Chapter Objectives After completing this chapter you will be able to: Describe the ATM Adaptation Layer Describe quality of service (QoS) categories Outline the function of the different AAL types and sublayers

Adaptation Layer The Adaptation Layer ATM Layer CS SAR TC Physical Layer Adaptation Layer PMD TC SAR CS Layer two Layer one

QoS Service Catagories CBR Constant Bit Rate VBR-RT Variable Bit Rate - Real Time VBR-NRT Variable Bit Rate - Non-Real Time ABR Available Bit Rate UBR Unspecified Bit Rate GFR Guaranteed Frame Rate (later)

ATM Service Classes Classes as defined by ITU-T rec. I 362 Class A Class B Class C Class D Timing between source and destination Required Not required Bit rate Constant Variable Connection mode Connection-oriented Connectionless AAL 1 AAL 2 AAL 3 AAL 4 AAL 5 Relevant Adaptation Layer

General Principles of Adaptation Layer SAR CS Higher layer data H The use of a CS is not required by all AALs Etc. T

Usage of Adaptation Layer AAL is used to adapt a source application to ATM ATM switching takes place in the ATM Layer. Source Destination Application Application ATM Network AAL AAL ATM ATM ATM ATM PHY PHY PHY PHY UNI NNI UNI

AAL 1 47 bytes 47 bytes 48 bytes 48 bytes 53 bytes

AAL 1 CSI CRC 1 bit 3 bit SN P 3 bit 1 bit 47 bytes 1 byte (8 bits)

AAL 2 Samples User 1 User 2 User 3 Initial AAL2 header 47 bytes Secondary AAL 2 header ATM cell header 48 bytes ATM cell header 53 bytes

AAL 2 47 bytes User 1 User 2 3 bytes (24 bits) CID LI UUI HEC 8 bits SN Offset 6 bits P 1 bit 1 byte (8 bits) 48 bytes

AAL 3/4 44 bytes 44 bytes 44 bytes 48 bytes 48 bytes 48 bytes 53 bytes

AAL 3/4 48 bytes Type Btag BASize Pad Control ETag Len Seg Seq MID CRC

8 byte trailer plus pad added, PDU now multiple of 48 bytes AAL5 (SEAL) Higher Layer Protocol Data Unit (PDU). Possible maximum size of 65535 bytes. 8 byte trailer plus pad added, PDU now multiple of 48 bytes Pad Trailer 48 bytes 48 bytes 48 bytes 48 bytes 48 bytes 48 bytes 53 bytes 53 bytes 53 bytes

8 byte trailer plus pad added, PDU now multiple of 48 bytes AAL5 Trailer 8 byte trailer plus pad added, PDU now multiple of 48 bytes UU Length CRC CPI 8 bytes 4 bytes 2 bytes byte 1 Pad Trailer 53 bytes

AAL5 Transmission AAL5 makes use of the PTI field in ATM cell header Bit 1 = 1 indicates this cell carries the AAL5 trailer 1 48-byte data field VPI VCI CLP HEC GFC PTI User cell O&M cell or EFCI SDU bit