5.4 Line Sharing
Multiplexing is a powerful technique for getting slow devices access to a fast communications link, however, this only works for devices that can easily be connected to the multiplexor, ie: the devices have to be in the same location. What can be done for devices that are in different locations?
The solution is to set up a multidropping system. This means that devices are connected to the fast line along its length. The drawback with multidropping is that more than one device may wish to use the line at the same time. This is a contention issue, ie: all of the devices and terminals are contending for the fast link. There are two ways of overcoming this problem:
- Polling: In this system all communication takes place under a monitoring computer’s control. This computer will poll or contact each terminal in turn at regular intervals to find out if the device wants to transmit. If it does then transmission can take place and if not the computer can continue polling.
- Interrupts: In this system if a device wants to transmit it will send a control message to the controlling computer to state that it is ready to send. This control message interrupts the computer from its processing, alerting it to the devices needs. The controlling computer will then, if appropriate, allow the device to transmit.