20.7
SERVICES: CONNECTION-ORIENTED AND CONNECTIONLESS
527
tables, which differ at every node, must be maintained in a quasi-real-time fashion. This
will have to be done by some kind of routing protocol.
One way to resolve this problem is to make it an internal network problem and use
a connectionless service as described in ITU-T Rec. I.364 (Ref. 11). We must keep in
mind that ATM is basically a connection-oriented service. Here we are going to adapt it
to provide a connectionless service.
20.7.1
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 support-
ing connectionless layer service. The CLNAP (connectionless network access protocol)
Figure 20.11
General protocol structure for the provision of CL data service in B-ISDN.
Figure 20.12
Protocol architecture for supporting connectionless service. CLNAP stands for connec-
tionless network access protocol.
Summary :
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. 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.
Tags :
network,clsf,bisdn,functional,data,figure,functions,layer,connectionoriented,adaptation,groups,proision,clnap