User's Guide

User’s Guide Modem Basics
2110059 Rev 1.0 Proprietary and Confidential Page 25
CSC and Wireline modes require that the host system provide the protocol stack for layers 3 and
above if they are needed. Windows Dial-up Networking can provide the TCP/IP stack. Other
Windows-based Internet clients can provide the higher layers for file transfer, e-mail, and other
services.
4.5. Modem Handshaking (SB320)
For two modems to communicate with each other, it is imperative that they speak the same
“language”, i.e. they use the same protocol(s). CDPD establishes a single protocol for its
communications, but CSC and Wireline modes can use any of several protocols. The following
discussion is for users of CSC and Wireline modes on the SB320 modem.
Because of the variety of protocols that are available for modem communications, a modem will
usually support several protocols to enable it to connect to a variety of modems. In the process of
a modem establishing a connection with another modem, there will be a negotiation phase during
which the two modems exchange basic information to determine which protocol(s) shall be used
for the present connection.
If the modems can agree on a communication scheme using one or more of their available
protocols, they will proceed to establish a connection. At this point, the modems are ready to pass
data to their respective DTEs where the application software runs. If the modems fail to agree on
a communication scheme due, for instance, to the lack of a common protocol, the connection
attempt is aborted and the modems disconnect, or go on hook.
4.5.1. Modem Protocol Layers
The communication protocols used by modems can be divided in 3 categories or layers as shown
in the figure below. Each layer operates independently of the layer above it. A modem
connection may be established using one or more of these layers, starting from the lowest layer
and including more layers as required by the situation.
Local Modem Remote Modem
Data Compression
(V.42bis, MNP5)
Error Control
(LAPM, MNP2-4, MNP10)
Data Communication
(V.34, V.32bis, V.32, ...)
Data Compression
(V.42bis, MNP5)
Error Control
(LAPM, MNP2-4, MNP10)
Data Communication
(V.34, V.32bis, V.32, ...)
Receive Data
Transmit Data
Figure 4-1: Modem Protocol Layers
4.5.1.1. Data Communication
The lowest layer is that which is closest to the hardware and it deals with the modulation scheme
used to transmit and receive the raw data that is presented to the layer above.
This layer, which we will call the data communication protocol, has many available standards:
V.34, V.32bis, V.32, V.22, etc. It is concerned mainly with establishing the modulation scheme
that will give the highest baud rate (bps) on a given physical point-to-point connection.