Networking means joining or linking devices such as computers. A local area network (LAN) is a collection of computers and other devices connected through some medium that transmits mostly data. The following characteristics are closely associated with LANs; however, one must be aware that LANs are continually changing.

LAN CHARACTERISTICS

Private ownership: LANs consist of devices such as computers, scanners, printers, and cables that are privately owned. Networks also consist of privately owned lines, wires, hubs, and servers. Lines can be made of twisted-pair or coaxial-cable wires, or they can be wireless (such as radio or infrared waves). Hubs connect LAN segments, which are lengths of wire that have one or several attached devices. Servers are computers that are dedicated to one function, such as managing printers or storing applications or data. In short, businesses create and manage their own networks.

LAN size: The size of a LAN is determined by the type of LAN configuration and specifications that are used. Generally, LANs range from several hundred yards up to several miles, but mostly they reside in an office or classroom, building, or several buildings. On one hand, an Ethernet network using 10Base2 ("Thin Ethernet") can have one segment 200 meters in length or five linked segments up to 1,000 meters in length. The 10 in 10Base2 stands for 10 megabits per second, Base means baseband, and 2 means 200 meters. On the other hand, FDDI (Fiber Distributed Data Interface) networks can be up to 200 kilometers (124 miles) in length; however, these are mostly used as backbones that link several LANs.

LAN speed: Another characteristic of a LAN is speed. LAN speed is most often measured in bits per second. For example, a byte (or one character or space) consists of 8 bits. If an average word length is 5 characters and an average double-spaced page is about 200 words, then a page (counting words and spaces) would consist of about 9600 bits [(200 words × 5 characters) 199 spaces] × 8 bits. If a network speed is 9600 bits per second (bps), then a normal double-spaced page is transmitted every second. Another LAN could operate at 56 kilobits (kbps)—or 56,000 bits—per second, about 6 pages of information per second. However, LANs are typically faster than 9600 and 56,000 bps. Many LANs are 10, 16, or 100 megabits per second (Mbps). A 100-Mbps LAN can send 100,000,000 bits in one second—or 10,416 pages per second. Some LANs can transmit 2.8 billion bits per second—gigabits (Gbps). At 2 Gbps (2,000,000,000/9600), 208,000 pages flash by every second—more than most people read in a lifetime! Engineers and LAN administrators are finding even faster ways to communicate data. LAN speeds are reaching terabits per second (Tbps). One Tbps equals 1000 gigabits—an almost unthinkable speed.

The medium used to carry signals in a LAN can be conducted or radiated. Electric signals over wire are conducted. Fiber-optic lines, microwaves, infrared waves, and radio waves are examples of radiated media.

Wire can be coaxial cable or shielded (STP) or unshielded (UTP) twisted pair. UTP is cheaper to install than STP or coaxial cable; therefore it is a popular network choice. However, STP or coaxial cable should be used if there is risk that a network might be harmed by electromagnetic interference. Other networks that overcome electromagnetic interference are fiber-optic lines and wireless media. They, too, are more expensive than UTP wiring.

Twisted-pair wires are rated by the American Wire Gauge (AWG) standard. The smaller the number, the thicker the wire. Regular telephone wire is rated a 28, which is too thin for most LANs. LANs use AWG that is between 22 and 26. Another characteristic of twisted-pair wires is the number of twists per foot. More twists may reduce crosstalk, but they also increase wire costs. Crosstalk occurs when one line picks up noise or voices from another line during a conversation or data transmission. Usually 2 twists per foot are minimum, while 4 are preferred.

The EIA(Electronic Industries Association) is another standard for rating wires. The EIA classifies LAN wires for different uses. For example, Category 3, or "Cat 3," must contain 3 twists per foot; it is commonly used in creating 10-Mbps LANs. "Cat 5" is good for 100 Mbps and up to 2 Gbps.

Low error rates: Older LANs had a difficult time eliminating electromagnetic interference, which created errors during data transmission. Therefore, error checking of data frames was necessary and performed by a protocol. On LANs, data are sent in separate frames controlled by protocols. Protocols are rules between sending and receiving LAN devices. Protocols used parity checks or algorithms to determine whether a data frame had transmission errors. As LAN technology has improved, less error checking is necessary. Less error checking converts to higher bit rate speeds. An example of new LAN technology, with less error checking, is the ATM (asynchronous transfer mode), or cell-relay, LAN.

Currently, most LANs use baseband transmission. Baseband means that there is one signal transmission per line. What this means is that only one device can use the line at a time. The channel is full when one device is sending data. Because baseband LANs are easily monitored, so errors can be detected and reduced. Broadband means that the line can handle several transmissions at one time. This is accomplished using different frequencies that act as separate channels; this is called frequency division multiplexing (FDM). Broadband LANs are more complex than baseband LANs, and they require expensive technology. Additionally, broadband is more susceptible to errors than baseband.

LAN USES

LANs have become an operational necessity for just about every business. Burgeoning information demands make it necessary to link all computers for efficient data sharing and storage. Furthermore, e-mail, electronic commerce, and video conferencing are enhanced services for today's LAN users.

In addition to sharing data and hardware, properly managed networks increase productivity. Things like standardized applications, controlled access, users' rights, companywide backups, and recovery strategies help businesses manage their information more efficiently than at any time in the past.

Standards also help LAN development and allow different devices from many manufacturers to be connected. Standards originate from businesses, organizations, and networking practices that are associated with electronics, communications, and computers.

When LANs are linked to other LANs, a network structure called a wide area network (WAN) is formed. WANs have many configurations and can extend globally. One aspect of a WAN is a gateway to the Internet. Gateways are special switches that allow LANs with different topologies, controls, and protocols to communicate.

LAN CONFIGURATIONS

Three common LAN configurations are bus, ring, and star topologies. All topologies have advantages and limitations.

A bus topology is a physical layout in which microcomputer workstations and other devices are connected to a UTP/STP or cable segment. Data travel in frames through a wire, called a bus, from the sending station to the receiving station. Bus wiring is looped together with terminators at each end. Terminators are used so that signals can recognize the bus end. When this happens, the signal is reflected back to the other end of the bus. This LAN topology is the most widely used configuration due to the popular Ethernet bus protocol.

Ring configurations are used, too. IBM is responsible for developing and promoting ring configurations. On a ring, all computers are connected to a continuous loop wire or cable. Data flow in one direction. Each workstation is a repeater. Repeaters charge up a signal and send it over the next segment to the next workstation; this process is repeated around the ring.

A star topology is a group of workstations connected to a controlling switch. Data packets always flow through the switch to get from one workstation to another. Switches control and manage all data flow. Switches can read the destination address on a data frame and route the frame to the line segment that contains the destination workstation.

LAN DATA FLOW

LANs need flow control to operate at high speeds. Flow control is another protocol function. Two common LAN protocols are CSMA/CD (carrier sense multiple access/collision detection) and token passing.

CSMA/CD is also called contention. CSMA/CD is used by one of the oldest and most used standards—Ethernet. Contention is a LAN term that means that any station can broadcast a data frame on the bus line at any time. A sending station creates a frame by putting a destination address along with its own address around a unit of information called a data unit. This frame is broadcast to every workstation on the bus. All stations check the destination address to see if the frame is for them.

Token-passing protocols can work on a ring or bus, but they are mainly used on a ring. A 3-byte "free" token is always circulating around a ring and stops at every station. When Station A wants to send data to Station B, it waits for the token. When the token stops at Station A, A changes the "free" token to "busy" and attaches a data frame. The data frame contains a header with the destination and source address around a data unit. The combined token and data frame go from one station to another until they get to Station B. Station B recognizes the busy token and checks the destination address. Since the frame has "its" destination address, it copies the data frame and changes a part of the header, acknowledging that it received the data. The to ken and data frame return to Station A and A deletes the data frame and changes the token back to "free." This process continues when any station wants to send a frame.

CONCLUSION

LANs are efficient and necessary business tools. They began as individual, stand-alone micro-computers that evolved not in any pattern but based on unique business needs. Technology and standards help networking grow. Perhaps some day a worldwide network of linked computers will exist to communicate data, voice, and video to everyone in the world.