d 20.3 interrelate the two. Figure 20.2 relates the B-ISDN access reference configuration with the ATM user-network interface (UNI). Note the similarities of this figure with Figure 12.15, the reference model for ISDN. The only difference is that the block nomenclature has a "B" placed in front to indicate broadband. Figure 20.3 is the traditional ITU-T Rec. I.121 (Ref. 2) B-ISDN protocol reference model showing the extra layer necessary for the several services. Returning to Figure 20.2, we see that there is an upper part and a lower part. The lower part shows the UNI boundaries. The upper part is the B-ISDN reference configuration with four interface points. The interfaces at the reference points UB, TB, and SB are standardized. These interfaces support all B-ISDN services. There is only one interface per B-NT1 at the U and one at the TB reference points. The physical media is point-to-point (in each case), in the sense that there is only one receiver in front of one transmitter. One or more interfaces per NT2 are present at the SB reference point. The interface at the SB reference point is point-to-point at the physical layer, in the sense that there is only one receiver in front of one transmitter, and may be point-to-point at other layers. Consider now the functional groupings in Figure 20.2. B-NT1 includes functions broadly equivalent to OSI layer 1, the physical layer. These functions include: Line transmission termination Interface handling at TB and UB OAM functions4 Legend: TA - Terminal adapter TE - Terminal equipment B-TE2 NT - Network termination X - Reference point ET - Exchange termination (B-ISDN/ATM switch) LT - Line termination at B-ISDN/ATM switch B-TE1 B-NT2 B-NT1 B-LT/ET Private Public ATM UNI Private ATM UNIs switch Public swirch Private network Public network Figure 20.2 ATM reference model and user-network interface configuration. (Sources: Refs. 2, 3, and 6.) 4 OAM stands for operations, administration, and maintenance. 20.3 USER-NETWORK INTERFACE (UNI) AND ARCHITECTURE Management plane U