ATM Adaptation Layer 2

ATM Adaptation Layer 2 (AAL2) is an ATM adaptation layer for Asynchronous Transfer Mode (ATM), used primarily in telecommunications; for example, it is used for the Iu interfaces in the Universal Mobile Telecommunications System, and is also used for transporting digital voice. The standard specifications related to AAL2 are ITU standards I.363.2 and I366.1.

What is AAL2?

AAL2 is a variable bitrate, connection-oriented, low latency service originally intended to adapt voice for transmission over ATM. Like other ATM adaptation layers, the purpose of AAL2 is to define how to segment and reassemble higher-layer packets into ATM cells, in this case packets of data containing voice and control information. AAL2 is further separated into two sub-layers that helps with the mapping from upper layer services to ATM cells: the Service Specific Convergence Sub-layer (SSCS) and the Common Part Sub-layer (CPS).

The objective of the AAL2 protocol, as compared to other ATM Adaptation Layers, is to pack lots of small packets efficiently into one standard-sized ATM cell (53 bytes). This way, if you have a one-byte packet, you don't have an overhead ratio of 52/53 (i.e. 98%). With this smallest packet size of 1 byte, there can be a total of 11 CPS packets (plus 3/4 of a 12th CPS packet) squeezed into a single cell. Of course, there can be a mixture of other CPS packet sizes with other CIDs, too, and when the transmission is ready, the CPS packets are all multiplexed together into a single cell which is then transported over existing standard ATM network infrastructure. The transport networks for ATM are well standardized fiber optic (SDH/Sonet, i.e. STM-1/OC-3 or higher) or copper cable (PDH, i.e. E1/T1/JT1 or higher bandwidth fixed lines) based synchronous networks with built-in redundancy and OAM-related network features which Ethernet networks never had originally (in order to keep things simple) but are sorely missed in metro Ethernet standard networks.

Efforts to improve Ethernet networks are in a sense trying to reinvent the wheel à la ATM. AAL2 is one example of a useful benefit of ATM, as a general standard for Layer 2 protocols. This is because ATM/AAL2 handles small packets efficiently, whereas with Ethernet, there is a minimum payload size of 48 bytes vs a 1-byte minimum size for an AAL2 CPS packet.

AAL2 is the standard layer 2 protocol used in all Iu interfaces, i.e. the interfaces between UMTS base stations and UMTS Radio Network Controllers (RNCs) (Iu-B), inter-RNCs (Iu-R), UMTS RNCs and UMTS Serving GPRS Support Nodes (SGSNs) (Iu-PS), and UMTS RNCs and media gateways (MGWs) (Iu-CS).

AAL2 and the ATM Cell

The basic component of AAL2 is the CPS packet. A CPS packet is an unanchored unit of data, that can cross ATM cells, and starts from any location within the payload of the ATM cell, other than the STF (start field) which is the first byte of the 48 byte ATM payload. The STF tells which byte index into the ATM cell (of 48 bytes) the first CPS packet in this cell begins. The data from byte 1 ... (STF+1), [where byte 0 is the location of the STF itself] would be the remaining straddled portion of the previous ATM cell's final CPS packet. If the STF is 0, then the first byte of the cell after the STF is also the location of the start of the first CPS packet.

The format for the 1 byte STF at the beginning of the ATM cell is the following:

• 6 bits - OSF (offset field)
• 1 bit - SN (sequence number)
• 1 bit - P (parity)

OSF

This is the Offset Field and carries the binary value of the offset in octets between the end of the P bit and the start of the CPCS-PDU Payload. Values of greater than 47 are not allowed.

SN

This is a Sequence Number used to number the stream of CPCS-PDUs.

P

This is a Parity bit used to detect error in the OSF and SN fields.

Additionally, if the ATM cell has less CPS packet data than 47 bytes, the remainder of the ATM cell will be filled by padding.

AAL2u

One common adaptation of AAL2 is known as AAL2u which doesn't use the STF field at all. In this case, there will be one single CPS packet which is aligned to the beginning of the cell. AAL2u is not used in standardized interfaces, but rather in proprietary equipment implementations where the multiplexing/demultiplexing, etc. that needs to be done for standard AAL2 is either too strenuous, unsupported, or too much overhead (i.e. the 1 byte of STF) from the internal system point of view. Most computer chips do not have AAL2 support in them. And therefore stripping this layer away makes it easier to interwork between the ATM interface and the rest of the network equipment computer system.

ATM AAL2 Cell Diagram

The following is diagram of the AAL2 ATM cell:

AAL2 and the CPS Packet

A CPS packet has a 3 byte header and a 1-45 octet payload. There is also a 64 octet mode defined by the standard, but it is not commonly used in real 3G networks.

The 3 byte CPS header has following fields

• 8 bits - CID (channel identifier)
• 6 bits - LI (length indicator)
• 5 bits - UUI (user to user indication)
• 5 bits - HEC (header error control)

CID

This is a Channel Identifier which identifies the user of the channel. The AAL2 channel is a bi-directional channel and the same value of channel identification is used for both directions.

LI

This is a Length Indicator that indicates the length of the CPCS information field between 1 and 45 (default) or 1 and 64 octets. For a given CID all channels must be of the same maximum length (either 45 or 64 octets)

UUI

This is User to User Indication. It conveys specific information transparently between the users. For example, in SSSAR, UUI is used to indicate that this is the final CPS packet for the SSSAR PDU.

HEC

This is Header Error Control and checks for errors in the CID, LI and UUI fields. The generator polynomial for the CPS HEC is the following:

$G(x)=x^{5}+x^{2}+1$

ATM AAL2 CPS Packet Diagram

The following is diagram of the CPS packet:

External links

• Broadband Forum - ATM Forum Technical Specifications
• AAL2 ITU Standard