Switched 384 and Switched 1536 are AT&T services. They increase bandwidth meter by combining multiple 64Kbps channels to achieve 384Kbps or 1536Kbps. Both of these services require ISDN Primary Rate Interface (PRI) lines--384 require one and 1536 requires two plus a D channel.

ISDN Multi rate allows users of ISDN PRI lines to specify any multiple of 64Kbps to create a custom channel. The effect of ISDN multi rate is the same as the effect of using an inverse multiplexer. The second aspect of digital dial-up services is access lines. There are four basic types of digital dial-up access lines. They are T-1, ISDN Basic Rate Interface (BRI), ISDN PRI, and Switched 56 lines.

T-1 access lines are not the same as the well-known dedicated T-1 lines used to connect two user network sites. T-1 access lines are, simply, T-1 lines that connect the user premises to the selected LEC or inter exchange carrier network. They carry 24 56Kbps channels using in band signaling. Switched 56 lines provide a 56Kbps channel with in-band signaling between the customer (user) premises and the LEC or inter exchange carrier. They connect only to Switched 56 services and circuits, although other types of lines, including T-1, ISDN PRI, or ISDN BRI, can also connect to Switched 56 circuits.

ISDN BRI access lines consist of two 64Kbps channels for data and an out-of-band signaling channel called the D channel. BRI lines can connect to Switched 56 or Switched 64 services. ISDN PRI lines are the "T-1 of ISDN." They carry 23 64Kbps channels, with out-of-band signaling on a D channel. ISDN PRI lines enable you to connect to Switched 56, 64, 384, and 1536 services, as well as to ISDN Multi rate. ISDN PRI lines cost about the same as T-1 access, with an additional cost for the D channel.

Enter the I-MUX Now that we have identified access and service types, a picture is beginning to emerge of a complex WAN. This is a WAN with high-speed backbone connections between major sites, lower-bandwidth circuits from branch offices, dial-up analog connections for road warriors, and switched connections for paths that have varying traffic loads or are intended as redundant circuits.

By using Switched 56, Switched 1536, ISDN, or the other combinations discussed, organizations can craft an economical mix of bandwidths unique to their requirements - provided they have an easy and relatively inexpensive method to take advantage of the switch opportunities. The inverse multiplexer, or I-Mux, will provide that method.

Combining several channels into a high-speed data stream is relatively simple, and today's breed of I-Mux goes beyond that capability, often allowing you to let the network decide how much traffic it must carry. For example, inverse multiplexers form Ascend Communications (Alameda, CA) allow automatic selection of a number of channels based on fixed requirements, time of day, or traffic load.

Given a large pipe, such as a T-1 access or ISDN PRI, you could dedicate a fixed number of channels for all calls (in other words, use enough smaller, cheaper channels to make up the larger bandwidth). Or, using the same pipe, you could designate a particular time of day--say, when traffic is always heavy--to get more low-bandwidth channels, and then assign fewer channels throughout the rest of the day.