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Packet switching was invented in the early 1960s by Paul Baran and his research associates for the RAND Corporation. Interestingly enough, the concept first was published in 1964 as a means of transmitting secure voice for military application. In the late 1960s, the U.S. General Accounting Office (GAO) issued a report suggesting that there existed a large number of data centers supported, at least in part, by the federal government. Further, the report indicated that many of those data centers were underutilized, and others were severely stressed. The imbalance was due largely to the lack of a wide area network technology that would allow the sharing of those resources on a cost-effective basis.
As a result of that study, the Advanced Research Project Agency Network (ARPANET), the first sophisticated packet switched network, was created in 1971. ARPANET was developed to link computers on a time-share basis in order to share computer resources on a cost-effectively [8-15]. Specifically, ARPANET was designed to support various defense, higher education, and research and development organizations [8-16]. In 1983 the majority of ARPANET users spun off to form the Defense Data Network (DDN), also called MILNET (MILitary NETwork), which included European and Pacific Rim continents. The United States and European locations which remained ARPANET then merged with the Defense Advanced Research Project Agency Network to become DARPA Internet [8-17].
Packet switching soon was commercialized and made widely available by Telenet, Graphnet (a facsimile-like service) and others. Packet switching was utilized very early on and extensively in Europe, as well. In fact, packet switching quickly became available in most countries, and currently is virtually ubiquitous. The CCITT (now the ITU-T) in 1976 internationally standardized X.25 as the interface for a packet-switched network.
The wide availability of packet switching has made it consistently popular over the last twenty years or so. Additionally, packet networks are highly cost-effective for applications that require many-to-many connectivity, and which involve relatively low data volumes. That popularity is growing and is ensured well into the future, largely through its historical deployment as the network technology of the Internet. It should be noted that X.25 is an interface specification, and does not define the internal operations characteristics of the data network.
The basic concept of packet switching is one of a highly flexible, shared network in support of interactive computer communications across a wide area network (WAN). Previously, large numbers of users spread across a wide area with only occasional communications requirements had no cost-effective means of sharing computer resources (time-share) from their remote terminals. In specific, the issue revolved around the fact that asynchronous communications are bursty in nature; data transmission is in bursts of keystrokes or data file transfers, with lots of idle time on the circuit between transmissions in either direction or relatively small amounts of data. Additionally, those early networks consisted of analog, twisted-pair facilities, which offered very poor error performance and relatively low bandwidth.
Existing circuit-switched networks certainly offered the required flexibility, as users could dial up the various host computers on which the desired database resided. Through a low-speed modem, data is passed over the analog network, although error performance was less than desirable. However, the cost of the connection was significant, because calls were billed based on the entire duration of the connection, even though the circuit was idle most of the time. Dedicated circuits could solve the cost issue, as costs are not usage-sensitive; however, dedicated circuits are expensive. Further, users tended not to be concentrated in locations where they could make effective use of dedicated circuits on a shared basis. Finally, large numbers of dedicated circuits would be required to establish connectivity to the various hosts.
Packet switching solved many of those problems, in the context of the limitations of the existing networksnamely, analog, twisted-pair. Packet-switched networks support low-speed, asynchronous, conversational and bursty communications between computer systems. As packet-switched network usage is billed to the user on the basis of the number of packets transmitted, they are very cost-effective for low-volume, interactive data communications. This cost advantage comes from the fact that the bursty nature of such interactive applications allows large volumes of data transmissions from multiple users to be aggregated in order to share network facilities and bandwidth [8-17]. Further, packet-switched networks perform the process of error detection and correction at each of the packet switches, or nodes, improving the integrity of the transmitted data considerably.
Understanding the concept and nature of packet switching requires the examination of a number of dimensions and characteristics of such networks including packet structure, switching and transmission, error control, connection-oriented service, latency, permanent virtual circuits versus switched virtual circuits, protocol conversion, and access techniques.
Packet Structure
Information is transported and switched through the network on the basis of packets (Figure 8.6). Each packet is of a fixed maximum size, typically containing 128B (B=byte or octet) or 256B of payload (user data); packet sizes of up to 4,096B can be accommodated in some networks. The typical upper limit of a packet is 1,024B [8-18], as is the case in many airline reservation networks.
Figure 8.6 Packet structure.
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