io Access Network Dimensioning for 3G UMTS, DOI 10.1007/978-3-8348-8111-3, Vieweg+Teubner Verlag | Springer Fachmedien Wiesbaden GmbH 2011 Appendix Key fields of the CPS packet are the Channel Identifier (CID), the Length Indicator (LI), and the User-to-User Indication (UUI) fields. These are defined below: CID Field: Uniquely identifies the individual user channels within the AAL2, and allows up to 248 individual users within each AAL2 structure. Coding of the CID field is shown below: Value Not Used Reserved for Layer Management Peer-to-Peer Procedures Reserved 8-255 Identification of AAL2 User (248 total channels) LI Field: Since AAL2 allows variable packet length, LI field is used to indicate the actual length of the CPS packet payload associated with each individual user. The value of the LI can vary from 1 to 45 octets (or 64 octets depending on the implementations). UUI Field: Provides a link between the CPS and an appropriate SSCS that satisfies the higher layer application. Different SSCS protocols may be defined to support specific AAL2 user services, or groups of services. The SSCS may also be null. Value Identification of SSCS entries 28, 29 Reserved for future standardization 30, 31 Reserved for Layer Management (OAM) The format of an AAL CPS-PDU block is shown in Figure A.3 [Mcl97]. The Offset Field identifies the location of the start of the next CPS packet within the CPS-PDU. For robustness, the Start Field is protected from errors by the party bit (P) and data integrity is protected by one bit sequence number (SN) used for identifying cell loss. A.3 ATM Service Categories and QoS Parameters Pointer CPS-Packet CPS-Packet Start Field CPS-PDU Payload 1 octet 47 octets CPS-PDU (48 octets) Offset Field (6 bits) Sequence Number (1 bit) Parity (1 bit) Padding (0-47 octets, all 0) Figure A.3: Format o the AAL2 CPS-PDU A.3 ATM Service Categories and QoS Parameters The ATM forum defines five different service categories. They are listed as follows: Constant Bit Rate (CBR) provides a deterministic bit rate 