Structure and Operation
of ATM Adaptation Layer
Abdul Wahab
Faculty of Computer Science
Griffith College Dublin
Abstract
This paper
is based on the concept of ATM Adaptation layer (AAL). The AAL connects the
higher layer (e.g. packet networks) to the ATM layer. AAL receive information
from higher layer services (packet data) and convert that information into
fixed sized ATM cells. This information can be of any type like voice, data,
audio or video and can be either of variable rate or fixed rate. On the receiving
side the AAL reassemble the ATM cells into their formats and passed onto the
receiver. The AAL is categorized into four layers.
Motivation
ATM is a cell relay protocols which is
designed by ATM Forum and International Telecommunication Union and adopted by International
Telecommunication Union -Telecommunication Standardization Sector (ITU-T). The
combination of ATM and B-ISDN will allow high speed interconnection of all the
networks. And the ATM can be thought be
thought as highway of the information superhighway [1].
And the use of ATM
technology and services creates the needs for an Adaptation Layer to support
the information from higher layers which are not based on ATM layer [2].
Introduction
Before the description of AAL I want to briefly
describe that what is ATM and what are the motives for designing ATM
Technology.
ATM stands for Asynchronous Transfer Mode.
This is an electrical digital data transmission technology. ATM was developed
in 1980s and implemented as network protocols. The base for designing the ATM
was to transport real-time video conference as well as audio and image files.
The ATM standards were created by ATM Forum and International Telecommunication
Union [4].
There are number of challenges faced by the
ATM designers [1].
·
First one is the need of such a transmission system
that can use the high data-rate media such as Fiber Optic.
·
Second one is to design a system that could
communicate with already existed systems like various packet networks.
·
Third one is the designing of such system which does
not cost too much.
·
Fourth one is that the new system must be able to
work and support the existing hierarchies.
·
And the last but not least is the speed of
transmission by moving most of the functions to hardware and avoid as much software
functions as possible.
ATM system uses cell as the basic unit of
data exchange (Cell is small fixed-sized
block of information) [1]. Cell is 53 bytes long with 5 bytes are allocated
to header and 48 bytes is payload. As Cell is small than size from the packet
that’s why ATM could transport real-time video conference. Let explain this by
a figure from [1].
In figure.01 shows a
multiplexer with two lines sending cells. Let packet X is segmented into three
cells X, Y, Z. Now first cell from the first line gets put on the link and then
first cell of the second line gets put and so on that no one gets suffer from
long delay. So now if the same scenario we use on high speed data link, then
cells will reach their respective destinations in an approximation of
continuous stream. And in this way the real-time transmission can be handled
[1].
Figure.02 ATM protocol
model.
Figure.02 shows the ATM Protocol Model [1,
6]. The upper layers represent the video standards and could be IP or Frame
Relay. The next is ATM Adaptation layer. The AAL is sandwiched between the ATM
layer and the Upper layers, as shown in Figure.02.
Figure.03 [1] shows that AAL does not use in
switches. It is use only at the end points i.e. at sending and the receiving
end point. At the send point it receives packet data from upper layer and
convert them into fixed-sized cell for transmission and at the receiving end
AAL again convert the cells into its original format [1].
Structure
and Operation of AAL
AAL is further divided into two sublayers,
the Convergence Sublayer (CS) and the Segmentation And Reassembly
sublayer (SAR).
Convergence Sublayer (CS)
The
CS provides service-specific functions, and this layer is further subdivided
into the Service Specific Convergence Sublayer (SSCS) and the Common
Part Sublayer (CPS) [2].
Segmentation And Reassembly Sublayer
(SAR)
SAR fragments and reassembles the packets so
as to allow them to be transported across ATM networks. The incoming packet
from another protocol to be transmitted across the ATM network is chopped up
into segments for to be fitted into 48-byte ATM cell payloads, the segment
could be less than 48 byte. At the far end, these chunks are fitted back
together to the original packet [4].
On the basis of services provided by AAL to upper
layer applications AAL were classified into four different classes, A, B, C and
D. This classification scheme was based on the following three attributes:
1st the need to transmit timing information
between source and destination, 2nd either the transmission is
constant bit rate or variable bit rate and 3rd either it is
connection-oriented or connectionless. Table.01 summarizes the values of these three
attributes for each class [2].
Attributes
|
Class A
|
Class B
|
Class C
|
Class D
|
Timing
between Source & Destination
|
Yes
|
Yes
|
No
|
No
|
Bit Rate
|
Constant
|
Variable
|
Variable
|
variable
|
Connection
mode
|
CO
|
CO
|
CO
|
CL
|
Table.01 Services
Classes
Class A
- services are constant-bit-rate and connection-oriented, and involve a strict
timing relationship between the two ends of the connection. AAL1 used to
support this class service. Examples of this service include 64 Kbit/sec voice,
fixed-rate uncompressed video and leased lines for private data networks.
Class B
- services are also Connection-Oriented with a bounded time relationship between
the two ends, but have a variable bit rate. The development of variable bit
rate services is still in its infancy, but typically they involve coding of
voice or video using compression algorithms that try to maintain the quality
associated with the peak bit rate of the information while exploiting the fact
that for much of the time the actual information rate is a lot less than the
peak rate. AAL2 support this class. Examples of this service include compressed
voice or video. The bounded delay for delivery is necessary for the receiver to
uncompressed voice or video.
Class C
- services are Connection-oriented and has variable bit rate and does not
require bounded delay for delivery. Two AAL protocols support this service
class, and have been merged into a single type, and that’s why are called
AAL3/4. But with its high complexity, the AAL5 protocol is used to support this
class.
Class D
- Connectionless data service: Examples of this service include datagram
traffic and generally, those data network applications where no connection set
up is required before data is transferred. Either AAL3/4 or AAL5 can be used to
support this class [7, 8].
Types of AAL
There are four types of
AAL.
AAL1 à Type 1
AAL2 à Type 2
AAL3 & AAL4 (AAL3/4)à Type 3/4
AAL5 à Type 5
All these types were defined
to support service classes A, B, C and D which were discussed previously and
shown in Table.02 below. There was a lot of commonality between AAL 3 and AAL 4,
and it was eventually decided to combine them into a single AAL which are referred
to as 3/4. It was realised that a lot of
the services in classes C and D did not need the complexity and associated
overheads of AAL3/4 and a simple and efficient adaptation layer was developed
to support them. Originally known as SEAL, it has now been standardised as AAL5
[8]
AAL
|
Services
Class
|
Type 1
|
A
|
Type 2
|
B
|
Type 3/4
|
C and D
|
Type 5
|
C and D
|
Table.02 AALs and
Services Class
AAL Type 1
This type supports
three specific services, 1st circuit transport, 2nd is
video signal transport, and 3rd is voice-band signal transport. The functions
of segmentation and reassembly sublayer are the same for all three, but there
are differences in the functions of convergence sublayer. AAL1 is in principle
also applicable to high-quality audio signal transport, but specific provisions
for this have not yet been standardised [8].
Circuit transport can
carry both asynchronous and synchronous signals.
Video signal transport
supports the transmission of constant bit rate video signals for both
interactive services and distribution services, the convergence sublayer (CS)
functions for the interactive and distributive services are not same because
one less tolerant to delay and the other is more tolerant to delay [8].
Operation of AAL1
Convergence sublayer
(CS) in the send direction accumulates 47 octets of user information which is
passed to the Segmentation and Reassembly (SAR) sublayer together with a 4-bit
sequence number (SN) consisting of a 3-bit sequence count (SC) and a 1-bit Convergence
Sublayer Indication (CSI). If the constant bit rate signal has a framing
structure (like the 8 kHz structure on an ISDN circuit-mode bearer service) the
frame boundaries to the remote peer CS indicates by the CS sublayer by inserting
a 1-Octet Pointer as the first octet of
the payload, and the CSI-bit indicate to the far end that this pointer is
present. It reduces the user information payload of this segment to 46 Octets. The
CSI bit can carry clock recovery information [1, 8].
Figure.04 [1] shown
that CS receive the bit stream from the upper layer and divides that stream
into 47 byte segments and then passes them to SAR.
SAR layer accepts 47
bytes payload from the CS and add 1 byte header to it.
Convergence Sublayer
(CSI) is 1 bit field of the header, used for signalling purpose that is not yet
clearly defined.
Sequence Counter (SC)
is 3 bit field. Which is a modulo 8 sequence number and used for ordering and
identifying the cells for errors and flow control.
Cyclic Redundancy Check
(CRC) is a 3 bits field which is calculated over the 1st four bits using
4 bits divisor. They not only detect single or multiple bits errors but also
correct single bit error.
Parity (P) is 1 bit
field which is calculated over the first 7 bits of the header. Parity bit
detects only odd number of errors but does not detect even number of errors.[1,8]
AAL Type 2
AAL2 supports variable
bit rate services, is still being developed, and has not yet been explicitly
defined.[1, 8]
Operation of AAL2
Figure.05 shows the
format of an AAL2.
Convergence Sublayer
(CS), the format for reordering the bit stream and adding overhead is not
defined and different applications may use different formats.
Segmentation and
Reassembly (SAR) layer accepts 45 byte payload form the CS and then add 1 byte
Header and 2 byte Trailer which is resulted
in 48-byte data unit which is then passed into the ATM layer. The
overhead consists of 3 fields Header and 2 fields Trailer.
One bit CSI will be
used for signally which is not defined yet.
Sequence Counter (SC) is
3 bit field. This is modulo 8 sequence number and used for ordering and
identifying the cells for errors and flow control.
Information Type (IT)
is a 4 bits field identify the position of the data segment, whether at the
beginning, middle or end of the message.
Length Indicator (LI)
is a 6 bits field of the Trailer, which is used with the final segment of the
message to identify the length of data and padding in the last cell.
CRC is a 10 bits field
of the Trailer, which is for the entire data and also used for single bit
correction.[1,8]
AAL Type 3/4
Initially AAL3
supported connection-oriented services and AAL3 supported connection-less
services. But as they were developed it became clear that the fundamental
issues for both were the same. And that was the reason that both of them were
combined into a single type and were called AAL3/4 [1].
Operation of AAL3/4
CS accepts data from the upper layer of maximum size 65.536 bytes and adds a header and a trailer. This header and trailer tells the start and end of the and also tells that how much of the last frame is data and how much is padding. When the header, trailer and padding are added the CS then pass in a 44 byte segment to SAR. CS adds header and trailer to the beginning and end of the original packet not to the middle and the middle segments are passed onto the SAR without the header and trailer. The header and trailer fields are as follow.
Type (T) is 1 byte
field of the header which is taken the previous AAL3 and is set to 0.
Begin Tag (BT) is also
1 byte field of the header acting as the beginning flag. This identifies the
1st cell of the segment and also synchronizes the receiving clock.
Buffer Allocation (BA)
is a 2 byte field of the header, tells the receiver the size of buffer needed.
Pad (PAD) padding is
added to the final cell or cells when needed. The size of padding varies and is
between 0 and 43 bytes. The following are three possible cases of padding:
1.
When the final segment is exactly of 40 bytes then
padding is required. Only header and trailer are added to it and passed on as
44 byte segment to SAR.
2.
When the final segment is less than (0-39) bytes
then add padding 40-1 bytes to bring the total to 40 bytes.
3.
When the final segment is 41-44 bytes that padding
bytes are 43-40 to bring to the total of 84 bytes. In which the 1st
44 bytes make a complete segment and the rest 40 bytes with header and trailer
makes total of 44 bytes.
Alignment (Al) is 1
byte field of the trailer which makes the rest of the trailer 4 bytes long.
Ending Tag (ET) is 1
byte field acting as a ending flag for synchronization.
Length (L) is 2 bytes
field tells the length of the data unit.
Segmentation and
reassembly (SAR) accepts a 44 bytes payload from the CS and also adds a two bye
header and a two byte trailer which is resulted into 48 bytes data unit and
that 48 bytes data unit is passed to the ATM layer.
The header and trailer
at this sublayer consist of:
Segmentation Type (ST)
is 2 bits field tells where does the segment belong to the message, either
start, middle or end or it is a single segment message.
One bit CSI will be
used for signally which is not defined yet.
Sequence Counter (SC)
is 3 bit field. This is modulo 8 sequence number and used for ordering and
identifying the cells for errors and flow control.
Multiplexing
Identification (MI) is 10 bit field shows the flow and multiplexing of cells on
the same link.
Length Indicator (LI)
is a 6 bit field of the trailer which is used in conjunction ST to show how
much of the segment is data and how much is padding.
CRC is a 10 bits field
of the Trailer, which is an error check for the entire data.[1]
AAL Type 5
AAL5 also provides data
transport service similar to the AAL3/4, but AAL5 does not include a
multiplexing capability and can only transfer one CS data unit at a time. The
service of AAL5 are much simple and with significantly fewer overheads. Error
detection in AAL5 is done in the CS which makes the SAR sublayer very simple
[8]. As Figure .07 shows [1].
Operation of AAL5
CS accepts data from
the upper layer of maximum size of 65.536 bytes and adds an 8 byte trailer and
also adds the padding when required. And a 48 byte segment passes to the SAR.
Pad (PAD) the padding for a packet are between 0-47 and uses the same rules as for AAL3/4.
User-to-User ID (UU) is
a 1 byte field of the trailer.
Type (T) is 1 byte
field, which is reserved but not defined yet.
Length Indicator (LI)
is a 2 bits field of the Trailer, which is used with to identify the length of
data and padding in the message.
CRC is a 4 bits field
of the Trailer, which is an error check for the entire data [1].
Summary
From the structure and operation
of the AAL it is cleared that the AAL connects the higher layer (e.g. packet
networks) to the ATM layer and that’s why the use of ATM technology and
services creates the needs for an Adaptation Layer to support the information
from higher layers which are not based on ATM layer.
References
[1] Behrouz A.
Forouzan, “Data Communication And Networking”, 2nd Edition, McGraw-Hill
Higher Education, June 2000
[2] Harry G. Perros,
“An Introduction to ATM Networks”,
Jhon Weily & Sons Ltd England, Feb. 2002.
[3]Gunnar Karlsson, “An ATM Adaptation Layer for
Reliable Transfers”, Swedish Institute of Computer Science.
[4] http://en.wikipedia.org/wiki/ATM_adaptation_layer
[5] http://www.techfest.com/networking/atm/atmaal.htm#aal34
[6] Michael
Duck and Richard Read, “Data Communications and Computer Networks for
Computer Scientists and Engineers”, 2nd Edition, Pearson
Prentice Hall.
[7] http://www.javvin.com/protocolAAL.html
[8] John
Atkins and Mark Norris, “TOTAL AREA NETWORKING”,
John
Wiley & Sons Ltd.
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