Enterprise Networks II: Wide Area Networks

Abstract

Early data networks were based on point-to-point or multipoint topologies. They were often of a star structure with a central processing unit (CPU) at the star's hub. Terminals in this network were “dumb” terminals and relied on the CPU's processing power for nearly all network activity. When the personal computer (PC) gained wide acceptance, distributed processing became the rule. As a result, data networks and data networking became much more complex. We now see complex multiple star, mesh, and tree like networks where many of the nodes have multiple inlets and outlets. Data rates were based on the limitations of the analog network. In the period of the 2nd edition of this book, this roughly equated to 9600 bps. Today, it is common to have 56-kbps access lines to a larger network where the basic channel data rate is 64 kbps. We can bring together these basic building blocks of 64 kbps to form aggregate channels of 128, 256, and 384 kbps up to 1.536 Mbps or 1.984 Mbps. Data protocols instilled order out of contentious chaos in data network operation. These protocols, applying to wide area networks (WANs), are the principal thrust of the chapter. Most are based on shipping user data in packets or frames. We discuss the transport of user data in cells in Chapter 28. Most packet networks are based on CCITT Rec. X.25. If we widen the definition of packet networks to include the transmission control protocol/internet protocol (TCP/IP), certainly X.25 will have a competitor. Both TCP/IP and X.25 have a comparatively high latency (latency is the time it takes from initiation to completion of a data transaction). Frame relay was introduced in the same period. This is a comparatively low latency system. This section provides design data and brief descriptions of wide area data networking as a whole with hypothetical reference circuits, the archetypical data link layer protocol, high-level data link control (HDLC), X.25 and its companion, link access protocol B-layer (LAPB), TCP/IP family of protocols, frame relay family of protocols, and a brief description of SMDS (switched multimegabit data service). Variants of LAPD are discussed in the frame relay section of this chapter. Keywords: enterprise networks; universal application; high-level data-link control; packet-switched; virtual connections; transmission control protocol; internet protocol; ISO transport protocol; frame relay; switched multimegabit data service; internetworking; ITU-T rec X.75; data network management