Notes on Interconnecting Networks


There is a limit to the maximum length of a LAN.

            Ethernet cables (segments) can be up to 500m.

            Restrictions due to power and propagation delay


Methods of Interconnecting Networks


Repeaters       Physical layer

            copy individual bits between cable segments.

            Every bit is visible on all segments, including collisions.

            Invisible to computers.

            Ethernet can be extended to 1500m and no more than 4 repeaters between hosts.


Bridges           Data Link level

            store and forward frames between LANs.

            Bridges receive a packet and then transmit it on the other side.

            Collisions are not seen on the other side.

            Bridges only forward frames that need to go to the other side (Frame Filtering).

            Bridges learn the location of hosts.

            Each segment is shielded from local traffic on the other segments (reduces collisions).



Routers           Network level

            store and forward frames between dissimilar networks


Gateways        higher levels

            protocol converters



Cycles of bridges have to be avoided.

            A packet could circulate forever.

            Sometimes it is advantageous to have extra bridges for robustness and performance.

            When a Bridge starts it communicates with other bridges to learn the configuration.

            Bridges form a Distributed Spanning Tree to determine how frames will be forwarded.


Long Distance Bridging can be done by connecting bridges.

Bridges (designed for this purpose) can be connected by a point to point connection (fiber optic line, leased phone line or satellite).