Communication PIC-Microcontroller Lab Course by JAOM Center, Feb. 2013 GSM Module Instructor: Mohsen Sarakbi
GSM Definition Short for Global System for Mobile Communications One of the leading digital cellular systems. GSM was first introduced in 1991 and as of the end of 1997; GSM service was available in more than 100 countries and has become the de standard in Europe and Asia. GSM uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency.
GSM Types Three types of GSM differ according to the frequency band used in each one and the number of ARFCNs;1. GSM: Use frequencies between 890-915 MHz UL and 935-960 DL (Band of 25 MHz) with 124 ARFCNs.2. EGSM: Use frequencies between 880-915 MHz UL and 925- 960 DL (Band of 35 MHz) with 174 ARFCNs.3. DCS1800: Use frequencies between 1710-1785 MHz UL and 1805-1880 MHz DL (Band of 75 MHz) with 374 ARFCNs Where ARFCN (Absolute Radio Frequency Channel Number) is a pair of frequencies, one to transmit (Up Link) and one to receive (Down Link).
GSM Features Compatibility that we can use the same mobile to make calls in several countries. Noise Robust such that digital GSM is better than analog because it suffer from noise therefore using digital will reduce the noise interference. Flexibility and increased capacity due to equipment is smaller in size. Security and confidentiality such that GSM offers increase defense against eavesdropping Flexible handovers in using GSM than by using analog systems. Enhanced range services such that the services available are speech services including Telephony and emergency calls, Data services including Short Message Service (SMS), cell Broadcast and supplementary services which charge extra including number identification, call Baring, call forwarding and call completion.
GSM Network Components Mobile Station (MS) o Mobile equipment (ME). Every ME has a unique identity number called International Mobile Equipment Identity (IMEI), it enables stolen mobile to be detected. o Subscriber Identity Module (SIM) Base Station System (BSS)It is linked to MS using air interface and contains the followingcomponents: o Base Transceiver System (BTS) which provides air interface connection to MS, it support one cell or more. o Base Station Controller (BSC) used to control one or more BTS, conveys information to/from BTS and connects terrestrial circuits and air interface.
GSM Network Components Network Switching System (NSS)It contains the function of the GSM Network as well as subscriber and Mobility Management (MM)database. It includes: o Mobile Station Centre (MSC) used as a call switcher exchange for mobile originated or terminated subscriber traffic. o Home Location Register (HLR) which adds information including subscriber ID (IMSI and MSISDN), o Visitor Location Register (VLR) creates a temporary local data base copied from the HLR for subscribers who are active in the area it covers avoiding the need to constantly refer back to the HLR (Roaming). Operational and Maintenance System (OMS)A central network entity that controls and monitors other entities, including the QoS provided by thenetwork. It is divided into two parts: o Network Management Centre (NMC) which provides global network management responsible for operation and maintenance at net level. Its function also includes: Oversees Alarms o Operational and Maintenance Center (OMC) which provides regional network management and should support events and alarms, faults, performance configuration and security. Monitors network service quality
GSM Cell StructureMacro-cells(3 to 35 km)Micro-cells(0,1 to 1 km)
Cell Mode Layout Omni-directional cell Adopt omni-directional antenna， O the overall directional propagation characteristic is the same. Directional cell In general, cell with multi-sector is in common use. Every directional cell adopts directional antenna.
Terrestrial Interfaces 2M bit/s trunks (E1): o Each 2M bit/s link provides 32*64 kbps timeslot. These timeslot carry: Speech or Data 155Mbit/s STM1 o 63xE1s Ethernet: o Max Ethernet frame size 1500Byte
PDH & SDHPDH: Plesiochronous Digital Hierarchy Plesio: near, Chronos: time E1 = 2.048 Mb/sSDH: Synchronized Digital Hierarchy Synchronized, Standard STM1= 155 Mb/s
Air Interface Phase modulation is easy to implement for digital signals and provide good noise tolerance when applied to digital it is known as (PSK). PSK require a wide BW for transmission because abrupt phase changes produce high frequency components, Gaussian PSK (GPSK) reduces the addition of high frequency components, therefore reducing the BW required. Such that GMSK filters the signal through a Gaussian filter, rounding the corners of the signal, the result signal is used to phase shift the carrier signal.
Channels on Air InterfaceThe digital air interface has two channel types: Physical: o The medium over which the information is carried and represent by the 8 timeslots occupy the ARFCN for exactly one eighth of the time. o The 8-timeslots sequence is called TDMA Frame Logical: o The information carried along the physical channel. GSM logical channels consist of Traffic Channels (TCHs) and Control Channels (CCHs).
Logical Channels Traffic Channels (TCHs): o The traffic channel consists of speech and data. o Control channels associated to the traffic channels are: o Slow Associated Control Channel (SACCH) used by the MS for reporting signal strength and quality measurements. o Fast Associated Control Channel (FACCH) used when rapid signaling is required between MS and BTS, example handover Control Channels (CCHs), and fall into 3 groups: o Broadcast Control Channel BCCH o Common Control Channel CCCH o Dedicated Control Channel DCCH
Outgoing Call1, MS sends dialed number to BSS2, BSS sends dialed number to MSC3,4 MSC checks VLR if MS is allowedthe requested service. If so, MSC asksBSS to allocate resources for call.4, MSC routes the call to GMSC6, GMSC routes the call to localexchange of called user7,8 Answer back9,10 Answer back(ring back) tone isrouted from called user to MS viaGMSC,MSC,BSS http://spvp.zesoi.fer.hr/predavanja/extra/GSMdemo.swf
Incoming Call1. Calling a GSM subscribers2. Forwarding call to GSMC3. Signal Setup to HLR4. 5. Request MSRN from VLR6. Forward responsible MSC to GMSC7. Forward Call to current MSC8. 9. Get current status of MS10.11. Paging of MS12.13. MS answers14.15. Security checks16.17. Set up connection http://spvp.zesoi.fer.hr/predavanja/extra/GSMdemo.swf
Data Rate Evolution 2G 3G GSM: Data rate: 9.6 Kbps WCDMA(Wide band CDMA) RSS: Data rate : 0.348 – 2.0 2.5G Mbps GPRS (General Packet Radio HSPA: Data rate: 7.2 Mbps service): Data rate: 14.4 - 115.2 Kbps 3.5G (3G+) HSPA+: Data rate: 21 – 42 Mbps 2.75G EDGE (Enhanced data rate 4G for GSM Evolution): Data LTE (Long Term Evaluation): Data rate: 547.2 Kbps rate: 100 Mbps
GSM Module Any GSM module is using the normal GSM network GSM modules can be communicated to PIC-microcontroller using normal serial USART protocol Communication is being done using regular GSM modem AT Commands.
AT Commands Call controlCommand DescriptionATA Answer commandATD Dial commandATH Hang up callATL Monitor speaker loudnessATM Monitor speaker modeATO Go on-lineATP Set pulse dial as defaultATT Set tone dial as defaultAT+CSTA Select type of addressAT+CRC Cellular result codes http://www.expertcore.org/viewtopic.php?f=18&t=3549
AT Commands MS Text mode Command Description AT+CSMS Select message service AT+CPMS Preferred message storage AT+CMGF Message format AT+CSCA Service centre address AT+CSMP Set text mode parameters AT+CSDH Show text mode parameters AT+CSCB Select cell broadcast message types AT+CSAS Save settings AT+CRES Restore settings AT+CNMI New message indications to TE AT+CMGL List messages AT+CMGR Read message AT+CMGS Send message AT+CMSS Send message from storage AT+CMGW Write message to memory AT+CMGD Delete message
AT Commands, CallsCommand CHECK COM Positive ResponseAT<CR><LV> OKCommand DIAL Positive ResponseATD<number><CR><LV> OKCommand ANSWER Positive ResponseATA<CR><LV> After RING, OKCommand end Positive ResponseATH<CR><LV> OK Parameters <CR> = ASCII character 13 <LV> = ASCII character 10
AT Commands, SMSCommand SET Positive ResponseAT+CMGF=<mode><CR><LV> <mode>: 0 = PDU Mode, 1 = Text Mode OKCommand DELETE Positive ResponseAT+CMGD=<index><CR><LV> <index>: Index number of the message , OKCommand SEND ResponseAT+CMGS=<number><CR><LV><message><CTRL-Z> +CMGS:<mr> OKParameters<CR> = ASCII character 13<LV> = ASCII character 10<CTRL-Z> = ASCII character 26<mr> = Message Reference