Aal

AAL 3/4 AAL 5

AAL 3/4 AAL 5

Timing relation between source and destination

Required

Required

Not required

Not required

Bit rate

constant

variable

variable

variable

Connection type

connection oriented

connection oriented

connection oriented

connectionless

Applications

voice communications

compressed audio or video

data

data

The CS converts the information stream into four types of packet streams called AAL Type 1, Type 2, Type 3/4 and Type 5. The packet formats should match the five types of ATM services already described. CS tasks are likely to be application dependent. Hence CS is further subdivided into the upper, service specific CS (SSCS) and the lower common part SS (CPCS).

SAR tasks are quite standard, depending only on traffic type. The SAR segments the SDU received from CS and add necessary overhead and convert them into 48 byte cells or protocol data units (PDU).

Types of Adaptation Layers

The types of ATM adaptation layers are AAL type 1 or AAL 1, AAL type 2 or AAL 2, AAL type 3/4 or AAL 3/4 and AAL type 5 or AAL 5. Fig. 11.34 shows the cell format of all these types.

AAL 1. It is designed to carry class A traffic. The field SN (4 bits) is used as cell sequence number to detect any missing cell or disordered cell. The sequence number protection field (SNP) is 4 bit and used for error detection on an SN field using a CRC polynomial X3 + X2 + 1.

The SAR sublayer takes the (periodic) packet stream generated by the CS sublayer, segments it into 47 byte SDU, and prepends to each SDU, a 4 bit sequence number (SN) protected by a 4 bit SNP field.

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