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Chapter 7. Backbone Networks

Business Data Communications and Networking Fitzgerald and Dennis,

7th EditionCopyright © 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 PhysicalLayer

Data Link Layer

NetworkLayer

 

BridgeData Link

LayerFiltered using data link layer

addresses

Same or

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RouterNetwork

LayerRouted using network layer

addresses

Same or

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Same or

Different

 

Same

 

GatewayNetwork

LayerRouted using network layer

addresses

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Same or

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Same or

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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.

9Figure 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.

11Figure 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

14Figure 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.

20Figure 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.

22Figure 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.

24Figure 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.

26Figure 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.

• Turn to page 217 in textbook for an example of Chassis switch. History repeats itself, looks like the old operator cord board!

28Figure 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 multi-switch 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.

31Fig. 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.

33Figure 7-13. Multi-switch VLAN-based Collapsed backbone design

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(Slide missing, turn to page 223 for example)

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).

38Figure 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.

40Figure 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.

48Figure 7-18 ATM Encapsulation

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ATM to the Desktop

• ATM-25 is a low-speed option that provides point-to-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