Synchronous Optical Network

The next WAN technology this chapter covers is Synchronous Optical Network (SONET), which is a physical layer specification for high-speed synchronous transmission of packets or cells over fiber-optic cabling. SONET was proposed by Bellcore in the mid-1980s and is now an international standard. SONET uses the SDH system with STS-1 as its basic building block. Service providers and carriers are making wide use of SONET in their internal networks. SONET is also gaining popularity within private networks to connect remote sites in a WAN or metropolitan-area network (MAN).

Both ATM and packet-based networks can be based on SONET. With packet transmission, SONET networks usually use PPP at the data link layer and IP at the network layer. Packet Over SONET (POS) is expected to become quite popular as Internet and intranet traffic grows, and as new applications, such as digital video, demand the high speed, low latency, and low error rates that SONET can offer.

One of the main goals of SONET and SDH was to define higher speeds than the ones used by the North American Digital Hierarchy and the European E systems, and to alleviate problems caused by incompatibilities in those systems. The creators of SONET and SDH defined high-speed capacities, starting with the 51.84-Mbps STS-1, that were approved by both North American and European standards bodies.

Another goal of SONET was to support more efficient multiplexing and demultiplexing of individual signals. With SONET (SDH), it is easy to isolate one channel from a multiplexed circuit (for example, one phone call from a trunk line that carries numerous phone calls). With plesiochronous systems, such as the North American Digital Hierarchy and the European E system, isolating one channel is more difficult.

Although isolating a 64-Kbps channel from a DS-1 circuit is straightforward, isolating a 64-Kbps channel from a DS-3 trunk requires demultiplexing to the DS-1 level first.

NOTE

The North American Digital Hierarchy and European E systems are called plesiochronous systems. Plesio means "almost" in Greek. A truly synchronous system, such as SONET, supports more efficient multiplexing and demultiplexing than a plesiochronous ("almost synchronous") system.

The SONET specification defines a four-layer protocol stack. The four layers have the following functions:

• The photonic layer specifies the physical characteristics of the optical equipment.

• The section layer specifies the frame format and the conversion of frames to optical signals.

• The line layer specifies synchronization and multiplexing onto SONET frames.

• The path layer specifies end-to-end transport.

Terminating multiplexers (implemented in switches or routers) provide user access to the SONET network. Terminating multiplexers turn electrical interfaces into optical signals and multiplex multiple payloads into the STS-N signals required for optical transport.

A SONET network is usually connected in a ring topology using two self-healing fiber paths. A path provides full-duplex communication and consists of a pair of fiber strands. One path acts as the full-time working transmission facility. The other path acts as a backup protection pair, remaining idle while the working path passes data. If an interruption occurs on the working path, data is automatically rerouted to the backup path within milliseconds. If both the working and protected pairs are cut, the ring wraps, and communication can still survive. Figure 11-6 shows a typical SONET network.

Figure 11-6. A Redundant SONET Ring

Continue reading here: Frame Relay Huband Spoke Topologies and Subinterfaces

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