Local Area Network Technology: Part 1 - OSI and Cable

by Jeffrey L. Carrell
Issue v1n1 - May 1997

I am embarking on a series of articles that will cover LAN technologies over the course of the next year or so. My goal is to make this series a form of tutorial that can be compiled into an introduction guide to LANs.

The series will follow the OSI Model, as defined by the ISO and including IEEE specifications. Although the chart depicts a top down view of operation, the technologies are discussed from the bottom up, as each subsequent layer builds on the previous layer.

The first area of discussion is the physical medium in which all the LAN data is transported - the cable. Cabling for LANs has evolved over the course of 24 years, from coax to fiber optics, with every imaginable option and type available.

Today the predominate cable type used is ANSI/TIA/EIA 568A - UTP Category (Cat) 5. This cable has four twisted pair conductors, and can support frequencies up to 350Mhz (at least so far) and transmission data rates of up to 1Gb (in trial at 1Gb). Depending on the topology type either two pairs or all four pairs may be used, and also pin-outs vary depending on topology. The predominant connector used is an RJ-45, 8 pin/8 conductor jack/outlet.

For most LAN topologies, the ANSI/TIA/EIA 568A specifications for Cat 5 allow for a total end-to-end distance of 100M. This is broken down into 90M for the horizontal (or main) run (MC/TC to WAO), 7M for the interconnection to the active electronics (in the MC/TC) and 3M for the connection of the device (usually the workstation) to the WAO.

Under the ANSI/TIA/EIA 586A specs, there are two supported pin-out configurations: T568A and T568B

Although the differences between T568A and B are subtle, if they are mixed within a cable plant most topologies will not operate when the pairs are "crossed". When installing a new cable plant, if the ANSI/TIA/EIA 586A spec is not included detailing T568A or B, T568A is the default. However I have seen many more new and upgraded cable plants installations using T568B.

The pinouts used for Ethernet and Token Ring are as follows:

Ethernet: pins 1,2 and 3,6

Token Ring : pins 4,5 and 3,6

I have also seen many cable plant installations where only the specific pins required for the topology were connected/terminated, thus allowing for either a duplex RJ-45 WAO to use a single cable run, or the extra conductors at the end of the cable were simply cut off. This does not follow the ANSI/TIA/EIA 586A specs, nor does it allow for some of the newer topologies to be used on the existing Cat 5 cable plant. Therefore, in order to be fully compliant with the ANSI/TIA/EIA 586A specs and allow for future topologies to be implemented, all 8 conductors must be terminated per the ANSI/TIA/EIA 586A specs, following either the T568A or B guidelines.


568A - ANSI/TIA/EIA Commercial Telecommunications Standard

ANSI - American National Standards Institute

EIA - Electronics Industries Association

ISO - International Standards Organization

MC - main cross connect room

OSI - Open System Interconnect

TC - terminal cross connect room

TIA - Telecommunications Industry Association

UTP - unshielded twisted pair

WAO - work station area outlet

In the next article I'll discuss the active electronics that the cabling connects to….hubs.

Copyright © 1997 Jeffrey L. Carrell. All Rights Reserved

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