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

REVISED 7/20/98 8
DOC. NUM. PRT/005A/0798
Inter-Office Trunking
Inter-office trunking is the part of the network that connects the CO facilities to each other, as well
as to other network systems and facilities. This is typically done over digital fiber-optic links which
are capable of carrying enormous amounts of data.
Long Distance and 1-800 numbers
Long distance and 1-800 connections pose a potential impairment to the modem due to the routing
of the call and the number of facilities through which the call must pass in order to reach its destina-
tion. This is especially true of international calls or other such connections which may pass through
a satellite relay. 1-800 numbers may also use a scheme known as ‘Least-Cost Routing’, which may
route the call through a more indirect path.
PBX Systems
PBX (Private Branch Exchange) systems can introduce their own set of problems for modem com-
munications. One important consideration is whether the available line at a particular location is
digital or analog – modems are generally not designed to operate over digital PBX lines, and in
some cases can be damaged by connecting to a digital line. While analog lines can be safely used
for modem communications, the quality of the connection will vary according to the PBX system
used and how it is connected to the PSTN. Many PBX installations are not digitally integrated into
the PSTN (line-side terminated instead of trunk-side), which can introduce impairments.
It’s also important to note that some PBX systems use non-standard call-progress signals (dial tone,
ring frequency and cadence, etc.), which can cause problems for modem operation. On the client
side of the PBX system, things like dial tone and ring cadences cannot be taken for granted; these
systems can and often do behave in a way that is quite unlike the PSTN that modems are designed
to operate on.
Analog Impairments
Noise
Noise is quite simply the presence of any interference or unwanted signal on the line. This can
sometimes be heard as pops, clicks, static, or other noise when using a normal telephone on the line.
The presence of such noise will degrade modem performance.
Signal Level
This is the level, expressed in decibels referenced to 1 milliwatt (dBm), at which the modem is
transmitting to the network (transmit level), as well as the perceived loudness of the remote signal
(receive level). Of these, the receive level is often the most variable and potentially harmful to
modem performance. If the receive signal level is too low, the modem will be more adversely
affected by any noise present on the line, and in extreme cases the level may be so low that the
modem cannot properly decode (or ‘hear’) the signal from the remote modem. Conversely, if the
receive level is too high, or too loud, the analog front-end of the modem may become saturated
(driven into clipping) or non-linear, resulting in poor signal quality. A typical receive level would
be somewhere around –22 to –26dBm, with an upper limit of approximately –18dBm and lower
limit of approximately –35dBm.