System for Mobile communications (GSM)

GSM – AN OVERVIEW

GSM, Global System for Mobile communications, is today the most successful digital mobile telecommunication system. This second-generation (2G) system provides voice and limited data services and uses digital modulation with improved audio quality.

The different versions of GSM are:

1. GSM 900 band (850-915MHz up-link frequency and 935-960MHz downlink frequency)
2. GSM 1800 band or digital cellular system (DCS) 1800 band (1710-1785MHz up-link frequency and 1805-1880MHz downlink frequency)
3. Personal Communication service (PCS) 1900 band (1850-1910MHz up-link frequency and 1930-1990MHz downlink frequency)

GSM Mobile communication system can be intelligently used by electronic devices that can collect some data and send it to the central place using SMS or GSM data call. It’s required In-Vehicle Tracking Systems because GPS (Global Positioning System) can normally only receive location information from the satellites but cannot communicate back with them. Hence we need some other communication system like GSM to send this location information to the central control room. Other technologies can also be used but they are more costly.

GSM Network Architecture

GSM Network consists of three main parts:

• Mobile Station (MS) carried by the subscriber
• Base Station Subsystem (BSS) controls radio link with mobile station
• Network & Switching Subsystem (NSS) mobility management and switching of calls between mobile users, and between mobile and fixed network users.

Mobile Station consists of:

• Mobile Equipment (ME) such as hand portable and vehicle mounted unit
• Subscriber Identity Module (SIM), which contains the entire customer related information (identification, secret key for authentication, etc).

Base Station Subsystem consists of:

• Base Transceiver Station (BTS) defines a cell and is responsible for radio link protocols with the Mobile Station
• Base Station Controller (BSC) controls multiple BTSs and manages radio channel setup, and handovers. The BSC is the connection between the Mobile Station and Mobile Switching Center.

Figure 1. Layout of generic GSM network

Network and Switching Subsystems consists of:

• Mobile Switching Center (MSC) is the central component of the NSS. Operates all switching functions for the mobiles within its  jurisdiction. Interface between mobile and other (including fixed) network. Its functions:
• Manages the location of mobiles
• Switches calls
• Manages Security features
• Controls handover between BSCs
• Resource management
• Interworks with and manages network databases
• Collects call billing data and sends to billing system
• Collects traffic statistics for performance monitoring

Network Databases – Home Location Register and Visitor Location Register together with MSC provides the call routing and roaming capabilities of GSM.

• Home Location Register (HLR) contains all the subscriber information for the purposes of call control, and location determination. There is logically one HLR per GSM network, although it may be implemented as a distributed database.
• Visitors Location Register (VLR) is only a temporary storage while the particular subscriber is located in the geographical area controlled by the MSC/VLR. Contains only the necessary information provision of subscribed services.
• Authentication Center (AuC) is a protected database that stores the security information for each subscriber (a copy of the secret key stored in each SIM).
• Equipment Identity Register (EIR) is a list of all valid mobile equipment on the network.

SMS is one of the unique features of GSM compared to older analog systems. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is sent to the sender. SMS also can be used in Cell Broadcast mode to send messages such as traffic or news updates. Messages can be stored on the SIM card for later retrieval. SMS is effective because it can transmit short messages within 3 to 5 s via the GSM network and doesn’t occupy a telephony channel. Moreover, the cost savings makes it a worthwhile choice. With SMS transmitting, gathering position data is easy and convenient.

THE GSM UNIT:

The GSM unit contains a GSM module along with a GSM transmitter antenna. The module functions according to its built and the antenna transmits the information to the Base Station wherein this is exposed to further processing. GPS is not a two-way system. It can either receive or transmit but not both. Due to its inability in doing so, GSM systems are used.

The GSM module that we are using in this unit is the SIMCOM SIM300 module. Designed for global market, SIM300 is a Tri-band GSM/GPRS engine that works on frequencies EGSM 900 MHz, DCS 1800 MHz and PCS1900 MHz. SIM300 provide RF antenna interface with two alternatives: antenna connector and antenna pad. The antenna connector is MURATA MM9329-2700. And customer’s antenna can be soldered to the antenna pad.

SMS is one of the unique features of GSM compared to older analog systems. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is sent to the sender. SMS also can be used in Cell Broadcast mode to send messages such as traffic or news updates. Messages can be stored on the SIM card for later retrieval. SMS is effective because it can transmit short messages within 3 to 5s via the GSM network and doesn’t occupy a telephony channel. Moreover, the cost savings makes it a worthwhile choice. With SMS transmitting, gathering position data is easy and convenient.

We use AT commands to control and program the SIMCOM SIM300 module. The data and control commands are exchanged between the PIC microcontroller and GSM module through the serial interface. There are many groups of AT commands, including: Call Control, Data Card Control, Phone Control, Computer Data Card Control, Reporting Operation, Network Communication Parameter, Miscellaneous, and Short Message Service. We use some of the SMS commands to communicate with the control center. The main AT commands for using SMS are listed below.

• A/                 -  Re-issues last AT command given
• ATD              -  Mobile originated call to dialable number
• ATH              – Disconnect existing connection
• AT+CSCA    – Set the SMS center address. Mobile-originated messages are transmitted through this service center.
• AT+CMGS   – Send SMS command
• AT+CMGF   – Select format for incoming and outgoing messages: zero for PDU mode, one for Text  mode.
• AT+CSMS    – Select message service
• AT+CRES     – Restore SMS settings
• AT+CSCB     – Select cell broadcast SMS messages
• AT+CSDH    – Show SMS text mode parameters

Let’s review an example of how to make a GSM module send and read a sample SMS in Text mode. First, initialize the GSM module with AT commands AT+CSCA and AT+CMGF. Using the former sets the SMS center number to be used with outgoing SMS messages. Remember, the number will be saved on the SIM card just like in normal mobile phones. There are two different modes—Text mode and Protocol Data Unit (PDU) mode—for handling short messages. The system default is PDU mode; however, Text mode is easier to understand. So, use the AT+CMGF=1 command to set the module to the GSM 07.05 standard SMS Text mode. The AT+CMGS command is used to send a short message. The GSM module can receive incoming short messages and save them on the SIM card automatically. You can use the AT+CMGR command to read an incoming short message from the SIM card storage, and then use the AT+CMGD command to delete it when you’re finished. If you want to read an SMS message, then send a AT+CMGR=x command to tell the GSM module which short message you want to read. Next, check the serial port to receive the message from the GSM module.