GSM Frame StructureThe GSM system has a defined GSM frame structure to enable the orderly passage of information. The

GSM frame structure establishes schedules for the predetermined use of timeslots.

By establishing these schedules by the use of a frame structure, both the mobile and the base station

are able to communicate not only the voice data, but also signalling information without the various

types of data becoming intermixed and both ends of the transmission knowing exactly what types of

information are being transmitted.

The GSM frame structure provides the basis for the various physical channels used within GSM, and

accordingly it is at the heart of the overall system.

Basic GSM frame structureThe basic element in the GSM frame structure is the frame itself. This comprises the eight slots, each

used for different users within the TDMA system. As mentioned in another page of the tutorial, the

slots for transmission and reception for a given mobile are offset in time so that the mobile does not

transmit and receive at the same time.

GSM frame consisting of eight slots

The basic GSM frame defines the structure upon which all the timing and structure of the GSM

messaging and signalling is based. The fundamental unit of time is called a burst period and it lasts for

approximately 0.577 ms (15/26 ms). Eight of these burst periods are grouped into what is known as a

TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the

definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.

In simplified terms the base station transmits two types of channel, namely traffic and control.

Accordingly the channel structure is organised into two different types of frame, one for the traffic on

the main traffic carrier frequency, and the other for the control on the beacon frequency. 

GSM multiframeThe GSM frames are grouped together to form multiframes and in this way it is possible to establish a

time schedule for their operation and the network can be synchronised.

There are several GSM multiframe structures:

Traffic multiframe:   The Traffic Channel frames are organised into multiframes consisting of

26 bursts and taking 120 ms. In a traffic multiframe, 24 bursts are used for traffic. These are

numbered 0 to 11 and 13 to 24. One of the remaining bursts is then used to accommodate the

SACCH, the remaining frame remaining free. The actual position used alternates between

position 12 and 25.

Control multiframe:   the Control Channel multiframe that comprises 51 bursts and occupies

235.4 ms. This always occurs on the beacon frequency in time slot zero and it may also occur

within slots 2, 4 and 6 of the beacon frequency as well. This multiframe is subdivided into

logical channels which are time-scheduled. These logical channels and functions include the

following:o Frequency correction burst   

o Synchronisation burst   

o Broadcast channel (BCH)   

o Paging and Access Grant Channel (PACCH)   

o Stand Alone Dedicated Control Channel (SDCCH)   

GSM SuperframeMultiframes are then constructed into superframes taking 6.12 seconds. These consist of 51 traffic

multiframes or 26 control multiframes. As the traffic multiframes are 26 bursts long and the control

multiframes are 51 bursts long, the different number of traffic and control multiframes within the

superframe, brings them back into line again taking exactly the same interval.

GSM HyperframeAbove this 2048 superframes (i.e. 2 to the power 11) are grouped to form one hyperframe which

repeats every 3 hours 28 minutes 53.76 seconds. It is the largest time interval within the GSM frame

structure.

Within the GSM hyperframe there is a counter and every time slot has a unique sequential number

comprising the frame number and time slot number. This is used to maintain synchronisation of the

different scheduled operations with the GSM frame structure. These include functions such as:

Frequency hopping:   Frequency hopping is a feature that is optional within the GSM system.

It can help reduce interference and fading issues, but for it to work, the transmitter and

receiver must be synchronised so they hop to the same frequencies at the same time.

Encryption:   The encryption process is synchronised over the GSM hyperframe period where

a counter is used and the encryption process will repeat with each hyperframe. However, it is

unlikely that the cellphone conversation will be over 3 hours and accordingly it is unlikely that

security will be compromised as a result.

GSM Frame Structure Summary

GSM Network Interfaces [4]

The network structure is defined within the GSM standards. Additionally each interface between the

different elements of the GSM network is also defined. This facilitates the information interchanges can

take place. It also enables to a large degree that network elements from different manufacturers can

be used. However as many of these interfaces were not fully defined until after many networks had

been deployed, the level of standardisation may not be quite as high as many people might like.

1. Um interface   The "air" or radio interface standard that is used for exchanges between a

mobile (ME) and a base station (BTS / BSC). For signalling, a modified version of the ISDN

LAPD, known as LAPDm is used.

2. Abis interface   This is a BSS internal interface linking the BSC and a BTS, and it has not been

totally standardised. The Abis interface allows control of the radio equipment and radio

frequency allocation in the BTS.

3. A interface   The A interface is used to provide communication between the BSS and the MSC.

The interface carries information to enable the channels, timeslots and the like to be allocated

to the mobile equipments being serviced by the BSSs. The messaging required within the

network to enable handover etc to be undertaken is carried over the interface.

4. B interface   The B interface exists between the MSC and the VLR . It uses a protocol known

as the MAP/B protocol. As most VLRs are collocated with an MSC, this makes the interface

purely an "internal" interface. The interface is used whenever the MSC needs access to data

regarding a MS located in its area.

5. C interface   The C interface is located between the HLR and a GMSC or a SMS-G. When a call

originates from outside the network, i.e. from the PSTN or another mobile network it ahs to

pass through the gateway so that routing information required to complete the call may be

gained. The protocol used for communication is MAP/C, the letter "C" indicating that the

protocol is used for the "C" interface. In addition to this, the MSC may optionally forward billing

information to the HLR after the call is completed and cleared down.

6. D interface   The D interface is situated between the VLR and HLR. It uses the MAP/D protocol

to exchange the data related to the location of the ME and to the management of the

subscriber.

7. E interface   The E interface provides communication between two MSCs. The E interface

exchanges data related to handover between the anchor and relay MSCs using the MAP/E

protocol.

8. F interface   The F interface is used between an MSC and EIR. It uses the MAP/F protocol. The

communications along this interface are used to confirm the status of the IMEI of the ME

gaining access to the network.

9. G interface   The G interface interconnects two VLRs of different MSCs and uses the MAP/G

protocol to transfer subscriber information, during e.g. a location update procedure.

10. H interface   The H interface exists between the MSC the SMS-G. It transfers short messages

and uses the MAP/H protocol.

11. I interface   The I interface can be found between the MSC and the ME. Messages exchanged

over the I interface are relayed transparently through the BSS.

Although the interfaces for the GSM cellular system may not be as rigorouly defined as many might

like, they do at least provide a large element of the definition required, enabling the functionality of

GSM network entities to be defined sufficiently.

GSM Radio Air Interface, GSM Slot and Burst [5]

One of the key elements of the development of the GSM, Global System for Mobile Communications

was the development of the GSM air interface. There were many requirements that were placed on the

system, and many of these had a direct impact on the air interface. Elements including the

modulation, GSM slot structure, burst structure and the like were all devised to provide the optimum

performance.

During the development of the GSM standard very careful attention was paid to aspects including the

modulation format, the way in which the system is time division multiplexed, all had a considerable

impact on the performance of the system as a whole. For example, the modulation format for the GSM

air interface had a direct impact on battery life and the time division format adopted enabled the

cellphone handset costs to be considerably reduced as detailed later.

GSM signal and GMSK modulation characteristics

The core of any radio based system is the format of the radio signal itself. The carrier is modulated

using a form of phase sift keying known as Gaussian Minimum Shift Keying (GMSK). GMSK was used for

the GSM system for a variety of reasons:

It is resilient to noise when compared to many other forms of modulation.

Radiation outside the accepted bandwidth is lower than other forms of phase shift keying.

It has a constant power level which allows higher efficiency RF power amplifiers to be used in

the handset, thereby reducing current consumption and conserving battery life.

Note on GMSK:

GMSK, Gaussian Minimum Shift Keying is a form of phase modulation that is used in a number of portable

radio and wireless applications. It has advantages in terms of spectral efficiency as well as having an almost

constant amplitude which allows for the use of more efficient transmitter power amplifiers, thereby saving on

current consumption, a critical issue for battery power equipment.

Click on the link for a GMSK tutorial

The nominal bandwidth for the GSM signal using GMSK is 200 kHz, i.e. the channel bandwidth and

spacing is 200 kHz. As GMSK modulation has been used, the unwanted or spurious emissions outside

the nominal bandwidth are sufficiently low to enable adjacent channels to be used from the same base

station. Typically each base station will be allocated a number of carriers to enable it to achieve the

required capacity.

The data transported by the carrier serves up to eight different users under the basic system by

splitting the carrier into eight time slots. The basic carrier is able to support a data throughput of

approximately 270 kbps, but as some of this supports the management overhead, the data rate

allotted to each time slot is only 24.8 kbps. In addition to this error correction is required to overcome

the problems of interference, fading and general data errors that may occur. This means that the

available data rate for transporting the digitally encoded speech is 13 kbps for the basic vocoders.

GSM slot structure and multiple access schemeGSM uses a combination of both TDMA and FDMA techniques. The FDMA element involves the division

by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart

as already described.

The carriers are then divided in time, using a TDMA scheme. This enables the different users of the

single radio frequency channel to be allocated different times slots. They are then able to use the

same RF channel without mutual interference. The slot is then the time that is allocated to the

particular user, and the GSM burst is the transmission that is made in this time.

Each GSM slot, and hence each GSM burst lasts for 0.577 mS (15/26 mS). Eight of these burst periods

are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26

ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst

period allocated in each TDMA frame.

There are different types of frame that are transmitted to carry different data, and also the frames are

organised into what are termed multiframes and superframes to provide overall synchronisation.

GSM slot structureThese GSM slot is the smallest individual time period that is available to each mobile. It has a defined

format because a variety of different types of data are required to be transmitted.

Although there are shortened transmission bursts, the slots is normally used for transmitting 148 bits

of information. This data can be used for carrying voice data, control and synchronisation data.

GSM slots showing offset between transmit and receive

It can be seen from the GSM slot structure that the timing of the slots in the uplink and the downlink

are not simultaneous, and there is a time offset between the transmit and receive. This offset in the

GSM slot timing is deliberate and it means that a mobile that which is allocated the same slot in both

directions does not transmit and receive at the same time. This considerably reduces the need for

expensive filters to isolate the transmitter from the receiver. It also provides a space saving.

GSM burstThe GSM burst, or transmission can fulfil a variety of functions. Some GSM bursts are used for carrying

data while others are used for control information. As a result of this a number of different types of

GSM burst are defined.

Normal burst   uplink and downlink

Synchronisation burst  downlink

Frequency correction burst  downlink

Random Access (Shortened Burst)   uplink

GSM normal burst

This GSM burst is used for the standard communications between the basestation and the mobile, and

typically transfers the digitised voice data.

The structure of the normal GSM burst is exactly defined and follows a common format. It contains

data that provides a number of different functions:

1. 3 tail bits:   These tail bits at the start of the GSM burst give time for the transmitter to ramp

up its power

2. 57 data bits:   This block of data is used to carry information, and most often contains the

digitised voice data although on occasions it may be replaced with signalling information in the

form of the Fast Associated Control CHannel (FACCH). The type of data is indicated by the flag

that follows the data field

3. 1 bit flag:   This bit within the GSM burst indicates the type of data in the previous field.

4. 26 bits training sequence:   This training sequence is used as a timing reference and for

equalisation. There is a total of eight different bit sequences that may be used, each 26 bits

long. The same sequence is used in each GSM slot, but nearby base stations using the same

radio frequency channels will use different ones, and this enables the mobile to differentiate

between the various cells using the same frequency.

5. 1 bit flag   Again this flag indicates the type of data in the data field.

6. 57 data bits   Again, this block of data within the GSM burst is used for carrying data.

7. 3 tail bits   These final bits within the GSM burst are used to enable the transmitter power to

ramp down. They are often called final tail bits, or just tail bits.

8. 8.25 bits guard time   At the end of the GSM burst there is a guard period. This is introduced

to prevent transmitted bursts from different mobiles overlapping. As a result of their differing

distances from the base station.

GSM Normal Burst

GSM synchronisation burstThe purpose of this form of GSM burst is to provide synchronisation for the mobiles on the network.

1. 3 tail bits:   Again, these tail bits at the start of the GSM burst give time for the transmitter to

ramp up its power

2. 39 bits of information:  

3. 64 bits of a Long Training Sequence:  

4. 39 bits Information:  

5. 3 tail bits   Again these are to enable the transmitter power to ramp down.

6. 8.25 bits guard time:   to act as a guard interval.

GSM Synchronisation Burst

GSM frequency correction burstWith the information in the burst all set to zeros, the burst essentially consists of a constant frequency

carrier with no phase alteration.

1. 3 tail bits:   Again, these tail bits at the start of the GSM burst give time for the transmitter to

ramp up its power.

2. 142 bits all set to zero:  

3. 3 tail bits   Again these are to enable the transmitter power to ramp down.

4. 8.25 bits guard time:   to act as a guard interval.

GSM Frequency Correction Burst

GSM random access burstThis form of GSM burst used when accessing the network and it is shortened in terms of the data

carried, having a much longer guard period. This GSM burst structure is used to ensure that it fits in

the time slot regardless of any severe timing problems that may exist. Once the mobile has accessed

the network and timing has been aligned, then there is no requirement for the long guard period.

1. 7 tail bits:   The increased number of tail bits is included to provide additional margin when

accessing the network.

2. 41 training bits:  

3. 36 data bits:  

4. 3 tail bits   Again these are to enable the transmitter power to ramp down.

5. 69.25 bits guard time:   The additional guard time, filling the remaining time of the GSM

burst provides for large timing differences.

GSM Random Access Burst

GSM discontinuous transmission (DTx)

A further power saving and interference reducing facility is the discontinuous transmission (DTx)

capability that is incorporated within the specification. It is particularly useful because there are long

pauses in speech, for example when the person using the mobile is listening, and during these periods

there is no need to transmit a signal. In fact it is found that a person speaks for less than 40% of the

time during normal telephone conversations. The most important element of DTx is the Voice Activity

Detector. It must correctly distinguish between voice and noise inputs, a task that is not trivial. If a

voice signal is misinterpreted as noise, the transmitter is turned off an effect known as clipping results

and this is particularly annoying to the person listening to the speech. However if noise is

misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased.

It is also necessary for the system to add background or comfort noise when the transmitter is turned

off because complete silence can be very disconcerting for the listener. Accordingly this is added as

appropriate. The noise is controlled by the SID (silence indication descriptor).

GSM handover or handoff [11]One of the key elements of a mobile phone or cellular telecommunications system, is that the system

is split into many small cells to provide good frequency re-use and coverage. However as the mobile

moves out of one cell to another it must be possible to retain the connection. The process by which

this occurs is known as handover or handoff. The term handover is more widely used within Europe,

whereas handoff tends to be use more in North America. Either way, handover and handoff are the

same process.

Requirements for GSM handoverThe process of handover or handoff within any cellular system is of great importance. It is a critical

process and if performed incorrectly handover can result in the loss of the call. Dropped calls are

particularly annoying to users and if the number of dropped calls rises, customer dissatisfaction

increases and they are likely to change to another network. Accordingly GSM handover was an area to

which particular attention was paid when developing the standard.

Types of GSM handoverWithin the GSM system there are four types of handover that can be performed for GSM only systems:

Intra-BTS handover:   This form of GSM handover occurs if it is required to change the

frequency or slot being used by a mobile because of interference, or other reasons. In this form

of GSM handover, the mobile remains attached to the same base station transceiver, but

changes the channel or slot.

Inter-BTS Intra BSC handover:   This for of GSM handover or GSM handoff occurs when the

mobile moves out of the coverage area of one BTS but into another controlled by the same

BSC. In this instance the BSC is able to perform the handover and it assigns a new channel and

slot to the mobile, before releasing the old BTS from communicating with the mobile.

Inter-BSC handover:   When the mobile moves out of the range of cells controlled by one

BSC, a more involved form of handover has to be performed, handing over not only from one

BTS to another but one BSC to another. For this the handover is controlled by the MSC.

Inter-MSC handover:   This form of handover occurs when changing between networks. The

two MSCs involved negotiate to control the handover.

GSM handover processAlthough there are several forms of GSM handover as detailed above, as far as the mobile is

concerned, they are effectively seen as very similar. There are a number of stages involved in

undertaking a GSM handover from one cell or base station to another.

In GSM which uses TDMA techniques the transmitter only transmits for one slot in eight, and similarly

the receiver only receives for one slot in eight. As a result the RF section of the mobile could be idle for

6 slots out of the total eight. This is not the case because during the slots in which it is not

communicating with the BTS, it scans the other radio channels looking for beacon frequencies that

may be stronger or more suitable. In addition to this, when the mobile communicates with a particular

BTS, one of the responses it makes is to send out a list of the radio channels of the beacon frequencies

of neighbouring BTSs via the Broadcast Channel (BCCH).

The mobile scans these and reports back the quality of the link to the BTS. In this way the mobile

assists in the handover decision and as a result this form of GSM handover is known as Mobile Assisted

Hand Over (MAHO).

The network knows the quality of the link between the mobile and the BTS as well as the strength of

local BTSs as reported back by the mobile. It also knows the availability of channels in the nearby cells.

As a result it has all the information it needs to be able to make a decision about whether it needs to

hand the mobile over from one BTS to another.

If the network decides that it is necessary for the mobile to hand over, it assigns a new channel and

time slot to the mobile. It informs the BTS and the mobile of the change. The mobile then retunes

during the period it is not transmitting or receiving, i.e. in an idle period.

A key element of the GSM handover is timing and synchronisation. There are a number of possible

scenarios that may occur dependent upon the level of synchronisation.

Old and new BTSs synchronised:   In this case the mobile is given details of the new

physical channel in the neighbouring cell and handed directly over. The mobile may optionally

transmit four access bursts. These are shorter than the standard bursts and thereby any

effects of poor synchronisation do not cause overlap with other bursts. However in this

instance where synchronisation is already good, these bursts are only used to provide a fine

adjustment.

Time offset between synchronised old and new BTS:   In some instances there may be a

time offset between the old and new BTS. In this case, the time offset is provided so that the

mobile can make the adjustment. The GSM handover then takes place as a standard

synchronised handover.

Non-synchronised handover:   When a non-synchronised cell handover takes place, the

mobile transmits 64 access bursts on the new channel. This enables the base station to

determine and adjust the timing for the mobile so that it can suitably access the new BTS. This

enables the mobile to re-establish the connection through the new BTS with the correct timing.

Inter-system handover

With the evolution of standards and the migration of GSM to other 2G technologies including to 3G

UMTS / WCDMA as well as HSPA and then LTE, there is the need to handover from one technology to

another. Often the 2G GSM coverage will be better then the others and GSM is often used as the

fallback. When handovers of this nature are required, it is considerably more complicated than a

straightforward only GSM handover because they require two technically very different systems to

handle the handover.

These handovers may be called intersystem handovers or inter-RAT handovers as the handover occurs

between different radio access technologies.

The most common form of intersystem handover is between GSM and UMTS / WCDMA. Here there are

two different types:

UMTS / WCDMA to GSM handover:   There are two further divisions of this category of

handover:o Blind handover:    This form of handover occurs when the base station hands off the

mobile by passing it the details of the new cell to the mobile without linking to it and

setting the timing, etc of the mobile for the new cell. In this mode, the network selects

what it believes to be the optimum GSM based station. The mobile first locates the

broadcast channel of the new cell, gains timing synchronisation and then carries out

non-synchronised intercell handover.o Compressed mode handover:    using this form of handover the mobile uses the gaps I

transmission that occur to analyse the reception of local GSM base stations using the

neighbour list to select suitable candidate base stations. Having selected a suitable

base station the handover takes place, again without any time synchronisation having

occurred.

Handover from GSM to UMTS / WCDMA:   This form of handover is supported within GSM

and a "neighbour list" was established to enable this occur easily. As the GSM / 2G network is

normally more extensive than the 3G network, this type of handover does not normally occur

when the mobile leaves a coverage area and must quickly find a new base station to maintain

contact. The handover from GSM to UMTS occurs to provide an improvement in performance

and can normally take place only when the conditions are right. The neighbour list will inform

the mobile when this may happen.

 Introducing GSM codec

The "Global System for Mobile communications" (GSM) is a digital mobile radio system which is extensively used throughout Europe, and also in many other parts of the world. GSM has used a variety of voice codecs to compress 3.1 kHz audio into between 6.5 and 13 kbps. Two codecs were designed originally, called Half Rate and Full Rate codecs. They were named after the data channel types that were used. Both Half Rate and Full Rate codecs use a system that is based on LPC (Linear Predictive Coding).

1. Half Rate (also called HR, GSM-HR or GSM 06.20). It is a speech coding system developed for GSM. 1.The codec operates at 5.6 kbps (meaning that it uses only the half bandwidth of the Full Rate codec); the network capacity used for voice transmission is doubled (however, it results in reduced audio quality); the sample rate is 8 kHz with 13 bit; frame length 160 samples (20 ms) and sub-frame length 40 samples (5 ms).

2. Full Rate (also called FR, GSM-FR or GSM 06.10). It was the first digital speech coding standard for GSM and it was developed in early 1990s. From this reason it does not ensure so high quality speech that is why it is gradually replaced by EFR and AMR codecs since they offer higher speech quality at lower bit rate. Full Rate codec is based on RPE-LTP (Regular Pulse Excitation - Long Term Prediction) speech coding paradigm.

3. Enhanced Full Rate (also called EFR, GSM-EFR or GSM 06.60). It is the enhanced development of GSM-Full Rate as it produces higher speech quality. Despite the high speech and call quality this codec needs about 5% more energy. It is based on Algebraic Code Excited Linear Prediction Coder (ACELP) algorithm.Read more at http://voip-sip-sdk.com/p_219-gsm-codec-voip.html

Introducing GSMThe "Global System for Mobile communications" (GSM) is a digital mobile radio system which is extensively used throughout Europe, and also in many other parts of the world. GSM has used a variety of voice codecs to compress 3.1 kHz audio into between 6.5 and 13 kbps. Two codecs were designed originally, called Half Rate and Full Rate codecs. They were named after the data channel types that were used. Both Half Rate and Full Rate codecs use a system that is based on LPC (Linear Predictive Coding).

Half Rate (5.6 kbit/s) and Full Rate (13 kbit/s) codecs used a system based upon linear predictive coding (LPC). LPC helps represent - in a compressed form - the spectral range of digital signal of speech. To achieve it, information of a linear predictive model is used.

In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal. These codecs were further enhanced with EFR (Enhanced Full Rate) codec. It operates at 12,2 kbps so it uses a full rate channel.

The GSM full rate speech codec operates at 13 kbits/s and uses a Regular Pulse Excited (RPE) codec. Basically the input speech is split up into frames 20 ms long, and for each frame a set of 8 short term predictor coefficients are found. Each frame is then further split into four 5 ms sub-frames, and for each sub-frame the encoder finds a delay and a gain for the codec's long term predictor. Finally the residual signal after both short and long term filtering is quantized for each sub-frame as follows. 

The 40 sample residual signal is decimated into three possible excitation sequences, each 13 samples long. The sequence with the highest energy is chosen as the best representation of the excitation sequence, and each pulse in the sequence has its amplitude quantized with three bits. At the decoder the reconstructed excitation signal is fed through the long term and then the short term synthesis filters to give the reconstructed speech. A postfilter is used to improve the perceptual quality of this reconstructed speech.

The GSM codec provides good quality speech. Its main advantage over other low rate codecs is its relative simplicity - it runs easily in real time on my 66 Mhz 486 PC for example, whereas CELP codecs need a dedicated DSP to run in real time.

AMR-NB (Adaptive Multi-Rate Narrowband) is a variable-rate codec that ensures high quality and robust against interference in case of full rate channels. If it is used on half-rate channels it is less robust but still quite high quality.

GSM codecs:

1. Half Rate (also called HR, GSM-HR or GSM 06.20). It is a speech coding system developed for GSM. The codec operates at 5.6 kbps (meaning that it uses only the half bandwidth of the Full Rate codec); the network capacity used for voice transmission is doubled (however, it results in reduced audio quality); the sample rate is 8 kHz with 13 bit; frame length 160 samples (20 ms) and sub-frame length 40 samples (5 ms).

Technology

o Encoded bandwidth: ~ 200-3400 Hz o Standardized: ETSI 1994o Coding type: VSELP (Vector Sum Excited Linear Prediction)o Bit rate: 6.5 kbpso Delay (ms):

Frame size: 20 Lookahead: 5

o Quality: < Tollo Complexity:

MIPS: 30 RAM (words): 4 K

2. Full Rate (also called FR, GSM-FR or GSM 06.10). It was the first digital speech coding standard for GSM and it was developed in early 1990s. From this reason it does not ensure so high quality speech that is why it is gradually replaced by EFR and AMR codecs since they offer higher speech quality at lower bit rate. Full Rate codec is based on RPE-LTP (Regular Pulse Excitation - Long Term Prediction) speech coding paradigm.

Technology

o Encoded bandwidth: ~ 200-3400 Hzo Standardized: ETSI 1987o Coding type: RPE-LTP (Regular Pulse Excitation with Long-Term Prediction)o Bit rate: 13 kbpso Delay (ms):

Frame size: 20 Lookahead: 0

o Quality: < Tollo Complexity:

MIPS: 4.5 RAM (words): 1K

3. Enhanced Full Rate (also called EFR, GSM-EFR or GSM 06.60). It is the enhanced development of GSM-Full Rate as it produces higher speech quality. Despite the high speech and call quality this codec needs about 5% more energy. It is based on Algebraic Code Excited Linear Prediction Coder (ACELP) algorithm.

Technology

o Encoded bandwidth: ~ 200-3400 Hzo Standardized: ETSI 1997o Coding type: ACELP® (Algebraic Code Excited Linear Prediction)o Bit rate: 12.2 kbpso Delay (ms):

Frame size: 20 Lookahead: 0

o Quality: Tollo Complexity:

MIPS: 15-20 RAM (words): 4K

Adaptive Multi-Rate (also called AMR or AMR-NB). It is an audio data compression scheme optimized for speech coding. Link adaptation is used for selecting one of eight different bit rates based on link conditions. This codec uses different techniques like ACELP (Algebraic Code Excited Linear Prediction), DTX (Discontinuous Transmission), VAD (Voice Activity Detection) and CNG (Comfort Noise Generation).

Summary for GSM codecs

Audio compression

formatAlgorithm

Sample Rate

Bit rate

Bits per

sampleLatency CBR VBR Stereo

Multi - channel

GSM-HR Lossy 8 kHz5.6 kbps

13 25ms Yes No No No

GSM-FR Lossy 8 kHz13 kbps

13 20-30ms Yes No No No

GSM-EFRACELP, Lossy

8 kHz12.2 kbps

13 20-30ms Yes No No No