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The SONET/SDH add/drop multiplexer (ADM) is a more complex version of the TM device. As shown in Figure 2.12(b), a SONET ADM receives an OC-N signal from which it can demultiplex and terminate any number of DSn and/or 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 ADMís 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. SONET/SDH 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 presentation 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 number 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
(a) Terminal multiplexer (b) Add-drop multiplexer
Figure 2.12 The SONET TM and ADM.
SONET/SDH AND THE GENERIC FRAME PROCEDURE (GFP)
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 Aís 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.
SONET/SDH 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 and/or OC-M signals, M < N, as in the case of an ADM device. Additionally, it can switch DSn and/or 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
Figure 2.14 A digital cross connect (DCS) node.
SELF-HEALING SONET/SDH RINGS
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.