5 ATM CONFIGURATIONIntroduction to ATMTechnologyATM Overview Asynchronous transfer mode (ATM) is a technology based on packet transmissionmode while incorporating the high speed of circuit transmission mode. It cansatisfy the need of various communication services. ATM was specified as abroadband ISDN transmission and switching mode by ITU-T in June 1992.Depending on its flexibility and support to multimedia services, it is regarded asthe core technology to implement broadband communications.As defined by ITU-T, ATM transmits, multiplexes, and switches information in ATMcells. An ATM cell has a fixed length of 53 bytes, among which 5 bytes is the cellheader and the remaining 48 bytes are payloads. The major function of the cellheader is to identify virtual connection, with limited functions on flow control,congestion control and error control.ATM is connection-oriented. The connection is a logical connection, or virtualconnection (VC). Each VC is identified by a pair of virtual path identifier (VPI) andvirtual channel identifier (VCI). One VPI/VCI pair has local significance only on asegment of the link between ATM nodes. It is translated on ATM nodes. When aconnection is released, the relevant VPI/VCI pair is released and put back into theresource table for other connections to use.ATM interfaces support permanent virtual circuits (PVCs).Hierarchical Structure ofATMThe basic ATM protocol framework consists of three planes: user plane, controlplane, and management plane.The user plane and the control plane is each subdivided into four layers, namely,physical layer, ATM layer, ATM adaptation layer (AAL), and upper layer, eachallowing further division.The control plane mainly uses signaling protocols to establish and releaseconnections.The management plane is subdivided into layer management and planemanagement. The former manages every layer in each plane and has a layeredstructure corresponding to other planes. The latter is responsible for systemmanagement and communications between different planes.The following figure presents the relationships between layers and planes:
