Lesson 2

Selecting Data-Link Layer Protocols

1-15

Unshielded Twisted Pair UTP is a type of copper cable that consists of four pairs of

wires, each of which is twisted together and contained inside a protective sheath. The

quality of a particular UTP cable is specified by its category rating. Category 5 (or

CAT5) UTP is the most commonly used today, although there are higher grades avail-

able for special applications (such as 1000Base-T Gigabit Ethernet networks). The con-

nectors on UTP cables are called RJ-45 and are similar in appearance to telephone

cable connectors, except that they have eight pins instead of four.

UTP is one of the cable types supported by all forms of Ethernet and by Token Ring as

well. As a network medium, it is the most cost-efficient selection because it is the same

type of cable used by telephone networks. In new construction, it is common for the

same contractor to install both the telephone and data network cables at the same time.

You install UTP cable using a star topology, in which you connect each workstation on

the network to a central hub (or repeater), as shown in Figure 1-2. You can then con-

nect hubs to create a larger and more complex network. On an Ethernet network, UTP

cable supports distances of up to 100 meters between each workstation and the hub.

For most LAN installations, this is more than enough. If greater distances are required,

you can modify the location of the hub in your network design or consider using fiber-

optic cable, which can span longer distances.

Hub

F01pm02

Figure 1-2

A star topology

Each form of Ethernet that supports UTP cable has its own limitations regarding the

number of hubs you can connect. This is one of the main reasons why you must select

the data-link layer protocol you intend to use before you start designing the layout of

your network infrastructure. For example, if you plan to use standard Ethernet running

at 10 megabits per second (Mbps), you can connect up to four hubs on a single LAN,

as shown in Figure 1-3. If you use Fast Ethernet (running at 100 Mbps), the Ethernet

guidelines dictate that you can use no more than two connected hubs.