1 Chapter 7. Backbone Networks Business Data Communications and Networking Fitzgerald and Dennis, 7th Edition Copyright © 2002 John Wiley & Sons, Inc.

2 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.

3 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

4 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?

5 Introduction

6 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.

7 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).

8 DeviceOperates atPacketsPhysical 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

9 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.

10 Figure 7-2. Bridge

11 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.

12 Figure 7-3 Router

13 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.

14 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

15 Figure 7-4

16 Backbone Architectures

17 Backbone Network Types There are four basic types of backbone networks: Routed Backbones Bridged Backbones Collapsed Backbones and Virtual LANs

18 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

19 LAN Access Layer Distribution Layer Core Layer Figure 7-5 Backbone network design layers

20 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.

21 Figure 7-6 Routed Backbone

22 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.

23 Figure 7-7 Bridged Backbone

24 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.

25 Figure 7-8 Collapsed Backbone

26 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.

27 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).

28 Layer-2 Switch Client Computer Client Computer Client Computer Client Computer Client Computer Client Computer 10/100 Ethernet 1000Base-T Layer-3 Switch Router to WAN Router to Internet Server 1GbE on fiber 1GbE on fiber 1GbE on fiber Figure 7-11 Central Parking’s collapsed backbone Layer-2 Switch Client Computer Client Computer Client Computer Client Computer Client Computer Client Computer 10/100 Ethernet 1000Base-T Server 10/100 Ethernet

29 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 multi- switch VLANs.

30 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.

31 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 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.

32 VLAN switch Client Computer Client Computer Client Computer Client Computer Client Computer Client Computer 10/100 Ethernet 1000Base-T VLAN switch 1GbE on fiber 1GbE on fiber 1GbE on fiber 1GbE on fiber Figure 7-14 IONA VLAN network

33 Backbone Technologies

34 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.

35 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).

36 Figure 7-15 FDDI Topology

37 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.

38 Figure 7-16 FDDI’s Self-healing Rings

39 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.

40 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).

41 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).

42 Figure 7-17 Addressing & Forwarding with ATM Virtual Circuits

43 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.

44 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).

45 Figure 7-18 LAN Encapsulation (LANE)

46 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.

47 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.

48 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 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.

49 Improving Backbone Performance

50 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.

51 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 Tips on Improving Performance

52 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.

53 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.

54 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.

55 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.

56 The Ideal Backbone?

57 End of Chapter 7