he phase relation using pointers, TU-11, TU-12, TU-2 and TU-3 being the lower level and AU-3 and AU-4 being the higher level. Higher Order SDH Multiplexing With the channel capacity increasing multifold every year, the SDH system can multiplex signals up to 40 Gbps (STM-256) if the need arises. This is done by taking the lower order STM signals and multiplexing them in the same way. Interleaving the individual STM-1 generates the STM-n multiplex signal frames byte by byte, as shown in Figure 3.25. The STM-1 frames are numbered in the sequence in which they appear in the STM-n frame. The third STM-1 frame (STM-1#3) starts, for example, in the third column of the STM-n frame. The de-multiplexing procedure (disassembling the multiplex signal into the individual STM-1 frames, TUG-3, TUG-2) is performed in the same way, but in the opposite order. Digital Hierarchies - PDH and SDH STM-N 140 OMbit/s TUG-3 34Mbit/s TUG-2 2Mbit/s Figure 3.24 Synchronous multiplex structure 3.3.3 Asynchronous Transfer Mode (ATM) ATM is a high speed network protocol for fast cell relay/transmission where different services like voice, video and data are transmitted in a special fixed size cell format. It is a dedicated connection-oriented switching and multiplexing technology which forms fixed size cells of 53 octets and transmits them over the physical medium using a digital signal technology (PCM, etc.) or multiplexing hierarchies like PDH and SDH. Speeds on ATM networks can reach up to 10 Gbps and such networks are sometimes also known as 'fast packet' networks. ATM is also used as another multiplexing technique like the PDH and SDH, but the major difference lies in the way their layers are implemented. The ATM systems mostly work in the data link layer as compared to the PDH and SDH, which are implemented in the physical layer. More will become known about the ATM layers when the broadband (B)-ISDN ATM protocol reference model is discussed. ISDN and B-ISDN With the exponential growing demands for different services at good enough bandwi