Layer 1 of the MSBTS Interface
Layer 1 of the OSI Reference Model (physical layer) contains all the functions necessary for the transmission of bit streams over the physical medium, in this case the radio channel. GSM Layer 1 defines a series of logical channels based on the channel access
- Figure 7.16: Layer 1 service interfaces
procedures with their physical channels. The higher layer protocols access these services at the Layer 1 service interface. The three interfaces of Layer 1 are schematically illustrated in Figure 7.16.
LAPDm protocol frames are transmitted across the service mechanisms of the data link layer interface, and the establishment of logical channels is reported to Layer 2. The communication across this interface is defined by abstract physical layer service primitives. A separate Service Access Point (SAP) is defined for each logical control channel (BCCH, PCH + AGCH, RACH, SDCCH, SACCH, FACCH).
Between Layer 1 and the RR sublayer of Layer 3 there is a direct interface. The abstract service primitives exchanged at this interface mostly concern channel assignment and Layer 1 system information, including measurement results of channel monitoring. At the third Layer 1 interface, the traffic channels for user (payload) data are provided.
The service access points (SAP) of Layer 1 as defined in GSM are not genuine service access points in the spirit of OSI. They differ from the PHY-SAPs of the OSI Reference Model insofar as these SAPs are controlled by Layer 3 RR sublayer (layer management, establishment and release of channels) rather than by control procedures in the link layer. Control of Layer 1 SAPs by RR comprises activation and deactivation, configuration, routing and disconnection of physical and logical channels. Furthermore, exchange of measurement and control information for channel monitoring occurs through service primitives.
7.4.1.1 Layer 1 Services
Layer 1 services of the GSM user-network interface are divided into three groups:
• Access capabilities
• Error detection
• Encryption
Layer 1 provides a bit transport service for the logical channels. These are transmitted in multiplexed format over physical channels which consist of elements defined for the transmission on the radio channel (frequency, time slot, hopping sequence, etc.; see Section 7.1). Some physical channels are provided for common (shared) use (BCCH and CCCH), whereas others are assigned to dedicated connections with single mobile stations (dedicated physical channels). The combination of logical channels used on a physical channel can vary over time, e.g. TCH + SACCH/FACCH replaced by SDCCH + SACCH (see Table 5.4).
The GSM standard distinguishes explicitly between access capabilities for dedicated physical channels and for common physical channels BCCH/CCCHs. Dedicated physical channels are established and controlled by Layer 3 RR management. During the operation of a dedicated physical channel, Layer 1 continuously measures the signal quality of the used channel and the quality of the BCCH channels of the neighboring base stations. This measurement information is passed to Layer 3 in measurement service primitives MPH. In idle mode, Layer 1 selects the cell with the best signal quality in cooperation with the RR sublayer based on the quality of the BCCH/CCCH (cell selection).
- Figure 7.17: State diagram of a mobile station's physical layer
GSM Layer 1 offers an error-protected bit transport service and therefore also error detection and correction mechanisms. To do this, error-correcting and error-detecting coding mechanisms are provided (see Section 6.2). Frames recognized as faulty are not passed up to Layer 2. Furthermore, security-relevant functions like encryption of user data is implemented in Layer 1 (see Section 6.3).
7.4.1.2 Layer 1: Procedures and Peer-to-Peer Signaling
GSM defines and distinguishes between two operational modes of a mobile station: idle mode and dedicated mode (Figure 7.17). In idle mode, the mobile station is either powered off (state NULL) or it searches for or measures the BCCH with the best signal quality (state SEARCHING BCH), or is synchronized to a specific base station's BCCH and ready to perform a random access procedure on the RACH for requesting a dedicated channel in state BCH (see Section 5.5.4).
In state TUNING DCH of the dedicated mode, the mobile station occupies a physical channel and tries to synchronize with it, which will eventually result in transition to state
Bit number
Bit number
|
e |
7 6 |
5 4 3 2 1 |
|
free |
Power level | |
|
free |
Timing Advance | |
|
Layer 2 data (21 octets) | ||
Figure 7.18: Format of an SACCH block
Figure 7.18: Format of an SACCH block
DCH. In this state, the MS is finally ready to establish logical channels and switch them through. The state transitions of Layer 1 are controlled by MPH service primitives of the RR interface, i.e. directly from the Layer 3 RR sublayer of the signaling protocol stack.
Layer 1 defines its own frame structure for the transport of signaling messages, which occur as LAPDm frames at the SAP of the respective logical channel. Figure 7.18 shows the format of an SACCH block as an example, which essentially contains 21 octets of LAPDm data.
Furthermore, the SACCH frame contains a kind of protocol header which carries the current power level and the value of the timing advance. This header is omitted in the other logical channels (FACCH, SDCCH, CCCH, BCCH) which contain only LAPDm PDUs.
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