ol) as parts of the payload of cells, to be able to detect missing cells. Figure 6.13 shows how an AAL1 element works. Media resource Continuous bitstream 1 octet 47 octets Segmentation/ reassembling -Cell loss detection 48 octet payloads in cells ATM actor Fig. 6.13 The AAL1 element. OH=overhead parameters in AAL1 messages; UD=media bits This AAL consists of single layer/protocol that takes one of the octets of the 48 payload octets in an ATM cell and uses it for parameters for cell loss detection. Note that no parameters are needed for sequencing received payload since the ATM switching stratum guarantees sequenced delivery of cells. Requirements on AAL2 functions are very different to AAL1 users of AAL2 deliver compressed media stream with a variable bit rate. The VC type used is therefore VBR. Contrary to AAL1, it is extremely important that the bits delivered to the receiving media resource are not corrupted since that would create strange results after encoding. The input to an AAL2 element is also not a continuous bit stream, but frames, i.e., the boundaries of frames must be preserved. To cope with these problems, the AAL2 layer adds a number of overhead parameters that, together, take three octets of the 48 payload octets in an ATM cell (see Fig. 6.14). 6.4 ATM Cell Switching Media resource Frame Frame Frame For each frame: 3 octet 45 octets -Segmentation/reassembling -Cell loss detection -Frame boundary detection -Payload error detection 48 octet payloads in cells ATM actor Fig. 6.14 The AAL2 element. OH=overhead parameters in AAL2 messages; UD=media bits The fact that segmentation and reassembling has to be performed also for ALL2 (and for most conceivable AALs) made the ATM standard committee conclude that this function (generically called SAR) should be defined by an internal "sublayer" in every AAL. The slip in this assumption was only that sometimes segmentation requires no extra overhead, and therefore defines no protocol (as, e.g., for AAL1). Sometimes it does, and therefore defines a protocol (a