ATM Adaptation Layer 2

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ATM Adaptation Layer 2 (AAL2)

is anATM adaptation layerforAsynchronous Transfer Mode(ATM), used primarily in

telecommunications; for example, it is used for the Iu interfaces in theUniversal 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.

Contents

1 What is AAL2? 2 AAL2 and the ATM Cell

o 2.1 OSFo 2.2 SNo 2.3 P

3 AAL2u 4 ATM AAL2 Cell Diagram 5 AAL2 and the CPS Packet

o 5.1 CIDo 5.2 LIo 5.3 UUIo 5.4 HEC

6 ATM AAL2 CPS Packet Diagram 7 References 8 External links

What is AAL2?AAL2 is avariable bitrate,connection-oriented,low latencyservice originally intended to adapt

voice for transmission over ATM. Like otherATM adaptation layers, AAL2 definessegmentation and reassembly of 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 help

with the mapping from upper layer services to ATM cells. These are named Service Specific

Convergence Sub-layer (SSCS) and Common Part Sub-layer (CPS).

The AAL2 protocol improves on otherATM Adaptation Layers, by packing lots of small packets

efficiently into one standard-sized ATM cell of 53 bytes. A one-byte packet thus no longer has

an overhead ratio of 52 unused bytes out of 53 (i.e. 98%). Potentially, total of 11 one-byte CPSpackets (plus 3/4 of a 12th CPS packet) could squeeze into a single cell. Of course, CPS packets

can come in other sizes with other CIDs, too. When the transmission is ready, the CPS packetsare all multiplexed together into a single cell and transported over standard ATM network

infrastructure.

The transport networks for ATM are well standardizedfiber 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
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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 inmetro Ethernetstandard networks.[citation needed]

Efforts to improve Ethernet networks are in a sense trying toreinvent the wheel la ATM.[citation

needed]

AAL2 is one example of a useful benefit of ATM, as a general standard forLayer 2protocols. ATM/AAL2's efficient handling of small packets contrasts with Ethernet's minimum

payload of 48 bytes vs the 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 UMTSRadio Network Controllers(RNCs) (Iu-B), inter-RNCs (Iu-R), UMTSRNCs and UMTSServing GPRS Support Nodes(SGSNs) (Iu-PS), and UMTS RNCs andmedia

gateways(MGWs) (Iu-CS).[1]

AAL2 and the ATM Cell

The basic component of AAL2 is the CPS packet. A CPS packet is an unanchored unit of datathat can cross ATM cells and can start from anywhere in the payload of the ATM cell, other thanthe start field (STF). The STF is the first byte of the 48-byte ATM payload. The STF gives the

byte index into the ATM cell where the first CPS packet in this cell begins. Byte 0 is the STF.

The data from byte 1 ... (STF+1), would be the straddled remainder of the previous ATM cell'sfinal CPS packet. If there is no remainder from the previous cell, the STF is 0, and 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:

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

OSF

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

SN

The Sequence Number numbers the stream of CPCS-PDUs.

P

TheParity bitis used to detect error in the OSF and SN fields.

If the ATM cell has fewer than 47 bytes, the remainder will be filled by padding.
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AAL2u

One common adaptation of AAL2, AAL2u, doesn't use the STF field at all. In this case, onesingle CPS packet 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 either is toostrenuous, is unsupported, or requires too much overhead (i.e. the 1 byte of STF) from the

internal system's point of view. Most computer chips do not support AAL2, so stripping this

layer away makes it easier to interwork between the ATM interface and the rest of the network.

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 payload of between one and 45 octets. The standard alsodefines a 64-octet mode, but this is not commonly used in real 3G networks. [citation needed]

The 3-byte CPS header has following fields:

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

The Channel Identifier identifies the user of the channel. The AAL2 channel is a bi-directional

channel and the same channel identification value is used for both directions.

LI

The Length Indicator indicates the length (in number of octets) of the CPCS information field,

and can have a value between 1 and 45 (default) or sometimes between 1 and 64. For a givenCID all channels must be of the same maximum length (either 45 or 64 octets) NB: the LI is one

less than the actual length of the payload, so 0 corresponds to the minimum length of 1 octet, and

0x3f to 64 octets.

UUI

User to User Indication 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 SSSARPDU.

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:

G(x) =x5

+x2

+ 1

ATM AAL2 CPS Packet DiagramThe following is a diagram of the CPS packet:
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ATM Adaptation Layer 2