SAR Segmentation and reassembly sublayer CNU User of a CN service Fig. 3.71 The adaption layer in ATM The AAL is generally separated into two sub-protocols. The SAR protocol pri- marily handles packing, unpacking, fragmentation and reassembling. It is supposed to be rather independent of the type of application. For example, for digital voice connection, the CNU sends and receives a synchronous stream of 8-bits SDUs, which must be packed and unpacked into a continuous stream of 48-octet ATM-- SDUs over the VC connection. The CS protocol is more dependent on the type of application. For example, in this case (denoted AAL 1), it is likely that timing information must be sent between AAL1 units SO that variations in time delay can be corrected. Some applications, e.g. compressed voice and video, also require that cell losses and biterrors become detected and corrected, since they may multiply through the compression algo- rithms that are used. 3 Logical Network Modelling 3.4.6 Resources in IP Networks In this section we will discuss transport, routing, relaying and discrimination in CNL networks in general, and IP networks in specific. There are numerous books available on IP and the Internet. Most of the information I have used in this section is from Tanenbaum (1966). In the previous sections we saw how links, switches and discrimination between user-data flows and process control flows are handled in circuit-switched CSPs (TE, CN and TR-CSPs), and in packet-switching connection-oriented CSPs (CON-CSPs). In this section we will look at IP networks as a prominent example of the last class of CSPs: CNL-CSPs (note, however that MTP in CCS is also a CNL-CSP, and that the X.25 standard defines a CNL-CSP variant as well). CNL-CSPs are packet switching, as are CON-CSPs. What distinguishes CNL- CSPs from other classes of CSPs is that, once a connectivity user is connected to the SAP of a CNL-CSP, this SAP can be used for sending and receiving at will, without having to request a connection (CN connection, virtual path, vi