Mobile Data Networks

The message receiver (addressee) responds to the call packet reception with transmitting the so-called confirmation packet that travels via the same nodes but in the opposite direction towards the sender. A circuit created this way virtually looks to an external observer like a real physical circuit even if it involves only the file of commands in „control computers“ of individual packet nodes. No other data or service packets contain full addresses and they get transferred via this so-called virtual circuit. After the successful transfer of all packets in a given message, such commutated virtual circuit gets automatically abolished. However this service enables to create fixed virtual circuits too, that keep the set-up and wait for another message transfer between these two subscribers (see the analogy with a fixed data circuit, e.g. in the telephone network. The virtual circuit service gives a smaller individual packet redundancy, the more efficient control during the data message transfer, but it is more sensitive for transfer parameter changes of transfer chain individual parts during the whole communication process. This service is more favourable principally at stable transmission parameters for longer data message transfers. One can say in general, that the majority of telecommunication systems, allowing packet communication, uses so far the service with link (fixed data networks packet switch-over, integrated services digital networks and others).

Few basic methods are used for packet routing:

• Random routing method is based upon the assumption, that the network node routes a received packet into quite randomly picked direction (excluding incoming direction, as long as into this node is not connected up already to a connection line to addressee). A shortcoming of this method is a fact that the optimum packet delivery time is not guaranteed. An advantage is the system simplicity and no chance of loosing the packet by sending it to a blocked direction.

• Fixed (static) routing method is based on establishing the shortest route algorithm, which is in every network node expressed by a fixed – routing table according to which the packets are routed. In case the specified direction is clogged, the packet gets lost.

• Adaptive routing methods are based again on establishment of routing tables in individual nodes, whose contents is however changing flexibly upon other criteria, that most often is the shortest packet delivery time.

• Node adaptive method is based admittedly on routing tables, but gives a possibility to alter them on the basis of certain local information, most often when the nominal direction is clogged.

• Centralised adaptive method is the most efficient one from the point of packet routing, but also the most complex one, it is based on the operational information exchange between the central supervising and control point of the packet network and individual nodes.