rate Priority (CLP) field to indicate whether the cell has a lower priority (constant or variable) and the connection mode (connection oriented and therefore can be discarded during overload conditions and a or connectionless) and these are shown in Figure 44.16. Header Error Control (HEC) field which covers the entire header. Two sub-layers have been defined in the adaptation layer, these The structure of the header is shown in Figure 44.15. are the Segmentation and Reassembly sublayer (SAR) and the Convergence Sub-layer (CS). The SAR divides the information to be carried by the ATM layer into segments suitable for carrying in the 48 octet information field of the ATM cell and vice versa. Other functions such as handling cell delays variation, cell loss, timing and error monitoring and control are handled by the service dependent Octet 44.8.5.4 Higher layers The higher layers in the user, signalling and management informa- tion are all supported on a common ATM bearer service with specific service dependent adaptation layers adapting the features of the higher layers of these services. Each service or application in the user plane for example will have differently specified higher layers, Figure 44.15 ATM header structure whilst the higher layers of the signalling in the control plane may be different again. For example signalling may be based upon the narrowband ISDN signalling with layer 3 signalling messages and protocols defined in 1.450/1 (Q.930/1). In which case decisions will 44.8.5.3 ATM Adaptation Layer need to be taken as to whether they should still be supported by the layer 2 specification (1.440/1) or whether these functions can be As the name implies this layer adapts or maps the functions or performed by the adaptation layer. services supported by the higher layers onto a common ATM bearer Signalling is established via a signalling circuit which is im- service. Different services therefore require different ATM Adapta- plemented by assigning two virtual channels, one for each direction, tion Layer