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ATM Adaptation Layer 2 (AAL2) is an ATM adaptation layer for Asynchronous Transfer Mode (ATM), 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. == What is AAL2? == AAL2 is a variable bitrate, connection-oriented, low latency service originally intended to adapt voice for transmission over ATM. Like other ATM adaptation layers, AAL2 defines segmentation and reassembly of higher-layer packets into ATM cells, in this case packets of data containing voice and control information. AAL2 is further separated into two sub-layers that help with the mapping from upper layer services to ATM cells. These are named Service Specific Convergence Sub-layer (SSCS) and Common Part Sub-layer (CPS). The AAL2 protocol improves on other ATM Adaptation Layers, by packing lots of small packets efficiently into one standard-sized ATM cell of 53 bytes. A one-byte packet thus no longer has an overhead ratio of 52 unused bytes out of 53 (i.e. 98%). Potentially, total of 11 one-byte CPS packets (plus 3/4 of a 12th CPS packet) could squeeze into a single cell. Of course, CPS packets can come in other sizes with other CIDs, too. When the transmission is ready, the CPS packets are all multiplexed together into a single cell and transported over standard ATM network infrastructure. The transport networks for ATM are well standardized fiber optic (SDH/Sonet, i.e. STM-1/OC-3 or higher) or copper cable (PDH, i.e. E1/T1/JT1 or higher bandwidth fixed lines) based synchronous networks with built-in redundancy and OAM-related network features which Ethernet networks never had originally (in order to keep things simple) but are sorely missed in metro Ethernet standard networks. Efforts to improve Ethernet networks are in a sense trying to reinvent the wheel à la ATM. AAL2 is one example of a useful benefit of ATM, as a general standard for Layer 2 protocols. ATM/AAL2's efficient handling of small packets contrasts with Ethernet's minimum payload of 48 bytes vs the 1-byte minimum size for an AAL2 CPS packet. AAL2 is the standard layer 2 protocol used in all Iu interfaces, i.e. the interfaces between UMTS base stations and UMTS Radio Network Controllers (RNCs) (Iu-B), inter-RNCs (Iu-R), UMTS RNCs and UMTS Serving GPRS Support Nodes (SGSNs) (Iu-PS), and UMTS RNCs and media gateways (MGWs) (Iu-CS). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ATM Adaptation Layer 2」の詳細全文を読む スポンサード リンク
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