ata traffic usually consists of short, bursty transactions from a few milliseconds duration to several seconds, depending on the data transmission rate (i.e., the number of bits per second). This is particularly true on local area networks (LANs). As the data rate slows down, such as we might find on a wide area network (WAN), transaction time increases, Note: Efficiency is total erlangs carried divided by size of group. This graph is based on full availability and on a loss probability of P = 0.02. Offered traffic (in erlangs) Figure 4.6 Group efficiency increases with size. 6 Carrier systems are frequency-division multiplex systems introduced in Section 4.5. 4.3 INTRODUCTION TO SWITCHING possibly to a minute or SO. For these reasons, it is dangerous to apply speech telephony traffic theory and practice to the data environment. There is an exception here-that is, when a data protocol specifies a permanent virtual circuit (PVC). This is a circuit that is set up in advance for one or several data trans- actions. One group of traffic engineers has proposed the milli-erlang for LAN and PVC applications. We think this idea bears merit. 4.3 INTRODUCTION TO SWITCHING In this section our concern is telephone switching, the switching of voice channels. We will deal with some switching concepts and with several specifics. Switching was defined in Section 4.1 in contraposition with transmission. Actual connectivity is carried out by the switching function. A connectivity may involve more than one switch. As we pointed out in Chapter 1, there are local switches, tandem switches, and transit switches. A transit switch is just a tandem switch that operates in the long distance or "toll" service. A local switch has an area of responsibility. We call this its serving area. All subscriber loops in a serving area connect to that switch responsible for the area. Many calls in a local area traverse no more than one switch. These are calls to neighbors. Other calls, destined for subscribers outside of that serving area, may traverse