th extremely low bit-error rates, compared to the case when X.25 was standardized. Note that there are also no parameters in the ATM-PDU for fragmentation, in order to optimize the network for transfer delays between terminal systems. Thus, both SDU error detection and error correction, as well as fragmentation (for SDUs exceeding 48 octets) must be done between terminal systems of an ATM network. The complete ATM-PDU has a fixed length of 53 octets; 5 of which are header information, 48 of which are the SDU. ATM calls this PDU a cell. 3 Logical Network Modelling An ATM-UDT maps cells into the frames of the underlying connection in a way that depends on the type of connection (PDH, SDH, ADSL, etc.). Since ATM networks can provide connections on two levels, VC and VP con- nections, ATM switches must be able to switch both virtual channels and virtual paths. As a matter of fact, the VP-level in ATM networks plays the same role as a transport network, such as SDH. The difference is that SDH transport networks are designed to provide transport connections between network elements (as ATM switches), while ATM is designed for providing transport connections between ter- minal systems. One natural application for ATM networks is, therefore, to interconnect nodes in private networks over VP connections. Such nodes can be local area networks (LAN) or e.g. 64K CS switches in the private teleservice network of a large corpo- ration. Figure 3.67 provides an intuitive picture of the connectivity relations between physical nodes in ATM based physical networks. Private VP connection (link in private network) Private Private access ATM access SDH transport link switching node SDH digital cross connect (DCC) Physical media connection TE-TS Teleservice terminal system PM-TS Physical media terminal system Fig. 3.67 There are links on four connectivity levels in ATM networks A VP connection between two switches in the private network in Fig. 3.67 could be used for over 2000 simultaneous voice calls, each call using one VC connection