 In the short term it can be shown that two coaxial Another issue is centred on the number of interfaces that need to be defined, in particular at Since different terminals will support cables are more cost effective for the 155Mbit/s interface giving a different services and applications, the throughput across the inter- reach of up to 100m. However the cost of optical transmission face will vary considerably. This could be from the few bit/s of a components is expected to fall and it is anticipated that an optical interface will become economic in the future if current cost trends telemetry service, through the tens of kbit/s required for telephony, to the hundreds of Mbit/s required for high definition visual service- continue and this could give a reach of up to 2km. S. However with the flexibility provided by an ATM interface, As well as the physical media the physical layer also includes the multi-service and multi-media terminals supporting various combi- framing structure. Two alternatives have currently been identified, the cell based structure and the SDH based structure. The first nations of services and applications should emerge. consists of a continuous stream of cells each containing 53 octets; in the second the stream of cells are mapped into the payload of an 44.8.4 Customer's access connection STM-1 frame of the SDH as shown in Figure 44.14. Maintenance messages are contained within the section and path overheads of the The customer's access connection between the B-NT1 and the first SDH frame in the latter case, but special operation and maintenance switching node in the network, often referred to as the "U" reference cells have been defined for inclusion in the former such that they point may take many forms. It may be a copper pair, coaxial cable, can easily be recognised at the physical layer from a unique pattern optical fibre cable, satellite or cellular radio. The transmission in the header. equipment may use existing plesiochronous multiplex or the new synchronous digital hierarchy stand