RSS
Twitter
Facebook
The purpose of the ATM Adaptation
Layer (AAL) is to adapt the PDUs passed down from the higher layer onto ATM
cells. As the higher level PDUs may in general be of an arbitrary size, so one
of the two sublayers in the AAL is responsible for segmentation and
reassembly (SAR) of the higher layer PDUs. The other sublayer, the convergence
sublayer (CS), is responsible for packaging the higher layer PDU with any
additional information required for the adaptation necessary and offering an
interface to the B-ISDN user. As mentioned previously, there are many and
varied requirements for applications using the B-ISDN and as there are several
AAL protocols defined. The classification of these protocols was guided by
considering the following parameters:
· Timing
relationship. The
(non-)requirement for synchronisation between the
receiver and sender.
· Bit rate. Constant or
variable.
· Connection mode.
Connection
oriented or connectionless.
A summary of the (sensible)
combinations of these parameters is given in Figure . Each of these classes may
be looseley associated with the the ATM Forum classes described previously:
Class A, CBR; Class B, VBR; Class C, ABR(ish); Class D, UBR. This mapping also
give example use of the various classes. The various AAL classes are provided
by several AAL protocols identified by a type number. The use of a particular AAL
type for the provision of a particular service is not wholy well defined.
Figure: AAL service
classification
AAL Type 0. This is
effectively a NULL AAL. It is not really an official AAL type but is mentioned
for completeness.
AAL Type 1. This AAL type is
normally used by Class A (CBR) services. The function performed by this AAL
are:
· Segmentation and
reassembly of user information.
· Handling of cell
delay variation (jitter).
· Handling of cell
reassembly variation.
· Handling of lost
and misinserted cells.
· Source clock
frequency recovery at receiver.
· Source data
structure recovery at receiver.
· Monitoring and
handling of AAL-PCI bit errors.
· Monitoring and
(possibly) correcting the bit errors in the user information field.
· For circuit
emulation, monitoring and maintenance of end-to-end QoS.
AAL Type 2. This AAL type
would be used with Class B (VBR). This type is not well defined and it seems
possible that the it may be merged with AAL Type 1 in the future. Some of its
functions are similar to ALL Type 1:
· Segmentation and
reassembly of user information.
· Handling of cell
delay variation (jitter).
· Handling of lost
and misinserted cells.
· Source clock
frequency recovery at receiver.
· Monitoring and
handling of AAL-PCI bit errors.
· Monitoring and
(possibly) correcting the bit errors in the user information field.
It also has the additional
functions:
· Handle SDUs from
a variable bit rate source.
· Transfer timing
information between source and destination.
· Notify the
higher layers of uncorrectable errors in AAL.
AAL Type 3/4. There was once
separate Type 3 and Type 4 AALs, but they have now been merged. This AAL is now
intended to support both Class C (ABR) and Class D (UBR) services.
In this AAL, the convergence
sublayer is split into two (Figure ), the service specific convergence
subslayer (SSCS) and the common part convergence sublayer (CPCS). The
SSCS is application dependent, i.e. it could be for a VBR video application.
The CPCS is responsible for constructing PDUs that can be sent to the other end
user. There are two modes of operation of ALL Type 3/4; message mode and
streaming mode.
Figure: AAL Type 3/4
sub-layering
The message mode is intended for
use framed data where the AAL-SDU is a logical unit of data with respect to the
B-ISDN user (Figure ). It allows the the transport of a single AAL-SDU in one
or (optionally) more than one CS-PDU. The CS-PDU may be then further spilt into
several SAR-PDUs. The AAL-SDU can be of an arbitrary size.
Figure: AAL Type 3/4
message mode service
In streaming mode, the AAL-SDUs
are of fixed size and one or more of them may be transported in a single CS-PDU
(Figure ). Each AAL-SDU is delivered in a separate SAR-PDU.
Figure: AAL Type 3/4
streaming mode service
In both cases, the SAR sublayer
provides error detection and both these modes can offer the following operational
procedures:
· Assured
operation. Flow
control and retransmission of missing or erroneous
AAL-SDUs. Flow control restricted
to point-to-point connections at the ATM
layer and point-to-multipoint
flow control possible.
· Non-assured
operation. No
retransmission of missing or erroneous SAR-PDUs.
Optionally deliver erroneous PDUs
to user. Allow flow control for point-to-point
connections but not
point-to-multipoint.
This AAL type also provides
multiplexing at the SAR sublayer. AAL Type 5.
This AAL type provides similar
services as AAL Type 3/4, but has a reduced overhead when compared to AAL Type
3/4. It is intended for use by VBR sources with timing relationship between
source and destination. It has identical modes and operational procedures as
AAL Type 3/4. The difference is that this AAL does not provide the AAL Type 3/4
multiplexing capability. The SAR sublayer accepts only AAL-SDUs that are an integer
multiple of 48 octets. So, it would be possible for this AAL to offer an
efficient cell based interface to the B-ISDN user.
AAL Types 1 and Type 2 are not
used much.
AAL Type 3/4 has been chosen for
the provision of the Bellcore switched multimegabit data service (SMDS),
the European version of which is the connectionless broadband data service
(CBDS).
AAL Type 5 has been selected by
the IETF to provide IP services over ATM.
0 comments