ATM Adaptation Layer

As an ATM Adaptation Layer ( AAL ) is called a protocol layer in ATM protocol. Through them, the coding within the Asynchronous Transfer Mode cells and the transfer class is defined.

Of the five that exist today ATM Adaptation Layer, the AAL 5 has been enforced in practice for typical data communication. The motion pictures that have a constant bit rate ( CBR) with, for example, AAL 1 presuppose in uncompressed form are usually highly compressed by methods such as MPEG -1 or MPEG-2. Thus, the bit - rate is highly variable in the transmission of changes from image to image and the information can be transported via the AAL 5.

AAL 1

The class of service allows the transmission of real-time service such as voice transmission with a constant data rate over an ATM network.

With AAL 1 is a circuit switching is emulated.

AAL 2

The AAL 2 service class exists in 2 versions. The old version was never used, so this designation was reassigned. But steeped in old textbooks with reference to video transmission still.

The new variant has been conceived mainly in terms of the transmission of mobile voice data between base stations and fixed network. Mobile voice data is encoded for much lower data rates than landline language. Thus, a transport class was desired, the small variable packets (length 1 bis 45/64 bytes) can be multiplexed on additional subchannels. These channels are 2 different packet headers using the Channel Identification (CID ) in the AAL. ATM cells can be filled with overlapping AAL 2 packets. In order to maintain a certain real-time capability, an only half-full cell can be sent after a predetermined time.

The substitution of ATM as such, and thus AAL 2 in mobile networks has been over ten years ago with the specification for 3G ( UMTS) started under the heading of All- IP ( example ) and in modern mobile networks ( 3G, 4G) since some time implemented consistently.

AAL 3/4

AAL 3 and 4 provide a service for traditional data transmission ready, ie without real-time capabilities and with a variable data rate. As a result of protocol overhead only 44 payload bytes per cell can be transferred, this is a bad utilization of the resources dar. The class of service is indeed specified, but no significance in practice.

AAL 5

5 illustrates an AAL unlike AAL 3/4 much more efficient class of service for the data transmission ready, in which there is only a relatively small overhead. The class of service is used to transmit IP packets over ATM networks. For example, transmits a DSL modem IP data using AAL5 to the DSLAM.

A packet is split into several cells. Only the last cell ( PT) field is especially characterized by the payload type, contains the last 8 bytes of the AAL5 trailer, which among other things, the length of the packet and a checksum ( Cyclic Redundancy Check ( CRC, Eng. CRC) ) includes.

• ATM network
• Network protocol