h speed transmission technology. ATM is a high bandwidth, low de- lay, connection-oriented, packet-like switching and multiplex- ing technique. Usable capacity is segmented into 53-byte fixed-size cells consisting of header and information fields allocated to services upon demand. The term "asynchronous" applies since each cell is presented to the network on a "start- stop" basis, in other words, asynchronously. The access de- vices, switches and interlinking transmission facilities, on the other hand, are highly synchronized. TCP/IP (Transmission Control Protocol/Internet Protocol) is a networking protocol (set of rules and conventions) that pro- vides communications across interconnected networks, between computers with diverse hardware architectures and various op- erating systems. TCP and IP are only two protocols in a family of Internet protocols. Over time they have come to refer to this entire family of protocols. Modems We mentioned the modem earlier. Let's talk about what a modem does and some of the different types of modems. Modem stands for MODulator and DEModulator. One type of modem sits at each end of an analog telephone line that converts digital signals to high and low tones. This is sometimes called a dial-up modem. These high and low tones represent zeroes and The Telecom Handbook x 237 TRANSMISSION ones, enabling the digital format to be carried on an analog line. Since most telephone lines in place are designed to accept analog signals, the modems enable digital signals to be transmitted over an analog line. The tones are discrete. If not kept discrete, a series of ones would be indistinguishable. This is achieved using varying techniques. The modem demodulates the stream of tones at the far end of the circuit, converting the ones and zeroes into a format that can be understood by the other device with which you are commu- nicating. There are many different modulation schemes. You may have seen (maybe in a museum!) one of the earliest types of modems developed called an acoustic coupler. The 