Networking: A Beginner’s Guide Fifth Edition- P12 pdf

Understanding Cable Topologies The word topology basically means shape, and the term network topology refers to the shape of a network—how all of the nodes points of a network are wired

Chapter 4

Understanding

Network Cabling

If you were to compare a computer network to the human body, the network cabling

system would be the nerves that make up the physical manifestation of the nervous system The network cabling system is what actually carries all the data from one point to another and determines how the network works How a network is cabled is of supreme importance to how the network functions, how fast it functions, how reliable the network will be as a whole, and how easy it will be to expand and change the network With any new network, your first task after assessing the needs for the network is to determine how the network should be wired; all the other components of the network are then built on that foundation This is much like the OSI seven-layer model you learned about in Chapter 3, in that the network cabling makes up layer 1 (the physical layer), and all the upper networking layers rely on it

Many people think that network cabling is relatively simple After all, what could

be simpler than running a wire between two points? However, as you will see, the topic of network cabling encompasses more than meets the eye, and it’s an extremely important area to get right If you make mistakes selecting or installing network cable, your network will likely be unreliable and may perform poorly Because of the labor costs involved in wiring a network, the best time to address any potential problems in

this area is well before they occur.

Understanding Cable Topologies

The word topology basically means shape, and the term network topology refers to the shape

of a network—how all of the nodes (points) of a network are wired together Networks may be wired in several different topologies, and the choice of a topology is often your most important decision when you plan a network The topologies have different costs (both to install and maintain), levels of performance, and levels of reliability

DEFINE-IT! Network Segment

The term network segment can mean somewhat different things depending on the

topology of the network, but the concept is simplest to understand when thinking

about a bus network, and is essentially the same for any topology A segment is

a single length of cable to which all the nodes in that segment are connected In truth, a segment is not a single continuous length of cable, because it is broken at each computer connection point with a connector that lets the node connect to the

network cable, but the cable is electrically one single cable

In any given segment, all the network traffic is “seen” by all the nodes on that segment You need to take this into account when planning how many nodes you will connect to any given segment If you have 20 computers, all fully using that segment at the same time, each computer will achieve only approximately one-twentieth of the available maximum bandwidth This is simplified; you will learn more about how this works later in this chapter and in following chapters

The main topologies in use today are bus, star, and ring These topologies are

described in this section

Bus Topology

A bus topology, more completely called a common bus multipoint topology, is a network

where, basically, a single network cable is used from one end of the network to the

other, with different network devices (called nodes) connected to the cable at different

locations Figure 4-1 illustrates a simple bus topology network

Different types of bus networks have different specifications, which include the

following factors:

N How many nodes can be in a single segment

N How many segments can be used through the use of repeaters

N How close nodes can be to each other

N The total length of a segment

N Which coaxial cable type is required

N How the ends of the bus must be terminated

Bus topology networks use coaxial cable, described later in this chapter Each end

of each segment of the network has a special cable terminator on it, without which

the network will not function Some bus topology networks, such as Thin Ethernet

(10Base-2) use BNC connectors to tie all the individual pieces of cable together Each

computer is connected to the network through the use of a BNC T-connector (called that

because it’s shaped like the letter T), which allows the network to continue its bus and

lets the computer connect to it Figure 4-2 shows several different BNC connectors

Figure 4-1 A simple bus topology network

Bus network topologies are by far the least expensive to install because they use much less cable than the other two topologies and, accordingly, use less material and need less installation labor

But bus networks have some big drawbacks Because all the subcables that make

up the segment and run from node to node must be connected at all times, and because

a failure in any part of the segment will cause the entire segment to fail, bus networks are prone to trouble And even more important, that trouble can take a long time to track down, because you must work your way through all the cable connections until you find the one causing the problem Often, the source of the problem isn’t visually apparent, so you need to use various techniques and equipment to find it (as discussed

in the “Troubleshooting Coaxial Networks” section later in this chapter)

Because of the tendency of bus networks to be unreliable, new network wiring installations do not use bus topologies, although many older networks still do

Figure 4-2 BNC connectors used in a coax-based bus topology network

DEFINE-IT! BNC Connectors

Depending on whom you ask, BNC stands for Bayonet Nut Connector, British Naval Connector, or Bayonet Neill-Concelman (with the latter two words standing for its inventors, Mr Paul Neill of Bell Labs and Carl Concelman of Amphenol Corporation) BNC is a bayonet-style connector that quickly attaches and detaches with a quarter turn A variety of different parts—T-connectors, barrel connectors, elbow connectors, cable ends that splice onto appropriate cable, and so forth—use BNC connectors, so you can achieve nearly any type of connection needed The BNC connector is extremely easy to use and makes a secure connection

By far, the most prevalent bus network used in the past (and in limited existence

today) is one called 10Base-2 Ethernet, or more commonly, Thin Ethernet This network

type has the following characteristics:

N Has a rated maximum speed of 10 Mbps

N Uses RG-58/AU or RG-58/CU coaxial cable and BNC connectors

N Requires a 50-ohm terminating connector at each end of each segment to function

N Can handle a maximum of 30 nodes per segment

N Can be run up to a maximum segment length of 185 meters (607 feet)

N Can use extended segments through the use of repeaters

N Requires each node to be at least 0.5 meter (1.5 feet) of cable distance from any

other node

If repeaters are used, you can connect a maximum of three segments together, and

each segment may have up to 30 nodes (with the repeater counting as a node) You can

also have two additional segments (a total of five) if those extra two segments are used

for distance only and do not have any nodes on them An entire repeated segment must

never exceed a total of 925 meters (3,035 feet) Remember the 5-4-3 rule: five segments,

four repeaters, three populated segments

NOTE Repeaters are hardware devices that electrically boost the signal on a cable so it can be

extended further; they do not route any of the data In fact, a repeater is “ignorant” of any of the

data it carries Repeaters are inexpensive and reliable However, remember that extending a cable

with a repeater means that all the network traffic on one side of the repeater is echoed to the cable

on the other side of the repeater, regardless of whether that traffic needs to go on that other cable

Repeaters are discussed in more detail in Chapter 6

Star Topology

A star topology is one in which a central unit, called a hub or concentrator, hosts a set of

network cables that radiate out to each node on the network Technically, the hub is

referred to as a multistation access unit (MAU), but that particular terminology tends

to be used with only Token Ring networks, which use a logical ring topology (see the

following section) Each hub usually hosts about 24 nodes, although hubs range in size

from 2 nodes up to 96 nodes Regardless of the hub size, you can connect multiple hubs

together to grow the network in any way that makes sense See Chapter 6 for more on

connecting hubs together in different configurations Figure 4-3 shows a simple star

topology network

All the network traffic used on any of the network connections to the hub is echoed

to all the other connected nodes on that particular hub Because of this, all the bandwidth

of any single node’s connection is shared with all other node’s connections For example,

if one of the nodes connected to the hub is using half the available bandwidth, all the