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FP Web-Server V2.11
15.5 Setup of an Individual Ethernet LAN
169
Hub or switch?:
If the IP address (and/or the data rate 10/ 100 MBPS) are not modified very often, a switch might
increase the performance of the network in comparison to a hub:
A switch is an intelligent unit with which you can subdivide the Ethernet in sub-segments that
are widely independent from each other. You can imagine it as a kind of switch matrix. If a piece
of information should be transferred to another segment, the switch automatically establishes
the respective connection. The other segments are not concerned by this data transfer and can
be run in parallel operation mode. With the help of switches you can cut the likelihood of
collisions drastically or even eliminate them totally (e.g. in the extreme case of a "totally
switched" network).
Unfortunately, the case that several stations want to send a message to the same recipient
cannot be defused by a switch. It has to be mentioned though that this special case cannot be
controlled with the conventional, deterministic bus system either.
A switch receives a data package on one side. Based on the destination address, the unit
decides via which output bus the message should be forwarded. You have to differentiate
between switches that receive whole messages at once, analyze them and forward them
afterwards ("Store and forward"), and other switches that are toggle controlled by the hardware
as soon as the recipient's address is defined ("Cut through"). Of course, the latter ones are
somewhat faster.
The price for this ideal way to avoid collisions has two aspects to consider:
First, switches are not cheap. There is a lot of processing effort behind them. Second, the
effective wiring of the bus system is abandoned again and one returns to the traditional
pier-to-pier connection. Consequently, the wiring effort is increased significantly.
For generalities on TCP/ IP also refer to IP and TCP/ IP (see page
166).