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ACCESS METHODS

Each topology requires that an access method be in place tocontrol when devices may transmit and receive signals. A startopology requires a central switching device. An example of acentral switch for a start wired network is a PBX, but this accessmethod and logical topology are not as commonly used in LANs. Thebus and ring are the most common LAN topologies and use twodifferent access methods: contention and token passing.

The ring topology uses the token passing access method in whicha single eight-bit token is passed around the ring from station tostation. An end device may transmit only when it receives an unusedtoken. The transmitting device changes the designation on the tokenfrom “free” to “buy,” usually by changing one bit in the token,reinserts it on the network, and then transmits a frame of dataimmediately after the token. The token delivers the accompanyinginformation frame to the destination station and returns to thesender. The sender then designates the token as “free” and sendsthe token to the next device on the ring. The token passing methodis a fair access method because each end device receives the samenumber of opportunities to transmit. This access mechanism iswell-suited to environments in which multiple users concurrentlytransmit bursty data.

The commonly used access method for bus topologies is carriersense multiple access with collision detection (CSMA/CD). This isknown as a contention based access method because there is nopredesignated time when an end device may transmit. When a devicehas information to transmit, it monitors the transmission mediumfor another device that is already sending information. If nosignal is detected on the line, the device begins transmitting.During transmission, the device continually monitors thetransmission facility for the presence of more than one signal. Ifit detects another signal on the line, the device terminatestransmission and waits a short amount of time before initiatingretransmission. CSMA/CD is an efficient access method in LANenvironments where multiple end devices are not all concurrently transmitting.

LAN TYPES

The wiring components described above are the building blocksfor the different types of LANs, which include:

  Ethernet.
  Token Ring.
  FDDI.

Each LAN requires a physical medium, a topology, and an accessmethod.

Ethernet

Ethernet’s logical topology is a bus and its access method isCSMA/CD. Ethernet can operate over unshielded twisted pair, coaxialcable, and fiber-optic cable. Various Ethernet standards exist. TheInstitute of Electrical and Electronic Engineers (IEEE) 802.3committee has developed several specifications, including 10Base-T, the specification defining 10M-bps Ethernet over UTP wiring (thisis the most popular type of network implementation, at present, inpart because it can take advantage of the large installed base ofUTP wiring). Other related standards include 10Base-5 (Ethernet onthick coaxial cable), 10Base-2 (Ethernet on thin coax), and10Base-F (Ethernet over fiber-optic cable).

Token Ring

Token Ring LANs use the token passing access method and areconfigured in a logical ring. Both unshielded and shielded twistedpair cable can be used, at 4M bps and 16M bps. The 16M-bps versionoffers higher bandwidth than Ethernet and tends to function betterthan Ethernet as bandwidth utilization increases. Because of itslogical ring structure, Token Ring wiring involves extra ring inand ring out connections (shown in Exhibit 1-3-2) that have to bemanaged and maintained whenever network devices (e.g., hubs andMAUs) are connected. The more complicated connections also makeToken Ring hardware more expensive than Ethernet equivalents.


Exhibit 1-3-2.  The Physical Ring In and Ring OutStructure of a Token Ring LAN and Its Logical Structure

FDDI

FDDI is an American National Standards Institute (ANSI) networkspecification for a fiber-optic LAN configured in a logical ringand using the token passing access method. The topology is actuallya dual ring, the second ring serving as a backup if a problemoccurs on the primary ring. FDDI transmits at 100M bps and takesadvantage of the high bandwidth and high reliability of fiber-opticcable. These qualities make FDDI well suited as a backbone LAN forconnecting smaller subnetworks.

WIRING STANDARDS

Several standards bodies have developed specifications for LANwiring systems and LAN design. The EIA/TIA, National Electric Code,IEEE, and Underwriters Laboratories are the major organizationsthat define how a wiring system should meet building, facilities,electrical, and physical LAN interface requirements. The focus isUTP, currently the dominant cabling for LAN applications.

The EIA/TIA-568 standard specifies 24 AWG 100 ohm UTP wire forLANs. IBM’s cable system specifies both 22 AWG UTP and STP,depending on the application. Also spelled out in the EIA standardare the allowable attenuation levels, cross talk levels, andcapacitance per foot of the cable. (Most of this information is ofconcern to the cable manufacturers. From the cable plant designer’sperspective, what is important is that the physical properties ofthe cable meet the EIA/TIA specifications for cable categories 3,4, or 5.) The cable plant designer must understand the maximum datarates and distances associated with each category cable.

In addition to the specifications for the wire’s physicalproperties, the form factor or polarization of wiring connectorsand the scheme or sequence of the wiring are also specified. Thetwo major form factors used in LAN wiring systems are the RJ-11(for two-pair cables) and RJ-45 (for four-pair cables). The RJ-45standard is used for Ethernet, Token Ring, and FDDI over copper.The RJ-11 form factor looks identical to a regular phone cord connector and is used primarily for Apple Local Talk LANs. Exhibit1-3-3 illustrates the RJ-11 and RJ-45 form factors and the AUIconnector.


Exhibit 1-3-3.  RJ-11 and RJ-45 Form Factors and the AUIConnector

At the wall plate, there are two major wiring schemes orsequences to determine on which wires which signals should beplaced, i.e., which pin numbers and what color wires should carrythe transmit signals and which carry the receive signals. Twopopular schemes are the Universal Standard Ordering Code (USOC) andthe AT&T 258A wiring specifications. The USOC standard has beenused by the phone industry and therefore has a very large installedbase. It specifies wiring configurations for 2, 3, or 4 pairs.


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