ATM Adaptation Layer 2

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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, AAL2 defines segmentation 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 other ATM 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 CPS packets (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 packets are all multiplexed together into a single cell and transported over 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 sorelmetro Ethernet 표준 네트워크에서 누락.^{[citation needed]}

이더넷 네트워크를 개선하려는 노력은 어떤 의미에서 ATM을 재창조하려는 것이다.^{[citation needed]} AAL2는 계층 2 프로토콜의 일반 표준으로서 ATM의 유용한 편익의 한 예다. ATM/AAL2의 작은 패킷의 효율적인 처리 방식은 이더넷의 최소 페이로드 46바이트 대 AAL2 CPS 패킷의 최소 1바이트 크기와 대비된다.

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).^[1]

AAL2와 ATM 셀

AAL2의 기본 구성요소는 CPS 패킷이다. CPS 패킷은 ATM 셀을 가로지를 수 있는 연결되지 않은 데이터 단위로서, 시작 필드(STF)를 제외한 ATM 셀의 페이로드 내 어디에서나 시작할 수 있다. STF는 48바이트 ATM 페이로드의 첫 번째 바이트다. STF는 이 셀의 첫 번째 CPS 패킷이 시작되는 ATM 셀에 바이트 인덱스를 제공한다. 바이트 0은 STF이다. 바이트 1 ... (STF+1)의 데이터는 이전 ATM 셀의 최종 CPS 패킷의 스트레이딩된 나머지일 것이다. 이전 셀로부터 잔량이 없는 경우, STF는 0이며, STF 이후의 셀의 첫 번째 바이트도 첫 번째 CPS 패킷의 시작 위치다.

ATM 셀의 시작 부분에서 1바이트 STF의 형식은 다음과 같다.

• 6비트 - 오프셋 필드(OSF)
• 1비트 - 시퀀스 번호(SN)
• 1비트 - 패리티(P)

OSF

오프셋 필드는 P 비트의 끝과 CPCS-PDU 페이로드의 시작 사이에 오프셋의 이진값(옥텟)을 전달한다. 47 이상의 값은 허용되지 않는다.

SN

The Sequence Number numbers the stream of CPCS-PDUs.

P

The Parity bit is 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.

AAL2u

One common adaptation of AAL2, AAL2u, doesn't use the STF field at all. In this case, one single 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 too strenuous, 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 also defines 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 - header error control (HEC)

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. The maximum number of multiplexed user channels is 248. As some channels are reserved for other uses, such as peer-to-peer layer management.

CE : Channel Element CID = CE -E + ID

LI

The Length Indicator indicates the length (in number of octets) of the CPS information field, and can have a value between 1 and 45 (default) or sometimes between 1 and 64. 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 0x3f to 64 octets.

UUI

User to User Indiction은 사용자들 사이에 특정한 정보를 투명하게 전달한다. 예를 들어 SSSAR에서 UUI는 이것이 SSSAR PDU에 대한 최종 CPS 패킷임을 나타내기 위해 사용된다.

HEC

헤더 오류 제어로 CID, LI, UUI 필드에서 오류를 확인한다. CPS HEC의 제너레이터 다항식:

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

ATM AAL2 CPS 패킷 다이어그램

다음은 CPS 패킷의 다이어그램이다.

참조

외부 링크

• 광대역 포럼 - ATM 포럼 기술 사양
• AAL2 ITU 표준