Connecting LANs and WANs:

dargspurNetworking and Communications

Oct 27, 2013 (4 years and 13 days ago)

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-
Hill Companies, Inc., 2000

Chapter 12

Connecting LANs

and WANs:

Making

Backbone Networks

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Differentiate between the different connecting devices and

the layers each supports.

Understand the functionality of a repeater and where it is used.

Understand the difference between a two
-
layer and a three
-

layer switch.

After reading this chapter, the reader should

be able to:

O
BJECTIVES

Understand the functionality of a bridge and where it is used.

Understand the functionality of a router and where it is used.

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O
BJECTIVES (continued)

Distinguish between a logical bus and logical star backbone.

Distinguish between a bridge
-
based and a router
-
based back
-

bone network.

Understand the functionality of a backbone network and

where it is used.

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CONNECTING

DEVICES

12.1

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Figure 12
-
1

Connecting devices

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Figure 12
-
2

Repeater

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A repeater connects segments of a

LAN together.

Note:

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A repeater forwards every packet;

it has no filtering capability.

Note:

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Technical Focus:


Repeaters and Amplifiers



It is tempting to compare a repeater to an amplifier, but
the comparison is inaccurate. An amplifier cannot
discriminate between the intended signal and noise; it
amplifies equally everything fed into it. A repeater does
not amplify the signal; it regenerates it. When it receives
a weakened or corrupted signal, it creates a copy bit for
bit, at the original strength. A repeater is a regenerator,
not an amplifier.

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Figure 12
-
3

Hubs

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Technical Focus:


Active versus Passive Hub



A passive hub is a hub that is used to create a connection; it
does not regenerate signals. An active hub, on the other
hand, is a connector as well as a repeater. Today, most hubs
are active hubs.

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A bridge has a table used in

filtering decisions.

Note:

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Figure 12
-
4

Bridge

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A bridge connects segments of a

LAN together.

Note:

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A router is a three
-
layer

(physical, data link, and network)

device.

Note:

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A repeater or a bridge connects
segments of LAN together

A router connects independent

LANs or WANs

to create an internetwork (internet).

Note:

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Figure 12
-
5

Routing example

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A router changes the physical addresses
in a packet.

Note:

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Technical Focus:


Multiprotocol Router



At the network layer, a router by default is a single
-
protocol
device. In other words, if two LANs are to be connected by a
router, they must use the same protocol at the network layer.
The reason behind this is that the routing table must use one
single addressing format.

However, multiprotocol routers have been designed to route
packets belonging to two or more protocols. For example, a
two
-
protocol router (for example, IP and IPX) can handle
packets belonging to either of the two protocols. It can receive,
process, and send a packet using the IP protocol or it can
receive, process, and send a packet using the IPX protocol. In
this case, the router has two routing tables: one for IP and one
for IPX.

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Technical Focus:


Brouters



A brouter (bridge/router) is a single
-
protocol or
multiprotocol router that sometimes acts as a router and
sometimes as a bridge. When a single
-
protocol brouter
receives a packet belonging to the protocol for which it is
designed, it routes the packet based on the network layer
address; otherwise, it acts as a bridge and passes the packet
using the data link layer address.

Likewise, when a multiprotocol brouter receives a packet
belonging to one of the protocols for which it is designed, it
routes the packet based on the network layer address;
otherwise, it acts as a bridge and passes the packet using the
data link layer address.

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Technical Focus:


Gateways



Today, the term gateway is used mostly as a synonym for a
router. However, in the past, the two terms had different
meanings. A gateway defined a device that could potentially
operate in all five layers of the Internet model. It was a
protocol converter. A router was a device that could
transfer, accept, and relay packets only across networks
using similar protocols. A gateway, on the other hand, could
accept a packet formatted for one protocol (e.g., AppleTalk)
and convert it to a packet formatted for another protocol
(e.g., TCP/IP) before forwarding it.

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BACKBONE

NETWORKS

12.2

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In a logical bus backbone, the logical

topology of the backbone is a bus;

the physical topology can be

a bus or a star.

Note:

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Figure 12
-
6

Bridged
-
base logical bus network

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Figure 12
-
7

Router
-
based logical bus backbone

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In a logical star backbone, the logical

topology of the backbone is a star;

the backbone is just one switch.

Note:

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Figure 12
-
8

Logical star backbone

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Technical Focus:


FDDI

Fiber distributed data interface

(FDDI) is a local area network
protocol standardized by ANSI and the ITU
-
T. It supports data
rates of 100 Mbps. FDDI is implemented as a dual ring as shown
below.