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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)
Bridges
• 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
LAN’s.
– 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
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