Structure and Operation of ATM Adaptation Layer


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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