A CDE Definition
(ATM Adaption Layer) The part of the ATM protocol that breaks up application packets into 48-byte payloads that become ATM cells when the 5-byte headers are attached. The AAL resides between the higher layer transport protocols and the ATM layer. The AAL comprises two layers: Convergence Sublayer (CS) and Segmentation & Reassembly Sublayer (SAR). There are four types of AALs, which are summarized below. See ATM.
AAL-1 Connection-oriented, Constant Bit Rate (CBR), such as DS1 and DS3.
AAL-2 Connection-oriented, Variable Bit Rate (VBR).
AAL-3/4 Connection-oriented and connectionless, Available Bit Rate (ABR).
AAL-5 Connection-oriented, Unspecified Bit Rate (UBR). Least amount of error checking and retransmission.
AAL-6 Connection-oriented, MPEG-2 video streams.
ATM Adaption Layer
(1) See also ATM machine and Adobe Type Manager.
(2) (ATMosphere) A measure of pressure. See atmosphere.
(3) "At the moment." See digispeak.
(4) (Asynchronous Transfer Mode) A network technology for both local and wide area networks (LANs and WANs) that supports real-time voice and video as well as data. The topology uses switches that establish a logical circuit from end to end, which guarantees quality of service (QoS). However, unlike telephone switches that dedicate end-to-end circuits, unused bandwidth in ATM's logical circuits can be appropriated when needed. For example, idle bandwidth in a videoconference circuit can be used to transfer data.
ATM is widely used as a backbone technology in carrier networks and large enterprises, but never became popular as a local network (LAN) topology (see below). ATM is highly scalable and supports transmission speeds of 1.5, 25, 100, 155, 622, 2488 and 9953 Mbps (see OC). ATM is also running as slow as 9.6 Kbps between ships at sea. An ATM switch can be added into the middle of a switch fabric to enhance total capacity, and the new switch is automatically updated using ATM's PNNI routing protocol.
ATM works by transmitting all traffic as fixed-length, 53-byte cells. This fixed unit allows very fast switches to be built, because it is much faster to process a known packet size than to figure out the start and end of variable length packets. The small ATM packet also ensures that voice and video can be inserted into the stream often enough for real-time transmission.
ATM works at layer 2 of the OSI model and typically uses SONET (OC-3, OC-12, etc.) for framing and error correction out over the wire. ATM switches convert cells to SONET frames and frames to cells at the port interface.
Quality of Service (QoS)
The ability to specify a quality of service is one of ATM's most important features, allowing voice and video to be transmitted smoothly. The following levels of service are available:
Constant Bit Rate (CBR) guarantees bandwidth for real-time voice and video.
Real-time variable Bit Rate (rt-VBR) supports interactive multimedia that requires minimal delays, and non-real-time variable bit rate (nrt-VBR) is used for bursty transaction traffic.
Available Bit Rate (ABR) adjusts bandwidth according to congestion levels for LAN traffic.
Unspecified Bit Rate (UBR) provides a best effort for non-critical data such as file transfers.
MPOA and LANE
MPOA (Multiprotocol Over ATM) is used to route protocols such as TCP/IP and IPX while preserving ATM quality of service. LANE (LAN Emulation) is used to interconnect Ethernet and Token Ring LANs by encapsulating their frames in LANE packets and converting them into ATM cells. MPOA route servers or traditional routers are used to internetwork LAN segments. See MPOA and LANE.
History of ATMWhen ATM came on the scene in the early 1990s, it was thought to be the beginning of a new era in networking, because it was both a LAN and WAN technology that could start at the desktop and go straight through to the remote office. Its ability to provide quality of service from end to end was highly praised as the perfect multimedia transport. In addition, ATM came from the telephone world, which had always delivered the highest quality communications.
It Never Happened
ATM never became the magic end-to-end solution. ATM adapters for the desktop were expensive, and standards for interconnecting existing networks to an ATM backbone were confusing and often delayed. When Gigabit Ethernet was announced, providing a 10-fold increase in speed and using a familiar technology, ATM's demise in the LAN arena was assured.
A Carrier's Transport
ATM succeeded in the carriers' networks, being deployed by major telephone companies and ISPs and sizable private enterprises. It was always installed for mission critical backbones because of its quality of service (QoS).
ATM in the Enterprise
ATM in the Internet
Where ATM Fits In
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