associated with a specific layer in the network. As we saw in Chapter 6, the SONET layer includes a path layer and a line layer. Both path layer and line layer. likely to change in the future with the advent of data services, as we shall see in Section 10.4. Best-effort data services, in particular, can use this capability. Another advantage is that the. switching is initiated if a signal fail or a signal degrade condition is detected on the line. A signal fail represents a hard failure and is detected typically as a loss of signal or as a loss of