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

REVISED 7/20/98 7
DOC. NUM. PRT/005A/0798
Central Office
The Central Office, or CO, is the telephone company switching location that directly services the
customer. Depending upon the size of an area, there may be several – or dozens – of CO locations
to serve all of the customers and telephone lines in an area. Central office provisioning has im-
proved considerably in the last several years, enabling such technology and features as Caller ID,
voice messaging, ISDN, call forwarding/call return, and other such features. Unfortunately, there
are still a number of CO facilities that utilize older technology, and which may prevent a modem
from operating at its peak performance. One example would be the facilities that are still using an
analog switch – this can introduce impairments for normal analog calls, and would make a PCM
connection impossible.
Copper Loop
The copper loop, or subscriber loop, is the run of wire between the telco CO and the customer
location. In its simplest form, this is nothing more than 22-guage wire from end to end. More
complex installations will use some combination of copper wire and digital subscriber loop
concentrators, or SLCs, to serve a large number of customers. The copper loop is the network
component that is most vulnerable to degradation. It is subject to physical damage due to being cut
or otherwise disturbed, environmental issues due to exposure, moisture, or other interference, and
degradation due to corrosion and insulation breakdown. The copper loop may also contain loading
coils or other devices if the length from the CO to the customer site is longer than normal –
generally 12,000 cable feet or greater.
Premises Wiring
This, of course, is the final link in the chain – the inside wiring of the house, business, or other facil-
ity. The premises wiring is the only part of the network topology that can be accessed or affected by
the end user. Some considerations to keep in mind are:
n Device loading – multiple devices on the same line as the modem can affect performance. This
would include such things as multiple extension phones, Caller ID boxes, answering machines,
satellite dish receivers, and so forth. In extreme cases, these devices could introduce a
sufficient load on the circuit that the modem may not operate properly.
n Cross-talk – this can be a problem when the premises is wired for two phone lines that are
being carried over the same wiring in adjacent pairs. Cross-talk is the condition where you can
hear interference (or in extreme cases, actual conversation) coming from one phone line while
using another. This is usually caused by poorly shielded cabling somewhere in the premises
wiring, and can make it very difficult for a modem to connect at the appropriate speed.
n Poor connections and corrosion – this can be a problem in older houses or facilities where the
telephone lines are old, the connections could be loose or corroded, or wiring exposed due to
deterioration of the insulation. It is also common to find corrosion in telephone network inter-
faces that are exposed to the elements.
n Interference – this can be caused by running the phone wiring parallel to electrical wiring,
fluorescent light fixtures, and other potential sources of interference.