en how long this is likely to 4 NETWORK TECHNOLOGY AND METHODOLOGY continue. The classical telephone companies don't want to cannibal- ize their bread and butter-lucrative voice services-and they des- perately want to tap into new revenue streams of their up and coming competitors-cable modems and DSL-capable ISPs. There- fore, initial DSL service is limited to Internet access. As the Internet matures-achieves a grade and quality of service capable of support- ing voice-over IP-this situation will change. Who knows when, but someday in the future it may be possible to call up the telephone company and ask them to discontinue POTS. Keep in mind that the main purpose of the switching function is to share use of the transmission function. Also, keep in mind the fact that change in the network is more a direct result of economic pressure than technological or regulatory forces. CELL-BASED SWITCHING Cell switching in communications networks can be one of two entirely different things. One use of the term applies to mobile com- munications backbones where the switching takes place as the mobile telephone passes from one coverage area to an adjacent area or cell. The other use of the term is more commonly known as ATM. One of the more astute and practical observations that can be made about ATM is that it's easy to get confused upfront by the name. The asynchronous part has to do with the interface between the network, which is truly synchronous and the nature of the disparate traffic, which is asynchronous, even though it maybe from a single, highly accurate digital clock source. Asynchronous also means the traffic, or data, is handled in a start-stop mode, similar to asynchronous serial interface with its start-and-stop bits. ATM functional elements include a fixed length 53-byte cell, trans- mission links, and a switching machine. Unlike circuit switching, where the intelligence is resident in an underlying common channel signaling system, ATM intelligence is embedded in each cell and distributed throughout the netwo