Introduction to Modems

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:

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 half-duplex channels.

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.

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 connection.

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 line.

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.

ATA Answer call
ATA/ Repeat last command
ATC Turn modems carrier signal ON (ATC1) or OFF (ATC0)
ATD Dial a telephone number (ATDT255-0789)
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' registers
ATT Touch tone dial
ATZ Reset the modem

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.

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