Broadband Integrated Services Digital Network
On the visual, the layers of the B-ISDN and ATM protocol stack are presented as a cube. Although confusing at first, this representation illustrates several important points about ATM.
First, note that ATM remains the transport mechanism for B-ISDN services. That is, ATM networks form the lower layers of the B-ISDN protocol stack. The total stack is not just ATM, but ATM plus the B-ISDN services the ATM network supports.
The lowest protocol layer is the Physical Layer (PHY), which is responsible for transmitting bits from the source to the destination. The PHY refers to the interface between user and network, as well as between switches.
The ATM Layer is responsible for switching the cells; this layer must create the cell and place switching information (i.e., virtual path identifiers and virtual channel identifiers) in the cell header. The ATM Layer provides a common cell relay switching service to the B-ISDN service layers above.
The ATM Adaptation Layer (AAL) is responsible for providing specific transport services to the higher layer protocols. 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 AAL has two sublayers. 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). The entire process must be controlled (i.e., control plane) by network administrators and users (i.e., signaling protocol for connections, and so on). In addition, the network must be managed (i.e., management plane). Both planes and layers must be managed (i.e., plane management and layer management).
The user plane transports user information. The AAL supports four generic types of services in the user plane: constant bit rate (CBR) services, connection-oriented data services, connectionless data services, and other variable bit rate (VBR) services. CBR services generate a constant number of bits per unit time, while VBR services vary the number of bits generated per unit time.
The control plane is responsible for call control and connection management. These signaling procedures will also be supported by a special signaling AAL function. The control plane operates over the same layered architecture as the user plane.