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

The Adaptation Layer

ATM Layer

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 betweensource and destination Required Not required

Bit rate Constant Variable

Connection mode Connection-oriented Connectionless

AAL 1 AAL 2 AAL 3 AAL 4

AAL 5

RelevantAdaptation Layer

General Principles of Adaptation

Adaptation Layer

SAR

CS

Higher layer data

H H

The use of a CS is not required byall AALs Etc.

H T H T H T

Usage of Adaptation Layer

AAL is used to adapt a source application to ATM– ATM switching takes place in the ATM Layer.

AAL 1

AAL 1

AAL 2

AAL 2

AAL 3/4

AAL 3/4

AAL5 (SEAL)

AAL5 Trailer

AAL5 Transmission

AAL5 makes use of the PTI field in ATM cell header– Bit 1 = 1 indicates this cell carries the AAL5 trailer

48-byte data field

VPI

VPI VCI

VCI

VCI CLP

HEC

GFC

PTI