The number of computers in use worldwide is in the hundreds of millions. Moreover, the expanding memory and processing power of these computers means that users can put the machines to work on new kinds of applications and functions. Accordingly, the pressure from the users of these systems for ways to communicate among all these machines is irresistible. It is changing the way vendors think and the way all automation products and services are sold. This demand for connectivity is manifested in two specific requirements: the need for communications software, which is previewed in the next section, and the need for networks.

Beyond the confines of a single office building, networks for voice, data, image, and video are equally important to business. Here, too, there are rapid changes. Advances in technology have led to greatly increased capacity and the concept of integration. Integration means that the customer equipment and networks can deal simultaneously with voice, data, image, and even video. Thus, a memo or report can be accompanied by voice commentary, presentation graphics, and perhaps even a short video introduction or summary. Image and video services impose large demands on wide area network transmission. Moreover, as LANs become ubiquitous and as their transmission rates increase, the demands on the wide area networks to support LAN interconnection have increased the demands on wide area network capacity and switching.

The opportunities for using networks as an aggressive competitive tool and as a means of enhancing productivity and slashing costs are great. The manager who understands the technology and can deal effectively with vendors of service and equipment is able to enhance a company’s competitive position.

In the remainder of this section, we provide a brief overview of various networks.

Parts Three and Four cover these topics in depth.

Wide Area Networks

Wide area networks generally cover a large geographical area, require the crossing of public right-of-ways, and rely at least in part on circuits provided by a common carrier. Typically, a WAN consists of a number of interconnected switching nodes. A transmission from any one device is routed through these internal nodes to the specified destination device. These nodes (including the boundary nodes) are not concerned with the content of the data; rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination.

Traditionally, WANs have been implemented using one of two technologies: circuit switching and packet switching. More recently, frame relay and ATM networks have assumed major roles.

Circuit Switching In a circuit-switching network, a dedicated communications path is established between two stations through the nodes of the network. That path is a connected sequence of physical links between nodes. On each link, a logical channel is dedicated to the connection. Data generated by the source station are transmitted along the dedicated path as rapidly as possible. At each node, incoming data are routed or switched to the appropriate outgoing channel without delay. The most common example of circuit switching is the telephone network.

Packet Switching A quite different approach is used in a packet-switching network. In this case, it is not necessary to dedicate transmission capacity along a path through the network. Rather, data are sent out in a sequence of small chunks, called packets. Each packet is passed through the network from node to node along some path leading from source to destination. At each node, the entire packet is received, stored briefly, and then transmitted to the next node. Packet-switching networks are commonly used for terminal-to-computer and computer-to-computer communications.

Frame Relay Packet switching was developed at a time when digital long distance transmission facilities exhibited a relatively high error rate compared to

today’s facilities. As a result, there is a considerable amount of overhead built into

packet-switching schemes to compensate for errors. The overhead includes additional bits added to each packet to introduce redundancy and additional processing at the end stations and the intermediate switching nodes to detect and recover from errors.

Frame relay was developed to take advantage of these high data rates and low error rates. Whereas the original packet-switching networks were designed with a data rate to the end user of about 64 kbps, frame relay networks are designed to operate efficiently at user data rates of up to 2 Mbps. The key to achieving these high data rates is to strip out most of the overhead involved with error control.

ATM Asynchronous transfer mode (ATM), sometimes referred to as cell relay, is a culmination of developments in circuit switching and packet switching. ATM can be viewed as an evolution from frame relay. The most obvious difference between frame relay and ATM is that frame relay uses variable-length packets, called frames, and ATM uses fixed-length packets, called cells. As with frame relay, ATM provides little overhead for error control, depending on the inherent reliability of the transmission system and on higher layers of logic in the end systems to catch and correct errors. By using a fixed packet length, the processing overhead is reduced even further for ATM compared to frame relay. The result is that ATM is designed to work in the range of 10s and 100s of Mbps, and in the Gbps range.

Local Area Networks

As with WANs, a LAN is a communications network that interconnects a variety of

devices and provides a means for information exchange among those devices. There are several key distinctions between LANs and WANs:

1. The scope of the LAN is small, typically a single building or a cluster of buildings.

This difference in geographic scope leads to different technical solutions, as we shall see.

2. It is usually the case that the LAN is owned by the same organization that owns

the attached devices. For WANs, this is less often the case, or at least a significant

fraction of the network assets is not owned. This has two implications. First, care must be taken in the choice of LAN, because there may be a substantial capital investment (compared to dial-up or leased charges for WANs) for both purchase

and maintenance. Second, the network management responsibility for a LAN falls solely on the user.

3. The internal data rates of LANs are typically much greater than those of WANs.

LANs come in a number of different configurations. The most common are switched LANs and wireless LANs. The most common switched LAN is a switched Ethernet LAN, which may consist of a single switch with a number of attached devices, or a number of interconnected switches. Two other prominent examples are ATM LANs, which simply use an ATM network in a local area, and Fibre Channel. Wireless LANs use a variety of wireless transmission technologies and organizations. LANs are examined in depth in Part Four.

Wireless Networks

As was just mentioned, wireless LANs are common are widely used in business environments. Wireless technology is also common for both wide area voice and

data networks. Wireless networks provide advantages in the areas of mobility and

ease of installation and configuration. Chapters 14 and 17 deal with wireless WANs and LANs, respectively.