Sep 1, 2000 12:00 PM, Kevin McNamara
It seems like only yesterday that I started writing articles on PC networking for BE Radio. But it was actually a few years ago. At that time, PC-based networks were not only being used for the storage of digital audio, but also to replace the clunky mainframe systems used for business and traffic. Today, PC networks are everywhere in the station. Installing Ethernet or fiber cables around the station is much easier than working with the bundles required for the previous systems. Although most of you have become experienced and, in general, quite proficient at installing and troubleshooting networks, it is always beneficial to step back and review the basic theory that makes your network run.
LAN basics The name Ethernet defines an access method, or a set of rules, that allows two or more computers to communicate over a common medium. Other types of access methods include Token Ring and the virtually obsolete Arcnet.
The specific access method used by Ethernet is called carrier sense multiple access/collision detection (CSMA/CD). In simple terms, this means that, when one PC attached to a network is talking, the others must be listening. If, however, two or more computers send data at the same time, the signals will interfere with each other and no connection will be achieved. Each PC will detect the collision and wait for a random period of time before sending again. The process will repeat until the destination PC successfully receives the data.
Protocols Protocols are a set of rules defining how the data will flow between two or more computers. Depending on which network operating system you choose, there are several possible protocols. IPX/SPX (Novell), NetBEUI (Microsoft), NetBIOS (IBM) and DECnet (Digital Equipment) are examples of LAN protocols. The most popular protocol currently in use is TCP/IP, which is used exclusively to communicate over the Internet. Protocols permit computers with different platforms to share files.
Protocols can be classified further into one of two groups: connection-oriented and connectionless. When two computers establish a successful contact, the protocol takes over. A connection-oriented, or reliable, protocol uses a three-step process: connection establishment, data-transfer and connection release. In unreliable, also called connectionless, protocols only the data transfer takes place without the connect and disconnect function. Reliable protocols ensure that the data reached the proper destination error-free. Unreliable protocols simply send data without any checks.
OSI The Open Standards Interconnect (OSI) model comprises a set of protocols that attempts to define and standardize the communications process. OSI breaks down the communications process into seven specific layers. During the communications process, data flows from one to the next successive layer, either from the bottom up (when sending) or from the top down (when receiving). Figure 1 shows how each layer depends on its surrounding layer. Each layer also has a specific purpose. The physical layer provides the electrical and mechanical interface to the network cabling. The data-link layer packages the data to send over the network and disassembles data sent to it. The network layer maintains the connection by translating logical address information into physical addresses. It also provides network routing and flow control across the network. The transport layer ensures data is successfully sent and received, asking for retransmission until it is successful. The session layer turns the communications process on or off as needed in order to maintain a clear communications path. The presentation layer translates data between different computer platforms. The application layer interfaces applications run on the computer and the network.
Other than the physical layer, the OSI is implemented through software drivers loaded at the start-up. You should note that certain protocols might not use all of the layers.
Topologies The topology of a network refers to the method in which the various PCs are connected. There are three basic types of topologies: star, bus and ring. These names describe the physical layout of the network cabling.
Most Ethernet networks use a star topology with a network hub at the center of the star with cables run to each PC. Hubs can be cascaded together to form other stars. Star topology allows you to centralize the placement of network cabling for ease of troubleshooting and reconfiguration.
The ring topology is not used with Ethernet but is used with Token Ring-type networks. Token Ring networks are characterized by the need to form two discreet signal paths for data flow, thus the cabling used to wire the network will contain two discreet cables. Fiber optic-based networks, even those used to carry Ethernet, will use a ring topology. Fiber network cabling is sold as two-pair zip cables, although you can purchase jacketed multipair cables.
Ring topologies are similar to star topologies in the sense that both physically connect to a central hub-type device. Logically, however, the ring network forms two separate data paths or rings.
TCP/IP Two protocols, Transport Control Protocol (TCP) and the Internet Protocol (IP) make up TCP/IP which encompasses other protocols as well.
The IP delivery process provides an unreliable connection. However, the basic function of TCP is to maintain reliable data transfer, therefore TCP/IP is considered a reliable protocol.
Basically, data sent over a TCP/IP protocol is broken down, sequentially numbered and encapsulated into packets that contain specific destination address information. If all of the data packets arrive at the specified destination, the TCP module will acknowledge their receipt. If the packets arrive out of sequence, TCP will attempt to put them in order. If a packet does not arrive, the destination computer will not acknowledge receipt and the source computer will attempt to resend.
Whether you are a certified network professional or someone who learned it the hard way, getting back to basic networking fundamentals always seems to help. It's time to disconnect my reliable connection.