Networking and Internetworking devices refer to the hardware components used to connect different network resources. Size and complexity of a network may demand internetworking. Simply adding new nodes and cables in the network does not help in expanding the network. Internetworking devices are used to expand the network. In order to connect a LAN to another LAN or a WAN, internetworking devices will be required. Similarly, internetworking devices can be used to link two-LANs via a WAN i.e. for a LAN-WAN-LAN internetwork. This allows a greater number of nodes to be connected in a network that can be expanded and extended over a large distance. On the other hand, internetworking devices help localize the traffic on the network which makes fault detection easier, faster and simpler. Therefore, in a network experiencing heavy traffic, internetworking devices can be used to break a LAN into segments and manage network traffic efficiently. Also, they can be used for security among different types of users. In this section, we will discuss various internetworking devices in detail and give a brief insight into which, when, where and why the internetworking device should be used. Different internetworking devices in their increasing order of power and complexity are:
- Repeaters (Bus Topology)
- Hubs (star Topology)
- Bridges (Bus topology)
- Switches (Star Topology)
- Routers
Repeaters and Hubs are Layer 1 devices. Bridges and Switches are Layer 2 devices. Routers are Layer 3 devices. Therefore, within a LAN, different network segments are connected using repeaters and hubs. In order to connect segments on two different networks i.e. for a LAN to LAN connection, bridges and switches are used. Routers are used to link a LAN with WAN. Table 4 shows Internetworking devices and their corresponding OSI Layers.
Repeaters
Repeaters are the internetworking devices used at the OSI layer 1, i.e. physical layer. Repeaters are used to connect different segments in a LAN and pass data as bits in both directions of the network segment. It thus helps to expand a network. As discussed earlier, in 10Base-5 thicknet, a single network segment can have a maximum length of 500 meters whereas in 10base-2 thinnet, the maximum length of a single network segment is limited to 185 meters. To increase the length of the network, a repeater may be used to connect network segments. An Ethernet network can span up to 2.5 km as Ethernet allows cascading of five network segments. However, as the length of the network segment increases, the signal weakens and deteriorates. Repeaters amplify and regenerate the signal while passing the signal to the other end. Also, repeaters do not separate one network segment from another Therefore, collision at one segment is regenerated and transmitted to the other network segment. All the devices connected to the repeater have the same collision domain and the broadcast domain.
Hub
A hub is an internetworking device with multiple ports. Multi-port
repeaters are better known as a Hub. It simply passes data received in the bit
format to all other ports. Multiple levels of the hierarchy of nodes can be
created by using the hub to realize a larger single LAN with a greater number
of nodes. RJ-45 is used to connect a node to the hub.
Hubs do not have the brain; they do not apply any logic nor do they filter network traffic. Therefore, data received by a hub is passed to all other network segments connected to the hub irrespective of whether the data is required or not. Implicitly it adds unnecessary traffic onto the network. Similar to the repeater, the collision in one segment is passed on to all other network segments by the hub. Devices connected through a hub are in the same collision domain and broadcast domain. A hub cannot connect unlike segments. Also, network segments working on different access methods cannot be joined using a hub.
Bridge
The bridge is an OSI Layer-2 device operating at the MAC sub layer of Data Link layer. It is used to connect two different LANs. The two LANs can be similar or dissimilar. It is protocol independent and is designed to store and forward frames. The bridge has the intelligence to isolate network traffic. It has the ability to examine source and destination addresses of the incoming packet and filter traffic based on MAC address. Also, a bridge has addressing and routing capability. Thus, a large geographic area can be connected using bridges. Each port has a different collision domain but the same broadcast domain. It provides high reliability and performance. But a bridge can match the QoS requirement of only one LAN. Availability, frame size and priority, frame lifetime, frame mishaps, transit delay, undetected bit errors are all examples of QoS parameters. Let us understand how a bridge works.
Switches
LAN Switch is an OSI Layer-2 device working at Data Link Layer. A switch allows a network to be segmented into smaller isolated LANs, i.e. different collision domains. This is because switches have intelligence. They can read source MAC address and destination MAC address. They forward the frame to a port only if it is needed to go there. Thus, a port receives only those frames that are intended for the device connected to it and it does not receive any frame addressed to devices connected to other ports. This, thus, helps in reducing the traffic experienced by a device. But, if the frames contain a broadcast address (frames intended for all) or multicast address (frames intended for a group), switch forwards it to all ports except from the one which it was received.
Devices connected to a port are all in the same collision domain
whereas devices connected to different ports are in different collision
domains. But all the devices connected to a switch, even though on different
ports, are in a single broadcast domain.
Ports on switches have a buffer. A collision can occur if two
devices try to send frames on a segment at the same time. This can be avoided
by buffering the frame of one of them for later access.
Let us have a look at how different is a switch from a hub:
- In a physical star topology, hubs work like logical bus whereas switches work like a logical star.
- All ports of a hub are in the same collision domain whereas all ports of a switch are in different collision domains.
- All ports share the bandwidth in the hub whereas each port has dedicated bandwidth in the switch.
- Ports in the hub do not have buffer whereas, in the switch, each port has a buffer.
- Even though logically equivalent, there are some-significant differences between bridges and switches:
- Switches are faster than bridges.
- Bridges cannot connect unlike bandwidth whereas switches can support more than one type of media. For example, two LANs viz. 10Mbps and 100Mbps Ethernet LANs, can be connected using switch but not bridge.
- Generally, switches have more ports than bridges.
Routers
A router is an OSL Layer 3 device working on Network Layer.
Different networks can be connected using a router. Devices on different LANs
can communicate with each other through router even though they may be
physically over a long distance. Routers have more intelligence than a hub and
a switch. Routers have the ability to forward packets to the best path on the
network for delivery to the destination. They can listen to a network to
identify whether the network is busy and transmit the data through the most
cost-effective path. The IP address of the destination is used along with the
routing tables in order to determine the best path. Hence, packets are
forwarded only where they need to go.
Broadcast messages and multicast messages are blocked by routers. They only support unicast messages. The collision domain is the same for all the devices connected through a single router port. The collision domain is different for devices connected through different router ports. Similarly, the broadcast domain is the same for all the devices connected through one router port. The broadcast domain is different for devices connected through different router ports.
Broadcast might be required by some protocols, for say, routing
protocol advertisement. But such broadcast is required only on local LAN and
not on other LANs. Moreover, for the advertisement of routing protocols,
routers can generate and broadcast packets, if necessary, but can avoid
broadcasting received packets. This helps in reducing traffic on the network.
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