ATM Adaptation Layer type 2

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AAL type 2 (AAL2) has a checkered history. Originally intended to support class B services, such as packet video or VBR voice, AAL2 was never fully defined and other AALs (e.g., AAL5) ultimately seemed better suited for this purpose. By 1995, it appeared AAL2 was dead for all practical purposes.

In 1996, a new AAL type, originally called AAL-CU, was proposed to support the transport of multiple, short, variable length packets (e.g., from compressed voice or video) over a single ATM virtual channel. The proposed format was assigned the AAL2 “position” in the B-ISDN protocol model because it has the appropriate service characteristics listed below.

  • Nonassured service (i.e., packet delivery is not guaranteed)
  • No automatic retransmission of cells (i.e., errored cells are discarded and error correction is an end-to-end responsibility)
  • Multiplexing of higher layer data so multiple VBR applications can share the VC
  • Sequence integrity so there is some assurance that packets arrive in order, if they arrive

The current AAL2 draft specification defines data unit formats as shown on the visual. All of these data units are part of the AAL2 Common Part Sublayer (CPS); higher protocol layers, if applicable, are specific to the application providing the VBR data. The most likely VBR video standard for ATM will be the MPEG standard. MPEG provides VBR video by using interframe compression—only changes from one frame to the next are encoded. Another candidate video compression scheme is the JPEG standard, which provides intraframe compression. In the JPEG scheme, compression is accomplished by removing redundancies from a single image.

The higher layer application generates CPS service data units (SDU), which are fixed- or variable-length byte streams of up to 45 octets (although up to 64 octets can be negotiated). An SDU is transformed into a CPS packet by adding a three-octet header to the SDU. The CPS packet header identifies the byte stream within the ATM VC, the length of the SDU, a header error check, and other information. A CPS packet contains exactly one SDU in the CPS packet payload.

A CPS PDU contains a 1-octet header plus a 47-octet payload, which carries one or more complete CPS packets plus zero, one, or two partial CPS packets. (The visual shows how packets are placed in a PDU. The first CPS PDU contains a complete CPS packet, plus a portion of a second CPS packet. The next CPS PDU contains the remainder of the second CPS packet, plus the complete third CPS packet; empty space in this CPS PDU is padded.) The header indicates where in the CPS packet byte stream the current payload begins, as well as a sequence number and parity bit.

AAL2 Applications

The original AAL2 protocol was abandoned because no applications could be found. In its place, the common user AAL was developed and standardized as the new AAL2 protocol.

The new AAL2 supports small-sized payloads (less than 48 octets) and multiplexing. Both features make for a more efficient network. The principal application for AAL2 is packetized voice. As voice calls are established, channels are added to the existing connection.

As currently fielded for 3G mobile computing devices, wireless data applications are well suited for AAL2 transport. An AAL2 connection between the wireless site and content provider could support multiple user sessions while minimizing overhead.

Today applications allow the integration and switching of wireless voice and data within an AAL2 format. One vendor supports up to four classes of service for information feeds and switching between channels of AAL2 payloads. Voice channel banks, PBXs, and audio servers supporting AAL2 voice (and audio) connections are available. These devices also support telephony signaling standards for interconnection and the creation of full-service packet voice networks.

The AAL2 protocol supports channel identification and defines four classes of service for intra-VC multiplexing and switching. Devices allow switching AAL2 data streams.