Computer Networks II

DEVELOPMENT OF NETWORK TECHNOLOGY

Research facilities sponsored by the U.S. Department of Defense were among the first to develop computer networks. Perhaps the most famous example of such a network is the Internet, which began in 1969 as Arpanet, part of a project to link computers at four research sites. One of the most significant developments in these early networks was the concept of packet switching, which encodes data for transmission over networks into small chunks of information that each carry meta-information about where the data are coming from, where they are going, and how each piece fits into the whole. Packet switching, the basis of all modern networking, enables a transmission to be routed through any number of computers to get to its destination, and provides an efficient means of retrieving lost information. If a packet is lost or corrupted, only a single packet need be re-sent, which is handled behind the scenes by the networking software, rather than starting the entire transmission over again.

TECHNICAL STANDARDS.

Several of the most defining advances occurred in the early 1980s. Coming on the heels of IBM's mid-1970s introduction of the Systems Network Architecture (SNA), a proprietary set of highly stable protocols for networking mainframes and mid-range systems, a few important industry wide standards were reached that cleared the path for widespread implementation of networking. The first of these was the debut of the Institute of Electrical and Electronics Engineers' (IEEE) 802.x series of standards, which prescribed the technical specifications for various types of network data exchanges. The IEEE standards, which are updated and expanded periodically, are still in force today. Next, a common architecture model called the Open Systems Interconnection (OSI, see below) was adopted by the International Organization for Standardization (ISO). Although the OSI was only a broad model, it provided network developers with an internationally accepted classification of the different network functions and processes and how they ought to work together. The OSI and the IEEE standards were complementary.

COMMERCIAL IMPLEMENTATION.

The Ethernet LAN protocols both influenced the formation of technical standards and became the most widespread embodiment of those standards. Ethernet was pioneered in the late 1970s by Xerox at its famous Palo Alto Research Center (PARC) with assistance from then-Digital Equipment Corp. (later part of Compaq Corp.) and Intel Corp. Indeed, the experimental Ethernet was the model on which the original IEEE standard was based, and Ethernet quickly became (and still is) the most common commercially produced LAN protocol.

Ethernet employs several hardware standards for various bandwidths and device connections, but it is perhaps best characterized by its use of a protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). CSMA/CD is essentially a set

•Ethernet—a series of widely used hardware/software protocols for local area networks

•Local area networks (LANs)—networks that are confined to a single building or part of a building and that employ technology to capitalize on the advantages of close proximity (usually speed)

•Metropolitan area networks (MANs)—networks that are accessed from multiple sites situated in a relatively concentrated area (within 50 km or so) and that function as a faster alternative to wide area networks

•Nodes—individual computers on a network

•OSt—Open Systems Interconnection model, a broadly defined international model for the hierarchy of data communications between networked computers

•Packets—also called datagrams, these are measured pieces of information (usually ranging 500 to 2,500 bytes in size) in a data transfer that are each separately addressed to their destination and reassembled into the full original message at the receiving end

•Protocols—a set of rules dictating how hardware and software communicate with other devices

•Storage area networks (SANs)—a high-performance network of storage/backup devices integrated with one or more primary computer networks

•Topology—the structure of how networked computers are actually connected to each other and to other network resources

•Wide area networks (WANs)—networks that are maintained over two or more separate buildings and use technologies that maximize the ease and cost-effectiveness of connections between distant locations (often at the expense of speed)

of rules for how competing devices can share finite network resources. Through this protocol a computer on the network can determine whether it can send data immediately or whether it must compete with another device for network services. Collision occurs when two devices attempt to use the same resource, and the CSMA/CD protocol provides a simple mechanism for resolving this contention: it halts the colliding operation (the one initiated last) and keeps trying to resend the data at specified intervals until either it succeeds or reaches a maximum number of attempts. If the maximum is reached, the operation may be aborted and data may be lost.

Since its inception Ethernet has enjoyed regular, albeit less rapid, advances in speed parallel to those in microprocessing. The latest generation of Ethernet standards, finalized in late 1998, is Gigabit Ethernet. This Ethernet standard supports transmission of up to I billion bits of data per second, representing a hundredfold improvement over the original Ethernet, which carried data at 10 million bits per second (Mbps). Gigabit Ethernet followed an enhanced 100 Mbps standard from the early 1990s known as Fast Ethernet.