电子信息类专业英语Unit23 Network.ppt

This is the hierarchy of digital signals that telephone networks uses. Note that the bit rate of a DS-1 signal is greater than 24 times the rate of a voice signal (64 kbps) because of the additional framing bits required. 　　Observe in the table, that the rates are not multiples of one another: the DS-1 signal carries 24 DS-0 channels, but its rates are more than 24 times 64 kbps. The additional bits are used to accommodate DS-0 channels with rates that deviate from the nominal 64 kbps because the signals are generated using clocks that are not perfectly synchronized. 　　Figure 23.2 Integrated Services Digital Network (ISDN). 　　　　　　　The basic access provided two bidirectional 　　　　　　　64-kbps links and one 16-kbps link. These links 　　　　　　　can be used to transmit voice or data 　　The last major innovation in telephony is the integration of voice and data signals through the introduction of the Integrated Services Digital Network (ISDN), illustrated in Figure 23.2. The ISDN basic access offered to a customer consists of two B channels and one D channel (both B and D channels are digital). Each B channel is a bidirectional, or full-duplex, channel at 64 kbps. One B channel can carry either a circuit- switched connection, a packet-switched transmission services (described below), or a permanent digital connection. The D channel carries a 16-kbps packet-switched service. ISDN makes available to subscribers the digital transmission facilities that were previously used between the switches of the network, thus extending the digital transmission all the way to the users.［2］ Applications of the ISDN services include computer communication, high-speed facsimiles, remote monitoring of buildings, videotext, and low bit rate microphones. With ISDN, the telephone system is transformed into a network that can transfer information in many forms, if at modest speeds. 　　A new technology for transmission of data over untwisted or twisted pair cable for distances up to 4000