AAL Type 2
Encyclopedia
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.
, 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 Controller
s (RNCs) (Iu-B), inter-RNCs (Iu-R), UMTS RNCs and UMTS Serving GPRS Support Nodes (SGSNs) (Iu-PS), and UMTS RNCs and media gateway
s (MGWs) (Iu-CS).
The format for the 1 byte STF at the beginning of the ATM cell is the following:
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.
The 3 byte CPS header has following fields
channel and the same value of channel identification is used for both directions.
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM), used primarily in telecommunications; for example, it is used for the Iu interfaces in the Universal Mobile Telecommunications System
Universal Mobile Telecommunications System
Universal Mobile Telecommunications System is a third generation mobile cellular technology for networks based on the GSM standard. Developed by the 3GPP , UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the CDMA2000 standard set for...
, 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 bitrateVariable bitrate
Variable bitrate is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate , VBR files vary the amount of output data per time segment...
, connection-oriented
Connection-oriented
Connection-oriented communication is a data communication mode in telecommunications whereby the devices at the end points use a protocol to establish an end-to-end logical or physical connection before any data may be sent. In case of digital transmission, in-order delivery of a bit stream or...
, low latency
Low latency
Low latency allows human-unnoticeable delays between an input being processed and the corresponding output providing real time characteristics. This can be especially important for internet connections utilizing services such as online gaming and VOIP....
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
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
(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
Metro Ethernet
A Metro Ethernet is a computer network that covers a metropolitan area and that is based on the Ethernet standard. It is commonly used as a metropolitan access network to connect subscribers and businesses to a larger service network or the Internet...
standard networks.
Efforts to improve Ethernet networks are in a sense trying to reinvent the wheel
Reinventing the wheel
To reinvent the wheel is to duplicate a basic method that has already previously been created or optimized by others.The inspiration for this idiomatic metaphor lies in the fact that the wheel is the archetype of human ingenuity, both by virtue of the added power and flexibility it affords its...
à la ATM. AAL2 is one example of a useful benefit of ATM, as a general standard for Layer 2
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
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 Controller
Radio Network Controller
The Radio Network Controller is a governing element in the UMTS radio access network and is responsible for controlling the Node Bs that are connected to it. The RNC carries out radio resource management, some of the mobility management functions and is the point where encryption is done before...
s (RNCs) (Iu-B), inter-RNCs (Iu-R), UMTS RNCs and UMTS Serving GPRS Support Nodes (SGSNs) (Iu-PS), and UMTS RNCs and media gateway
Media gateway
A Media gateway is a translation device or service that converts digital media streams between disparate telecommunications networks such as PSTN, SS7, Next Generation Networks or PBX...
s (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.P
This is a Parity bitParity bit
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code....
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 controlError detection and correctionIn information theory and coding theory with applications in computer science and telecommunication, error detection and correction or error control are techniques that enable reliable delivery of digital data over unreliable communication channels...
)
CID
This is a Channel Identifier which identifies the user of the channel. The AAL2 channel is a bi-directionalTwo-way communication
Two-way communication - uses communication to negotiate with the public, resolve conflict, and promote mutual understanding and respect between the organization and its public. Two-Way Communication in Public Relations...
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) NB: the LI is one less than the actual length of the payload, so 0 corresponds to the minimum length of 1 octet, and thus allowing 0x3f to represent 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: