Originally Published: Monday, 30 October 2000 Author: Chris Campbell
Published to: enhance_articles_sysadmin/Sysadmin Page: 1/1 - [Printable]

Introduction to Networking, Part 1: Physical Media

Chris Campbell, Project Manager for the Sysadmin section of Linux.com, is back with a vengeance! Welcome to the first part of Chris' Networking series, focusing on physical media, the nuts and bolts of your Linux network. Enjoy!

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The Local Area Network must rank as the technological legacy of the 1990's.

The technology existed long before the decade, but much like the Internet, LAN's came into vogue in the 90s, both in the corporate world and in the home user spectrum. Networks can be rather complex, and documentation about networks, even more so. These articles approach the subject in a more simplified manner.

Most networking books refer to a chart called the OSI model, which was developed to aid in the conception of a network. Unfortunately, in its eagerness to simplify, the chart confuses things more. The OSI model is more of an ideal and often doesn't really match practical application. For the intentions of this series, suffice it to say that there are several layers to network connectivity, but the three necessary for simple perception are:

Physical Media

Physical Media refers to the actual wires of the network that are used to carry the signal. This is the primary focus of this article.


Protocol refers to the agreed-upon method of transmission. A good analogy would be to a language. Something may be spoken (data may be transmitted), but if the person speaking is using a language (protocol) not understood by the listener, then the conversation will cease to function. By using the same agreed-upon language (protocol) the word spoken (data) will be understood (transmitted). An example of a protocol is TCP/IP, the common grounds of the Internet.


To continue the language conversation analogy, the application would be the speaker of the words spoken (data). An example of an application would be e-mail messages of Web pages.

Media is the first part necessary for the network. The wires are crucial for communication. There are many different types of wire available today. The oldest is coax, a wire similar to cabling used by cable TV companies. It is an old form no longer used much. This sort of wire would connect machines in series to network them. The disadvantage to this is similar to a series circuit used in things such as Christmas lights. One bulb breaks and there will be no power to the bulbs beyond it. So is the same with coaxial networks. One machine fails and no machine beyond it may be seen. Coaxial also has serious speed limitations, reaching only 3-6 Mbps (megabits per second) depending on local traffic.

One of the newest forms of network is fibre optics. Fibre optics are strands of flexible glass-like material that can route light signals. This has the disadvantage of not being entirely flexible and it's very expensive. The transmission speeds though are fast, and for all practical purposes limited only by the speed of the transmitting and receiving NICs (network interface card) on either end. This implies that as technology improves only the NICs ever need be replaced.

Due to the price and durability, the most common form of network media is twisted pair or unshielded twisted pair. This technology employs using 8 wires twisted in the casing to shield itself. This wire looks similar to phone wire, except that it has twice as many wires.

In addition to the physical media of the wires, there also are other forms of media, such as NICs, hubs, routers and switches. Simply they are:


Network Interface Card. A card placed inside a computer that enables it to communicate with the physical wire that comprises the network. NICs come in different types, depending on the network. 10baseT are NICs that function at 10Mpbs, 100baseT at 100 Mbps. Some cards are capable of doing either. Other architectures exist, such as HP's proprietary network architecture, VG. VG NICs have a low rate of collisions and can function up to 100mbs, but unfortunately for HP, the technology was not embraced by many and is rather rare.


Hub is the common signal interchange with ports that other computers or hubs may plug into. The hub is essentially the heart of the network, where all of the devices meet together. All of the devices and the hub must be of a compatible format, in the form of architectures (10baseT, 100baseT, 100VG, etc.)


A switch is a device that takes different segments of the network and combines them. Modular Switches are an excellent way to conjoin different network architectures (10baseT, 100VG, etc.) on a single network. They also route network transmissions intelligently and are often used for key servers to plug into for high availability. Switches are like a super-fancy HUB. Switches usually can handle traffic in parallel, which HUBs don't do.


Routers are the mainstay of WANs (Wide Area Networks) and the Internet. These network appliances are used to route specific protocols to different network segments, or to join a LAN as a segment to a larger network, such as the Internet. These devices have their own operating system and must be configured as to the routing needed. These devices may also function as Firewalls, filtering incoming traffic for potentially dangerous transmissions.

A basic network:

To create a basic network, first we need to acquire the physical media. NICs can be purchased at local computer stores for varying prices. A Linksys 10/100baseT card sells for about $20 at most places. 3Com 3c5/0xx series 10/100baseT cards retail for around $70. The difference? Preference, typically. Although I can't say that I've had bad luck with Linksys cards, I have extreme faith in 3Com cards. $20 cards are perfectly fine for this example network. Install the cards into the machines, following the directions that come with the devices.

A hub also may be purchased. Hubs retail from $40 on up. A small $40 four port hub will be fine for this demonstration network. If more than 4 machines are to be added to the network, a larger hub should be purchased. If two machines, the bare minimum for a network, are to be used, a hub isn't necessary. It is preferred, however.

Next stop, the wires. All sorts of wires may be used, but typically Category 5 wires. UTP are preferred. Again available at most computer stores, the wires can often be purchased pre-'modded', that is, with the wires already arranged and crimped with modular heads. Prices for these crimped wires vary greatly by length. Ten feet, is about $15 at the local store. If pre-done wires are not available, then uncut wire may be purchased. Typically, raw wire is sold in 1000ft spools for about $100. Crimpers may be acquired usually for about $15 and the modular heads vary in price from $.05 to $.25 each.

If the wires are pre-modded, simply plug the cables in, one end into the NIC in the machine and the other into the hub. If not, mod the heads. This is done with the crimper. First, measure and cut the length between the machine and the hub. Leave a little extra, as it is always better to have too much than too little. Cut the wire so that all of the internal wires are equal in length.

Next, strip away the outer casing of the wire for about an inch. Arrange the wire in the order desired, from the descriptions below. Flatten and straighten the wires so that they will slide easily into the modular head.

Cut the internal wires again, this time just to trim them to all equal length. Leave exposed only slight less wire than the length of the modular head. The casing must go into the modular head so that the head may crimp the casing also for added strength. Failure to do so may result in the wires falling loose of the modular head.

Slide the wires into the mod head. There is little section in the mod head for each wire. Take care to confirm that the wires are in proper order, and match at both ends. (Unless using a diagram that indicates differently). The wires must slide all the way in. With the wires in the correct order and fully in the mod head, place the mod head into the crimper and squeeze it with the might of God until you hear a satisfying click. Here, two things are happening. A wedge is being crimped into the wire casing to secure the mod head and the wires are being pierced by the mod head's copper interface. If either part fails to happen, redo the mod head.

10baseT Connector Signals:

Pin Number Signal
1 TD+
2 TD-
3 RD+
6 RD-

The technology employs only 4 wires, so standard phone wire theoretically could be used, but it is not preferred. As long as both ends are the same, the wire should function going into a hub. If a hub is not to be used, a simple crossover of the TDs (transmit) and RDs (receive) should do.

There are standards in the actual wire placement, but one of the most robust configurations, that works with 10 and 100baseT is as follows: (100baseT needs all 8 wires, btw.)

Straight Through Cable:

Pin Number Wire Color
1 White w/ Orange
2 Orange
3 White w/ Green
4 Blue
5 White w/ Blue
6 Green
7 White w/ Brown
8 Brown

Cross-Over Cable

A crossover cable is for connecting two machines directly (without a hub or switch). On one of the ends you will have to swap wires 1&3, and 2&6 so they become:

Pin Number Wire Color
1 White w/ Green
2 Green
3 White w/ Orange
4 Blue
5 White w/ Blue
6 Orange
7 White w/ brown
8 Brown

After the wires are arranged and crimped, again, all devices should be plugged in.

At this point, the physical foundation of the network should be in place and capable of functioning. We will move onto the protocols and applications in Part 2.

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