【文章內(nèi)容簡介】 stems acmodate a large number of users over a large geographic area, within a limited frequency spectrum. Cellular radio systems provide high quality service that is often parable to that of the landline telephone systems. High capacity is achieved by limiting the coverage of each base station transmitter to a small geographic area called a cell so that the same radio channels may be reused by another base station located some distance away. A sophisticated switching technique called a handoff enables a call to proceed uninterrupted when the user moves from one cell to another. A basic cellular system consists of mobile station, base stations and a mobile switching center (MSC). The Mobile Switching Center is sometimes called a mobile telephone switching office (MTSO), since it is responsible for connecting all mobiles to the PSTN in a cellular system. Each mobile municates via radio with one of the base stations and may be handedoff to any number of base stations throughout the duration of a call. The mobile station contains a transceiver, an antenna, and control circuitry, and may be mounted in a vehicle or used as a portable handheld unit. The base stations consists of several transmitters and receivers which simultaneously handle full duplex munications and generally have towers which support several transmitting and receiving antennas. The base station serves as a bridge between all mobile users in the cell and connects the simultaneous mobile calls via telephone lines or microwave links to the MSC. The MSC coordinates the activities of all the base stations and connects the entire cellular system to the PSTN. A typical MSC handles 100000 cellular subscribers and 5000 simultaneous conversations at a time, and acmodates all billing and system maintenance functions, as well. In large cities, several MSCs are used by a single carrier. Broadband Communication As can be inferred from the examples of video phone and HDTV, the evolution of future munications will be via broadband munication centered around video signals. The associated services make up a diverse set of highspeed and broadband services ranging from video services such as video phone, video conferencing, video surveillance, cable television (CATV) distribution, and HDTV distribution to the highspeed data services such as highresolution image transmission, highspeed data transmission, and color facsimile. The means of standardizing these various broadband munication services so that they can be provided in an integrated manner is no other than the broadband integrated services digital work (BISDN). Simple put, therefore, the future munications work can be said to be a broadband telemunication system based on the BISDN. For realization of the BISDN, the role of several broadband munication technologies is crucial. Fortunately, the remarkable advances in the field of electronics and fiber optics have led to the maturation of broadband municatio n technologies. As the BISDN bees possible on the optical munication foundation, the relevant manufacturing technologies for lightsource and passive devices and for optical fiber have advanced to considerable levels. Advances in highspeed device and integrated circuit technologies for broadband signal processing are also worthy of close attention. There has also been notable progress in software, signal processing, and video equipment technologies. Hence, from the technological standpoint, the BISDN has finally reached a realizable state. On the other, standardization activities associated with broadband munication have been progressing. The Synchronous Optical Network (SONET) standardization centered around the T1 mittee eventually bore fruit in the form of the Synchronous Digital Hierarchy (SDH) standards of the International Consultative Committee in Telegraphy and Telephony (CCITT), paving the way for synchronous digital transmission based on optical munication. The standardization activities of the integrated services digital work (ISDN), which menced in early 1980’s with the objective of integrating narrowband services, expanded in scope with the inclusion of broadband services, leading to the standardization of the BISDN in late