Understanding LAN Switching
Switching Technology: Full Duplex
Another concept that we have in LAN switching
that allows us to dramatically improve the scalability, is
something known as full duplex transmission. And that effectively
doubles the amount of bandwidth between nodes.This can be
important, for example, between high bandwidth consumers such
as between a switch and a server connection, for example.
It provides essentially collision free transmissions in the
And what this provides, for example, in 10
megabit per second connections, it effectively provides 10
meg of transmit capacity, and 10 megabit of receive capacity,
for effectively 20 megabits of capacity on a single connection.Likewise,
for a 100 megabit per second connection, we can get effectively
200 megabits per second of throughput
Switching Technology: Two Methods
Another concept that we have in switching
is that we have actually two different modes of switching.
And this is important because it can actually effect the performance
or the latency of the switching through our network.
First of all we have something known as cut
through switching. What cut through switching does, is, as
the traffic flows through the switch, the switch simple reads
the destination MAC address, in other words we find out where
the traffic needs to go through, go to.And as the data flows
through the switch we don't actually look at all of the data.
We simply look at that destination address, and then, as the
name implies, we cut it through to its destination without
continuing to read the rest of the frame.
And that allows to improve performance over
another method known as store and forward. With store and
forward switching, what we do is we actually read, not only
the destination address, but we read the entire frame of data.As
we read that entire frame we then make a decision on where
it needs to go, and send it on it's way. The obvious trade-off
there is, if we're going to read the entire frame it takes
longer to do that.
But the reason that we read the entire frame
is that we can do some error correction, or error detection,
on that frame, that may increase the reliability if we're
having problems with that in a switched network.So cut through
switching is faster, but the trade-off is that we can't do
any error detection in our switched network.