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Chapter 7. Backbone Networks
Business Data Communications and
Networking Fitzgerald and Dennis,
7th Edition
Copyright © 2002 John Wiley & Sons, Inc.
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Chapter 7. Learning Objectives
• Understand the types of internetworking
devices used in backbone networks
• Understand several common backbone
architectures
• Be aware of FDDI
• Be familiar with ATM
• Be aware of ways to improve backbone
network performance
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Chapter 7. Outline
• Introduction
• Backbone Network Components
– Bridges, Routers, Brouters, Gateways, A Caveat
• Backbone Architectures
– Backbone Architectures, Routed backbone, Bridged
backbone, Collapsed backbone, Virtual LAN
• Backbone Technologies
– FDDI, ATM
• Improving Backbone Performance
– Improving Computer and Device Performance, Improving
Circuit Capacity, Reducing Network Demand
• The Ideal Backbone?
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Introduction
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Backbone Networks
• Backbone networks are high speed networks that
perform that linking an organization’s LANs,
making information transfer possible between
departments.
• Such a network is also sometimes referred to as an
enterprise network.
• A backbone network that connects LANs in
several buildings is sometimes referred to as a
campus-wide network.
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Backbone Network Components
• Backbone networks have two basic components:
– the network cable
– the hardware devices connecting it to the other
networks.
• The backbone network’s cable functions in the
same way as in LANs.
• Optical fiber is more commonly chosen since it
provides higher data rates.
• The hardware devices can be computers or special
purpose devices used for interconnecting networks
including bridges, routers and gateways (Fig. 7-1).
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Device
Operates at
Packets
Physical
Layer
Data Link
Layer
Network
Layer
Bridge
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
Figure 7-1 Backbone Network Devices
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Bridges (Figure 7-2)
• Bridges are data link layer devices. The networks
they connect together must be similar, but they can
connect different types of cable.
• Bridges operate in a similar way to layer 2
switches: they learn which computers are on each
side of the bridge by reading the source addresses
on incoming frames and recording this
information in forwarding tables.
• Once popular, bridges are losing market share to
layer 2 switches as the latter become cheaper and
more powerful.
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Figure 7-2. Example of a Bridge
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Routers (Figure 7-3)
• Routers operate at the network layer, connecting two or more
network segments that may different data link layer protocols,
but the same network layer protocol.
• They can also connect different types of cabling.
• Router operations involve stripping off the header and trailer
of the incoming data link layer frame and then examining the
destination address of the network layer packet. The router
then builds a new frame around the packet and sends it out
onto another network segment.
• Another important router feature is that they choose the “best”
route for a packet to follow, hence the name ‘router’.
• This also means that routers need to perform more processing
than bridges or layer 2 switches.
• Another important difference is that, unlike a bridge, a router
only processes messages that are specifically addressed to it.
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Figure 7-3 Example of a Router
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Gateways
• Like routers, gateways also operate at the
network layer, but they are more complex
than routers because they provide an
interface between more dissimilar networks.
• Like routers, gateways only process
messages that are specifically addressed to
them.
• Some gateways operate at the application
layer as well.
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Hybrid Internetworking Devices
• In the real world, a number of hybrid networking
devices exist that fill market niches beyond those
provided by the “pure” bridges, routers and
gateways.
• These include:
– Multiprotocol routers
– Brouters that combine operations of both
routers and bridges
– Layer 3 Switches that make switching decisions
based on IP addresses
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Figure 7-4 Example of a Gateway
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Backbone Architectures
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Backbone Network Types
• There are four basic types of backbone
networks:
• Routed Backbones
• Bridged Backbones
• Collapsed Backbones
• Virtual LANs
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Backbone Architecture Layers (Figure 7-5)
• Network designs are made up of three
technology layers:
• The access layer which is the technology
used in LANs
• The distribution layer connects LANs
together
• The core layer connects different backbone
networks together
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Figure 7-5 Backbone network design layers
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Routed Backbones
• Routed backbones move packets using network
layer addresses, typically using a bus topology.
• Each LAN is a separate and isolated network.
• LANs can use different data link layer protocols.
• Main advantage: LAN segmentation.
• Main disadvantages:
– routers tend to impose time delays compared to
bridging and (layer 2) switching
– routers require more mgmt. than bridges & switches.
• Figure 7-6 shows an example of a distribution
layer routed backbone.
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Figure 7-6 Routed Backbone
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Bridged Backbones (Figure 7-7)
• Bridged backbones move packets between
networks using a bus topology, forwarding of
packet is based on their data link layer addresses.
• The entire bridged backbone network forms just
one subnet.
• Formerly common in the distribution layer, their
use is declining due to performance problems.
• Bridged backbones are cheaper (since bridges are
cheaper than routers) and easier to manage than
routed backbones.
• For small networks, a bridged backbone performs
well, but for large networks broadcast messages
can lower performance.
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Figure 7-7 Bridged Backbone
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Collapsed Backbones (Figure 7-8)
• Collapsed backbones use a star topology, usually
with a switch at the center.
• This replaces the many routers or bridges of the
previous designs, so the backbone has more cable,
but fewer devices.
• Each connection to the switch becomes a separate
point-to-point circuit.
• Advantages are: 1) simultaneous access and much
higher performance (from 200-600% higher) and
2) a simpler more easily managed network.
• Two minor disadvantages are: 1) use more cable
and the cable runs for longer distances, 2) if the
central switch fails, the network goes down.
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Figure 7-8 Collapsed Backbone
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Rack-based Collapsed Backbones
• Rack-based backbones collapse the
backbone into a single room, called a main
distribution facility (MDF) where
networking equipment is connected and
mounted on equipment racks (Figure 7-9).
• Devices are connected using short patch
cables.
• Moving computers between LANs is
relatively simple since equipment is all in
the same location.
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Figure 7-9. Rack-based Collapsed backbone network design.
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Chassis-based Collapsed Backbones
• Uses a large chassis switch that has slots
into which modules (i.e., card-mounted
networking devices) can be inserted.
• Chassis switch designs include a number of
open slots and have an internal capacity
capable of supporting all active modules.
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Figure 7-11 Central Parking’s collapsed backbone
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Virtual LANs
• VLAN are a new type of LAN/BN architecture
using high-speed intelligent switches.
• In a VLAN, computers are assigned to LAN
segments by software.
• VLANs are often faster and provide more flexible
network management than traditional LAN and
BN designs.
• They are also more complex and so far usually
used for larger networks.
• The two basic designs are single switch and multiswitch VLANs.
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Single Switch VLANs (Figure 7-12)
• This VLAN design connects computers using a
single switch acting as a large physical switch.
• Computers are assigned to individual VLANs
through software in one of four ways:
– Port-based VLANs assign computers according to the
VLAN switch port to which they are attached
– MAC-based VLANs assign computers according each
computer’s data link layer address
– IP-based VLANs assign computers using their IP-address
– Application-based VLANs assign computers depending
on the application that the computer typically uses. This
has the advantage of allowing precise allocation of
network capacity.
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Fig. 7-12. VLAN-based Collapsed backbone design
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Multi-switch VLANs (Figure 7-13)
• Multi-switch VLANs use multiple VLAN switches, sending
packets among themselves, making new types of VLANs
possible, such as VLANs in separate locations.
• Two approaches to implementing multi-switch VLANs are
now in use. In one case proprietary protocols are used to
envelope the Ethernet frame, which is then sent to its
destination switch, where the Ethernet packet is released and
sent to its destination computer.
• The other approach is to modify the Ethernet packet to include
VLAN information. The IEEE 802.1q standard adds 16 bytes
of overhead onto the IEEE 802.3 Ethernet packet. When an
Ethernet packet reaches a VLAN switch, it is set inside an
IEEE 802.1q packet. When the IEEE 802.1q packet reaches its
destination switch, its header is stripped off and the Ethernet
packet inside is sent to its destination computer.
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Figure 7-13. Multi-switch VLAN-based
Collapsed backbone design
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Figure 7-14 IONA VLAN network
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Backbone Technologies
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Fiber Distributed Data Interface (FDDI)
• FDDI (standardized as ANSI X3T9.5) backbone
protocol was developed in the 1980s and popular
during the 80s and 90s.
• FDDI operates at 100 Mbps over a fiber optic
cable.
• Copper Distributed Data Interface (CDDI) is a
related protocol using cat 5 twisted wire pairs.
• FDDI’s future looks limited, as it is now losing
market share to Gigabit Ethernet and ATM.
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FDDI Topology (Figure 7-15)
• FDDI uses both a physical and logical ring
topology capable of attaching a maximum
of 1000 stations over a maximum path of
200 km. A repeater is need every 2 km.
• FDDI uses dual counter-rotating rings
(called the primary and secondary). Data
normally travels on the primary ring.
• Stations can be attached to the primary ring
as single attachment stations (SAS) or both
rings as dual attachment stations (DAS).
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Figure 7-15 FDDI Topology
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FDDI’s Self Healing Rings
• One important feature of FDDI is its ability
to handle a break in the ring to form a
temporary ring out of the pieces of the two
rings.
• Figure 7-16, show an example of a cable
break between two dual-attachment stations.
• After the cable break is detected, a single
ring is formed out of the primary and
secondary rings until the cable break can be
repaired.
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Figure 7-16 FDDI’s Self-healing Rings
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FDDI Media Access Control
• FDDI uses a token passing system. Computers wanting to
send packets wait to receive a token before transmitting.
• Multiple packets can be attached to the token as it moves
around the network.
• When a station receives the token, it looks for attached
packets addressed to it and removes them from the incoming
packet.
• If the station wants to send a packet it attaches it to the token
and sends the token with its attached packets to the next
station.
• This controlled access technique provides a higher
performance level at high traffic levels compared to a
contention-based technique like Ethernet.
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Asynchronous Transfer Mode (ATM)
• Asynchronous Transfer Mode (ATM) (also
called cell relay) is a technology originally
designed for use in wide area networks that
is now often used in backbone networks.
• ATM backbone switches typically provide
point-to-point full duplex circuits at 155
Mbps (total of 310 Mbps).
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ATM vs. Switched Ethernet
• ATM is a switched network, but differs from
switched Ethernet in four ways:
1. ATM uses small, fixed-length packets of 53 bytes
(called cells). Ethernet frames are variable and can
be up to about 1 kilobyte in length.
2. ATM provides no error correction on the user data.
Switched Ethernet does error correction.
3. ATM uses virtual channels instead of the fixed
addresses used by traditional data link layer
protocols such as switched Ethernet (see Fig. 7-17).
4. ATM prioritizes transmissions based on Quality of
Service (QoS), while switched Ethernet does not.
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Figure 7-17 Addressing & Forwarding
with ATM Virtual Circuits
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ATM’s Virtual Circuits
• ATM is connection-oriented, meaning all
packets travel in order through the same
virtual circuit.
• There are two types of ATM virtual circuits:
– Permanent Virtual Circuits (PVCs) - defined
when the network is established or modified.
– Switched Virtual Circuits (SVCs) - defined
temporarily for one transmission and deleted
when the transmission is completed.
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ATM and Traditional LANs
• Since ATM’s small fixed length cells are so
different from Ethernet frames, frames must be
translated before being sent over ATM networks.
• The main approaches for this is LAN
Encapsulation (LANE). A second approach
Multiprotocol Over ATM (MPOA) is an
extension of LANE to include network layer
addresses.
• LANE essentially involves splitting the frame into
small pieces without changing it and then
reassembling the original frame when it reaches its
destination LAN.
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LAN Encapsulation
• The first step in LAN Encapsulation is to create an
ATM virtual circuit identifier for the virtual circuit
that will connect the “gateway” ATM edge switch
to the ATM edge switch nearest the frame’s
destination (see Figure 7-18)
• Once the virtual circuit is ready, the Ethernet
frame is broken up into a series of ATM cells and
sent over the ATM backbone using the ATM
virtual circuit identifier.
• At the receiving edge switch the frame is
reassembled. Unfortunately LAN has very high
overhead and so network performance suffers as a
consequence.
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Figure 7-18 ATM Encapsulation
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ATM to the Desktop
• ATM-25 is a low-speed option that provides pointto-point full duplex circuits at 25.6 Mbps in each
direction. It is an adaptation of token ring that runs
over cat 3 cable and can even use token ring
hardware if modified.
• ATM-51 is designed for the desktop allowing
51.84 Mbps from computers to the switch.
• Both these ATMs appear to be good choices for
desktop connections when ATM backbone
networks are used. However, industry has been
very slow to accept either and have instead moved
to Fast Ethernet which is both cheaper and faster.
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Improving Backbone Performance
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Improving Backbone Performance
• Improving the performance of backbone networks
is similar to improving LAN performance. First
find the bottleneck, then solve it, or move it
somewhere else.
• You can improve performance by improving the
computers and other devices in the network, by
upgrading the circuits between computers, and by
changing the demand placed on the network.
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Figure 7-19 Backbone Performance
Checklist
Increase Computer and Device Performance
 Change to a more appropriate routing protocol
(either static or dynamic)
 Buy devices and software from one vendor
 Reduce translation between different protocols
 Increase the devices’ memory
Increase Circuit Capacity
 Upgrade to a faster circuit
 Add circuits
Reduce Network Demand
 Change user behavior
 Reduce broadcast messages
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Improving Computer and Device
Performance
• The primary functions of computers and devices
in backbone networks are routing and protocol
translations. They can be improved with a faster
routing protocol.
• Static routing is faster than dynamic, but can
impair circuit performance in high traffic
situations.
• Many of the newer backbone technologies have
standards that are not fully developed.
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Improving Computer and Device
Performance (cont.)
• FDDI and ATM require the translation or
encapsulation of Ethernet packets before they can
flow through the backbone.
• Translating protocols typically requires more
processing than encapsulation, so encapsulation
can improve performance if the backbone devices
are the bottleneck.
• Most backbone devices are store and forward
devices.
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Improving Circuit Capacity
• If network circuits are the bottleneck there
are several options:
– Increase overall circuit capacity.
– Add additional circuits alongside heavily used
ones.
– Replace shared circuit backbones with a
switched circuit backbone.
• If the circuit to the server is the problem:
replace the Ethernet hub with a switch and
change one NIC on the server.
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Reducing Network Demand
• Restrict applications that use a lot of
network capacity, like video-conferencing,
imaging, or multimedia.
• Reduce the number of broadcast LAN
messages on non-switched LANs.
• Filter broadcast LAN messages so they do
not exit their native LAN.
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The Best Practice Backbone
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Current Backbone Technology Trends
• The following trends in backbone technologies have
been taking place in recent years:
• Organizations are moving to Ethernet-based collapsed
backbones with switched LANs or VLANs.
• Gigabit Ethernet use is growing.
• FDDI seems to be on its way out.
• ATM, while still popular in WANs, is also losing
ground to Gigabit Ethernet.
• Taken together, it appears that Ethernet use will
dominate the LAN and backbone.
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The Ideal Backbone? (Figure 7-20)
• The ideal network design is likely to include the
following characteristics:
– Combined use of layer 2 and layer 3 Ethernet switches.
– The access layer (LANs) uses 10/100 Layer 2 Switches
running Cat 5 or Cat 6 twisted pair cables (Cat 6
enables the move to 1000BaseT).
– The distribution layer uses Layer 3 Ethernet Switches
that use 1000BaseT or fiber, Cat 6 or Cat 7 TP.
– The core layer uses Layer 3 Ethernet Switches running
10GbE or 40GbE over fiber.
– Reliability is also increased in the network by using
redundant switches and cabling.
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Figure 7-20 A best practice network design
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End of Chapter 7
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