Data Communications Networking Devices - 4th Ed.pdf

2.7 ATM _________________________________________________________________________ 251Figure 2.50The general structure of APPNencapsulation or tunneling. In Chapter 5 when we examine LAN internetworkingdevices we will turn our attention to several gateway solutions, such as techniquesto encapsulate SNA and APPN such that they can be routed over an IP network, atechnique referred to as Data Link Switching DLSw).2.7 ATMIn concluding this chapter covering the fundamentals of wide area networks, wewill focus our attention upon an emerging technology which represents a mechanismto transport voice, data, video, and images between private and publicnetworks. This emerging technology is known as Asynchronous Transfer Mode, orATM, and offers the ability to remove the barriers between local and wide areanetworks, providing a seamless interconnection for LAN internetworking betweengeographically separated areas. Although ATM represents a ubiquitous networkingtechnology that can be used for both local and wide area networking, in this sectionwe will primarily focus our attention upon its technology and use in a wide areanetworking environment. When we cover the fundamentals of local area networksin Chapter 3, we will then turn our attention to the operation of ATM in a LANenvironment.OverviewATM represents a joining of packet switching and multiplexing technology toprovide a transport facility for the movement of voice, data, video, and images overa common network infrastructure. Packet switching enables the use of networkresources only when it becomes necessary to transmit information, while multiplexingenables communications from multiple sources to share the use of the ATMinfrastructure by time. However, unlike conventional packet switching that dividesmessages into variable-length segments, ATM breaks all messages into ®xed-length53-byte cells. Through the use of ®xed length cells, switching and multiplexingfunctions become more ef®cient. In addition, as we will shortly note, the use ofrelatively short cells makes it possible to transport a mixture of voice, video, dataand image messages on a common infrastructure without the delay associated with