Functional Architecture

The provision of connectionless data service in the B-ISDN is carried out by means of ATM switches and connectionless service functions (CLSF). ATM switches support the transport of connectionless data units in the B-ISDN between specific functional groups where the CLSF handles the connectionless protocol and provides for the adaptation of the connectionless data units into ATM cells to be transferred in a connection-oriented environment. It should be noted that CLSF functional groups may be located outside of environment. It should be noted that CLSF functional groups may be located outside of the B-ISDN, in a private connectionless network or with a specialized service provider, or inside the B-ISDN.

The ATM switching is performed by the ATM nodes (ATM switch/cross-connect), which are a functional part of the ATM transport network. The CLSF functional group terminates the B-ISDN connectionless protocol and includes functions for the adaptation of the connectionless protocol to the intrinsically connection-oriented ATM layer protocol. These latter functions are performed by the ATM adaptation layer type 3/4 (AAL-3/4), while the CLSF group terminations are carried out by the services layer above the AAL called the CLNAP (connectionless network access protocol). The connectionless (CL) protocol includes functions such as routing, addressing, and QoS (quality of service) selection. In order to perform the routing of CL data units, the CLSF has to interact with the control/management planes of the underlying ATM network.

The general protocol structure for the provision of connectionless (CL) data service is illustrated in Figure 20.11. Figure 20.12 shows the protocol architecture for supporting connectionless layer service. The CLNAP (connectionless network access protocol)

Customer ATM switched terminal capabilities equipment plus CLSF

Customer ATM switched terminal capabilities equipment plus CLSF

CLNAP

ATM

CLNAP

ATM

AAL 3/4

node

AAL 3/4

node

ATM

ATM

ATM

ATM

Physical

Physical

Physical

Physical

Figure 20.11 General protocol structure for the provision of CL data service in B-ISDN.

Figure 20.11 General protocol structure for the provision of CL data service in B-ISDN.

CLNAP user layer CLNAP Type 3/4 AAL ATM Physical

Figure 20.12 Protocol architecture for supporting connectionless service. CLNAP stands for connectionless network access protocol.

layer uses the type 3/4 AAL, employs unassured service, and includes the necessary functionality for the connectionless layer service.

The CL service layer provides for transparent transfer of variable-sized data units from a source to one or more destinations in a manner such that lost or corrupted data units are not retransmitted. This transfer is performed using a CL technique, including embedding destination and source addresses into each data unit.

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