HILI subcommittee (IEEE802.1) of the IEEE identified the following possible internet working scenarios.

• A single LAN
• Two LANs connected together (LAN-LAN)
• A LAN connected to a WAN (LAN-WAN)

• Two LANs connected through a WAN (LAN-WAN-LAN)

  Various internetworking devices such as hubs, bridges, switches, routers and gateways are required to link them together.

i. Repeaters

Repeaters are used to extend the length of the Network. Repeaters were created to regenerate and amplify weak signals, thus extending the length of the network. The basic function of a repeater is to retime, reshape, and reamplify the data signal to its original level.

Important features of a repeater are as follows:

• A repeater connects different segments of a LAN
• A repeater forwards every frame it receives
• A repeater is a regenerator, not an amplifier
• Repeaters operate at the physical layer of the OSI model.

ii. Hubs

• Hub is a generic term, but commonly refers to a multiport repeater. It can be used to create multiple levels of hierarchy of stations.
• The stations connect to the hub with RJ-45 connector having maximum segment length is 100 meters. This type of interconnected set of stations is easy to maintain and diagnose. Hubs operate at the physical layer of the OSI model.

iii. Bridges

• The device that can be used to interconnect two separate LANs is known as a bridge. It is commonly used to connect two similar or dissimilar LANs as shown in Fig. 1.3.
• The bridge operates in layer 2, that is data-link layer and that is why it is called level-2 relay with reference to the OSI model. It links similar or dissimilar LANs, designed to store and forward frames, it is protocol independent and transparent to the end stations. A bridge must contain addressing and routing capability.

• Key features of a bridge are mentioned below:

  a. A bridge operates both in physical and data-link layer

.b. A bridge uses a table for filtering/routing

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

• Types of bridges:

a. Transparent Bridges

b. Source routing bridges

iv. Switches

A switch is essentially a fast bridge having additional sophistication that allows faster processing of frames.

Some of important functionalities are:

• Ports are provided with buffer
• Switch maintains a directory: #address - port#
• Each frame is forwarded after examining the #address and forwarded to the proper port#

• Three possible forwarding approaches: Cut-through, Collision-free and Fully- buffered as briefly explained below.

  a. Cut-through: A switch forwards a frame immediately after receiving the destination address. As a consequence, the switch forwards the frame without collision and error detection.

  b. Collision-free: In this case, the switch forwards the frame after receiving 64 bytes, which allows detection of collision. However, error detection is not possible because switch is yet to receive the entire frame.

  c. Fully buffered: In this case, the switch forwards the frame only after receiving the entire frame. So, the switch can detect both collision and error free frames are forwarded.

v. Router

• Routers link two or more different networks together, such as an Internet Protocol network. These networks can consist of various types of LAN segments, for example, Ethernet, token ring, or Fiber Distributed Data Interface (FDDI).
• A router receives packets and selects the optimum path to forward the packet across the network.
• Routers build a table of all the device addresses (routing table) across the networks.
• Using this table, the router forwards a transmission from the sending station to the receiving station across the best path. Routers operate at the network level of the OSI model.

vi. Gateways

• Gateways are multi-purpose connection devices. They are able to convert the format of data in one computing environment to a format that is usable in another computer environment (for example, AppleTalk and DEC net).
• The term gateway is sometimes used when referring to a router.
• For example, gateways translate different electronic mail protocols and convey email across the Internet.

Gateways Translate Different Network Protocols

Gateways can operate at all layers of the OSI model since them:

• Can provide a physical link between networks.
• Create junctions between dissimilar networks.
• Translate different network protocols and/ or applications (for example, electronic mail between the Internet and a commercial online service with its own mail protocol).