FP Web-Server V2.11

15.5 Setup of an Individual Ethernet LAN

 

 

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).

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