SONET

Applications

SONET was originally designed for the public telephone network. In the early 1980's, the forced breakup of AT&T in the United States created numerous regional telephone companies, and these companies quickly encountered difficulties in networking with each other. Fiber optic cabling already prevailed for long distance voice traffic transmissions, but the existing networks proved unnecessarily expensive to build and difficult to extend for so-called long haul data and/or video traffic.

The American National Standards Institute (ANSI) successfully devised SONET as the new standard for these applications. Like Ethernet, SONET provides a "layer 1" or interface layer technology (also termed physical layer in the OSI model). As such, SONET acts a carrier of multiple higher-level application protocols. For example, Internet Protocol (IP) packets can be configured to flow over SONET.

Rings

One of SONET's most interesting characteristics is its support for a ring topology. Figure 1 illustrates the concept of a SONET ring. Normally, one piece of fiber -- the working ring -- handles all data traffic, but a second piece of fiber -- the protection ring remains on standby. Should the working ring fail, SONET includes the capability to automatically detect the failure and transfer control to the protection ring in a very short period of time... often in a fraction of a second. For this reason, SONET can be described as a self-healing network technology.

One of SONET's most interesting characteristics is its support for a ring topology. Figure 1 illustrates the concept of a SONET ring. Normally, one piece of fiber -- the working ring -- handles all data traffic, but a second piece of fiber -- the protection ring remains on standby. Should the working ring fail, SONET includes the capability to automatically detect the failure and transfer control to the protection ring in a very short period of time... often in a fraction of a second. For this reason, SONET can be described as a self-healing network technology.

Rings normally will help SONET service to reach the "five nines" availability level. However, the usefulness of rings also depends on their physical location. Figure 2 shows two instances of SONET ring topology. In (2a), the cables take distinctly different routes to reach the same destination. Geographically speaking, one path turns north first and then east, the other first turns east. Being physically separated, the likelihood of an excavation or natural disaster breaking both cables lessens dramatically.

In (2b), however, the cables follow essentially the same route. Imagine in this case two strands of fiber set only a few feet apart from each other... possibly even in the same trench! The likelihood of one problem disabling both fiber strands increases dramatically, effectively defeating the advantage of SONET rings. Note that SONET does not require rings: many SONET networks have been deployed in single-strand linear architectures.

Management and Maintenance

The term OAM&P often appears in conjunction with optical network technologies like SONET (and ATM). OAM&P -- Operations, Administration, Maintenance, and Provisioning -- refers to the support built into the technology for ease of network management. In the case of SONET, a significant number of bytes inside the data frame have been reserved for this "management overhead." At the expense of some bandwidth, problems can be more quickly detected, isolated, and repaired.

The Future of SONET

Because SONET can carry very large amounts of traffic, it would seem on the surface to be an ideal technology for future voice and data broadband networks. SONET competes with several other viable technologies including ATM and Gigabit Ethernet for this role.