Subject: Advanced Network Technologies

Topic: WAN Technologies

Difficulty: High

AAL5 is the adaptation layer for UNI signaling, ILMI, PNNI signaling, and for IP PDUs. It is the most widely used AAL type to date. AAL5 requires two distinct processes. The first is the encapsulation, on the transmit side, and de-encapsulation, on the receive side, of the higher layer PDU into the AAL5 CPCS PDU which requires the computation of the length and the CRC-32. The time required for this process depends on whether the CRC-32 computation is done on the interface (on-board) or in machine central memory (in core). On-board computation should produce only a small, constant delay; however, in core computation will produce variable delay, which will negatively effect TCP RTT computations. The second process is segmentation and re-assembly.

AAL5 provides the following options:

Reliable service with guaranteed delivery with flow control

No guaranteed delivery

Unicast. Sending messages between single ends.

Multicast. Sending messages to multiple destinations, where there is no guaranteed delivery.

Message/stream mode. A possible drawback is that there is no distinction between data and control messages.

A PDU is accepted by The Convergence Sublayer which itself is divided into Common Part (CPCS) and Service Specific Part (SSCS). The SSCS processes the PDU if necessary which is then passed to CPCS which attaches pads of 0-47 bytes and 1-byte trailer to produce a CPCS PDU that is a multiple of 48 bytes. SAR sublayer divides the received PDU into blocks of 48 bytes which is passed to ATM layer which adds its own header and cells are formed. The SAR layer also directs the ATM layer to set the PTI field in the header of the last cell of CPCS PDU. This allows boundary between groups of cells corresponding to different messages to be distinguished at the destination. These cells are transmitted over the network.

Maximum length allowed for CPCS PDU is of 65,535 bytes. In AAL 5 header is not added by CPCS while in other AAL services header is added.

Fig shows CPCS PDU for AAL5. The trailer contains 1 byte of UU which is User to User identifier. This byte is passed transparently between end system entities.

There is Common Part Identifier (CPI) which aligns the trailer to 8 bytes

There is two-byte LI (Length Indicator) which specifies the number of bytes of user data in the CPCS PDU payload. 4 byte of CRC checks to detect the errors in the PDU.