AAL - ATM Adaptation Layer

AAL - In order for ATM to support many kinds of services with different
traffic characteristics and system requirements, it is necessary to adapt
the different classes of applications to the ATM layer.  This function is
performed by the AAL, which is service-dependent.  Four types of AAL were
originally recommended by CCITT.  Two of these (3 and 4) have now been
merged into one. 

Briefly the four AALs are:

AAL1 - Supports connection-oriented services that require constant bit rates
and have specific timing and delay requirements.  Example are constant
bit rate services like DS1 or DS3 transport.

AAL2 - Supports connection-oriented services that do not require constant
bit rates.  In other words, variable bit rate applications like some video
schemes. 

AAL3/4 - This AAL is intended for both connectionless and connection
oriented variable bit rate services.  Originally two distinct adaptation
layers AAL3 and 4, they have been merged into a single AAL which name is
AAL3/4 for historical reasons. 

AAL5 - Supports connection-oriented variable bit rate data services.  It
is a substantially lean AAL compaired with AAL3/4 at the expense of error
recovery and built in retransmission.  This tradeoff provides a smaller
bandwidth overhead, simpler processing requirements, and reduced
implementation complexity.  Some organizations have proposed AAL5 for use
with both connection-oriented and connectionless services. 

AAL5 (like any other AAL) is composed of a convergence sublayer (CS) and a
segmentation and reassembly (SAR) sublayer. The CS is further composed of
a common part (CPCS) and a service specific part (SSCS). 

+--------------------+
|       SSCS         |
| ------------------ | CS
|       CPCS         |
+--------------------+
|                    | SAR
+--------------------+

SAR segments higher layer PDUs into 48 byte chunks that are fed into the
ATM layer to generate 53 byte cells. 

CPCS provides services such as padding and CRC checking. It takes an SSCS
PDU, adds padding if needed, and then adds an 8-byte trailer such that the
total length of the resultant PDU is a multiple of 48. The trailer consist
of a 2 bytes reserved, 2 bytes of packet length, and 4 bytes of CRC. 

SSCS is service dependent and may provide services such as assured data
transmission based on retransmissions. (See also SAAL for more on assured
data transmission). 

A recent document which describes these AALs (except AAL2) with frame
formats is: "Asynchronous Transfer Mode (ATM) and ATM Adaptation Layer
(AAL) Protocols Generic Requirements", Bellcore Technical Advisory,
TA-NWT-001113, Issue 1, August 1992.  This can be obtained by writing to: 

Bellcore
Document Registrar
445 South Street - Rm. 2J125
P.O. Box 1910
Morristown, NJ  07962-1910

AAL 5 is described in CCITT document I.363 Temp Doc 10 (XVIII) "AAL Type 5 , 
Draft Recommendation text for section 6 of I.363" 06/93

The ATM Forum is currently working on a sixth AAL for supporting MPEG2
video streams.