band ISDN. Considerable effort has therefore gone into the defini- ISDN has been adopted in CCITT with the reference points being tion of this interface. labelled SB and TB instead of S and T as shown in Figure 44.5. As with N-ISDN the functions of B-NT1 are to terminate the trans- mission systems in the local line plant and support the interface to 44.8.5.1 Physical layer the B-NT2. In Europe this function is seen as part of the public network and also provides maintenance features for use by the Two interfaces are to be defined at both the reference points at TB network operator. The B-NT2 contains a local switching element and SB and the bit rates have been standardised at 155.52Mbit/s equivalent to the narrowband PABX or LAN. 622.08Mbit/s by CCITT (although the use of 622.08Mbit/s at the SB One of the issues currently being debated is the relationship reference point is for further study). These rates were chosen to between the interfaces at the T, and reference points and align with the synchronous digital hierarchy (SDH) rates at levels 1 whether these interfaces should be point-to-point or point-to-multi- and 4 (STM-1 and 4 X STM-1) and at the same time provide point. It is desirable that the two interfaces are compatible to ensure sufficient capacity for carrying encoded HDTV channels. However terminal portability between the large business customers and the the whole of these bit rates are not available to the user; of the small domestic customer who may not need an NT2. However it is 155Mbit/s a maximum of only 135Mbit/s can be used due to the maintenance overheads and the ATM header. imperative that the interface at which may be in the majority, is of minimum cost. The cost of any additional features required at the Several different types of media can be considered for the physical interface at TB should therefore not have to be borne by the interface layer, but only coaxial cable and optical fibres are serious conten- ders since they will both satisfy the performance requirements for both interfaces.