This chapter discusses cables used on various kinds of networks. The topics of this chapter are:
Concepts:This chapter begins with a concept that belongs to the Physical layer of the OSI model: bandwidth use. The bandwidth topic is split into two methods for using the bandwidth of a medium.
The next topic, Dialog Control, belongs on the Session layer of
the OSI model, but it relates to how cables are used on a network. It
concerns three ways a dialog can be conducted:
Computers use electric currents and various forms of electromagnetic
waves to communicate. We can class networks as being cable or wireless,
for obvious reasons. We will discuss five attributes for each type of
medium in this chapter:
The text describes several types of cable media (follow the link to a Microsoft TechNet article about media. It is not perfect, but it is pretty good.):
(For the purists among you, I will note that the speed of light through these media is about two thirds the speed of light in a vacuum.) The graphic on page 5-5 shows several twisted pairs of wires. Each wire
is covered with an insulator, and the two wires in the pair complete a
circuit. These wires suffer from crosstalk, leakage of signal.
The twists help cancel out such leaks. The graphic shows a UTP cable with
eight wires in it, making four pairs. The wires in each pair are twisted around each other. This type of cable comes in several varieties: two pair, three pair and four pair are common. Also, each variety may be available in grades, such as CAT 1 (Category 1) and CAT 5 (Category 5). There are several such categories, and a major difference between them is the number of twists per foot in each pair. CAT 1 will have less than 5 twists per foot, CAT 5 will have 25 or more twists per foot (so it is better, and costs more). Note that the better the class of cable, the more bits per second can be passed across it. Connecting a system with twisted pair wiring is easy. It is illustrated on page 5-6. A possible problem is that the wiring closets in any building are often in need of being "cleaned up". The "closet" on each floor of a building contains punch-down blocks, patch panels, and hubs (or switches). Many are disorganized and messy. People who try to clean them up, however, must be careful not to disconnect circuits that are needed. The factors for UTP:
An STP (Shielded Twisted Pair) cable is illustrated on page 5-7. This cable is more expensive than unshielded cable, and is less flexible due to the stiff shielding. The shield, however, makes it more EMI resistant than UTP. The factors for STP:
The wiring standards used for network coax are different from those
used for cable TV. You may want to know this list:
The number associated with each RG specification tells you the relative size of the center conductor. Smaller numbers mean thicker wires. The coaxial line is essentially a single bus, going from one station to the next. At each end of the line, the cable has to have a terminator on it. At one end, it also has to be grounded. If using thin Ethernet, T-connectors are used. If using thick Ethernet, vampire taps are used. They are called vampire taps because little teeth bite into the cable (to contact the shield), and a big tooth bites deeper to contact the central conductor when you screw the clamp down. Note that the vampire tap provides a place to tap into the cable, and a transceiver to translate between the PC and the network. The workstation also needs a patch cable to connect to the tap. The factors for Coax:
Fiber optic can be glass or plastic, and is meant to conduct light instead of electricity. The conductor is called a waveguide, and is covered with cladding, a material to reflect the signal back into the center of the conductor. Two configurations exist. Loose configuration has a liquid filler between the outer sheath and the conductor. Tight configuration has wire or stiff fibers around the conductor to add strength to the cable. Fiber optic comes in two modes: single mode conducts a single signal, while multi-mode conducts many signals simultaneously. You may want to know that the most common type used is 62.5 micron core with 125 micron cladding, multimode. Fiber optic is much harder to install and splice than electrical conductors. As illustrated on page 5-19, this type of connection requires two connectors for each station, a line in and a line out. The factors for fiber optic:
Ethernet hubs are discussed on page 5-22. The term concentrator
is often used for a hub, since the hub is used to collect connections
at one point. Four types of hubs are listed:
Token ring concentrators, are called Multistation Access Units (MSAUs, or MAUs). The nodes on the ring are actually wired to several MSAUs in a star-wired manner. The MSAUs are wired to each other through their ring in and ring out ports. This brings up a phrase your author has not used yet. This sort of network is wired as a star. That is its physical topology. It works like a ring. That is its logical topology. All networks can be described in terms of each of these two properties. A network's physical topology is how it is shaped or wired. Its logical topology is how it is used: typically like a bus or a ring. This text does not discuss the classic wireless network media types in this chapter. Since they have been mentioned in other chapters already, I will provide you with some notes here. It may be redundant to remind you that wireless media means that there is no cable of any sort between certain parts of the network. (There are still wires inside lots of components). Radio is the label used for frequencies from 10 KHz to 1 GHz. Several bands are used. Frequencies that are used for networks can be divided into regulated and unregulated frequencies. Only a few frequencies are unregulated in the United States. It is not possible to guarantee error free transmission in the unregulated frequencies. This is because anyone else can broadcast in those frequencies, causing errors in your transmissions. For this reason, broadcasts are usually limited to low power in unregulated bands, to minimize interference. Three types of radio usage:
The comparison factors for wireless media are different from those for
wired media. The factors for low power, single frequency:
The factors for high power, single frequency:
Spread spectrum radio usage puts the incoming data stream on
several frequencies at once. This discourages eavesdropping. Using direct
sequence modulation, the signal is put on several frequencies, some
of which may contain false signals. Using frequency hopping, the
frequency being used is changed on a preset pattern, which the sender
and receiver know. The factors for spread spectrum:
Microwave signals are used in two formats: terrestrial
(earth-based) and satellite systems. Terrestrial systems
are used in line of sight connections where it is not possible to put
a wire, such as across several city blocks. The factors for terrestrial
microwave:
Satellite systems are used to connect sites that are widely separated.
Usually, signals are sent to geosynchronous satellites, orbiting
22,300 miles above the earth. This orbit puts the satellite in
the same part of the sky relative to a ground based observer at all times.
The factors for satellite microwave:
Infrared systems come in two types: point-to-point and
broadcast. Point-to-point systems are like the remote controls
we use for televisions. Some systems also use lasers. The factors for
point-to-point infrared:
Broadcast infrared systems are used in single room settings,
as these waves will bounce off walls, but not penetrate them. The advantage
is that you can put a system in each room where required, and the users
may move their machines around as they like. The factors for broadcast
infrared:
|
UTP
cables are usually connected to devices with RJ-45 connectors. Your text
does not show an RJ-45 connector (or any other) very well. In the enlarged
picture on the right, note the eight gold-colored connections for the
eight wires usually found in UTP cables. The wires are used in pairs to
form circuits. See the Networking Technologies notes for more information
on this sort of connector.
IBM
Data connectors are typically used with Shielded Twisted Pair cable on
a Token Ring network.
The example on the right shows a typical T-connector with BNC
fittings. The fitting on the bottom of the image might attach to a port
on a NIC that looks like the barrel on either end of the top of the T.
Attachment is achieved by pushing the connector onto the barrel of the
port, then twisting the collar of the connector to lock onto the pin that
is part of the port. In other words, it mounts like a bayonet. 
The
next (enlarged) picture shows a BNC connector attached to a thin Ethernet
cable. Such a connector would be used to attach to one of the T-connector
barrels in the photo above. The other end of the cable would run to the
next node on the network.
