ATM Adaptation Layer 5

Last updated July 25, 2023

ATM Adaptation Layer 5 (AAL5) is an ATM adaptation layer used to send variable-length packets up to 65,535 octets in size across an Asynchronous Transfer Mode (ATM) network.

Unlike most network frames, which place control information in the header, AAL5 places control information in an 8-octet trailer at the end of the packet. The AAL5 trailer contains a 16-bit length field, a 32-bit cyclic redundancy check (CRC) and two 8-bit fields labeled UU and CPI that are currently unused.

Each AAL5 packet is divided into an integral number of ATM cells and reassembled into a packet before delivery to the receiving 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 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 CRC to ensure that all pieces arrived correctly, and passes the resulting block of data to the host software. The process of dividing a block of data into cells and regrouping them is known as ATM segmentation and reassembly (SAR).

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.

The AAL5 on the receiving side knows how many cells comprise a packet because the sending AAL5 uses the low-order bit of the "PAYLOAD TYPE" field of the ATM cell header to mark the final cell in a packet. This final cell header can be thought of as an "end-to-end bit". Thus, the receiving AAL5 collects incoming cells until it finds one with an end-of-packet bit set. ATM standards use the term "convergence" to describe mechanisms that recognize the end of a packet. Although AAL5 uses a single bit in the cell header for convergence, other ATM adaptation layer protocols are free to use other convergence mechanisms.

Packet type and multiplexing

The AAL5 trailer does not include a type field. Thus, an AAL5 frame does not identify its content. This means that either the two hosts at the ends of a virtual circuit must agree a priori that the circuit will be used for one specific protocol (e.g. the circuit will only be used to send IP datagrams), or the two hosts at the ends of a virtual circuit must agree a priori that some octets of the data area will be reserved for use as a type field to distinguish packets containing one protocol's data from packets containing another protocol's data.

RFC 2684, Multiprotocol Encapsulation over ATM, describes two encapsulation mechanisms for network traffic, one of which implements the former scheme and one of which implements the latter scheme.

The former scheme, in which the hosts agree on the high-level protocol for a given circuit, is referred to in RFC 2684 as "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 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 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. The agreement may involve manual configuration.

Datagram encapsulation and IP MTU size

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 RFC 2225. Before data can be sent, a virtual circuit (PVC or SVC) must be in place to the destination host and both ends must agree to use AAL5 on the circuit. To transfer a datagram, the sender passes it to AAL5 along with the VPI/VCI identifying the circuit. AAL5 generates a trailer, divides the datagram into cells, and transfers the cells across the network. At the receiving end, AAL5 reassembles the cells, checks the CRC to verify that no bits were lost or corrupted, extracts the datagram, and passes it to the IP layer.

Related Research Articles

Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and ITU-T for digital transmission of multiple types of traffic. ATM was developed to meet the needs of the Broadband Integrated Services Digital Network as defined in the late 1980s, and designed to integrate telecommunication networks. It can handle both traditional high-throughput data traffic and real-time, low-latency content such as telephony (voice) and video. ATM provides functionality that uses features of circuit switching and packet switching networks by using asynchronous time-division multiplexing.

Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version deployed for production on SATNET in 1982 and on the ARPANET in January 1983. It is still used to route most Internet traffic today, even with the ongoing deployment of Internet Protocol version 6 (IPv6), its successor.

The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.

Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses. Whereas network addresses identify endpoints the labels identify established paths between endpoints. MPLS can encapsulate packets of various network protocols, hence the multiprotocol component of the name. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

In computer networking, Point-to-Point Protocol (PPP) is a data link layer communication protocol between two routers directly without any host or any other networking in between. It can provide loop connection authentication, transmission encryption, and data compression.

In computer networking, the User Datagram Protocol (UDP) is one of the core communication protocols of the Internet protocol suite used to send messages to other hosts on an Internet Protocol (IP) network. Within an IP network, UDP does not require prior communication to set up communication channels or data paths.

A frame is a digital data transmission unit in computer networking and telecommunication. In packet switched systems, a frame is a simple container for a single network packet. In other telecommunications systems, a frame is a repeating structure supporting time-division multiplexing.

A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is first established across the network between two endpoints. The network, rather than having a fixed data rate reservation per connection as in circuit switching, takes advantage of the statistical multiplexing on its transmission links, an intrinsic feature of packet switching.

In computing, Internet Protocol Security (IPsec) is a secure network protocol suite that authenticates and encrypts packets of data to provide secure encrypted communication between two computers over an Internet Protocol network. It is used in virtual private networks (VPNs).

In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end communication services for applications. It provides services such as connection-oriented communication, reliability, flow control, and multiplexing.

The Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol for encapsulating Point-to-Point Protocol (PPP) frames inside Ethernet frames. It appeared in 1999, in the context of the boom of DSL as the solution for tunneling packets over the DSL connection to the ISP's IP network, and from there to the rest of the Internet. A 2005 networking book noted that "Most DSL providers use PPPoE, which provides authentication, encryption, and compression." Typical use of PPPoE involves leveraging the PPP facilities for authenticating the user with a username and password, predominately via the PAP protocol and less often via CHAP. Around 2000, PPPoE was also starting to become a replacement method for talking to a modem connected to a computer or router over an Ethernet LAN displacing the older method, which had been USB. This use-case, connecting routers to modems over Ethernet is still extremely common today.

In computer networking, the Point-to-Point Protocol over ATM (PPPoA) is a layer 2 data-link protocol typically used to connect domestic broadband modems to ISPs via phone lines. It is used mainly with DOCSIS and DSL carriers, by encapsulating PPP frames in ATM AAL5. Point-to-Point Protocol over Asynchronous Transfer Mode (PPPoA) is specified by The Internet Engineering Task Force (IETF) in RFC 2364.

The use of Asynchronous Transfer Mode (ATM) technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM. This adaptation layer defines how to segment higher-layer packets into cells and the reassembly of these packets. Additionally, it defines how to handle various transmission aspects in the ATM layer.

In computer networking and telecommunications, a pseudowire is an emulation of a point-to-point connection over a packet-switched network (PSN).

Segmentation and reassembly (SAR) is the process used to fragment and reassemble variable length packets into fixed length cells so as to allow them to be transported across Asynchronous Transfer Mode (ATM) networks or other cell based infrastructures. Since ATM's payload is only 48 bytes, nearly every packet from any other protocol has to be processed in this way. Thus, it is an essential process for any ATM node. It is usually handled by a dedicated chip, called the SAR.

Virtual circuit multiplexing or VC-MUX is one of the two mechanisms for identifying the protocol carried in ATM Adaptation Layer 5 (AAL5) frames specified by RFC 2684, Multiprotocol Encapsulation over ATM.

Multiprotocol Encapsulation over ATM is specified in RFC 2684. It defines two mechanisms for identifying the protocol carried in ATM Adaptation Layer 5 (AAL5) frames. It replaces RFC 1483, a standard data link access protocol supported by DSL modems.

In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload.

ATM Adaptation Layer 2 (AAL2) is an Asynchronous Transfer Mode (ATM) adaptation layer, used primarily in telecommunications; for example, it is used for the Iu interfaces in the Universal Mobile Telecommunications System, and is also used for transporting digital voice. The standard specifications related to AAL2 are ITU standards I.363.2 and I366.1.

An IPv6 packet is the smallest message entity exchanged using Internet Protocol version 6 (IPv6). Packets consist of control information for addressing and routing and a payload of user data. The control information in IPv6 packets is subdivided into a mandatory fixed header and optional extension headers. The payload of an IPv6 packet is typically a datagram or segment of the higher-level transport layer protocol, but may be data for an internet layer or link layer instead.

References

B-ISDN ATM Adaptation Layer specification: Type 5 AAL (PDF) (Technical report). ITU-T. August 1996. I.363.5. Retrieved 2023-04-17.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.