er Mode from guaranteeing the negotiated QOS to the calls in progress 41.4.7 I.327 B-ISDN functional architecture in the network. This provides a statement of the basic functional architecture of 41.4.6 I.321 B-ISDN Protocol Reference Model B-ISDN to complement that for ISDN given in I.324. The architec- (PRM) ture model defines reference configurations for B-ISDN, breaking down these into connection elements such as private access, public This is based on the ISDN PRM defined in 1.320. This recommen- access, national transit etc. and identifying the locations of refer- dation takes into account the functionalities of B-ISDN and defines ence points for interfaces. The way in which functional groups a B-ISDN PRM, reflecting the principles of layering of communi- relate to the connection elements is defined. cations as defined in the Reference model of Open Systems Inter- connection (CCITT Rec. X.200). 41.4.8 I.361 B-ISDN ATM layer specification This defines the cell structure and the coding of the fields of the 41.4.6.1 Protocol reference model ATM cell header, and also addresses the ATM protocol procedures. (Figure 41.2.) The B-ISDN PRM is represented as a cube with a number of The cell header fields at the UNI are defined in Table 41.1, and horizontal layers and with three vertical planes: the user plane, the the cell header fields at the NNI are defined in Table 41.2. control plane and the management plane. (Figure 41.15.) The con- trol plane and user plane are layered as follows: physical layer; ATM Table 41.1 Cell header fields at the UNI layer; ATM adaptation layer; higher layers. The Management Plane contains both layer related and plane related functions. Header field Number of bits 41.4.6.2 PRM Layer Functions Generic Flow Control (GFC) Virtual Path Indicator (VPI) The basic functions in the PRM Layers are as follows: Virtual Channel Indicator (VCI) 1. Physical Layer: Payload Type (PT) Physical medium sublayer; bit timing; physical medium. Reserved (Res) Transmission Convergence sublayer: cell rate decoup