User's Guide

The Operational View SB300 Series
Page 24 Proprietary and Confidential 2110059 Rev 1.0
4.4. Communication Protocols and Stacks
Data communication protocols are generally discussed in relation to the Open Systems
Interconnection (OSI) model. The OSI model was introduced in 1978 as a long-term project of
the International Organization for Standardization (ISO). It divides the communications process
into seven distinct layers, stacked one above the other, hence the term protocol stack. This is very
different from the computer processing use of the term stack for a LIFO buffer.
Table 4-1: The OSI Model
Layer Description Common Protocols
7 – Application
This is the window between the end-user
application and the communications process. It
includes functions such as login and password
checks, and resource allocation.
Application programs
for file transfer,
e-mail, etc.
6 – Presentation
Responsible for terminal management such as
character set interpretation (i.e. ASCII, ANSI) and
code conversion.
5 – Session
Session management includes data-flow
synchronization, mapping addresses with names,
handling graceful and abrupt disconnections, and
data buffering. This layer organizes data into
Session Protocol Data Units (SPDUs).
Telnet, FTP, SMTP,
etc.
4 – Transport
Transport Protocol Data Units (TPDUs) are
assembled at this layer. This can include
multiplexing and demultiplexing, error detection
and recovery, and packet reordering where the
network can receive packets along different routes
which can arrive out of sequence.
UDP, TCP
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 by April 1999 or earlier.