AT of adaptation Layer (AAL) one calls a protocol layer in at minutes. The coding is specified by it within at cells and the transmission class.

From the five AT existing today adaptation Layers practically already became generally accepted the AAL 5 for typical data communication. The moving pictures, which presuppose a constant bit rate (CBR) with for example AAL 1 in unkomprimierter form, are normally compressed with procedures such as MPEG-1 or MPEG-2 strongly. Thus the bit rate is very variable during the transmission of the changes from picture to picture and the information can over the AAL 5 be transported.

AAL 1

This service class permits the transmission of a real timable service such as speech transmission with constant data rate over an at net.

With AAL 1 a circuit switching is emulated.

AAL 2

The AAL 2-Dienstklasse exists in 2 variants. The old variant was never used, so that this designation was again assigned. It emerges however in old text books with the reference to video transmission still.

The new variant was above all conceived under the criterion of the transmission of portable radio data (UMTS) between basis stations and fixed net: Portable radio data are coded for substantially smaller data rates than fixed net language. Thus a transportation class is necessary, which knows small variable packages (length 1 to 45/64 byte) on additional multiplex. These channels are differentiated on the basis the Channel identification (CID) with respect to the AAL 2 package headers. At cells can be filled up overlapping with AAL 2-Paketen. In order to protect a certain real time ability, a only half-full cell can be sent after a given time. This technology is used today within the portable radio range.

AAL 3/4

AAL 3 and 4 make a service available for the traditional data communication, i.e. without real time abilities and with a variable data rate. Since due to the minutes Overheads only 44 utilizable data bytes per cell will transfer can, this represents a bad extent of utilization of resources. This service class is specified, but without meaning in practice.

AAL 5

AAL 5 makes a service class available clearly more efficient contrary to AAL 3/4 for the data communication, with which there is only a very small Overhead. This service class is used, in order to transfer IP packages over at nets. For example a DSL modem transmits IP data by means of AAL5 to the DSLAM.

A package is divided on several cells. Only the last cell, which is particularly characterized by Payload type (PT) the field, contains in the last 8 bytes the AAL of 5 Trailer, that among other things the length of the package and a check sum (cyclic redundancy check (ZRP, English (carriage return character))) contains.

Articles in category "AT of adaptation Layer"

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• AAL: ATM Adaptation Layer (AAL0, AAL1, AAL2, AAL3/4, AAL5)
  AAL: ATM Adaptation Layer (AAL0, AAL1, AAL2, AAL3/4, AAL5) relays ATM cells between ATM Layer and higher layer.


• ATM Adaptation Layer
  The purpose of the ATM Adaptation Layer (AAL) is to adapt the PDUs passed down from the higher layer onto ATM cells. As the higher level PDUs may in general ...


• ATM Adaptation Layer Protocols Overview
  The following ATM Adaptation Layer protocols (AALs) have been defined for ... The ATM Adaptation Layer Protocols are formally defined in ITU-T Standard ...


• Citations: Multiprotocol Encapsulation over ATM Adaptation Layer ...
  J. Heinanen. Multiprotocol Encapsulation over ATM Adaptation Layer 5. Internet RFC1483, July 1993.


• Network Working Group Juha Heinanen Reguest for Comments: 1483 ...
  Instead, the PDUs are carried in the Payload field of Common Part Convergence Sublayer (CPCS) PDU of ATM Adaptation Layer type 5 (AAL5) [2]. ...