Networking: A Beginner’s Guide Fifth Edition- P18 ppsx

You most often see repeaters on Thin Ethernet networks, but they are available for virtually any network connection.. In practice, repeaters are usually used with 10Base-2 networks Thin

Putting together all the necessary pieces in the proper way is the art of network

design Chapter 15 discusses important aspects of assembling these devices so they work

together optimally, but first you need to know what they are and what they can do The

following sections discuss these essential network devices

Repeaters

A repeater is a device that extends the distance of a particular network run It takes a

weak network signal in on one side, boosts the signal, and then sends it out its other

side You most often see repeaters on Thin Ethernet networks, but they are available for

virtually any network connection For instance, if you need to run a 100Base-T Cat-5

cable longer than 100 meters (328 feet), a repeater enables you to double that distance

Repeaters operate at the physical layer of the OSI networking model They do

not have the intelligence to understand the signals they are transmitting Repeaters

merely amplify the signal coming in either side and repeat it through their other side

(Remember that they also amplify any noise on the cable!) Repeaters are used to connect

only the same type of media, such as 10Base-2 Thin Ethernet to 10Base-2 Thin Ethernet,

or Token Ring twisted-pair to Token Ring twisted-pair In practice, repeaters are usually

used with 10Base-2 networks (Thin Ethernet), which are discussed in Chapter 4

Repeaters do have a small amount of intelligence that can be useful They can

separate one of their connections from the other when there is a problem For example,

consider two segments of Thin Ethernet that are connected using a repeater If one of

those segments is broken, the repeater allows the good segment to continue working

within itself Users on the good segment will be unable to connect to resources on

the broken segment, but they can still use the good segment without trouble (But

remember that this capability does you little good if your servers are on the broken

segment and your workstations are on the good segment!) Figure 6–1 shows a network

extension using repeaters

Figure 6-1. Using repeaters to extend network length (10Base-2 Thin Ethernet shown)

Hubs and Concentrators

Intelligent LAN concentrators—usually just called concentrators or, even more simply, hubs—are used to connect network nodes to network backbones Nodes are connected

to hubs in a physical star fashion (cables fan out from the hub to each node), whether they are used for a star topology or a ring topology network (these topologies are discussed in Chapter 4) A simple network might consist of just a hub or two; smaller networks usually don’t require a network backbone

Hubs are available for virtually any network media type, with the higher-end units using replaceable modules to support multiple media types For example, you can purchase a high-end hub chassis that can house both Ethernet and Token Ring modules You can purchase hubs in a variety of sizes, ranging from those that support only

2 workstations to those that support more than 100 workstations Many network designers use stackable hubs, which usually support 24 node connections each These hubs are often used in concert with switches, which are discussed in the next section Hubs have two important properties:

N Hubs echo all data from each port to all the other ports on the hub Although hubs are wired in a star fashion, they actually perform electrically (logically) more like a bus topology segment in this respect Because of this echoing, no filtering or logic occurs to prevent collisions between packets being transmitted

by any of the connected nodes

N Hubs can automatically partition (in this context, cut off) a problematic node

from the other nodes—in effect, shutting down that node Such partitioning occurs if a cable short is detected, if the hub port is receiving excessive packets that are flooding the network, or if some other serious problem is detected for a given port on the hub Automatic partitioning keeps one malfunctioning connection from causing problems for all of the other connections

Hubs are becoming much more sophisticated They often have a number of

advanced built-in features, including the following:

N Built-in management, where the hub can be centrally managed over the

network, using SNMP or other network management protocols and software

N Autosensing of different connection speeds For example, Ethernet hubs that can automatically detect and run each node at either 10 Mbps (10Base-T) or

100 Mbps (100Base-T) are common

N High-speed uplinks that connect the hub to a backbone These usually operate

at ten times the basic speed of the hub (For example, for a 100 Mbps hub, the uplink ports might run at 1 Gbps.)

N Built-in bridging and routing functions, which make it unnecessary to use separate devices to perform bridging and routing

N Built-in switching, where nodes on the hub can be switched instead of shared

When ordering a hub, it’s important to know how many nodes you want to

connect, how much bandwidth each requires, and what type of network backbone is

being used Backbones can be anything from shared 10 Mbps Thin Ethernet, to 100

Mbps 100Base-TX, to higher-speed backbones Your choice of a backbone technology

depends on the total amount of bandwidth that you need and the various other

network design criteria that you must meet

Each hub is a separate collision domain, or an area of the network in which collisions

can occur Connecting all hubs together in some fashion generally results in a larger

collision domain, encompassing all the hubs The exception to this rule is a configuration

where all the separate hubs are connected to a switch (see the next section), which keeps

each hub in its own collision domain Figure 6–2 shows an example of a network

using hubs

Switches

Switches, as their name implies, can switch connections from one port to another, and

they can do so rapidly They are connection-oriented and dynamically switch among

their various ports to create these connections Think of a train yard, with many trains

coming in on some tracks and leaving on other tracks The yard manager orders the

Figure 6-2. A typical hub arrangement

Common backbone cable

track “switches” to take place so the trains can get to their destination A network switch is much like the yard manager, except that the switch directs packets rather than trains and uses Ethernet cabling rather than train tracks to transport its cargo

NOTE Switches are a lot like bridges, except that they have many ports and otherwise look like hubs You might think of a switch as a bridge with multiple ports

Because switches form one-to-one connections between any two ports, all the ports coming into a switch are not part of a single collision domain In this sense, the switch acts as a sort of super bridge (bridges are discussed in the next section)

Switches are often used to connect a number of hubs to a much faster backbone For example, suppose that you have 10 hubs, each with 24 workstation nodes connected If you simply connect all the hubs together on a common backbone, all 240 workstations would share a single collision domain, which could hurt performance quite a bit Instead, a much better approach is to install a 12-port switch and connect each hub to one of the ports on the switch For instance, it is common to use 100Base-T Ethernet for workstation connections, but 1000Base-T (or some other faster network connection) for the backbone This allows all the traffic being generated by each of the 10 hubs to continue to run at about a 100 Mbps connection speed to the servers, even though all the hubs are sharing the backbone Figure 6–3 illustrates this approach

NOTE Switches often are used simply to connect two given ports (such as traffic from port 5 to port 21, for instance), but they are also intelligent enough to echo certain types of broadcast packets

to all ports simultaneously

Figure 6-3. A network built using hubs and switches

Switch

Hubs

Server

Switches have become inexpensive and are blazingly fast For local area network

(LAN) connections, switches make more sense than hubs, partly because of their cost

and their relative simplicity In fact, purchasing bridges has become difficult, as switches

now dominate the market

Additionally, most new networks eschew hubs in favor of a 100 percent–switched

approach In fact, it’s virtually impossible to purchase hubs any longer, because

manufacturers typically offer only switches (You may still be able to purchase very

small hubs, with four to eight ports, but even in these small applications, switches are

preferable and not much more expensive.)

It’s important that you understand the difference between hubs and switches, because

you may still encounter hubs installed in existing networks For new networks, you will

use switches exclusively Doing so dramatically reduces the opportunity for network

packet collisions, which are more likely in a hub arrangement

Bridges

Bridges are, in a nutshell, more intelligent versions of repeaters Bridges can connect

two network segments together, but they have the intelligence to pass traffic from one

segment to another only when that traffic is destined for the other segment Bridges are used

to segment networks into smaller pieces Some bridges can span different networking

systems and media, such as from coaxial Thin Ethernet to twisted-pair Token Ring

As you might recall, repeaters operate at the physical layer (layer 1) of the OSI

networking model Bridges operate one layer higher, at the data-link layer (layer 2)

Bridges examine the media access control (MAC) address of each packet they encounter

to determine whether they should forward the packet to the other network Bridges

contain address information about all the parts of your network, through either a static

routing table that you program or a dynamic, learning-tree system that discovers all the

devices and addresses on the network automatically

Is It Better to Use Fewer Large Switches or More Small Switches?

Larger switches that can host hundreds of connections within a single chassis are

generally more powerful than their smaller 24-port siblings, and they tend to

have more built-in redundancy, such as redundant power supplies in the unit and

so forth However, sometimes it’s easier and less expensive to build a network

using smaller 24-port switches You can simply purchase an extra 24-port unit as

a hot-swap backup (a backup unit that can be quickly swapped in to take the place

of a failed unit) that you can manually implement at a moment’s notice The only

real disadvantage to this approach is that the redundancy is not automatic If one

24-port switch fails, you’ll need to move its connections to the backup switch In

contrast, a larger unit can switch to redundant features automatically As always,

consider such trade-offs carefully for your particular company and its needs