1. GSM upper layer protocol
The design of the GSM protocol itself refers to the OSI (Open System Interconnection Model), and the layering of the protocol stack is carried out according to the functional plane, layered on top of one layer. The bottom layer completes the physical transmission of information between two long-distance entities, depending on the physical medium. In each layer, the entities exchange information and work together to provide the services required by the upper layer. These services are enhancements to the services provided by the next layer. The exchange of information between layers and entities is carried out through a series of primitives at the service access point. The routing function of information exchange is realized through PD (Protocol Discriminator, protocol discriminator) and TI (TransacTIonIdenTIfier, transaction identifier). The exchange rules of information flow through reference points at interfaces between different entities are called signaling protocols.
Figure 1 shows the signaling structure of the GSM digital mobile communication system. The horizontal is based on the physical setting, starting from the leftmost mobile station, and sequentially accessing various ground facilities of the system; the vertical corresponds to each functional level, from the lowest transmission The layer starts and gradually reaches various high levels.
Figure 1 Signaling structure of GSM digital mobile communication system
1.1 Wireless Resource Management
The role of the radio resource management layer is to establish and release a stable connection between the mobile station and the MSC (Mobile Services Switching Center) during the call.
The function of the radio resource management layer is mainly performed by the mobile station and the BSC (base station controller). In addition, since the responsibility for the handover process is located in the radio resource management layer, some functions implemented in the MSC also belong to the radio resource management layer.
The sub-functions of the radio resource management layer are as follows:
(1) Initialization function
Access and initial allocation, the function of the mobile station to switch between the two main states, namely from "idle" mode to "dedicated" mode. There are three reasons for the mobile station to apply for initialization: performing cell location update, answering a paging, and establishing a call (including additional service request, sending short message request).
(2) Paging function
There are three modes, normal paging mode, full paging mode, and extended paging mode.
(3) Transmission mode and encryption mode management functions
The method of carrying user information between the mobile station and the interconnection point with the cooperative network using GSM transmission includes: wireless channel mode, encryption mode, discontinuous transmission mode.
(4) Switch execution function
When the cell changes, the handover procedure is used to completely modify the channel allocated to the MS (mobile station).
(5) Radio resource management dialogue release function
(6) Load management function
The load on the common channel, the load on the traffic channel, and the general load.
(7) SACCH (Slow Speed ​​Channel) function
â— Wireless transmission control: In the downlink direction, carry commands related to power control and timing advance; in the uplink direction, carry measurement reports and corresponding responses from the mobile station.
â— SMS.
â— General information: parameters for monitoring measurement process (frequency table for monitoring, BSIC identification, BCCH frequency indication); parameters for monitoring wireless link failure monitoring; requirements for uplink discontinuous transmission applications; not directly related to the current state of the specification Additional information.
(8) Frequency redefinition function
It is used to change the frequency characteristics of frequency hopping channels used by mobile stations synchronized with other mobile stations to adapt to changes in the cell frequency table.
(9) General information broadcasting function
Cell selection information, idle mode function information, access required information, dedicated mode mobile station information, cell identification code, message arrangement and content.
1.2 Mobility and security management
(1) Location management and cell selection
(2) Location area division and structure
(3) Location update
(4) Security management
Prevent unauthorized users from accessing and provide confidentiality for authorized users to communicate.
1.3 Communication Management
(1) Call control function
Mobile terminated call routing, mobile initiated call setup, mobile terminated call setup.
(2) Inquiry
Call release and intra-call functions.
(3) Additional business management functions
Unconditional call forwarding for phones, call forwarding when all basic services are busy, etc.
(4) Short message function (idle state and dedicated state)
Mobile start short message, mobile end short message.
Figure 2 shows the procedure for establishing a mobile terminated call.
Figure 2 Mobile terminated call establishment procedure
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