 its frequency and the position of its corresponding burst within a TDMA frame. GSM has two types of channels: The traffic channels used to transport speech and data information The control channels used for network management messages and some channel maintenance tasks The Physical Layer Each physical channel supports a number of logical channels used for user traffic and signaling. The physical layer (or Layer 1) supports the functions required for the transmission of bit streams on the air interface. Layer 1 also provides access capabilities to upper layers. The physical layer is described in the GSM Recommendation 05 series (part of the ETSI docu- mentation for GSM). At the physical level, most signaling messages carried on the radio path are in 23-octet blocks. The data-link layer functions are Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Chapter 1 multiplexing, error detection and correction, flow control, and segmentation to enable long messages on the upper layers. The radio interface uses the Link Access Protocol on Dm channel (LAPDm). This protocol is based on the principles of the ISDN Link Access Protocol on the D channel (LAPD) protocol. Layer 2 is described in GSM Recommendations 04.05 and 04.06. The following logical channel types are supported: Speech traffic channels (TCHs) Full-rate TCH (TCH/F) Half-rate TCH (TCH/H) Broadcast channels (BCHs) Frequency correction channel (FCCH) Synchronization channel (SCH) Broadcast control channel (BCCH) Common control channels (CCCHs) Paging channel (PCH) Random access channel (RACH) Access grant channel (AGCH) Cell broadcast channel (CBCH) Cell broadcast channel (CBCH) (the CBCH uses the same physical channel as the DCCH) Dedicated control channels (DCCHs) Slow associated control channel (SACCH) Stand-alone dedicated control channel (SDCCH) Fast associated control 