ser plane Control plane VBR Services Other Signalling Services Services Figure 20.3 B-ISDN reference model. (From Ref. 6, reprinted with permission.) The B-NT2 functional group includes functions broadly equivalent to OSI layer 1 and higher OSI layers. The B-NT2 may be concentrated or distributed. In a particular access arrangement, the B-NT2 functions may consist of physical connections. Examples of B-NT2 functions are: Adaptation functions for different media and topologies Cell delineation Concentration; buffering Multiplexing and demultiplexing OAM functions Resource allocation Signaling protocol handling The functional group B-TE (TE stands for terminal equipment) also includes functions of OSI layer 1 and higher OSI layers. Some of these functions are: User/user and user/machine dialog and protocol Protocol handling for signaling Connection handling to other equipment Interface termination OAM functions B-TE1 has an interface that complies with the B-ISDN interface. B-TE2, however, has a noncompliant, B-ISDN interface. Compliance refers to ITU-T Recs. I.413 and I.432 as well as ANSI T1.624-1993 (Refs. 4-6). ASYNCHRONOUS TRANSFER MODE The terminal adapter (B-TA) converts the B-TE2 interface into a compliant B-ISDN user-network interface. Four bit rates are specified at the UB, TB, and SB interfaces based on Ref. 5 (ANSI T1.624-1993). These are: 1. 51.840 Mbps (SONET STS-1) 2. 155.520 Mbps (SONET STS-3 and SDH STM-1) 3. 622.080 Mbps (SONET STS-12 and SDH STM-4) 4. 44.736 Mbps (DS3) These interfaces are discussed subsequently in this chapter. The following definitions refer to Figure 20.3. User Plane (in other literature called the U-plane). The user plane provides for the transfer of user application information. It contains physical layer, ATM layer, and mul- tiple ATM adaptation layers required for different service users such as CBR and VBR service. Control Plane (in other literature called the C-plane). The control plane protocols deal with call-establishment and call-release and other connection-control 