5 prepares a cell for transmission in three steps. Firstly, the CS appends a variable length pad and an 8-byte trailer to a frame. The pad ensures that the resulting PDU falls on the 48-byte boundary of an ATM cell. The trailer includes the length of the frame and a 32-bit cyclic redundancy check (CRC) computed across the entire PDU. This allows the AAL5 receiving process to detect bit errors, lost cells or cells that are out of sequence. Secondly, the SAR sublayer segments the CS-PDU into 48-byte blocks. A header and trailer are not added (as is in AAL3/4), SO messages cannot be interleaved. Finally, the ATM layer places each block into the payload field of an ATM cell. For all cells except the last, a bit in the payload type (PT) field is set to zero to indicate that the cell is not the last cell in a series that represents a single frame. For the last cell, the bit in the PT field is set to one. Performance of the ATM (QoS Parameters) The following are classes defined by the ATM forum: Constant bit rate (CBR). Under this class, the user traffic is continuous and steady. Supported by AAL1, this class has tight delay and delay variation bounds. Voice and video conferencing are typical examples of CBR traffic. Real-time variable bit rate (rt-VBR). Supported by AAL3/4 and AAL5, the nature of traffic is bursty and insensitive to smaller delay variations. Non-real-time variable bit rate (nrt-VBR). This is similar to rt-VBR, except that the traffic is non- real time. This class is used for frame relay interworking. Both rt-VBR and nrt-VBR have a variable bandwidth. Unspecified bit rate (UBR). This is the best effort transmission. The user may request a maximum traffic rate, but the network provides the best rate possible. As in the previous two cases, AAL3/4 and AAL5 support this type of traffic. Available bit rate (ABR). Under this, there is no guarantee of bandwidth. Also there is no timing relationship between the source and destination. Sustainable cell rate (SCR). This is the upper bound on the average rate of t