Chapter 6. Modems, LAPM, PPP, and the V.100 Series

In this chapter, we examine the newer high-speed modems, with an emphasis on V.34 and V.90 (the 56 kbit/s modem). Many Internet users are using the layer-2 link protocol, Link Access Procedure for Modems (LAPM), so we will examine LAPM as well. The Point-to-Point Protocol (PPP) is used in most dial-up connections to the Internet, so we must examine the PPP also. Some PPP-based products do not use LAPM, and instead utilize a similar protocol, named Link Access Procedure, Balanced (LAPB); it is covered, too. Since ISDN services are available in most cities today, we will take a look at how the V.100 Series supports the ISDN user.

The reader may wonder why I am including information on modems in a VoIP book. After all, modems are designed to transport data over analog links, and VoIP is voice. We need to keep in mind that VoIP voice samples are represented as data bits, and these data bits are contained in data packets. Therefore, if a telco analog local loop is used by the VoIP customer, the modem is the instrument used to get the VoIP data packets across this link. If the local loop is ISDN based, the modem may be still involved at the customer site.

Of course, if the VoIP packets are being transported across a native digital network such as an Ethernet, then the modem is not employed. But for point-to-point dialup links, the modem is quite important, and the VoIP gateways stipulate the types of modems the customer must use for the gateway interface.

One other point before we proceed into the chapter. I have chosen to explain outboard modems in this chapter; that is, modems that are not housed inside the user's terminal. These examples are relevant to an installation that has a modem pool, with banks of modems in use. An in board modem may not implement all the operations that are described in this chapter, specifically the use of some of the V.24 interchange circuits.