Jun 00 Getting Started
Volume Number: 16
Issue Number: 6
Column Tag: Getting Started
Networks 201 - Part 2
by John C. Welch, Edited by Ilene Hoffman
Topologies, Signals, and Wires, Oh My!
In the first article of the series, we went through the OSI seven-layer network model,
and discussed the purpose of each layer. We also covered the philosophy and reasons
for the OSI model. Finally, we pointed out that there is a pseudo-layer 0 in the model,
that deals with the areas between stations or computers. This layer 0 is the actual
wiring or RF that connects the stations together, and this is what we will delve into
this month.
Scope
As we talked about in the OSI article, Layer 0 is the media between stations in a
network. Although Layer 1 is the Physical Layer, it actually only deals with the
signals on the network, and ceases to function once you leave the transmitter, only
picking up its function at the receiver. Layer 1 is blind to everything in between nodes
on a network. This is actually good, because it gives us a lot of flexibility we might not
otherwise have in media types. This is not to say that Layer 1 is completely
independent of the media. If this were the case, then we could use Ethernet wiring and
Token Ring network cards. Rather, as long as the connection interface between the
media and the end node is correct, Layer 1 assumes that everything else is correct. So
Layer 0's scope is all the wired and wireless media between node A and node B. In this
article, we will stay on wired media. I covered 802.11 wireless networking in detail
in the December 1999 MacTech, and recommend you read that issue for information on
wireless media and topologies.
Wire Types
Logically, the first thing to discuss in a wired network is the wire. Wire is a bit of a
misnomer, as when talking about copper network cabling, we are actually speaking of
cable bundles. If we talk about fiber optic cable, then wire doesn't apply at all, beacuse
in a fiber network, there is no wire. For ease of use, we'll use wire to mean any
physical path for a network transmission to travel on.
Copper network cables are the most common, even though fiber optic cable has
superior characteristics. Primarily, copper is cheaper than fiber on a per-foot basis.
Considering the total length of even a small network can easily reach well over a mile
when all is said and done, this is an important issue. Also, even with Gigabit networks
being implemented, copper still works quite well. Copper is more simple to work with
than fiber, and it's much easier for a network administrator to quickly build copper
cable when needed, than it would be for fiber.
In spite of being somewhat harder to work with, and more expensive to buy, fiber has
some clear advantages. It is capable of much higher speeds than copper, it is physically
smaller, and it is immune to most electromagnetic situations that will cause great
harm to copper networks. As an example, when I was an Administrator for a city in
Florida, we had a building that was poorly grounded. Every time there was a lightning
storm, (in Florida that means daily), we had to replace the 56K WAN modems. After a
year of this, we talked the city council into budgeting for a fiber optic connection to the
building. Once that was done, it was the most stable building in the city.
Coaxial Cable
Since copper is the most prevalent wiring type, let's look at that first. There are three
basic types of copper wiring for networks: Coaxial cable, shielded twisted pair or STP,
and unshielded twisted pair or UTP. Coaxial cable, referred to simply as coax, is a
round, fairly thick cable with two conductors laid along the same axis, (hence the
name Co-Ax). The inner conductor is a single copper strand, with a layer of insulating
material around it. This is covered by a third layer of braided conducting material,
such as aluminum, and this layer is covered by a coating of PolyVinylChloride (PVC),
or Teflon. A cross-section is shown in Diagram 1 below
Diagram 1. Coaxial Cable Cross-Section.
Coaxial cable has several advantages because of its construction. It is virtually
immune to noise from outside sources such as fluorescent light fixtures and power
lines. Coax also has excellent bandwidth capabilities. It has a ceiling of 2Gbps as long
as the total cable length is less than 1km. These characteristics made coax a popular
choice for network cable for many years. In fact, the original media specifications for
Ethernet specifically required coax cable. Unfortunately, coax has a number of
disadvantages which have led to its removal as a common network media. Coax is
relatively fragile because of all its layers. It can be damaged or broken if it is bent on
too small of a radius, or kinked. If it is subjected to even moderate crushing force,
from say, a file cabinet, it easily breaks. Coax is also heavy, which made it difficult to
use in a large network, due to the cost of supporting the wire bundles, whereas twisted
pair wire can normally be laid on top of a suspended ceiling. Also, coax tends to be
fairly thick, with diameters from 3/8 of an inch or more being common. This causes
problems in dense installations, as available space for wiring runs and closets is
quickly consumed. In addition, the complexities of manufacturing coax made its overall
cost much higher per foot than twisted pair wiring. Finally, coax cable is not flexible
in its implementation. Due to the need for impedance matching and proper termination
of the lines, coax networks are very sensitive to breaks in the line. Since most coax
networks use a daisy chain style implementation, any break in the line can easily
bring down an entire physical segment of the network. This sensitivity also means that
adding stations to an existing segment either requires bringing down part, or all of