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'''ATM Adaptation Layer 5''' (AAL5) is an [[ATM adaptation layer]] used to send variable-length [[Packet (information technology)|packet]]s up to 65,535 [[octet (computing)|octet]]s in size across an [[Asynchronous Transfer Mode]] (ATM) network.▼
▲'''ATM Adaptation Layer 5''' ('''AAL5''') is an [[ATM adaptation layer]] used to send variable-length [[Packet (information technology)|
Unlike most network frames, which place control information in the [[Header (information technology)|header]], AAL5 places control information in an 8-octet [[Trailer (information technology)|trailer]] at the end of the packet. The AAL5 trailer contains a 16-bit length [[Field (computer science)|field]], a 32-bit [[cyclic redundancy check]] (CRC) and two 8-bit fields labeled ''UU'' and ''CPI'' that are currently unused.▼
▲Unlike most network frames, which place control information in the [[Header (
Each AAL5 packet is divided into an integral number of ATM cells and reassembled into a packet before delivery to the receiving [[host (network)|host]]. This process is known as [[Segmentation and Reassembly]] (see below). The last cell contains padding to ensure that the entire packet is a multiple of 48 octets long. The final cell contains up to 40 octets of data, followed by padding bytes and the 8-octet trailer. In other words, AAL5 places the trailer in the last 8 octets of the final cell where it can be found without knowing the length of the packet; the final cell is identified by a bit in the ATM header (see below), and the trailer is always in the last 8 octets of that cell.
==Convergence, segmentation, and reassembly==
When an [[Application software|application]] sends data over an ATM connection using AAL5, the host delivers a block of data to the AAL5 interface. AAL5 generates a trailer, divides the information into 48-octet pieces, and transfers each piece across the ATM network in a single cell. On the receiving end of the connection, AAL5 reassembles incoming cells into a packet, checks the
By separating the functions of segmentation and reassembly from cell transport, AAL5 follows the layering principle. The ATM cell transfer layer is classified as "machine-to-machine" because the layering principle applies from one machine to the next (e.g., between a host and a switch or between two switches). The AAL5 layer is classified as "end-to-end" because the layering principle applies from the source to the destination - AAL5 presents the receiving software with data in exactly the same size blocks as the application passed to the AAL5 on the sending end.
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==Packet type and multiplexing==
The AAL5 trailer does not include a ''type'' field. Thus, an AAL5 frame
The former scheme, in which the hosts agree on the high-level protocol for a given circuit, is referred to in RFC <nowiki /> 2684 as "[[Virtual Circuit Multiplexing|VC Multiplexing]]". It has the advantage of not requiring additional information in a packet, which minimises the overhead. For example, if the hosts agree to transfer IP, a sender can pass each datagram directly to AAL5 to transfer, nothing needs to be sent besides the datagram and the AAL5 trailer. The chief disadvantage of such a scheme lies in duplication of virtual circuits: a host must create a separate virtual circuit for each high-level protocol if more than one protocol is used. Because most carriers charge for each virtual circuit, customers try to avoid using multiple circuits because it adds unnecessary cost.
The latter scheme, in which the hosts use a single virtual circuit for multiple protocols, is referred to in RFC <nowiki/> 2684 as "LLC Encapsulation". The standards suggest that hosts should use a standard [[IEEE 802.2]] ''[[Logical Link Control]]'' (LLC) header, followed by a ''[[Subnetwork Access Protocol]]'' (SNAP) header if necessary. This scheme has the advantage of allowing all traffic over the same circuit, but the disadvantage of requiring each packet to contain octets that identify the protocol type, which adds overhead. The scheme also has the disadvantage that packets from all protocols travel with the same delay and priority.
RFC <nowiki/> 2684 specifies that hosts can choose between the two methods of using AAL5. Both the sender and receiver must agree on how the circuit will be used
== Datagram encapsulation and IP MTU size==
[[Internet Protocol|Internet Protocol (IP)]] can use AAL5, combined with one of the encapsulation schemes described in RFC 2684, to transfer datagrams across an ATM network, as specified in
== References ==
* {{cite
[[Category:Asynchronous Transfer Mode]]
[[Category:Network protocols]]
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