Internetworking Devices

Introduction

HILI subcommittee (IEEE802.1) of the IEEE identified the following possible internetworking 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. These internetworking devices are introduced in this lesson.

Repeaters

A single Ethernet segment can have a maximum length of 500 meters with a maximum of 100 stations (in a cheaper net segment it is 185m). To extend the length of the network, a repeater may be used as shown in Fig. Functionally, a repeater can be considered as two transceivers joined together and connected to two different segments of coaxial cable. The repeater passes the digital signal bit-by-bit in both directions between the two segments. As the signal passes through a repeater, it is amplified and regenerated at the other end. The repeater does not isolate one segment from the other, if there is a collision on one segment, it is regenerated on the other segment. Therefore, the two segments form a single LAN and it is transparent to rest of the system. Ethernet allows five segments to be used in cascade to have a maximum network span of 2.5 km. With reference of the ISO model, a repeater is considered as a level-1 relay as depicted in Fig. It simply repeats, retimes and amplifies the bits it receives. The repeater is merely used to extend the span of a single LAN. 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

· It can be used to create a single extended LAN

                    Repeater connecting two LAN segments

              Operation of a repeater as a level-1 relay

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. Figure 6.1.3 shows how several hubs can be connected in a hierarchical manner to realize a single LAN of bigger size with a large number of nodes.

Hub as a multi-port repeater can be connected in a hierarchical manner to form a single LAN with many nodes