WANs

The best known wide area network is, of course, the Internet. Its area is now pretty much all of the populated world. In addition, however, many businesses have created wide area networks of their own. Most of these use IBM's SNA protocol suite, while others use TCP/IP with wires, routers, and switches that are physically separate from the Internet. Nearly all wide area connections currently use the SONET protocols, but there is intense interest in moving to simpler approaches. We'll discuss SONET first.

SONET

SONET, which stands for Synchronous Optical Network, takes us back to Layer 1. This is a physical transport that uses (exclusively) optical cable and synchronous time slot data management. SONET, also known in international standards as SDH (Synchronous Digital Hierarchy), is a brilliant design that has been adopted very rapidly by telephone companies and a few of their competitors (remember Qwest, described in Chapter 11).

SONET's topology is the self-healing, dual counter-rotating ring. The redundancy is especially important for SONET networks because in a WAN, lines don't live safe and secure in the walls of buildings. Rather, they are outdoors, mostly buried underground and occasionally suspended in air. As a result, the danger of damage to a cable is much higher than in other systems. The "self-healing" aspect of SONET's ring architecture means that it can survive a cut without the network going down—the other cable takes over automatically, becoming the main circuit until the original cable is restored.

SONET's synchronous nature makes it much easier to switch. When two SONET rings meet, the fact that their slots are synchronized at the same speed means that it is relatively easy to move the contents of a slot from one ring to another. SONET is also easier to multiplex and demultiplex. T-carrier frames have to be completely torn apart to extract one channel, then rebuilt before they can be sent on. SONET, by contrast, allows information from within a slot to be pulled out without interrupting the flow of data. Incredibly, synchronization flows from a central clocking system. At one point, there was a single master clock for telephone and connected SONET networks. Currently, networks use something called Stratus 3 clocks in central offices or network hubs. These clocks are accurate to a level of just over one part in a billion. These are fairly technical points; you may recall that SONET is considered to be a first-rate technology, one with no significant flaws other than overhead.

The speed of SONET is specified in Optical Carrier (OC) levels. The major levels defined to date are

Unlike T-carrier and DS levels, SONET/SDH's OC specifications are the same around the world. This standardization helps both in developing a competitive market for equipment and in ensuring that international links are efficient and easy to effect. Remember, though, that SONET is just a carrier. While SONET does provide some error detection and some control functions, it isn't capable on its own of setting up and maintaining end-to-end communications. For telephone connections, the world's telcos have settled on a protocol called Signaling System 7. For data, and for mixed data, video, and voice, ATM is the technology of choice. SONET is flexible enough that the two protocols can run on the same links simultaneously. National and international Internet backbone connections (currently at speeds up to OC-48) are now primarily TCP/IP over ATM over SONET.

Wave Division Multiplexing

SONET is circuit switched so that someone buying an OC-3 link is, in effect, getting a dedicated pipe within the SONET ring. If it isn't filled, SONET will just waste the excess capacity. To improve efficiency and reduce the overhead involved in doing things like putting TCP/IP over ATM over SONET, telecommunications vendors are planning to strip out some of the layers. The original idea was to put ATM directly on the optical waves, dispensing with SONET. Some carriers, though, are planning to also drop ATM and put TCP/IP directly on the light carrier. It's probable that both these approaches will be used extensively.