CE : Channel Element CID = CE -E ID The Length Indicator indicates the length (in number of octets) of the CPS information field, and can have a value between 1 and 45 (default) or sometimes between 1 and 64.
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.
One common adaptation of AAL2, AAL2u, doesn't use the STF field at all.
In this case, one single CPS packet is aligned to the beginning of the cell.
ATM/AAL2's efficient handling of small packets contrasts with Ethernet's minimum payload of 46 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.In effect, the AAL provides a convergence function for the ATM Layer; the AAL makes the ATM Layer appear to the higher layers as the higher layers want the underlying cell relay function to appear. The Convergence Sublayer (CS) formats higher layer information so the Segmentation and Reassembly (SAR) Sublayer can segment it for transport in the Payload field of a cell.However, networking is more than just the transfer of information (i.e., user plane).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. 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.