Time was, when building a WAN meant spending a lot of money on T-1 and fractional T-1 lines. Time was, when WANs equaled high costs. But today, network managers can reverse the multiplexing process and save significant money on high bandwidth tester WANs. A technique that's increasing in popularity, called inverse multiplexing, offers bandwidth when you need it, without the attendant cost of continuous high-capacity connections. Inverse multiplexing reverses the traditional multiplexing process.

Instead of combining many signals into a single, composite channel, inverse multiplexing takes a single data channel and breaks it up into several lower-bandwidth channels, for transmission on not one, but several lines. This process, known colloquially as rubber bandwidth, lets your network decide how much bandwidth a data channel needs for the amount of data it is carrying and connects to the requisite number of fixed-bandwidth circuits to obtain it.

Digital Dial-Up You can connect LANs over a wide area network in a couple of different ways. You can dedicate lines such as T-1 or T-3 to full-time use. Alternatively, you can use a dial-up service. The advantage of fixed, dedicated lines is that they are always there and they are always yours. You don't have to share them and you can depend on them being in place. This traditional method of building WAN results in a physical backbone.

The disadvantage, for many systems, is that you pay for the dedicated lines even when your traffic volume doesn't warrant their use. Typical network traffic in a well-architected WAN averages a relatively low level. However, periodic bursts of traffic will heavily tax a low-bandwidth connection, such as a 56Kbps circuit. Usually, WAN designers look at the total enterprise and try to determine which segments have high-bandwidth requirements and which have lower traffic volumes.

This approach usually results in some trade-offs between price and performance. With dial-up systems, you can, in many cases, have access to just as much bandwidth as you need when you need it. You pay, essentially, only for what you use. While dial-up may not be appropriate for all circuits in an enterprise, it does allow you to accommodate bursty traffic requirements economically. Because of these trade-offs, you will likely use a combination of dedicated and dial-up lines.

Several types of digital dial-up services exist. Probably the most talked about is Integrated Services Digital Network (ISDN). Like many services, ISDN should be viewed from two perspectives: network services and access lines. These two aspects may seem confusing, because many terms are used interchangeably.

Let's start with network services. These services are available generally from either inter exchange carriers, such as AT&T and MCI, or Local Exchange Carriers (LECs) such as the Baby Bells. These services include Switched 56, Switched 64, Switched 384, Switched 1536, and ISDN Multi rate.

Switched 56 and Switched 64 services are similar in that they use 64Kbps channels for transport, but Switched 56 allocates one bit out of every eight for signaling, which is called in-band signaling. Switched 64, however, uses all eight bits of every octet for data, so it requires a separate channel for signaling. This technique is called out-of-band signaling and is available only with ISDN. ISDN systems have a special channel, the D channel, for signaling information.