Connecting LANs, Backbone Networks Chapter 15 Connecting LANs, Backbone Networks Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Connecting Devices LANs usually do not operate in isolation. They are connected to one another or to the internet. To connect LANs, or segments of LANs, we use connecting devices. Connecting devices can operate in different layers of the internet model. After discussing some connecting devices, we show how they are used to create backbone networks.

Topics discussed in this section: 15-1 CONNECTING DEVICES In this section, we divide connecting devices into five different categories based on the layer in which they operate in a network. Topics discussed in this section: Passive Hubs Active Hubs Bridges Two-Layer Switches Routers Three-Layer Switches Gateways

Figure 15.1 Five categories of connecting devices

Continued…. Passive hubs operate below the physical layer. A repeater or an active hub operates at the physical layer. A bridge or a two-layer switch operate at the physical and data link layer. A router or a three-layer switch operate at the physical and data link layer and network layer. A gateway can operate at all the five layers.

Passive Hubs A passive hub is just a connector. It connects the wires coming from different branches. In a star topology Ethernet LAN, a passive hub is just a point where the signals coming from different stations collide. The hub is a collision point. This type of a hub is part of the media; its location in the internet model is below the physical layer.

Passive Hubs Passive hubs do not provide any additional feature except for working just as an interface between the topology. These types of hubs do not help in rectifying /enhancing the signals Thus they do not help in enhancing the performance of the network/LAN. It is very hard to get any help from the passive hubs while troubleshooting in case if there is any fault in the hardware or the network.

Continued…. A passive hub is ideal for home use because it can only accommodate a limited amount of shared information without bogging down the whole network. The good thing about passive ethernet hubs is that it can be used in conjunction with intelligent or active hubs. Passive hubs are typically used only to keep all the cables organized by making them originate from only one source. This makes physical maintenance a lot more convenient.

Passive Hubs

Repeater The signal on LANs attenuate after travelling a specific distance. To solve the above problem we use repeaters. The repeater receives the signal and regenerates it. A repeater can also extend the physical length of a LAN. A repeater actually does not connect two LANs using different protocols; it connects two segments of the same LAN. The segments connected are still part of one single LAN.

Figure 15.2 A repeater connecting two segments of a LAN

Active Hubs An active hub is actually a multiport repeater. It is normally used to create connections between stations in a physical star topology.

Continued…..

Figure 15.3 Function of a repeater

Figure 15.4 A hierarchy of hubs

Bridge A bridge operates in both the physical and the data link layer. As a physical layer device, it regenerates the signal it receives (the task of repeater). As a data link layer device, the bridge can check the physical MAC address contained in the frame.

filtering So what is the difference between a bridge and a repeater? A bridge has the capability of filtering. It can check the destination address of a frame and decide if the frame should be forwarded or dropped. If the frame should be forwarded, the decision must specify the port. A bridge has a table that maps addresses to ports.

A bridge has a table used in filtering decisions. Note A bridge has a table used in filtering decisions.

Figure 15.5 A bridge connecting two LANs

A bridge does not change the physical (MAC) addresses in a frame. Note A bridge does not change the physical (MAC) addresses in a frame.

Bridge

TWO LAYER SWITCHES When using the term switch, we must be careful as it should be mentioned on which layer it is used. We can have a two layer or a three layer switch. A two layer switch is used on the data link and physical layer. A two layer switch is a bridge with many ports. It is designed to allow better (faster) performance.

Continued…. A bridge with a few ports can connect a few LANs together. A bridge with many ports may be able to allocate a unique port to each station, with each station on its own independent entity. This means no competing traffic (no collision, as we saw in Ethernet). A switch is more sophisticated than a bridge. It can have a buffer to hold frames for processing. It can have a switching factor that forward the frames faster.

Layer 2 Switch

Layer 3 Switch A three-layer switch is used at the network layer. A router is a layer 3 Switch. A router is a device that routes packets based on their logical addresses (host to host addressing). A router connects LANs and WANs in the internet. A router has a routing table that is used for making decisions about the route. The routing tables are normally dynamic and are updated using routing protocols.

Continued…..

Figure 15.11 Routers connecting independent LANs and WANs

Gateway A gateway is normally a computer that operates in all five layers of the internet or seven layers of OSI model. A network gateway is an internetworking system capable of joining together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both.

Continued….. Because a network gateway, by definition, appears at the edge of a network, related capabilities like firewalls tend to be integrated with it. On home networks, a broadband router typically serves as the network gateway although ordinary computers can also be configured to perform equivalent functions. The job of a gateway is much more complex than that of a router or switch.

Continued…. Example: In homes, the gateway is the ISP that connects the user to the internet.

Topics discussed in this section: 15-2 BACKBONE NETWORKS A backbone network allows several LANs to be connected. In a backbone network, no station is directly connected to the backbone; the stations are part of a LAN, and the backbone connects the LANs. Topics discussed in this section: Bus Backbone Star Backbone Connecting Remote LANs

In a bus backbone, the topology of the backbone is a bus. Note In a bus backbone, the topology of the backbone is a bus.

Figure 15.12 Bus backbone

In a star backbone, the topology of the backbone is a star; Note In a star backbone, the topology of the backbone is a star; the backbone is just one switch.

Figure 15.13 Star backbone