 then a new VCC is established for communication. Several other aspects of the virtual channels and paths will be covered in the ATM layer of the B-ISDN ATM protocol reference model. ATM Cell Format ATM transfers information in fixed size units called cells. Each cell consists of 53 octets, or bytes. The first 5 bytes contain cell header information, and the remaining 48 contain the 'payload' (user information). Small fixed length cells are well suited to transferring voice and video traffic because such traffic is intolerant of delays that result from having to wait for a large data packet to download. This eliminates the need to wait for long for packets of some services. Thus services like video and voice queuing delays should virtually be absent, taking advantage of the small fixed size cell technology. Another important factor in favour of small size cells is that they can be switched more efficiently as compared to large packets, thus supplementing the very high data rates of the ATM. The importance of the small size cells can be depicted using the graph shown in Figure 3.28. Of all sizes why was only a 53-byte cell used? Initially two cell sizes of 32 and 64 bytes were put forward. The 32-byte cell had less queuing delays whereas the 64-byte cell had better transmission efficiencies. A compromise, between Europe who wanted a 32-byte cell as the standard and the US and Japan who were in favour of a 64-byte cell, resulted in a 48-byte cell for the ATM, which was extended to a 53-byte cell after the header information was attached to the cell. A small sized header meant that Digital Hierarchies - PDH and SDH Packetizer Digital Pipe Voice Cells ilitii Video Multiplexer Figure 3.29 The multiplexing process of the cells there would be a few provisions and limited functions for multiplexing information and error control. Due to virtual connections the information about cell number and the source-destination address was not needed in the header. The information of different types of services is broken into fixed s