VoIP Voice and Fax Signal Processing

18.1 ATM CELLS AND TRANSMISSION

On a DSL interface, IP packets are sent as asynchronous transfer mode (ATM) cells [Black (1999)]. ATM uses smaller byte cells to reduce delay variation while multiplexing multiple streams. Each ATM cell is of fixed 53 bytes consisting of a 5-byte mandatory header and maximum 48-byte payload. An ATM 5-byte header carries information for end-to-end ATM packet delivery. Application adaption layer 5 (AAL5) accepts transmitted data packets and maps them to ATM cells. The AAL5 process consists of protocol data unit (PDU), segmentation and reassembly (SAR), and 8 bytes of an AAL5 trailer. The trailer consists of 2 bytes for length, 4 bytes for cyclic redundancy check (CRC), 1 byte for user-to-user (UU), and 1 byte for common part indicator (CPI). To incorporate trailer bytes, the last cell of the total bytes will have a maximum of 40 bytes of payload and 8 bytes of trailer bytes. In many applications, packet sizes are much larger than 40 bytes. On an ATM network, packet sizes of 65,535 bytes can be sent [Gross et al. (1998)]. In the process of fragmentation and ATM cell creation, every original IP packet is appended with interface-specific headers, an 8-byte trailer, and zero padding to create an integer number of ATM cells. The number of ATM cells = the smallest integer of cells that satisfies the condition, AAL5 PDU bytes + zero padded bytes + 8 bytes AAL5 trailer = (smallest integer) (48).

As an example, an IP packet of size 87 bytes after adding an ...