Wide Area Network WAN

A Token Passing LAN is a local computer network in which a uniquely defined data pattern, called a token, travels continuously through the network, passing from one host to another. Only the host that currently possesses the token has the right to transmit data if the need exists at all, and its data transmission is terminated by the token, which passes the right to talk to another host. If there is no data to transmit, the host retransmits the token alone, preserving the continuous token circulation in the network. The token size and the time needed for a token transmission are constant; consequently, the network throughput is independent of the network load. The network response time is deterministic; the worst−case scenario determines its guaranteed value. Control of token passing is more complex. A logical ring must be provided to keep up the repeated sequential token passing from one host to another, but keeping the logical ring operable is not an easy task the token could be lost or doubled, or something else could happen. A special procedure is required to add a host to or to remove a host from the network. This is especially true in the case of the bus network structure. It is much easier to handle networks with ring topology; the token circulation is preserved by the network structure itself. Depending on the implemented network topologies, Token Passing LANs are divided into Token Bus and Token Ring networks. Token Bus LANs have disappeared from the network arena due to the complexities of their control, but Token Ring LANs are widely used. Physical rings automatically provide the logical ring required for token passing; each host in the ring receives data from the preceding host, and transmits data to the succeeding host each host knows and communicates with two neighboring hosts only, regardless of who they are. Inserting a new host into the ring is not a problem; removing a host means simply bypassing its previous connection. All these characteristics have contributed to the wide implementation of this type of LAN.

14.2.2 Wide Area Network WAN

Local area networks have revolutionized data transmission, and a number of new technologies for fast data transmission have been developed and implemented. An enormous quantity of data unimaginable in the past has become available through networking, promoting distributed data processing. Processing resources, and power, could be spread over the network and used more efficiently. Therefore, a distributed environment and the long−held dream of the processors pool that could handle incoming requests in an optimal way has become a reality. Despite all of their advantages, LANs can only connect computers that are geographically close. How could two computers on two sides of a city communicate, or two computers in two distant cities, or states, or even continents? LAN technologies are not very useful in these cases, and more traditional and expensive telecommunication techniques must be implemented. If two distant LANs are connected with a fast link, all participating hosts think that they share an equivalent fast computer network, regardless of what the actual distance between those hosts is. In this fashion, we now have a Wide Area Network WAN, as seen in Figure 14.2. In WANs there is always more traffic between neighboring hosts within the same LAN than among distant ones because many hosts run strictly local network applications; this means that the inter−LAN link could be even slower than LANs themselves, while WAN throughput remains acceptable. 324 Figure 14.2: Wide area network. WANs are widely used, and they are constantly evolving. Today, numerous WANs are connected into a unique worldwide WAN, a global computer network known as the Internet. Multiplying LANs within WANs had two positive effects: it decreased the costs of the requested links which became shorter and were shared among more participating LANs and provided alternative routes to reach any host in the network. However, the problem of reaching a distant host in a network still had to be solved. Special addressing mechanisms to uniquely identify each host in the WAN were implemented, and appropriate routing algorithms to transmitretransmit data in the network were developed. Special, dedicated hosts, known as routers have become a part of each LAN, with their only mission to route data toward distant LANs and hosts. There are many different LANs currently in use, and they are often mutually incompatible: different media, modulation techniques, protocols, etc. just compare Ethernet and Token−Ring LANs for a look at the variety possible. However, such different and incompatible LANs often need to be connected together. Therefore, special devices to overcome such incompatibility must be implemented, and in the case of protocol conversion, these devices are known as gateways. The terms gateway and router are often used interchangeably, mostly because gateways also provide routing services. However, the two terms are not interchangeable — routing and protocol conversions are two independent concepts, and they are not necessarily complementary. 325 The common thread that ties the enormous Internet community together is TCPIP network software. The name TCPIP refers to an entire suite of data communication protocols that define how different types of computers talk to each other. The suite gets its name from two of the protocols that belong to it: the transmission control protocol TCP and the Internet protocol IP. Although there are a number of other protocols in the suite, TCP and IP are the best known. In 1969 the Defense Advanced Research Projects Agency DARPA started a research and development project to create an experimental packet switching network. This network was named A R P A N E T , and was built to explore techniques for providing robust, reliable, and vendor−independent data communications. The project outputs far surpassed all expectations, and a number of modern data communication techniques were developed, or at least conceptually solved. The experimental ARPANET was so successful that many of the organizations attached to it began to use it for their daily needs. In 1975 the ARPANET was converted from an experimental network to an operational one, and the responsibility for administering the network was given to the Defense Communications Agency DCA, later renamed the Defense Information System Agency DISA. However, the development of ARPANET did not stop once the network became operational; the basic TCPIP protocols were developed after ARPANET was operational. In 1983 the TCPIP protocols were adopted as MIL standards, and all hosts computers connected to the network were required to convert to the new protocols. To ease this conversion, DARPA funded an expert team to implement TCPIP in BSD UNIX, thus beginning the marriage of UNIX and TCPIP and their triumphant, long−lasting journey. BBN Bolt, Beraneck and Newman, was chosen to facilitate the implementation. The company, located in Boston, MA, was well known in the field of acoustics. As it was located close to MIT the Massachusetts Institute of Technology, it attracted talented MIT graduates and very quickly gained a strong reputation in computer technologies. The project was a real success and was completed extraordinarily well. Todays Internet fully relies on the solutions introduced by BBN. UNIX itself also contributed to the development of inter−computer communication. Besides the TCPIP protocols used primarily to communicate throughout the local area network and more widely, UNIX also provides UUCP for communication with remote, isolated computer sites.

14.3.1 TCPIP and the Internet