The STS-1 Payload SONETSDH

SONETSDH DEVICES 33

2.5 SONETSDH DEVICES

Several different types of SONETSDH devices exist. The SONETSDH terminal multi- plexer TM device multiplexes a number of low-speed signals into a higher-speed signal. It also works in the opposite direction as a demultiplexer. That is, it demultiplexes a high- speed signal into a number of lower-speed signals. For instance, a SONET TM device multiplexes a number of DSn signals into a single OC-N signal see Figure 2.12a. It can also demultiplex an OC-N signal into a number of DSn signals. It consists of a controller, low-speed interfaces for DSn, an OC-N interface, and a time slot interchanger TSI, which is used to map the incoming time slots of the DSn signals into the STS-N SPE. The SONETSDH adddrop multiplexer ADM is a more complex version of the TM device. As shown in Figure 2.12b, a SONET ADM receives an OC-N signal from which it can demultiplex and terminate any number of DSn andor OC-M signals, where M N. At the same time, it can add new DSn and OC-M signals into the OC-N signal. Specifically, the incoming OC-N signal is first converted into the electrical domain, and then the payload is extracted from each incoming frame. Recall that the payload consists of a fixed number of bytes, or time slots. These time slots carry different virtual tributaries, such as DSn and OC-M signals, some of which are dropped i.e., terminated. That is, the information is first extracted from the appropriate time slots that carry these virtual tributaries, and then it is transmitted to local users through the ADM’s low-speed DSn and OC-M interfaces. An OC-M interface is typically connected to a TM device. This termination process frees up a number of time slots in the frame, which, along with other unused time slots, can be used to carry traffic that it is locally generated. That is, DSn and OC-M signals received from its low-speed DSn and OC-M interfaces can be added into the payload of the frame using these unused time slots. The final payload is transmitted out at the same SONET level as the incoming OC-N signal. SONET or SDH ADM devices are typically interconnected to form a SONET or an SDH ring. SONETSDH rings are self-healing; that is, they can automatically recover from link failures. Self-healing rings consist of two or four fibers, and are discussed in the following section. In Figure 2.13, we show a SONET ring interconnecting four ADM devices. For pre- sentation purposes, we assume that these four ADM devices are connected by a single fiber and that the direction of transmission is clockwise. Each ADM device serves a num- ber of local TM devices and other users. User A is connected to TM 1, which in turn is connected to ADM 1. User A has established a connection to user B, who is attached to ADM 3 via TM 2. In Figure 2.13, this connection is signified by the dotted line. Let us assume that user A transmits a DS1 signal. This is multiplexed with other DS1 signals in DSn DSn OC-N TM DSn OC-M OC-N OC-N ADM a Terminal multiplexer b Add-drop multiplexer .... Figure 2.12 The SONET TM and ADM. 34 SONETSDH AND THE GENERIC FRAME PROCEDURE GFP A B TM 1 TM 2 ADM 1 ADM 2 ADM 3 ADM 4 Figure 2.13 A SONET ring. TM 1, and the output is transmitted to ADM 1. Let us assume that the output signal of TM 1 is an OC-3 signal, and that the speed of the ring is OC-12. ADM 1 adds the OC-3 signal it receives from TM 1 into the STS-12 payload and transmits it out to the next ADM. The OC-12 signal is transmitted to ADM 2, where it is terminated and converted to the electrical domain. ADM 2 adds and drops various signals, and then transmits the resulting STS-12 frames to ADM 3. At ADM 3, the DS1 signal belonging to A is dropped from the payload and transmitted with other signals to TM 2. TM 2 then demultiplexes the signals and transmits A’s DS1 signal to B. The connection from to A to B is a good example of a circuit-switching connection. It is set up manually using network management software and by appropriately configuring each SONET device along the path. The connection is permanent, in the sense that it lasts for a long time. The connection is up all of the time, independently of whether A is transmitting continuously to B. A similar connection might also exist from B to A. SONETSDH rings are interconnected to cover a wide geographical area via digital cross connect systems DCS . A DCS is a more complex version of an ADM device. As shown in Figure 2.12, an ADM device receives an OC-N signal from the incoming fiber of the working ring. It then transmits out a new OC-N signal on the outgoing fiber of the ring. A DCS node has a similar functionality, but it is connected to multiple incoming and outgoing OC-N interfaces. For each incoming OC-N signal, it can drop and add any number of DSn andor OC-M signals, M N, as in the case of an ADM device. Additionally, it can switch DSn andor OC-M signals from an incoming interface to any outgoing interface. Figure 2.14 shows a DCS node interconnecting two rings Ring 1 and Ring 2. The DCS node receives STS-N frames from Ring 1. For each frame, the DCS node then drops predefined virtual tributaries. It then adds new virtual tributaries – those that are from the local SONET devices i.e., that are directly attached to the DCS, and those that are from Ring 1 Ring 2 ADM ADM ADM ADM ADM ADM DCS Figure 2.14 A digital cross connect DCS node. SELF-HEALING SONETSDH RINGS 35 Ring 2. The resulting STS-N frames are transmitted out to the adjacent ADM device on Ring 1. The dropped virtual tributaries are either delivered to the local SONET devices or are switched to Ring 2. Likewise, the DCS receives STS-N frames from Ring 2 – from which it drops some virtual tributaries and adds new ones generated from local SONET devices that are attached to the DCS – and from Ring 1. The resulting STS-N frames are transmitted out to the adjacent ADM device on Ring 2. The dropped virtual tributaries are either delivered to the local SONET devices or are switched to Ring 1. 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. SONETSDH 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 SONETSDH 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 SONETSDH RINGS

SONETSDH rings have been specially architected so that they are highly reliable. Specif- ically, 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 SONETSDH 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. SONETSDH 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 SONETSDH links, and then we describe several self-healing SONETSDH ring architectures. The simplest SONETSDH network is a point-to-point fiber link that connects two SONETSDH 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 SONETSDH 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 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.