User's Guide

The Operational View SB300 Series
Page 26 Proprietary and Confidential 2110059 Rev 1.1
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.
The more advanced protocols, such as V.34 for example, are smarter in that they are capable of
simultaneously supporting different rates on the transmit and receive lines, and allow two modems
to exchange information to fine-tune their modulation parameters to optimize performance.
4.5.1.2. Error Control
The next layer is the Error Control Protocol Layer which implements one of a number of error
encoding and detection schemes on outgoing and incoming data frames respectively.
By appending extra encoding bits to outgoing data packets, it allows the receiving modem using
the same Error Control protocol to check whether the data has been corrupted during transmission
and demodulation by the lower layer. Similarly, it processes data frames received from the other
modem by using the appended encoding information to check for corrupted data.
In addition to error coding and detection, this layer also takes care of the retransmission of
corrupted packets to ensure that data delivered to the layer above it is error free.
This layer is also provided in several standards: LAPM, MNP2 through 4, MNP10, etc.
4.5.1.3. Data Compression
The third layer is the Data Compression and Decompression Protocol. On outgoing data, this
layer effectively reduces (or compresses) the number of bits coming from the application layer
above it to a smaller number of bits without losing any information. The receiving modem then
decompresses this data and reconstructs it to its original form.
Considering that the slowest link between two modems is always the wireline or cellular link, this
scheme reduces the amount of data flowing on that link and consequently increases the overall
data throughput.
This layer is implemented by a variety of protocols, two of the most widely used ones being
V.42bis and MNP5.