Network Modems

Network modems (modulators-demodulators) are devices or programs that allow computers to transmit data over telephone lines. They convert digital computer data to analog sound waves and then demodulate the carrier signals to decode the transmitted information. There are two form factors for network modems: internal and external. Internal modems are computer cards that attach to a computer through a slot. External modems are modules or devices that connect to a larger system through a physical port. In terms of connectivity, several device types are commonly available. Public switched telephone network (PSTN) modems transmit analog data across copper telephone wires. Integrated services digital network (ISDN) devices can transmit data over regular telephone lines or digital telephone lines. Network modems that use digital subscriber lines (DSL) support standard xDSL or variants such as asynchronous DSL (ASDL), symmetric DSL (SDSL), or high data rate DSL (HDSL). Cable modems operate over cable television lines and can achieve very high data rates. Fiber optic modems are also available.

Selecting network modems requires an analysis of bus, card, connector and interface types. Industry standard architecture (ISA) buses can handle 16-bit data transfers at clock speeds of 8 MHz. Peripheral component interconnect (PCI) buses can transfer 32 or 64 bits of data at clock speeds of 33 MHz. Universal serial bus (USB) 2.0 supports data transfer rates of up to 480 Mbps. Type II modem cards are 54 mm x 85.4 mm and up to 5.5 mm thick. Type III cards are also 54 mm x 85.4 mm, but up to 10.55 mm thick. Some network modems plug into RS232, RS422, or RS485 ports. Others interface directly with a programmable logic controller (PLC). Several connector types are commonly available. Medium interface connectors (MIC) are duplex devices used in fiber distributed data interface (FDDI) networks. RJ-45 connectors are similar in appearance to standard telephone connectors, but are twice as wide with eight wires. Straight tip (ST) connectors feature a quick release, bayonet coupling that requires only a quarter turn to engage or disengage. Many ISDN devices have a U interface that allows a direct connection to a wall jack, or an S/T interface for connections to other ISDN devices. Network modems that use IEEE 1394 or FireWire^® connectors are also available. FireWire is a registered trademark of Apple Computer, Inc.

Important specifications for network modems include modem speed, fax speed, modem technology, and fax mode. Modem speed, the maximum rate at which devices can deliver data, is normally measured in kilobits per second (kbps). Typical fax speeds are 7.2 kbps, 9.6 kbps, and 13.4 kbps. Common modem technologies include V.120, V.92, V.90, V.90, V.34 and V.34+. Fax modes include V.17, V.29, V.27ter, and V.21.

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Part Numbers for Network Modems