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22.3 LOWER LAYERS 729Figure 22.2-3. ATM layers and ATM nodes in the User Plane and in the Control Plane.The CS has to adapt to the characteristics of the sources, so is itself subdivided into aservice-specific CS (SSCS) and a common part CS (CPCS). The only differencebetween AAL and SAAL is that, for SAAL, the SSCS is further subdivided intotwo sublayers: the Service-Specific Connection-Oriented Protocol (SSCOP) [11]and the Service-Specific Coordination Function (SSCF) [12,13].Figure 22.2-3(a) shows the layers of the ATM reference model involved in VCCpayload transfer (bearer traffic in the user plane) in the different types of ATM entities.For signaling (i.e., in the control plane, during connection setup or release), endpointsand intermediate nodes are involved at all layers of the mode (Fig. 22.2-3(b)),for reasons that will become clear in the next sections.22.3 LOWER LAYERS22.3.1 ATM LayerThe ATM layer is responsible for cell formatting. An ATM cell consists of a 5-byteheader and a 48-byte information field, for a total of 53 bytes. The size of the informationfield is the result of a compromise reached at the then CCITT betweenEuropean and North American proposals. Both sides wanted a short cell forspeed and efficiency, but Europeans advocated a 32-byte payload, while NorthAmericans favored a 64-byte payload; so it was agreed to split the difference.The header contains the information necessary for switching cells and the informationfield contains the data payload, which is transferred transparently fromsource to destination. The cell headers for the UNI and the NNI interfaces areslightly different. Figures 22.3-1 and 22.3-2 show the ATM cell formats for eachtype of interface.