Ans :
Having studied the features of bridges and why we require multiple LANs, let’s learn
how they work. Assume that, there are two machines A and B. Both of them are
attached to a different LANs. Machine A is on a wireless LAN (Ethernet IEEE 802.3).
While B is on both LANs are connected to each other through a bridge. Now, A has a
packet to be sent to B. The flow of information at Host A is shown below:
1) The packet at machine A arrives at the LLC sublayer from the application
layer through the transport layer and network layer.
2) LLC Sublayer header get attached to the packet.
3) Then it moves to the MAC Sublayer. The packet gets MAC sublayer header
for attachment.
4) Since the node is part of a wireless LAN, the packet goes to the air using
GRF.
5) The packet is then picked up by the base station. It examines its destination
address and figures that it should be forwarded to the fixed LAN (it is
Ethernet in our case).
6) When the packet arrives at the bridge which connects the wireless LAN and
Ethernet LAN, it starts at the physical layer of the bridge and moves to its
LLC layer. At the MAC sublayer its 802.11 header is removed.
7) The packet arrives at the LLC of a bridge without any 802.11 header.
8) Since the packet has to go to 802.3 LAN, the bridge prepares packets
accordingly.
Note that a bridge connecting k different LANs will have K different MAC
sublayers and k different physical layers. One for each type.
So far we have presented a very simplistic scenario in forwarding a packet from one
LAN to another through a bridge. In this section, we will point out some of the difficulties that one encounters when trying to build a bridge between the various 802 LANs due to focus on the following reasons:
1) Different frame Format : To start with, each of the LANs uses a different
frame format. Unlike the differences between Ethernet, token bus, and token
ring, which were due to history and big corporate egos, here the differences
are to some extent legitimate. For example, the Duration field in 802.11 is
there, due to the MACAW protocol and that makes no sense in Ethernet. As a
result, any copying between different LANs requires reformatting, which
takes CPU time, requires a new checksum calculation, and introduces the
possibility of undetected errors due to bad bits in the bridge’s memory.
2) Different data rates: When forwarding a frame from a fast LAN to a slower
one, the bridge will not be able to get rid of the frames as fast as they come in.
Therefore, it has to be buffered. For example, if a gigabit Ethernet is pouring
bits into an 11-Mbps 802.11 LAN at top speed, the bridge will have to buffer
them, hoping not to run out of memory.
3) Different frame lengths: An obvious problem arises when a long frame must
be forwarded onto a LAN that cannot accept it. This is the most serious
problem. The problem comes when a long frames arrives. An obvious solution
is that the frame must be split but, such a facility is not available at the data
link layer. Therefore the solution is that such frames must be discarded.
Basically, there is no solution to this problem.
4) Security: Both 802.11 and 802.16 support encryption in the data link layer,
but the Ethernet does not do so. This means that the various encryption
services available to the wireless networks are lost when traffic passes over
the Ethernet.
5) Quality of service: The Ethernet has no concept of quality of service, so
traffic from other LANs will lose its quality of service when passing over an
Ethernet.
