Local Area Networks (LAN) by Ansi-Christ A local area network connects devices that are within a short range. Most LANs are unable to connect to public communications services. LANs were developed because most communications within a business either occur in the very same building or within a short distance of each other. This allows for the sharing of information and equipment and thus a large savings for the company. This files is the first in a series of files. It is not meant to be a complete systems manual for all LANs, but a basic reference manual on the terminology and general set up. In later files there will be more specific information. This file can be distributed freely as long as none of the information is changed. If I have made a mistake, please remember that we are all human and this does happen. Contact me on one of the two boards listed at the end to point out an error. Topologies: Ring - A ring type set up goes in one direction around a group of devices until it reaches the device that is meant as the destination. The ring will be connected by a cable to form the ring. The ring topology uses a method known as token passing. A token is like a bucket in which information can be dumped. If a certain device does not contain this "bucket" then it can not send out the data until it gets it. This will prevent data collision from occurring. A LAN will usually transmit data in packets (buckets = packets) and add the necessary routing information to the packet. (this tells the packet where it's destination is) Because a ring system has a circular shape the information is routed in one direction and it will still reach its destination. One note - I have altered the description of a token slightly to avoid confusion. A token is usually sent before a packet and will tell the next device if the following packet is full or is capable of receiving data. Diagram of a ring topology: O---------O / \ O = nodes | | O O (I apologize for the shitty diagram) \ | \ | O-----O----O Tree - The tree set up links components to the network via connectors. The network can be tapped at any available point. Data can pass from one device to another and does not have to route through a centralized point. Just the way a tree network is set up should make it much faster than a ring type network with the same computers and devices connected to it. This type of network is also very useful because you could add any additional devices as they were need via a connector without having to go through major modifications to the network itself. A tree network does occasionally, just because of the nature of it's set up, have certain problems with controlling the passing of data. There is a safety factor involved with a tree network though. If one of the systems along the network go down it will not adversely affect the network as a whole, just that certain part. Diagram of a tree network: again I apologize for the shitty diagram. O / \ O \ | O | O | | | O O | / | / O Star - The star network is connected through a central processing device. This device usually takes the form of a PBX or a host computer system. All lines in this network are connected to this central unit. When information must go to another node on this network it must first pass through the center. The star topology is very common and is probably the most common set up for a network. There is a problem with the star though. If the central unit goes down then the whole network is useless. Diagram of a star topology: O O | O \ | / \ | / \ | / O--------O--------O / |\ / | \ / | \ O | O O Transmission media - Twisted pair wire is commonly used to connect all kinds of equipment. This is used in older buildings where it may be very expensive to convert to newer types of cable. Problems with this include distortion of signals especially at higher transmission speeds. Coaxial cable consists of a single conductor surrounded by a flexible metallic shielding to minimize signal loss and interference. Coaxial permits a high transmission speed. Twin - Axial cable uses two Coaxial cables, one receiving and one transmitting. These cables are better than twisted pair cable because of the higher transmission speed and longer distance. Baseband and Broadband Systems - A baseband LAN assigns its entire capacity to a single user for a brief time. Only one device can use its pathway. These networks usually use 3/8 inch coaxial cable. Advantages are low cost, easy installation, and considerable capacity. Baseband does, though, not lend itself to voice quality communication. A broadband LAN is sophisticated and uses common television cable and a modem. Although it is expensive it will allow the network to handle voice, video, teleconferencing, and graphics transmission. It does this by using different bandwidths so one signal does not interfere with another signal. It is good for high volume office communication. Fiber Optics are the next step although very expensive it will eventually replace the other cables. Fiber Optics use light to transmit large amounts of information in a very short time. A single cable can carry 240 thousand calls at once. Interconnecting LANs - Bridges are one way of connecting similar networks. Gateways are a way of connecting dissimilar networks. It acts as a translator between these networks. It understands different file structures, data types, and access methods. Vendors - Ungermann-Bass is a large independent vendor. Offering both base and broadband services and coaxial cable and fiber optic LANs. Systek is the OEM supplier of IBM's broadband network. 3Com is a major supplier of PC LANs. Corvus sells Omninet. They offer a system based on twisted pair wiring. Interlan Inc. provides Ethernet based products. Wangnet by Wang laboratories. Codex 4000 series by Codex corp. Information systems network by none other than AT&T information systems. (commonly abbreviated ISN) Novell network LAN terms and what they mean - (note - originally printed in LAN magazine) Access method: a way to determine which workstation or PC will be next to use the LAN. A set of rules by network hardware and software that direct the traffic over the network. Examples include Token passing and Carrier Sense Multiple Access for Collision Detection (CSMA/CD) Collision: The result of two workstations trying to use a shared transmission medium (cable) at the same time. The electronic signals collide and ruin both signals. The whole process takes seconds. Collision detection: The process of detecting when a collision has occurred. Workstations know a collision has occurred if the other station sends no reply that the signal was received. Disk Server: a device equipped with disks and a program that allows users to make and store files on the disks. Allows each user to have an increase in storage space normally not accessible at their PC. Drop cable: The cable which allows connection and access to the trunk cables of a network. Locking: prevents people from changing the same data at the same time. Polling: used in star networks. allows the central "hub" system to know when a node wants to transmit. LAN Magazine Call Anarchia at 518-869-6035 12 West 21 Street TSD at 518-377-6487 New York, NY 10010 1-800-LIBRARY or boards for those who take their 212-691-8251 telecom seriously.