the need for efficient transport of come drop and insert (Add/Drop) multiplexers, which attempt to the existing North American PDH rates, the most important of strike a balance, that leads to adequate, rather than comprehensive which are the 1.5Mbit/s and 45Mbit/s rates. This leads to a standard routing flexibility, with an attendant reduction in equipment costs. where a 45Mbit/s signal is loaded into the equivalent of a VC3, which, in turn, is loaded into the SONET counterpart of an STM, known as a Synchronous Transport Signal - 1 (STS-1), which runs at 51.84Mbit/s i.e. precisely one third of the STM-1 rate. As with STM signals TTF = Transport Termination Function SDH, there is also a concatenation mechanism for dealing with within NNI customer signals which do not easily fit inside any of the already defined VC payload areas. This mechanism is particularly useful for carrying rates such as 140Mbit/s, where the concatenation results in a SONET NNI signal known as an STS-3c, which has a bit rate that is identical to an STM-1, and an SOH and payload area of exactly CONNECTION FUNCTION the same size as well. Other terminology within SONET is also different, in that SONET refers to Virtual Tributaries (VTs) rather than VCs, and the equival- ent of the VC3, referred to above, is known as a Synchronous Plesiochronous interfaces Payload Envelope (SPE). It also makes a distinction between the STS logical signal and its optical manifestation at an NNI, in which case it is referred to as an Optical Carrier (OC). North American transmission systems are usually referred to by their transmission rates in OCs e.g. OC3 or OC48. Figure 42.21 Generalised representation of the traffic 42.4.8 NNI Optical Interface standardisation paths through an SDH network element Although recommendations G.707/8/9 have done an excellent job All of the above mentioned types of network element normally in standardising the logical signal presented at an NNI, this alone is assume that any interconnection will be based on optical fibres. It not eno