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CHAPTER
8
BACKBONE NETWORKS
Backbone
Networks
Fundamental Concepts
Network Technologies
Application Layer
LAN WLAN
Backbone
Transport Layer
Network Layer
Data Link Layer
Physical Layer
WAN
Internet
M
k
Network Management
The Three Faces of Networking
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CHAPTER 8
BACKBONE NETWORKS
T
HIS CHAPTER
examines backbone networks (BNs) that are used to link LANs
together and to link BNs to WANs. We begin with the various types of devices used in
BNs and discuss several backbone architectures. We then turn to two technologies
designed primarily for use in the BN (ATM and gigabit Ethernet). The chapter ends with
a discussion of how to improve BN performance and of the future of BNs.
OBJECTIVES
Understand the internetworking devices used in BNs
■
Understand several common backbone architectures
■
Be familiar with ATM
■
Be familiar with gigabit Ethernet
■
Understand the best practice recommendations for backbone design
■
Be aware of ways to improve BN performance
■
CHAPTER OUTLINE
INTRODUCTION
BACKBONE NETWORK COMPONENTS
Switches
Routers
Gateways
A Caution
BACKBONE NETWORK ARCHITECTURES
Backbone Architecture Layers
Routed Backbone
Collapsed Backbone
Virtual LAN
BACKBONE TECHNOLOGIES
Asynchronous Transfer Mode
THE BEST PRACTICE BACKBONE DESIGN
Architectures
Effective Data Rates
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BACKBONE NETWORK COMPONENTS
Conversion between Protocols
Recommendations
IMPROVING BACKBONE PERFORMANCE
Improving Computer and Device Performance
Improving Circuit Capacity
Reducing Network Demand
IMPLICATIONS FOR MANAGEMENT
SUMMARY
INTRODUCTION
Most business organizations realize that information must be stored, retrieved, analyzed,
acted on, and shared with others at a moment’s notice. Without an enterprisewide network
or an Internet connection, moving information from one department LAN to another or to
customers is difficult.
Interconnecting the organization’s diverse networks is critical. A
backbone network
(BN)
is a high-speed network that connects many networks. BNs typically use higher-
speed circuits to interconnect a series of LANs and provide connections to other BNs,
MANs, WANs, and the Internet. A backbone that connects many BNs spanning several
nearby buildings for a single organization is often called a
campus network.
A BN also
may be called an
enterprise network
if it connects all networks within a company, regard-
less of whether it crosses state, national, or international boundaries.
We begin this chapter by describing several commonly used devices in the BN and
then showing how those can be used to create different backbone architectures with differ-
ent performance capabilities. Next, we focus on the high-speed network technologies
often used in BNs.
BACKBONE NETWORK COMPONENTS
There are two basic components to a BN: the network cable and the hardware devices that
connect other networks to the BN. The cable is essentially the same as that used in LANs,
except that it is usually fiber optic to provide higher data rates. The hardware devices can
be computers or special-purpose devices that just transfer messages from one network to
another. These include switches, routers, and gateways (Figure 8.1).
Switches
Most
switches
operate at the data link layer. They connect two or more network segments
that use the
same
data link and network protocol. They understand only data link layer
protocols and addresses. They may connect the
same or different
types of cable. These are
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CHAPTER 8
BACKBONE NETWORKS
Physical
Layer
Data Link
Layer
Network
Layer
Device
Operates At
Packets
Switch
Data link layer
Filtered using data
link layer addresses
Same or
different
Same
Same
Router
Network layer
Routed using network
layer addresses
Same or
different
Same or
different
Same
Gateway
Network layer
Routed using network
layer addresses
Same or
different
Same or
different
Same or
different
Backbone network devices.
FIGURE 8.1
the same
layer-2 switches
discussed in Chapter 6 in that they use the data link layer ad-
dress to forward packets between network segments (Figure 8.2). They learn addresses by
reading the source and destination addresses.
Routers
Routers
operate at the network layer. Routers connect two or more network segments that
use the
same or different
data link protocols but the
same
network protocol. They may
connect the
same or different
types of cable. Routers are the “TCP/IP gateways” that we
first introduced in Chapter 5. Routers strip off the data link layer packet, process the net-
work layer packet, and forward only those messages that need to go to other networks on
the basis of their network layer address (Figure 8.3).
Hub
Hub
Server
Computer
Switch
Computer
Use of switches to connect local area network segments.
FIGURE 8.2
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BACKBONE NETWORK COMPONENTS
Server
Hub
Hub
Computer
Server
Router
Computer
Use of routers to connect local area networks.
FIGURE 8.3
Routers may be “black boxes,” computers with several NICs, or special network
modules in computers or other devices. In general, they perform more processing on each
message than switches and therefore operate more slowly.
One major feature of a router is that it can choose the “best” route between net-
works when there are several possible routes between them. Because a router knows its
own location, as well as the packet’s final destination, it looks in a routing table to identify
the best route or
path.
One other important difference between a router and a switch is that a router
processes only those messages that are specifically addressed to it. Switches process all
messages that appear on the network and forward them to the appropriate network on the
basis of their data link layer address. Switches simply forward the message unchanged to
the other network. In contrast, because routers operate at the network layer, the router’s
data link layer must first recognize that the incoming message is specifically addressed to
the router at the data link layer level before the message is passed to the network layer for
processing. The router will then process the message by building an entirely new data link
layer packet, then transmit it to the other network.
The router attempts to make no changes to the network layer packet and user data it
receives. (As noted previously, it creates a new data link layer packet.) Sometimes, how-
ever, changes are needed, such as when the maximum data link layer packet size on one
network is different from another, which forces the router to split a message into several
smaller messages for transmission.
Gateways
Gateways
operate at the network layer and use network layer addresses in processing mes-
sages. Gateways are more complex than switches or routers because they are the interface
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