Wireless systems GSM Simon Sörman

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1 Wireless systems GSM Simon Sörman

2 Contents 1 Introduction Channels Physical channels FDMA/TDMA Bursts Logical channels Mapping of logical channels to physical channels Radio path Synchronization Time synchronization Frequency synchronization Connection and random access Power management Handoff Equalization Modulation GMSK Other modulations Coding Channel coding Full rate TCH SACCH, SDCCH, FACCH, BCCH, PCH, AGCH, NCH and CBCH Source coding (Full Rate speech) Summary References... 15

3 Abstract In the 1980s work started to develop the second generation mobile phone system, GSM. It was at first commercialized in 1991 and has as of today spread world-wide. Even though the fourth generation is currently deployed in some places, GSM still is in use in many of those areas. In this paper we will delve into some basic wireless aspects of this cellular technology and present the foundations of this cellular system. Abbreviations AB Access Burst AGCH Access Grant Channel BCCH Broadcast Control Channel BTS Base Transceiver Station DB Dummy Burst EFR Enhanced Full Rate FACCH Fast Associated Control Channel FB Frequency Correction Burst FCCH Frequency Control Channel FN Frame number GMSK Gaussian Minimum Shift Keying MS Mobile Station NB Normal Burst NCH Notification Channel PCH Paging Channel RACH Random Access Control Channel RPE-LTP Regular Pulse Excitation Long Term Prediction SACCH Slow Associated Control Channel SB Synchronization Burst SCH Synchronization Channel SDCCH Stand-alone Dedicated Control Channel SF Stealing flag TCH Transport Channel TN Time slot Number

4 1 Introduction This paper is part of the course TSKS03 Wireless Systems given at Linköping University during the spring term of It is aimed at describing typical wireless telecommunications aspects, at the lower layers of the OSI model, of the widespread GSM standard; such as modulation techniques, channel coding, radio characteristics etc. The GSM standard is however quite broad and this paper will only be able to go into details of a few parts of it. The interested reader is referred to the specification 3GPP TS for further information and reading about the GSM system. When reading about GSM and comparing to other wireless systems, one should keep in mind that GSM was designed for mobile telephony, i.e. it was heavily customized towards telephone calls and only small amounts of other data. A larger support for other data (e.g. internet data) was added later in the GSM development. Importantly, in terms of infrastructure GSM is a cell based system with a Base Transceiver Station (BTS) in each cell handling all Mobile Stations (MS) committed to that particular BTS. 1

5 2 Channels In this chapter we present the different channels of the GSM system. The chapter begins with an explanation of the physical channel partitioning, followed by a presentation of the numerous logical channels and their function. 2.1 Physical channels This section treats the physical channel arrangement with multiplexing both in the frequency domain (FDMA) and the time domain (TDMA), and how the partitioning is used FDMA/TDMA A GSM system uses 2 frequency bands of equal width, one for uplink and one for downlink. Since the GSM standard has spread throughout the entire globe, different bands are allocated at different places in the world. For the sake of this paper we will limit the discussion to P-GSM. This allocation uses MHz for uplink traffic and MHz for downlink. This is split into radio channels of 200 khz width, and placed such that the first and last 100 khz of the bands are unused (this is for protection). In GSM-900 this allows for 124 duplex channels. However, each base station is allocated only a set of these, called the Cell Allocation (CA). (Eberspächer et al., 2009) Apart from this frequency division multiplexing, GSM also uses time division multiplexing by splitting each channel into 8 time slots of s 577 μs each. These 8 slots together form what is called the TDMA frame with duration of 4.62 ms. At the base transceiver station the timing of the uplink frame is delayed by 3 time slots as compared to the downlink frame. At the mobile station (MS) this delay is variable, which allows the MS to adjust for the signal propagation delay. The delay also makes it possible to build a cheaper and more efficient MS since it doesn t need a high-frequency duplexing unit. The organization of the FDMA and the TDMA is presented graphically in figure 1. (3GPP TS ) Figure 1: FDMA and TDMA in GSM (Eberspächer et al., 2009) The TDMA frames are further grouped into multiframes, comprising either 26 or 51 normal frames. A larger grouping, consisting of = 1350 frames is called a superframe, i.e. this contains multiframes and/or multiframes. The duration of 2048 superframes is called a hyperframe and 2

6 each TDMA frame of a hyperframe has a unique frame number (FN) that is used for encryption purposes. (3GPP TS ) Bursts The physical use of a time slot is called a burst (i.e. modulated data), and for this discussion we will present five types of bursts. The timeslot is divided into 156,25 symbol periods, and in this section we will assume that the modulation used is GMSK (see Section 4) which results in one bit per symbol. The structure of the bursts is visible in figure 2: Figure 2: The structure of bursts in GSM (Eberspächer et al., 2009) The 3 tail bits at the start and end of each burst are all set to 0, and these periods can be used to ramp up/down power (the 8 tail bits at the start of the Access Burst are not all 0). Different bursts are separated from each other by a guard period. The Normal Burst (NB) is used for data transfer and its stealing flags signal what type of data it is. The Frequency Correction Burst (FB) lets a MS correct its frequency since all bits are 0 which is equivalent to an unmodulated carrier with a constant frequency offset. The Synchronization Burst (SB) is used for time synchronization. The Dummy Burst (DB) is only transmitted by the BTS on the BCCH carrier frequency and allows the MS to do signal power measurements. The Access Burst (AB) is used for random access before the MS is time-synchronized to the BTS or it hasn t got a connection yet. (Eberspächer et al., 2009) 2.2 Logical channels GSM defines several different channels for various uses. This paper won t go much more into detail than the absolute basics. The BTS uses a set of Broadcast Channels (BCH) which all allows the BTS to broadcast information to all MSs in the cell. The Broadcast Control Channel (BCCH) is transmitted on the first frequency of the CA and broadcasts information about the cell (e.g. radio configuration) and its neighbours. The Frequency Correction Channel (FCCH) is used to send FBs. And the Synchronization Channel (SCH) is used to identify the BTS and for frame synchronization by transmitting SBs. Traffic channels (TCH) are duplex channels used for circuit switched data transfer between a MS and the BTS. There are several types, but most importantly a TCH can be split into two half-rate channels to support more users. 3

7 For signalling between single MSs and the BTS there are four simplex Common Control Channels (CCCH). The Paging Channel (PCH) is used by the BTS to page a specific MS. The MS uses the Random Access Channel (RACH) to request the assignment of a SDCCH or TCH with a random access burst, using the principle of slotted Aloha. This principle basically means that the MS sends in a slot of the RACH as soon as possible. If no acknowledgement is received in an expected time, a random back-of time is used and then the transmission is resent. The BTS uses the Access Grant Channel (AGCH) to assign a SDCCH or a TCH to a MS (this constitutes the acknowledgement). And at last, the BTS also uses the Notification Channel (NCH) to notify MSs of group and broadcast calls. Another type of channel is the Dedicated Control Channels (DCCH) which are duplex signalling channels between a single MS and the BTS, and there are three main types of them. The Stand-alone Dedicated Control Channel (SDCCH) is used when there is no active connection (e.g. a call). The Slow Associated Control Channel (SACCH) is always used together with a TCH or SDCCH to signal synchronization commands and power control. Transmission of SACCH data is also taken as proof of the connection. The last channel is the Fast Associated Control Channel (FACCH) which only can be used when using a TCH. When transmitting NBs on a TCH, setting the stealing flags (SF) signals that the data is not user data but FACCH data, i.e. the FACCH allows to send signals immediately but at the expense of the user data rate. Lastly, there is a Cell Broadcast Channel (CBCH) used by the BTS to send Short Message Service Cell Broadcast messages at the same physical channel as the SDCCH. When a mobile phone gets a call the following occurs: The MS gets paged on the PCH and uses the RACH to request a SDCCH. This is granted through the AGCH, and the SDCCH is then used to respond to the page, setup authentication and encryption, and to confirm the call. The BTS then uses the SDCCH to assign a TCH to the MS, which in turn uses the FACCH of the TCH to acknowledge this and finish the call setup. After this, voice data can be transferred on the TCH, see figure 3. (3GPP TS ) (Eberspächer et al., 2009) Figure 3: GSM call setup (Eberspächer et al., 2009) 2.3 Mapping of logical channels to physical channels A full rate TCH occupies one specific time slot of each frame on one uplink and one downlink frequency. This is however multiplexed together with the SACCH using the 26-multiframe. In one 4

8 such multiframe 1 frame is for the SACCH, 24 for the TCH and one frame is idle in this configuration. This means that the resulting data rate (before encryption and channel coding) for a full rate TCH is / bps = 22.8 kbps. Other channels (i.e. broadcast and control channels, SDCCH and SACCH) are all using only the BCCH carrier frequency (except CBCH, SDCCH and SACCH which also can be put on other frequencies). Most of these channels are only allowed to use timeslot 0 of a frame, but BCCH, PCH, AGCH and RACH can also use timeslots 2, 4 and 6. The channels are mapped in time on this frequency using the 51-multiframe, and the mapping is dependent of the current channel usage. This means that only part of the BCCH carrier frequency is reserved for signalling in the system and the rest of the time and spectrum can be used for TCHs (with associated SACCH and FACCH) and SDCCHs. (3GPP TS ) 5

9 3 Radio path This chapter deals with the physical procedures that are done to ensure successful communication on the radio path for all units. 3.1 Synchronization Time synchronization There are two reasons to why it is very important for a MS to be synchronized in time to the BTS. The first is the TDMA structure with slots and numbered frames; the MS needs to know which frame and time slot is which to be able to interpret received communication and know when to transmit. The second reason is that inside a GSM cell, MSs are mobile and can be at different distances from the BTS. But at the BTS the bursts from different MSs must not overlap by more than the guard period. To resolve the first problem, the BTS uses the SCH with SBs to transmit the current FN, so the MS knows the current frame. The solution to the second problem is to let the BTS detect if the MS needs to transmit earlier or later and send commands about this on the SACCH. The MS can at most advance the transmission of burst by 63 symbol times, which then is supposed to adjust for the round trip-time delay. This means that the maximum distance between the MS and the BTS that is supported corresponds to 31.5 symbol periods which equals approximately μs or in actual distance 35 km. This is therefore the maximum radii of a GSM cell. (Eberspächer et al., 2009) Frequency synchronization GSM allows for cheap MSs to be produced that can utilize cheap oscillators which aren t very stable. It is thus important to continuously synchronize the frequency of the MS to the BTS to assure that the MS does not drift too much. The BTS provides this synchronization by frequently sending FBs on the FCCH. The FB is a burst with only 0s, which due to GMSK modulation (see section 4.1) renders in an unmodulated carrier of frequency khz more than the frequency that should be set. 24 (Eberspächer et al., 2009) 3.2 Connection and random access Before a MS is synchronized at all to a BTS it may be that it does not have any information about the BTS at all (e.g. BCCH carrier, BTS identification, FN). Still it should be able to synchronize to it. The first thing the MS needs to do is to find the BCCH carrier to be able to discern information needed for registration with that particular BTS. Specification 3GPP TS specifies that the MS should monitor all GSM frequencies during a period of 3-5 s, but not how to determine the BCCH carrier with this information. However this could be done by realizing that the BCCH carrier is most probably the frequency with the most energy since the BTS is required to send a DB during any unused time slot, while other frequencies can remain silent. The MS will probably find several BCCH frequencies from different close cells, and have to choose the one where it will have the highest probability of being able to communicate. When the MS is synchronized to a BTS (called camping) it can start communicate with the BTS (registration with the network, receiving pages for calls etc.). Before a MS can transmit any information to the BTS, it must request a SDCCH (as in the call setup in figure 3). This is requested with the use of the RACH, sending an AB, which is much shorter than a 6

10 regular burst. This is because at this time, the MS maybe hasn t transmitted anything yet, and so the BTS haven t sent any time synchronization data yet. The shortened AB ensures that there is no overlap at the BTS to the next burst even if the MS is at the edge of the cell (35 km from the BTS). (Eberspächer et al., 2009) 3.3 Power management Another important part of the GSM system is power management. This function has two benefits; a MS can save battery, and it decreases the interference between adjacent channels. The MS always measures the signal level (RXLEV) and the signal quality (RXQUAL, which is related to the received bit error rate) received from the BTS. Whenever the MS has a SACCH it will continuously send these values to the BTS including the currently used power level. The BTS can use these values to adjust its own power level for bursts to that particular MS, although it never changes the power of the BCCH carrier which always shall be constant so that a MS can compare the received signal from different BTSs. However, the BTS always sends commands to the MS about changes that should be made to the MSs power level, which can be adjusted in steps of 2 db. Before any power level commands have been received, or when transmitting on RACH, the MS uses a maximum allowed power level that is broadcast by the BTS. (3GPP TS ) 3.4 Handoff When not in an active connection (i.e. a SDCCH or a TCH) the MS itself monitors other BCCH carriers from nearby BTSs and chooses if it should synchronize to another one. But during an ongoing connection this is not as easily done. During a connection the MS shall the entire time measure RXLEV and RXQUAL for the connected BTS and for surrounding BTSs and send all these values on the SACCH. To know which BTSs these values come from, the MS needs to read an identification value from each BTS which is broadcast on the SCH. This is the reason to the idle frame in the 26-multiframe for full rate TCH. During the idle frame the MS should perform measurements on other BCCH carriers. The control channels uses the 51- multiframe which means that the position of this multiframe always shifts as compared to the 26- multiframe, and at some point there will be a SCH burst from a neighbouring cell in the idle frame. Moreover, the BTS also measures RXLEV and RXQUAL, and the current interference on its idle channels. With all this information, the BTS can (optionally supported by the Mobile Switching Center) decide if a handover should be done and signal this to the MS. A handover can be made both in the current BTS to another channel, or to a different BTS. The criteria for a handover to be made and the entire handover process are not specified by the GSM standard but are left to the network operator. The specification only provides a default algorithm for decision and handover procedure that could be used. This allows operators to configure their network more freely, and perhaps also use handover to try to level out traffic distribution. (3GPP TS ) 3.5 Equalization Since GSM is a narrowband system used all over the world with different environments and moving MSs, it is absolutely vital for adaptive equalizers as to undo the effect of the filtering of the signal between sender and receiver. 7

11 GSM does not specify anything about equalizers but provides means that are to be used by them. Each and every burst contains a training sequence that is known by the receiver, moreover the tail bits in the beginning and end of each burst is also known. These two properties of the bursts makes it possible for all receivers to adaptively perform equalization of received signals. As an example of how adaptive equalization can be performed we can look at an example developed by Agilent Technologies (2008). In their library an equalizer is available that consists of an adjustable matched filter and a modified Viterbi processor. The matched filter tries to provide an optimum signal-to-noise ratio either by help of a channel estimator or by adjusting the filter using a gradient algorithm. The output of the filter is fed into the modified Viterbi processor. A Viterbi decoder is used to get a Maximum Likelihood Sequence Estimate (MLSE) of a received signal. Together the two parts provide an adaptive equalization of the received GSM signals. 8

12 4 Modulation In GSM there are many different modulation schemes allowed: APQSK, QPSK, 16-QAM, 32-QAM, 8- PSK and GMSK. In this section we will mostly focus on GMSK since this technique is characteristic for GSM. For the discussion in this chapter we denote the symbol time as 1 = 1625 ksymbols/s T ksymbols/s, in accordance with section GMSK GMSK stands for Gaussian Mean Shift Keying which comes from that this modulation is a variation of Mean Shift Keying combined with a Gaussian filter as explained further in this section. The internal state of the modulator both before and after the burst (which is to be modulated) is as if continuous ones enter the modulator right before and right after the actual bits of the burst. We call the input bit sequence d i {0, 1}, and thus for bits outside the burst we have d i = 1. The first step of the modulation is to encode this differentially: d i = d i d i 1 where is the XOR operation. This sequence generates another sequence according to α i = 2d i 1 which implies that α i { 1, +1}, and it is this sequence as Dirac pulses that is actually modulated. At this point we need to define a couple of functions: with rect ( t T ) = { 1 T, for t < T 2 0, otherwise h(t) = 1 t2 exp ( 2πδT 2δ 2 T 2) δ = ln (2), BT = 0.3 2πBT This implies that B is the 3 db bandwidth of a linear filter with h(t) as impulse response. The Dirac sequence α i now excites the filter with impulse response g(t) = rect ( t ) h(t). The output of this T filter is then used to produce the phase of the modulation signal according to: t it φ(t) = a i πh i g(s)ds Here h is called the modulation index and has a constant value of 1, which means that the maximum 2 phase change is π per data interval. And the final transmitted signal is then 2 x(t) = 2E c T cos(2πf 0t + φ(t) + φ 0 ) 9

13 Where E c denotes the energy per bit, f 0 the carrier frequency and φ 0 a random phase that is constant for the entire burst. The transmitter filter impulse response g(t) is presented in figure 4. (3GPP TS ) Figure 4: Impulse response of the transmitter filter This entire procedure produces Mean Shift Keyed modulation with an extra Gaussian lowpass filter in the process. The Mean Shift Keying results in a constant amplitude envelope of the signal which makes receivers cheaper, since the amplifiers don t have much demand on linearity. The Gaussian filter makes the spectrum of the signal more narrow, but at the cost of increasing the inter-symbol interference (see figure 4, where the impulse response lasts more than one symbol interval). The benefit is that interference from adjacent channels is reduced. (Eberspächer et al., 2009) 4.2 Other modulations The other allowed modulation techniques are the standard AQPSK, 8-PSK, QPSK, 16-QAM and 32- QAM with some small modifications. For a symbols sequence s i that is to be modulated, GSM creates a new sequence before modulation which is a continuous rotation of the input sequence: s i = s i e jiφ, where φ is a constant that depends on the modulation scheme. The rotated sequence is then pulse shaped with a linear filter with impulse response c 0 (t): y(t) = s ic 0 (t it + 2T) i c o (t) is a linearized GSMK pulse. The expression for this is quite complicated and will not be presented here, but can be found in the specification 3GPP TS , section 3.5. The transmitted signal is then as usual: x(t) = 2E s T Re[y(t)ej(2πf 0t+φ 0 ) ] Where E s is the energy per symbol. (3GPP TS ) 10

14 5 Coding In this chapter coding schemes that are used in GSM are presented. In the first section methods for error correction and detection are explained, and in the second we will discuss compression methods used when transmitting voice. 5.1 Channel coding Since there are numerous types of channels there are also many channel codes, each channel has its own. In this document we will only present two of the most important ones. However, the general structure is the same for all channels. The first step is to create some parity bits using a block code on a block of data mostly used for error detection. This is followed by encoding both information and parity bits with a convolutional code or a turbo code for error correction. Finally, these bits are reordered and interleaved before they are sent over the channel. These techniques can reduce a very high bit error probability of order 10 1 to 10 3 down to error probabilities in the range of 10 5 to 10 6 (Eberspächer et al., 2009) Full rate TCH The input to the channel coder for full rate TCH is either blocks of 260 bits from a Full Rate speech coder, or blocks of 244 bits from an Enhanced Full Rate (EFR) speech coder. If EFR is used an extra step is done at first to produce a block of 260 bits. This extra step is to first use an 8-bit CRC code on the most important 65 bits, these parity bits are placed last in the block. The generator polynomial of this code is g(d) = D 8 + D 4 + D 3 + D Also four important bits that are not otherwise protected by the channel coding are repeated two times each. This adds the extra 16 bits needed to get a block of the same length as with full rate speech encoding, and concludes the extra coding step needed for EFR. The block is divided into two classes; 182 bits for class 1, and 78 bits for class 2. Only the bits in class 1 are protected by the channel coding, the bits of class 2 are not equally important and are left unprotected. The block code used to create the parity bits in the first step is a degenerate cyclic block code with parameters (53, 50, 2) which is used on the first 50 bits of class 1, and produces 3 parity bits. Then the information bits of class 1 and the parity bits are reordered and appended with four tail bits, all 0, making a total of 189 new class 1 bits. The bits of class 1 are then encoded by a rate ½ convolutional code with memory of order m = 4 and free distance of d free = 7. The coded bits followed by the bits of class 2 now constitute a block of 456 bits. Each coded block of 456 bits are then interleaved over 8 bursts, with 57 bits in each. This means that every 4 th TCH burst a new block starts, which is spread over 8 bursts. (3GPP TS ) SACCH, SDCCH, FACCH, BCCH, PCH, AGCH, NCH and CBCH For these channels, the information blocks that are to be sent are 184 bits long. Apart from the first block code which for these channels is a shortened cyclic block code generating 40 parity bits, the 11

15 channel coding is the same as for a full rate TCH. I.e. the same rate ½ convolutional code is used, and the interleaving procedure is the same. The special case is for the full rate FACCH which steals bits from the TCH which, due to the same interleaving scheme used, all belong to the same block. The stealing of bits in bursts is indicated by the SFs in the NB. For the first four bursts the first SF is set, and for the last four the last SF is set. (3GPP TS ) 5.2 Source coding (Full Rate speech) The standard full rate speech encoding used in GSM is called Regular Pulse Excitation Long Term Prediction (RPE-LTP). The speech is sampled at 8 khz with 13 bits uniform quantization and is grouped into frames of 160 samples (20 ms). These frames are also divided into 4 sub-frames of 40 samples each. The general structure of the algorithm is presented in figure 5. Figure 5: The structure of the RPE-LTP algorithm (3GPP TS ) The pre-processing step is to remove offset of the signal and to filter it with a pre-emphasis FIR filter. In the short term Linear Prediction Speech (LPC) analysis block, reflection coefficients are calculated from the frame. These are a model of the human speech. The coefficients are transformed into Log Area Ratios (LAR) according to a bijective mapping. These are quantized and coded with different amounts of bits depending of their importance, and totals in 36 bits. The short term analysis filter uses the LPC coefficients to calculate residuals for the frame and forwards on a sub-frame basis (i.e. the rest of the system operates on sub-frames). The long term analysis filter and LTP analysis produces a long term correlation lag (using history from previous sub-frames) and an associated gain factor. These are coded with 7 and 2 bits respectively, making a total of 36 bits per frame. The RPE part of the coding first filters each residual sub-frame with a weighting filter. Then it does an adaptive sample rate decimation which is also called RPE grid selection, since it finds the optimal decimation grid out of four candidates. The samples of the selected sequence are then quantized. The information that is transmitted is the RPE grid selection and the quantization of the samples which totals in 47 bits per sub-frame, or 188 bits per frame. 12

16 In total this gives 260 bits per frame to be sent to the channel coding procedure. The original frame size was = 2080 bits, which means that this is a compression of factor = 8 (the compressed stream takes up only 12.5 % of the original data size). The uncompressed data stream that is transmitted with some distortion by the GSM has a rate of = 104 kbps. (3GPP TS ) 13

17 6 Summary In this paper we have described and discussed some basic parts of the widespread cellular mobile telephony standard GSM. The division of time and frequency to create several channels and the structure of logical channels was presented. We have explained some procedures that are used to ensure the quality of the communication in an efficient manner. Also the characterizing modulation for GSM, GMSK was thoroughly described and lastly an overview of the compression of voice data that is employed was made. Together this is aimed at giving the reader a fundamental understanding of the wireless aspect of GSM, and if interest arises there is much more to read and learn in the references of this report. 14

18 7 References Internet ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Technical Specifications and Technical Reports for a GERAN-based 3GPP system (3GPP TS version Release 11). Available at: [PDF] ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); LTE; Network architecture (3GPP TS version Release 12). Available at: [PDF] ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Radio transmission and reception (3GPP TS version Release 12). Available at: [PDF] ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Multiplexing and multiple access on the radio path (3GPP TS version Release 12). Available at: [PDF] ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Physical layer on the radio path; General description (3GPP TS version Release 12). Available at: [PDF] ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Full rate speech; Transcoding (3GPP TS version Release 12). Available at: [PDF] ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Channel coding (3GPP TS version Release 12). Available at: [PDF] ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Modulation (3GPP TS version Release 12). Available at: [PDF] 15

19 ETSI TS V ( ) Digital cellular telecommunications system (Phase 2+); Radio subsystem link control (3GPP TS version Release 12). Available at: [PDF] Agilent Technologies, Advanced Design System 2008 Documentation (2008) GSM Equalizer. Available at: er-gsmequalizer [WWW] Books J. Eberspächer, C. Bettstetter, H. Vögel, C. Hartmann GSM Architecture, Protocols and Services (2009). John Wiley & Sons, New Jersey. S. M. Redl, M. K. Weber, M. W. Oliphant An introduction to GSM (1995). Artech House, Boston. 16

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