Chapter 7. Backbone Networks
Business Data Communications and
Networking Fitzgerald and Dennis,
7th Edition
Copyright © 2002 John Wiley & Sons, Inc.
1
Copyright John Wiley & Sons, Inc. All rights reserved.
Reproduction or translation of this work beyond that named in
Section 117 of the United States Copyright Act without the
express written consent of the copyright owner is unlawful.
Requests for further information should be addressed to the
Permissions Department, John Wiley & Sons, Inc. Adopters of
the textbook are granted permission to make back-up copies for
their own use only, to make copies for distribution to students of
the course the textbook is used in, and to modify this material to
best suit their instructional needs. Under no circumstances can
copies be made for resale. The Publisher assumes no
responsibility for errors, omissions, or damages, caused by the
use of these programs or from the use of the information
contained herein.
2
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
3
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?
4
Introduction
5
Backbone Networks
• Backbone networks are high speed networks that
perform the critical function of linking an
organization’s LANs together making information
transfer between departments possible.
• 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.
6
Backbone Network Components
• Backbone networks have two basic components:
– the network cable and
– the hardware devices connecting it the other networks.
• The cable is the same as that used in LANs except
that optical fiber is usually used to provide 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).
7
Device
Operates at
Packets
Physical
Layer
Data Link
Layer
Network
Layer
Bridge
Data Link
Layer
Same
or
Different
Same
Same
Router
Network
Layer
Filtered using
data link
layer
addresses
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
8
Bridges (Figure 7-2)
• Bridges are data link devices that connect two or
more similar networks together, but they can
connect different types of cable.
• Bridge operate in a similar way to layer 2 switches
in that they learn which computers are on each
side of the bridge by reading the source addresses
on incoming frames and record this information in
forwarding tables.
• Once popular, bridges are losing market share to
layer 2 as the latter become cheaper and more
powerful.
9
Figure 7-2. Bridge
10
Routers (Figure 7-3)
• Routers operate at the network layer, connecting two or more
network segments that use the same or 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 data link layer of
the incoming frame and then examining the destination
address of the network layer packet. Then build a new frame
around the packet and send 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.
11
Figure 7-3 Router
12
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.
13
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 are routers that
– Brouters are devices that combine operations of both
routers and bridges
– Layer 3 switches
14
Figure 7-4
15
Backbone Architectures
16
Backbone Network Types
• There are four basic types of backbone
networks:
• Routed Backbones
• Bridged Backbones
• Collapsed Backbones and
• Virtual LANs
17
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
18
LAN
LAN
LAN
LAN
LAN
LAN
Core Layer
Distribution Access
Layer
Layer
Figure 7-5 Backbone network design layers
19
Routed Backbones
• Routed backbones move packets using network
layer addresses, typically using a bus topology.
• Each LAN is separate and isolated the network
• LANs can use different data link layer protocols.
• Main advantage is LAN segmentation.
• Main disadvantages are:
– 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.
20
Figure 7-6 Routed Backbone
21
Bridged Backbones (Figure 7-7)
• Bridged backbones move packets using data link
layer addresses using a bus topology. Entire
network forms just one subnet.
• Formerly common in the distribution layer, their
use is declining due to performance problems.
• Advantages are that they are cheaper (since
bridges usually 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, since they travel
everywhere on the entire network.
22
Figure 7-7 Bridged Backbone
23
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 (often 2-600% higher) and 2)
a simpler more easily managed network.
• Main Disadvantages are: 1) most still use layer 2
switching, so broadcast traffic can be a problem
and it is harder to isolate network segments.
24
Figure 7-8 Collapsed Backbone
25
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
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.
26
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 support all active modules
(Figure 7-11).
27
Client
Computer
Client
Computer
10/100 Ethernet
Client
Computer
Layer-2
Switch
Client
Computer
Client
Computer
Router
to WAN
1GbE
on fiber
1000Base-T
Client
Computer
10/100 Ethernet
Router
to Internet
Layer-3
Switch
Client
Computer
1GbE
on fiber
1GbE
on fiber
Client
Computer
10/100 Ethernet
Client
Computer
Layer-2
Switch
Client
Computer
Client
Computer
Server
Server
1000Base-T
Client
Computer
Server
Server
Figure 7-11 Central Parking’s collapsed backbone
28
Virtual LANs (Figure 7-12)
• 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.
29
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 use the 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.
30
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 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, it is stripped off and the Ethernet packet
inside is sent to its destination computer.
31
Client
Computer
VLAN switch
VLAN switch
Client
Computer
VLAN switch
10/100 Ethernet
Client
Computer
VLAN switch
Client
Computer
VLAN switch
Client
Computer
1GbE
on fiber
1000Base-T
Client
Computer
VLAN switch
1GbE
on fiber
VLAN switch
VLAN switch
VLAN switch
1GbE
on fiber
VLAN switch
VLAN switch
VLAN switch
1GbE
on fiber
VLAN switch
VLAN switch
VLAN switch
VLAN switch
VLAN switch
Figure 7-14 IONA VLAN network
32
Backbone Technologies
33
Fiber Distributed Data Interface (FDDI)
• FDDI (standardized as ANSI X3T9.5) is 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.
• Its future looks limited, as it is now losing market
share to Gigabit Ethernet and ATM.
34
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).
35
Figure 7-15 FDDI Topology
36
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
repair.
37
Figure 7-16 FDDI’s Self-healing Rings
38
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.
39
Asynchronous Transfer Mode (ATM)
• Asynchronous Transfer Mode (ATM) (a.k.a.
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).
40
Asynchronous Transfer Mode (ATM)
• ATM is a switched network but differs from
switched ethernet and switched token ring in four
ways:
1. ATM uses fixed-length packets of 53 bytes.
2. ATM provides no error correction on the user data.
3. ATM uses a very different type of addressing from
traditional data link layer protocols such as ethernet or
token ring.
4. ATM prioritizes transmissions based on Quality of
Service (QoS).
41
Figure 7-17 Addressing & Forwarding
with ATM Virtual Circuits
42
Asynchronous Transfer Mode (ATM)
• Asynchronous Transfer Mode (ATM) is
connection-oriented so all packets travel in
order through the virtual circuit. A virtual
circuit can either be a:
– Permanent Virtual Circuit (PVC) - defined
when the network is established or modified.
– Switched Virtual Circuit (SVC) - defined
temporarily for one transmission and deleted
with the transmission is completed.
43
ATM and Traditional LANs
• ATM uses a very different type of protocol than
traditional LANs. It has a small 53-byte fixed
length packet and is connection-oriented.Ethernet
and token ring use larger variable length packets
and are typically connectionless.
• Translation must be done to enable the LAN
packets to flow over the ATM backbones. There
are two approaches LAN encapsulation (LANE)
and Multiprotocol over ATM (MPOA).
44
Figure 7-18 LAN Encapsulation (LANE)
45
ATM and Traditional LANs
• Translating from Ethernet or Token Ring into
ATM is not simple.
• First the ethernet address must be translated into
an ATM virtual circuit identifier for the circuit that
leads from the edge switch to the edge switch
nearest the destination.
• Once the virtual circuit address for the destination
data link layer address has been found, it can be
used to transmit the packet through the ATM
backbone.
46
ATM and Traditional LANs
• Once the virtual circuit is ready, the LAN packet is
broken into the series of ATM cells, and
transmitted over the ATM backbone using the
ATM virtual circuit identifier.
• Unfortunately this process can cause quite a delay
(a reduction of 40 to 50 %).
• Multiprotocol over ATM (MPOA) is an extension
to LANE.
47
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.
48
Improving Backbone Performance
49
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.
50
Tips on Improving Performance
Increase Computer and Device Performance
Change to a more appropriate routing protocol
(either a static or dynamic)
Reduce translation between different protocols
Increase Circuit Capacity
Upgrade to a faster circuit
Add circuits
Reduce Network Demand
Change user behavior
Reduce broadcast messages
51
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.
52
Improving Computer and Device
Performance
• FDDI and ATM require the translation or
encapsulation of ethernet and token ring 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.
53
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.
54
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.
55
The Ideal Backbone?
56
End of Chapter 7
57