ata, for connectionless data, for compressed video, and SO on: AAL 0-When a customer's network equipment takes care of all the AAL- related functions, the network uses a Null AAL (also known as AAL 0). This means that no services are performed and that cells are transferred transparently between the service interface and the ATM network. AAL 1-AAL 1 is designed to meet the needs of isochronous, constant-bit- rate (CBR) services, such as digital voice and video, and is used to support applications that are sensitive to both cell loss and delay and to emulate conventional leased lines. It requires an additional byte of header informa- tion for sequence numbering, leaving 47 bytes for payload. This adaptation layer corresponds to fractional and full T-1/E-1 and T-3/E-3. AAL 1 provides a timing recovery functional to maintain the bit timing across the ATM net- work and to avoid buffer overflow/underflow at the receiver. AAL 2-AAL 2 is for isochronous variable-bit-rate (VBR) services such as packetized video. It allows ATM cells to be transmitted before the payload is full to accommodate an application's timing requirements. AAL 3/4-AAL 3/4 supports VBR data, such as LAN applications, or bursty connection-oriented traffic, such as error messages. It is designed for traffic that can tolerate delay but not cell loss. This type performs error detection on each cell by using a sophisticated error-checking mechanism that con- sumes 4 bytes of each 48-byte payload. AAL 3/4 allows ATM cells to be multiplexed, and it supports the process of segmentation and reassembly required to carry variable-length frames over the ATM network. It also pro- vides a per-cell CRC to detect transmission errors and a per-frame length check to detect loss of cells in a frame. Packet-Switched Networks AAL 5-AAL 5 is intended to accommodate bursty LAN data traffic with less overhead than AAL 3/4. It is also known as SEAL (simple and efficient adaptation layer). Its major feature is that it uses information in the cell header to identify the first 