User's Guide

The Operational View SB300 Series
Page 24 Proprietary and Confidential 2110059 Rev 1.1
4.3.6. Result Code Formats
The modems normally issue result codes on completion of an AT command. This can be disabled
with quite mode (Q1). The default is to issue results in verbose (English text) form. Numeric
codes can be selected by disabling the verbose option (V0).
If echo is enabled (E1) the modem will append a <LF> to the echoed <CR> from the host. If
echo is off, then there is no <CR> or <LF> prior to the response to the command.
Verbose result codes are returned with a trailing <CR><LF>.
Numeric result codes are returned with a trailing <CR> without the line feed.
Where the modem issues a response (data) prior to the result code, the response will have a <CR>
and <LF> appended regardless of the echo, quiet, and verbose settings.
A full diagram of the framing characters for both result codes and modem responses is provided in
the AT Command Reference (document 2110031) provided with the OEM Developer’s Toolkit.
NOTE
Allow the modem a few seconds to respond to some commands. Where mode changes
are involved the modem may require several seconds to reconfigure before responding.
On power up, following initial internal configuration, the modem will issue the OK
result code to the host whether the host is actively receiving or not.
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