|  | 2.1 | WANs
| 2.1.3 | WAN technologies
Following is a brief description of the most common WAN technologies. They have been grouped into circuit-switched, cell-switched, dedicated digital, and analog services. For more information click on the Web links that are included. Circuit-Switched Services POTS (Plain Old Telephone Service) -- not a computer data service, but included for two reasons: (1) many of its technologies are part of the growing data infrastructure, (2) it is a model of an incredibly reliable, easy-to-use, wide-area communications network; typical medium is twisted-pair copper wire Narrowband ISDN (Integrated Services Digital Network) -- a versatile, widespread, historically important technology; was the first all-digital dial-up service; usage varies greatly from country to country; cost is moderate; maximum bandwidth is 128 kbps for the lower cost BRI (Basic Rate Interface) and about 3 Mbps for the PRI (Primary Rate Interface); usage is fairly widespread, though it varies considerably from country to country; typical medium is twisted-pair copper wire Packet-Switched Services X.25 -- an older technology, but still widely used; has extensive error-checking capabilities from the days when WAN links were more prone to errors, which makes it reliable but which limits its bandwidth; bandwidth may be as high as 2 Mbps; usage is fairly extensive; cost is moderate; typical medium is twisted-pair copper wire Frame Relay -- a packet-switched version of Narrowband ISDN; has become an extremely popular WAN technology in its own right; more efficient than X.25, but with similar services; maximum bandwidth is 1.544 Mbps; 56kbps and 384kbps are extremely popular in the U.S.; usage is widespread; cost is moderate to low; Typical media include twisted-pair copper wire and optical fiber Cell-Switched Services ATM (Asynchronous Transfer Mode) -- closely related to broadband ISDN; becoming increasingly important WAN (and even LAN) technology; uses small, fixed length (53 byte) frames to carry data; maximum bandwidth is currently 622 Mbps, though higher speeds are being developed; typical media are twisted-pair copper wire and optical fiber; usage is widespread and increasing; cost is high SMDS (Switched Multimegabit Data Service) -- closely related to ATM, and typically used in MANs; maximum bandwidth is 44.736 Mbps; typical media are twisted-pair copper wire and optical fiber; usage not very widespread; cost is relatively high Dedicated Digital Services T1, T3, E1, E3 -- the T series of services in the U.S. and the E series of services in Europe are extremely important WAN technologies; they use time division multiplexing to "slice up" and assign time slots for data transmission; bandwidth is: T1 -- 1.544 Mbps T3 -- 44.736 Mbps E1 -- 2.048 Mbps E3 -- 34.368 Mbps other bandwidths are available The media used are typical twisted-pair copper wire and optical fiber. Usage is extremely widespread; cost is moderate. xDSL (DSL for Digital Subscriber Line and x for a family of technologies) -- a new and developing WAN technology intended for home use; has a bandwidth which decreases with increasing distance from the phone companies equipment; top speeds of 51.84 Mbps are possible near a phone company office, more common are much lower bandwidths (from 100s of kbps to several Mbps); usage is small but increasing rapidly; cost is moderate and decreasing; x indicates the entire family of DSL technologies, including: HDSL -- high-bit-rate DSL SDSL -- single-line DSL ADSL -- asymmetric DSL VDSL -- very-high-bit-rate DSL RADSL -- rate adaptive DSL SONET (Synchronous Optical Network) -- a family of very high-speed physical layer technologies; designed for optical fiber, but can also run on copper cables; has a series of data rates available with special designations; implemented at different OC (optical carrier) levels ranging from 51.84 Mbps (OC-1) to 9,952 Mbps (OC-192); can achieve these amazing data rates by using wavelength division multiplexing (WDM), in which lasers are tuned to slightly different colors (wavelengths) in order to send huge amounts of data optically; usage is widespread among Internet backbone entities; cost is expensive (not a technology that connects to your house) Other WAN Services dial-up modems (switched analog) -- limited in speed, but quite versatile; works with existing phone network; maximum bandwidth approx. 56 kbps; cost is low; usage is still very widespread; typical medium is the twisted-pair phone line cable modems (shared analog) -- put data signals on the same cable as television signals; increasing in popularity in regions that have large amounts of existing cable TV coaxial cable (90% of homes in U.S.); maximum bandwidth can be 10 Mbps, though this degrades as more users attach to a given network segment (behaving like an unswitched LAN); cost is relatively low; usage is small but increasing; the medium is coaxial cable. wireless -- no medium is required since the signals are electromagnetic waves; there are a variety of wireless WAN links, two of which are: terrestrial -- bandwidths typically in the Mbps range (e.g. microwave); cost is relatively low; line-of-sight is usually required; usage is moderate satellite -- can serve mobile users (e.g. cellular telephone network) and remote users (too far from any wires or cables); usage is widespread; cost is high
| Web Links
| ISDN
| What is X.25?
| The Frame Relay Forum
| The ATM Forum
| Standards Committee T1 Telecommunications