| 单词 | ATM reference model |
| 释义 | ATM reference model • Physical Layer (PHY) functions are addressed through two sublayers: the Physical Medium and Transmission Convergence.The ATM Forum specifications for various User Network Interfaces (UNIs) address the implementation of the Physical Layer.The B-UNI, or Public UNI, is the specification for carrier internetworks.The UNI and DXI are Private UNIs, describing the implementation specifics for user access to the ATM network. Physical Medium (PM) sublayer specifies the physical and electro-optical interfaces with the transmission medium.The PM also provides timing functions. The Transmission Convergence (TC) sublayer handles frame generation, frame adaption, cell delineation, header error control (HEC), and cell rate decoupling. • ATM Layer (ATM) functions include multiplexing of cells, selection of appropriate Virtual Path Identifiers (VPIs) and Virtual Channel Identifiers (VCIs), generation of headers, and flow control. At this layer, all multiplexing, switching, and routing takes place for presentation to the appropriate Virtual Paths (VPs) and Virtual Channels (VCs) of the SONET fiber optic transport system, which interfaces through the Physical Layer. • ATM Adaptation Layer (AAL) functions are divided into sublayers.The Convergence Sublayer (CS) functions are determined by the specifics of the service class supported by that particular AAL. The Segmentation and Reassembly (SAR) sublayer functions to segment the user data into 48-byte payloads for insertion into cells, on the transmit side. On the receive side, the SAR extracts the payloads from the cells and reassembles the data into the information stream as originally transmitted, e.g. IP packets. The planes include the Control Plane, User Plane, and Management Plane. See also AAL, B-UNI, cell, CS, flow control, frame, header, HEC, OSI Reference Model, Physical Layer, PM, Private UNI, Public UNI, SAR, TC, VC, VCI, VP, and VPI. |
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