87-01-45.1 Control of Wide Area Networks
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87-01-45.1 Control of Wide Area Networks Previous screen Steven Powell Frederick Gallegos Payoff This article describes the various types of wide area networks and their access methods and connective devices, communications protocols, network services, and network topologies. It also describes the automated tools that are currently available for network monitoring, and provides a brief introduction to the Internet. Introduction Local area networks (LANs) have become commonplace in most medium and large companies. Now, wide area networks (WANs) have become the next communications frontier. However, WANs are much more complicated than LANs. In most WAN environments, the more devices an individual has to manage, the more time-consuming is the process of monitoring those devices. Complexity also increases very rapidly due to the fact that each new device on the network invariably has to interface with many existing devices. In order to get a further understanding of WANs, it is useful to explore the differences between WANs and LANs. LANs are defined as communications networks in which all components are located within several miles of each other and communicate using high transmission speeds, generally 1M-bps or higher. They are typically used to support interconnection within a building or campus environment. WANs connect system users who are geographically dispersed and connected by means of public telecommunications facilities. WANs provide system users with access to computers for fast interchange of information. Major components of WANs include CPUs, ranging from microcomputers to mainframes, intelligent terminals, modems, and communications controllers. WANs cover distances of about 30 miles, and often connect a group of campuses. WANs are usually static in nature. Changes to them require rerouting telephone lines and installing modems. LANs on the other hand can be quickly reconfigured; communications lines are set up and rerouted more easily and gateways to host computers can be quickly added. Types of Wide Area Networks There are two basic types of WANs: centralized and distributed. Centralized WANs consist of a mainframe or minicomputer that serves remotely distributed dumb terminals. Network managers lease communications channels from a common long-distance carrier and tie together terminals and the central computer using a star (or other) topology. (WAN topologies are described in more detail later in this article.) Communications is fairly straightforward; the smart computer polls the dumb terminals to find out if they have anything to transmit, and then it controls data transmission so that there are no collisions. Distributed networks provide an environment that allows independent computers to have equal levels of control in the communications architecture. Distributed networks have grown as smart computers have increased throughout organizations. Today's packet WAN technologies are capable of supporting worldwide transmission at rates that are less than LAN transmission rates. (LANs support transmission rates of 100M-bytes and higher over relatively short distances.) Benefits of Wide Area Networks Previous screen WANs provide a number of benefits, including: · Allowing network expansion and terminal changes to be accomplished through plug-in connections over wide area locations. · Support of a variety of applications and a large number of terminals. · Facilitating interconnectivity by means of gateways, bridges, and routers. · Distributing terminals to the most convenient locations rather than forcing them to remain in a more centralized area. · Centralized network management and monitoring of use and performance to ensure maximum reliability and availability. These benefits help make users more effective in their jobs. Networking over wide areas also helps reduce or eliminate the expensive costs of gathering teams of people together. A US manufacturing team can now work closely with a team in Germany using such services as electronic mail and computer video conferencing. Elements of Wide Area Networks WANs differ according to their access methods, connective hardware and software, communications protocols, types of network services, and network topologies. These differences affect network installation, growth, and operating costs. In addition, WANs depend on the network management system for efficient and reliable operation. The following sections describe these elements. Access Methods Connections to remote networks may be accomplished over public data networks or private lines provided by long-distance and interexchange telephone carriers. The Internet requires the use of 32-bit addresses, which are administered by the Network Information Center. Locally administered private networks should encourage use of addresses that are compatible with Internet addresses to facilitate connection to the Internet. Connective Devices Information is transmitted over WANs in packets. In addition to user data, these packets contain information necessary for network management and protocols that permit local and remote devices on the network to recognize one another. For example, each packet contains address information which is necessary to ensure the correct routing of the packet. Bridges and routers are the primary connective devices used to handle these transmissions. Bridges. A bridge is a hardware and software device used to connect networks using various media and signalling systems. Bridges operate at the data-link layer of the open systems interconnection model. Bridges read and filter data packets and frames, passing traffic only if the address is on the same segment of the network cable as the originating station. (Frames contain information that is necessary for reassembling the messages contained in packets after they reach their destination. There are two types of frames: control frames for Previous screen link management and information frames for the transfer of information.) Routers. A router is a sophisticated hardware and software device that connects local and wide area networks. It serves packets or frames containing certain protocols, and it routes packets using network layer protocols. Multiprotocol routers can operate in heterogeneous environments by simultaneously using multiple protocols. Protocols WAN protocols are designed to provide connections for many devices within a wide area. Their purpose is to support a peer network of terminals, microcomputers, and hosts. A number of WAN protocols are available, including TCP/IP (which is the combined acronym for a pair of networking protocols, the Transmission Control Protocol [TCP] and the Internet Protocol [IP]). The TCP/IP protocols provide the prim ary communications procedures for the Internet. IBM's Systems Network Architecture (SNA) is designed to provide communications compatibility among microcomputers, minicomputers, and mainframes. For example, it can be used to connect IBM token-ring LANs to a host environment. Network Services Frame relay and asynchronous transfer mode (ATM) are technologies used to support network traffic. Their method of operation is described in the following paragraphs. The appendix at the end of this article provides a listing of selected vendors of frame relay and ATM products. Frame Relay Network Services. Frame relay is an extremely flexible and cost-effective technology that supports variable network traffic. Service bandwidth is scalable from 56K bps to 2.048M bps, and it offers a variable-length frame size from 262 bytes to 8K bytes. Frame relay allows users to gain the advantages of high-speed circuits without having to run dedicated links between all the endpoints on a private network. The other major advantage of frame relay is its minimal packet overhead. Asynchronous Transfer Mode Network Services. Asynchronous transfer mode (ATM) refers to a high-bandwidth, low-delay switching and multiplexing technology. ATM network services provide a foundation for high-speed data transmission, LAN connectivity, imaging, and multimedia applications. ATM is based on cell-switching technology that is equally effective at transmitting voice, video, and data at high speeds. ATM is better suited than packet-switching to real-time communications (e.g., video) because it uses standard-length cells with small headers containing packet and address information. ATM supports transmission speeds of up to 622M-bps. It supports services requiring both circuit-mode information transfer capabilities (characterized by a constant bit rate) and packet-mode capabilities (characterized by a variable bit rate). The Network Management System Previous screen Every major business wants to have the most efficient and economical operation of the corporate network. In order for a business to achieve this, it must effectively manage the computer and communications resources over the wide area network. Because most businesses buy their networking products from more than one vendor, network management systems must be able to support a wide variety of equipment on the same network. This diversity makes the task of management and troubleshooting more challenging. (Network management software is discussed later in this article.) Network Topologies Although the star topology is the most popular WAN topology, a number of other network topologies are available. Each of the these topologies has consequences with respect to reliability and availability. Star Topology. The star topology is highly reliable; loss of one node results only in the loss of a single