PC Troubleshooting FAQ
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Introduction To Modems
The modem is a device that converts digital
information to analog by MODulating it on the sending end and
DEModulating the analog information into digital information at the
receiving end. Modems are known as Data Circuit-Terminating
Equipment(DCE) while the computer using the modem is often referred to
as Data Terminal Equipment(DTE). Modems have different transmission
modes as follows:
Modems can also be classified by their speed which
was measured by the BAUD rate. One baud is one electronic state change
per second. Since a single state change can involve more than a single
bit of data, the Bits Per Second(BPS) unit of measurement has replaced
it as a better expression of data transmission speed. Common modem
speeds are V.34 at 28.8 kbps, V.34+ at 33.6 kbps and V.90 at 56 Kbps.
- Simplex - Simplex means that signals can be
passed in one direction only which means that communication only
happens in one direction.
- Half Duplex - Half duplex means that signals
can be passed in either direction, but not in both simultaneously.
Half-duplex modems can work in full-duplex mode.
- Full Duplex - Full duplex means that signals
can be passed in either direction, simultaneously. Full duplex
operation on a two-wire line requires the ability to separate a
receive signal from the reflection of the transmitted signal. This
is accomplished by either FDM (frequency division multiplexing) in
which the signals in the two directions occupy different frequency
bands and are separated by filtering, or by Echo Canceling (EC). The
implication of the term full-duplex is usually that the modem can
transmit and receive simultaneously at full speed. Modems that
provide a low-speed reverse channel are sometimes called split-speed
or asymmetric modems. Full duplex modems will not work on
Error correction is the method by which modems verify that the
information sent to them has been undamaged during the transfer.
Error-correcting modems break up information into small packets, called
frames. The sending modem attaches a checksum to each of these frames.
The receiving modem checks whether the checksum matches the information
sent. If not, the entire frame is resent. Though error correction may
slow down data transfer on noisy lines, it does provide greater
reliability. As with data compression protocols, for an error correction
protocol to be used, it must be supported by both modems in the
Sometimes one modem in a connection is capable of sending data at a
faster rate than the other can receive. Flow control allows the
receiving modem to tell the other to pause while it catches up. Flow
control exists as either software(XON/XOFF) flow control or hardware(RTS/CTS)
flow control. With software flow control, when a modem needs to tell the
other to pause and when to resume. Hardware, or RTS/CTS, flow control
uses wires in the modem cable or, in the case of internal modems,
hardware in the modem. This is faster and much more reliable than
software flow control.
Most modern modems are internal, however, they can be internal or
external. External modems are connected to the back of the system board
via a RS-232 serial connection. Internal modems are installed in one of
the motherboard's PCI or ISA expansion slots depending on the modem. The
modem contains an RJ-11 connection that is used to plug in the telephone
Hayes Corporation developed a smart modem which accepted AT type
commands. This is now a widely accepted standard. The following is a
brief list of the AT command set.
Don't get to attached to modems as they probably
won't be around in their current form for too many more years. Broadband
solutions such as cable modems and DSL are rapidly replacing old dial-up
connections. For those on the road, expect high-speed wireless solutions
to replace your 56k modem in the near future.
- ATA Answer call
- ATA/ Repeat last command
- ATC Turn modems carrier signal
ON (ATC1) or OFF (ATC0)
- ATD Dial a telephone number
- ATE Enable (ATE1) or disable
(ATE0) the echo of characters to the screen
- ATH Hang up the phone (ATH0)
or pick up the phone (ATH1)
- ATM Turn on modem speaker
(ATM1) or turn off speaker (ATM0)
- ATO Place modem on-line
- ATP Pulse dial
- ATS Set values in modem 'S'
- ATT Touch tone dial
- ATZ Reset the modem
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