An Overview of Analog Dialup Modem Performance, Environments, and Impairments.

REVISED 7/20/98 6
DOC. NUM. PRT/005A/0798
Error Correction
In order to correct for any errors that may occur on the line while the modems are connected, the
modems will negotiate an error-correcting protocol. In most applications, the protocol used will be
LAPM, although MNP4 may also be negotiated – both protocols are specified in the ITU-T Rec-
ommendation V.42. The error-correction performed should be transparent to the user, but over
heavily impaired lines, the overall throughput of the modem may decrease as the EC protocol is
forced to re-send data blocks to correct the errors. The primary function of error correction is to de-
tect and correct any bit errors that occur between the two modems, so that the integrity of the data is
maintained.
Network Topology
In order to better understand how modems are supposed to function during a data call, it is important
to understand the basic building blocks of the telephone network and how each piece can affect
modem performance. The PSTN is a complex network, consisting of both old and new technology,
and is a dynamic environment – enough so that it is even possible to see different performance from
one call to the next, depending on network traffic and how the call is routed. The basic components
of the PSTN are described in the following sections.