User's Guide

User’s Guide Modem Basics
2110059 Rev 1.1 Proprietary and Confidential Page 25
3 – Network
Data is organized into packets which are data frames
with network headers and trailers added including
network addressing. The duties include flow control,
handling network service data units, notifying the
transport layer of errors, and possibly sequenced
delivery.
IP, SLIP, PPP
2 – Link
This layer arranges the bits into frames. It also
establishes and releases one or more link connections.
Network Interface
Cards, Modems
1 – Physical
This is the physical connection layer concerned with
carrying the communication of digital data.
Transmission Media:
Twisted Pair, Fibre
Optics, Cellular
Radio, etc.
4.4.1. SB300 Series Internal Protocol Stacks
The SB300 Series modems are designed to provide the bottom two layers, Link and Physical. The
modem will handle acquiring a telephone line or cellular channel to provide the physical
connection.
In CDPD mode, the SB300 Series modems are also capable of supporting the next two layers at
the user’s option. Both UDP and TCP Packet Assembly/Disassembly (PAD) services are
available along with IP and SLIP network support. The modems do not currently support PPP but
it will be implemented in a later firmware release.
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 into three 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.