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Typical SONET Topology

Figure 1–4 shows a typical topology for a SONET network. This topology is a dual ring. Each ring is an optical fiber cable. One ring is the working facility. The other ring is the protection facility, which acts as a standby in the event of fiber or system failure on the working facility.

Figure 6-4Figure 1–4 SONET topology.

End-user devices operating on LANs and digital transport systems (such as DS1, E1, etc.) are attached to the network through a SONET service adapter. This service adapter is also called an access node, a terminal, or a terminal multiplexer. This machine is responsible for supporting the end-user interface by sending and receiving traffic from LANs, DS1, DS3, E1, ATM nodes, etc. It is really a concentrator at the sending site because it consolidates multiple user traffic into a payload envelope for transport onto the SONET network. It performs a complementary, yet opposite, service at the receiving site.

The user signals (such as T1, E1, and ATM cells) are converted (mapped) into a standard format called the synchronous transport signal (STS), which is the basic building block of the SONET multiplexing hierarchy. The STS signal is an electrical signal. The notation STS-n means that the service adapter can multiplex the STS signal into higher integer multiples of the base rate. The base rate is 51.84 Mbit/s in North America and 155.520 Mbit/s in Europe. Therefore, from the perspective of a SONET terminal, the SDH base rate in Europe is an STS-3 multiplexed signal (51.84 x 3 = 155.520 Mbit/s).

The terminal/service adapter (access node) shown in Figure 1–4 is implemented as the end-user interface machine, or as an add-drop multiplexer (ADM). The ADM implementation multiplexes various STS input streams onto optical fiber channels. The optical fiber channels are now called the optical carrier signal and designated with the notation OC-n, where n represents the multiplexing integer. OC-n streams are demultiplexed as well as multiplexed with the ADM.

The term add-drop means that the machine can add or drop payload onto one of the fiber links. Remaining traffic that was not dropped passes straight through the multiplexer without additional processing.

The digital cross-connect (DCS) machine usually acts as a hub in the SONET network. It can not only add and drop payload, but it can also operate with different carrier rates, such as DS1, OC-n, E1, etc. The DCS can make two-way cross-connections between the payload and can consolidate and separate different types of payloads.

The DCS is designed to eliminate devices called back-to-back multiplexers. As we learned earlier, these devices contain a plethora of cables, jumpers, and intermediate distribution frames. SONET does not need all these physical components because cross-connection operations are performed by hardware and software.

The topology can be set up either as a ring or as a point-to-point system. In most networks, the ring is a dual ring, operating with two or more optical fibers. As noted, the structure of the dual ring topology permits the network to recover automatically from failures on the channels and in the channel/machine interfaces. This is known as a self-healing ring and is explained in later chapters.

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