1. 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

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

3. 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)

4. 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

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

6. 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

7. AAL 1
47 bytes
47 bytes
48 bytes
48 bytes
53 bytes

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

9. 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

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

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

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

13. 8 byte trailer plus pad added, PDU now multiple of 48 bytes
AAL5 (SEAL)
Higher Layer Protocol Data Unit (PDU). Possible maximum size of 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

14. 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

15. 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