LAN bridges

Limitations of hubs
• single collision domain results in no increase in max throughput
– multi-tier throughput same as single segment throughput
• individual LAN restrictions pose limits on number of nodes in same collision
domain and on total allowed geographical coverage
• cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)
• A network component connecting LAN’s together.
• Operates only in the data link layer, thus is can handle any network protocol used.
• May be used
– to divide the large expensive and hard to manage network into smaller
– split networks that became loaded over time.
– to handle larger distances.
– to block some traffic leaking outside the network.
Link Layer devices: operate on Ethernet frames, examining frame header and selectively
forwarding frame based on its destination
Bridge isolates collision domains since it buffers frames
When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment
and transmit
• Bridge advantages:
– Isolates collision domains resulting in higher total max throughput, and
does not limit the number of nodes nor geographical coverage
– Can connect different type Ethernet since it is a store and forward device
– Transparent: no need for any change to hosts LAN adapters
• bridges filter packets
– same-LAN -segment frames not forwarded onto other LAN segments
• forwarding:
– how to know which LAN segment on which to forward frame?
– looks like a routing problem (more shortly!)
• Reasons for bridges
– Limited number of stations on a LAN segment or ring
– Limited distance for executing CSMA / CD algorithm or distance one
wants a token traveling on a ring
– Limited traffic on a single LAN: available bandwidth must be shared by
all stations
• Interconnecting networks
– Networks connected at the physical layer are connected by a repeater
– Networks connected at the MAC or link layer are connected by bridges
– Networks connected at the network layer are connected by routers
– Higher layer interconnection devices that perhaps execute additional
functions such as protocol conversion are often called gateways
• Bridges
– Devices for gluing together LANs so that packets can be forwarded from

one LAN to the other