ATM adaptation layer
The ATM Adaptation Layer ( AAL , sometimes also ATM Adaptation Layer ) is a protocol layer in the ATM protocol. It defines the coding within the ATM cells and the transmission class.
Of the five ATM adaptation layers that exist today, the AAL 5 has practically established itself for typical data communication. Moving images, which require a constant data rate ( CBR ) in uncompressed form , as implemented, for example, by AAL 1, are now normally heavily compressed using methods such as MPEG-1 or MPEG-2 . The data rate for the transmission of changes from picture to picture is therefore very variable and the information can also be transported via the AAL 5.
Service classes
AAL 1
This service class allows the transmission of a real-time capable service such as voice transmission with constant data rate over an ATM network. AAL 1 emulates a line switch.
AAL 2
The AAL 2 service class exists in two variants. The old variant was never used, so the name was reassigned. But it still appears in old textbooks with the reference to video transmission.
The new variant was designed primarily from the point of view of the transmission of cellular voice data between base stations and the fixed network. Cellular voice data is encoded for significantly lower data rates than landline voice. As a result, a transport class was desired that could multiplex small variable packets (with a length of 1 to 45/64 bytes) onto additional subchannels. These channels are differentiated on the basis of the Channel Identification ( CID ) in the AAL-2 packet header. ATM cells can be filled with overlapping AAL-2 packets. In order to maintain a certain real-time capability, a cell that is only half full can be sent after a specified time has elapsed.
The replacement of ATM as such, and thus also AAL 2 in mobile radio networks, began over ten years ago with the specification for 3G ( UMTS ) under the keyword All-IP ) and has now been implemented consistently in modern mobile radio networks (3G, 4G) .
Eel 3/4
AAL 3 and 4 provide a traditional data transfer service, i.e. H. without real-time capabilities and with a variable data rate. Since only 44 useful data bytes can be transmitted per cell due to the protocol overhead , this represents a poor utilization of the resources. This service class is specified, but has no practical significance.
AAL 5
In contrast to AAL 3/4, AAL 5 provides a significantly more efficient service class for data transmission. The layer structure corresponds to the AAL 3/4, but in a significantly simplified form. Since all cells are transmitted strictly sequentially, it is no longer possible to multiplex several AAL connections onto one ATM connection. In addition, there is only a relatively low overhead.
This service class is used to transmit Ethernet or IP packets over ATM networks. For example, a DSL modem transmits PPPoE data to the DSLAM using AAL 5 .
A packet is divided into several cells. Only the last cell, which is specially identified by the payload type field ( PT ), contains the AAL-5 trailer in the last 8 bytes, which includes the length of the packet and a checksum ( cyclical redundancy check - ZRP) . CRC).
Individual evidence
- ↑ cf. The ATM adaption layer , Department of Computer Science, Trinity College Dublin
- ↑ ETSI TR 125 933 V5.4.0 (2003-12). Universal Mobile Telecommunications System (UMTS); IP transport in UTRAN. (3GPP TR 25.933 version 5.4.0 Release 5)
- ↑ ATM adaptation layer