and Optimisation Transmission Path Figure 3.26 VCs concatenate to create VPs variable length packets in the X.25. The main reason for the fixed size cells was the reduction in the queuing delay. If the packet size had been variable then a bigger video packet might have kept the smaller voice or data packets waiting in the queue for a longer time than usual. The term 'asynchronous' shows that the multiplexing carried out is not synchronous, as was the case in B-ISDN. There were two reasons for this. Firstly, there could be a variety of applications with different data rates that needed to be switched. Also many data applications like voice and video are bursty in nature and can be handled more efficiently with some sort of packet switching technique. Secondly, the use of the synchronous approach for high speed transmission would complicate the switching system as compared to the N-ISDN, where just the 64-kbps data stream had to be switched. The ATM is a virtual channel technology, i.e. connection is made before the communication com- mences, just like in connection-oriented circuit switching. Thus, although the fixed size cells are trans- mitted, they are carried using the virtual channels. The ATM therefore enjoys the advantages of both the circuit as well as packet switching. Packet switching ensures fast switching of user information and circuit switching makes sure of the cell integrity (ordering of cells at the destination) and the quality of service (QoS). Virtual Channels and Virtual Paths A very important feature provided by the ATM is the combination of both circuit and packet switching. The question arises as to how both types can exist for the same service. It is known that packet switching is done using fixed size packets called cells. The other type of switching, i.e. circuit switching, is accomplished by what is known as virtual channel connections (VCCs) and virtual path connections (VPCs), as shown in Figure 3.26. The concept of these virtual paths (VPs) and channels is taken from virtual circuits/