Connecting LANs and WANs: Making Backbone Networks

Chapter 12Connecting LANs and WANs:Making Backbone NetworksDifferentiate between the different connecting devices and the layers each supports.Understand the functionality of a repeater and where it is used.Understand the difference between a two-layer and a three-layer switch. After reading this chapter, the reader should be able to:OBJECTIVESUnderstand the functionality of a bridge and where it is used.Understand the functionality of a router and where it is used.OBJECTIVES (continued)Distinguish between a logical bus and logical star backbone.Distinguish between a bridge-based and a router-based back-bone network.Understand the functionality of a backbone network andwhere it is used.CONNECTINGDEVICES12.1Figure 12-1Connecting devicesFigure 12-2RepeaterA repeater connects segments of aLAN together.Note:A repeater forwards every packet;it has no filtering capability.Note:Technical Focus: Repeaters and AmplifiersIt is tempting to compare a repeater to an amplifier, but the comparison is inaccurate. An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not amplify the signal; it regenerates it. When it receives a weakened or corrupted signal, it creates a copy bit for bit, at the original strength. A repeater is a regenerator, not an amplifier.Figure 12-3HubsTechnical Focus: Active versus Passive HubA passive hub is a hub that is used to create a connection; it does not regenerate signals. An active hub, on the other hand, is a connector as well as a repeater. Today, most hubs are active hubs. A bridge has a table used infiltering decisions. Note:Figure 12-4BridgeA bridge connects segments of aLAN together. Note:A router is a three-layer(physical, data link, and network)device. Note:A repeater or a bridge connects segments of LAN togetherA router connects independent LANs or WANsto create an internetwork (internet).Note:Figure 12-5Routing exampleA router changes the physical addresses in a packet. Note:Technical Focus: Multiprotocol RouterAt the network layer, a router by default is a single-protocol device. In other words, if two LANs are to be connected by a router, they must use the same protocol at the network layer. The reason behind this is that the routing table must use one single addressing format. However, multiprotocol routers have been designed to route packets belonging to two or more protocols. For example, a two-protocol router (for example, IP and IPX) can handle packets belonging to either of the two protocols. It can receive, process, and send a packet using the IP protocol or it can receive, process, and send a packet using the IPX protocol. In this case, the router has two routing tables: one for IP and one for IPX. Technical Focus: BroutersA brouter (bridge/router) is a single-protocol or multiprotocol router that sometimes acts as a router and sometimes as a bridge. When a single-protocol brouter receives a packet belonging to the protocol for which it is designed, it routes the packet based on the network layer address; otherwise, it acts as a bridge and passes the packet using the data link layer address. Likewise, when a multiprotocol brouter receives a packet belonging to one of the protocols for which it is designed, it routes the packet based on the network layer address; otherwise, it acts as a bridge and passes the packet using the data link layer address.Technical Focus: GatewaysToday, the term gateway is used mostly as a synonym for a router. However, in the past, the two terms had different meanings. A gateway defined a device that could potentially operate in all five layers of the Internet model. It was a protocol converter. A router was a device that could transfer, accept, and relay packets only across networks using similar protocols. A gateway, on the other hand, could accept a packet formatted for one protocol (e.g., AppleTalk) and convert it to a packet formatted for another protocol (e.g., TCP/IP) before forwarding it.BACKBONENETWORKS12.2In a logical bus backbone, the logicaltopology of the backbone is a bus;the physical topology can be a bus or a star. Note:Figure 12-6Bridged-base logical bus networkFigure 12-7Router-based logical bus backboneIn a logical star backbone, the logicaltopology of the backbone is a star;the backbone is just one switch.Note:Figure 12-8Logical star backboneTechnical Focus: FDDIFiber distributed data interface (FDDI) is a local area network protocol standardized by ANSI and the ITU-T. It supports data rates of 100 Mbps. FDDI is implemented as a dual ring as shown below.