Abuzer Study

7/31/2019 Abuzer Study

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There are two types of multiframe. They

are: 26-TDMA frame multiframe which is used to carry TCH,SACCH

and FACCH

51-TDMA frame multiframe which is used to carry BCCH,CCCH,

SDCCH and SACCH

A superframe consists of 51 or 26 multiframes and a hyperframe

consists of 2048 superframes.

The information contained in one time slot on the TDMA frame iscalled a burst. There are five different types of bursts:

GSM uses Frequency Division Duplexing (FDD) where the uplink and

downlink of each channel operates on a different frequency.


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The access burst always

starts with the bit sequence00111010 followed by 41

bit synchronization

sequence


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CBCH is used only on the downlink to carry short massage service cell

broadcast (SMSCB). CBCH uses the same physical channel as theSDCCH.

The 26 frame multi-frame lasts 120ms and the traffic channels for the downlink and

uplink are separated by three bursts. As a consequence the mobiles will not need

to transmit and receive at the same time which simplifies considerably the

electronics of the system ( single synthesizer chip in MS) and preventing high level

transmitted power leakage back to the sensitive receiver. Half rate traffic (TCH/H)

double the capacity of the system are also grouped in a 26 frame multi-frame. The

net bit rate, block length, block recurrence forfull rate and half rate traffic channels

are 13Kbps, 260 bits, 20ms and 6.5Kbps, 112 bits, 20 ms. For full rate speech theblock is divided into two classes according to the importance of the bits (182 bits

for class I and 78 bits for class II). For half rate speech, the block is divided into two

classes as 95 bits for Class I and 17 bits for class II. The TCH/F consists of one

time slot in each TDMA frame i.e. one slot every 4.615ms.


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As we have already seen, GSM uses eight time slots per carrier frequency. If an

MS transmits data with the classical GSM circuit switched data (CSD), then one

of these time slots is allocated to the MS for the whole duration of the data

connection. Each time slot offers a gross data rate of22.8 kbit s1 but not all of

this bandwidth is available to transfer user data. To ensure that the data is

transmitted reliably, a copy of the data is made and sent separately across the

air interface, i.e. each time slot contains both original data and a back-up copy of

data from other time slots. As a result of this built-in redundancy, the net rate of a

CSD connection is 9.6 kbit s1 (or less if the quality of the air interface is poor).


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If the call is for another network, the originating MSC will route it to the

gateway (GMSC) where it will be passed to the other networks gateway.


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LAI and IMSI

or TMSI.

IMSI or TMSI, the paging group,

TRU identity, the channel type

and the time slot number.

IMSI or TMSI.

access delayis registered.

access delay value

selects an idle SDCCH

time slot number, SDCCH, TA

Set AsynchronousBalance Mode (SABM)

is used in order toestablish a multipleframe operation. Thisis a type of LAPDmsignaling.UnnumberedAcknowledged (UA) isused to acknowledgeSABM.

the IMSI or TMSI and the MS classmark.

The BSC adds the CGI and F/W to

MSC/VLR

RAND

SRES

The MSusesRANDtogetherwith itsown Ki,

which isstored inthe SIMcard, tocalculatethe Kc andSRES

Mobile-Terminated Call

T3120

P d t i d b


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A request for Authentication can be initiated by the MS or the network.

Procedure triggered by:

A change in subscriber profile data at the HLR/VLR

Accessing a service

activation or deactivation of a supplementary service

First network access after MSC/VLR restart

Cipher key sequence number mismatch

IMSI/TMSI


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Call initiation isstarted when theMSC sends theSetup messageto the MS. Thismessage contains

bearer serviceinformation forthis call, that is, if itis a speech, data orfax, call.

If the MS canhandle therequested service,

it will send a Callconfirm messageto the MSC/VLR.


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Cell selection


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CALCULATION OF C1


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The hysteresis term prevents unnecessary re-selection on a location area

boundary which would require extra signaling to perform the location update


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1. MS receives and measures SS on serving cell, TS2.2. MS transmits.

3. MS measures SS for at least one of the surrounding cells.

4. MS reads BSIC on SCH (TS0) for one of the surrounding cells. during the

idle frame on the TCH, BSICs for neighboring BTSs are read

there is a gap of 3 Time slot between the reception & transmission at MS

this is called 3 Time slot offset


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The MS measures the signal strength of neighboring cells on the BCCH carrier. The

MS reads the SCH of each neighboring cell. On the SCH the MS receives the BSIC.

When power is turned on or when the MS enters a new cell, the MS isprovided with a list of neighboring cells to measure. This list is stored in both the MS

and in the BSC. In the MS, there is a list with 124, 374 or 299 multiple positions

which are equal to the number of carrier frequencies depending on the system

(GSM 900/1800/1900). Each neighboring cell the MS is told to measure is noted by

setting a flag on the list.

The MS sends complete measurement reports to the BSC, on SACCH every

480ms (4 SACCHs x 4.615ms x 26 TCHs). One measurement report contains the

signal strength and quality measured on the downlink for the serving cell and the

measured signal strength for a maximum of six neighboring cells. These

measurement reports are received at the BTS, where the BTS adds the signal

strength and quality of this connection on the uplink. The reports are then received

at the BSC where they are used as an input to the locating algorithm.

After processing the reports, the locating algorithm result is a list of possible

handover candidates called the PO-cell list. Each neighboring cell is ranked using

the reported signal strength. If the serving cell is at the top of the list (that is, has the

strongest signal strength) then no handover takes place.

MEASUREMENT REPORT


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MEASUREMENT REPORT in TEMS


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SYSTEM INFORMATION


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SYSTEM INFORMATION

The purpose of this function is to supply the BTS with system information messages.

System information messages are continuously sent by the BTS to all MSs in a cellon a BCCH (idle MS) or SACCH (busy MS). The parameters sent in these

messages are either controlled internally in the BSC or are set externally via

commands by the operator.

In a GSM system, eight different System Information message

types are used, as follows:

Type 1 (optional) messages are used for frequency hopping Types 2, 3 and 4 are broadcast on the BCCH

Types 5 and 6 are broadcast on an SACCH

Types 7 and 8 (optional) are an extension to type 4 and broadcast on the BCCH


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SYSTEM INFORMATION 5


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SYSTEM INFORMATION 6


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Cell Identity


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Cell Identity

CI Cell Identity within a LA

Cell Options

DTX Discontinuous transmission indicator.PWRC Power control indicator.RLINKT Radio link time-out is the time before an MS

disconnects due to failure in decoding SACCH

messages

Cell Selection Parameters

ACCMIN This is the minimum permitted received signalstrength for the MS to access the system.

CCHPWR Maximum transmission power an MS may usewhen accessing the system.

CRH This is the Cell Reselect Hysteresis. If theneighboring cell belongs to a new LA, the

measured signal strength of the serving cell is

artificially increased to make handover to the

neighboring cell more difficult


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CBCH Description (Optional)

CHN This is the channel number for CBCH. It is

controlled internally in BSC.

TSC Training Sequence Code,

BCC part of BSIC is used.

CBCHNO Absolute RF channel number for CBCH.

MAC Mobile Allocation in the cell, describes the

frequencies to be used in the hopping.

Neighbor Cells Description

CANO Cell Allocation Number. Band 0 for GSM.

ARFCN This indicates which neighboring BCCH

frequencies the SS should be measured on.

SYSTEM INFORMATION TYPE 1


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SYSTEM INFORMATION TYPE 1When frequency hopping is used in a cell, the MS needs to know which frequency

band and what frequencies within the band it should use in the hopping algorithm.

This information is given in the Cell Channel Description in System Information

Type 1. The BSC is adapted to handle more than one system type at the same time,

e.g. GSM 900 and GSM 1800 cells can be supported by the same BSC.

RACH Control Parameters

Cell Channel Description


Neighboring Cells DescriptionPLMN PermittedRACH Control Parameters

Cell Identity

Location Area Identity (LAI)Control Channel DescriptionCell OptionsCell Selection ParametersRACH Control Parameters



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CBCH Description (Optional)Location Area IdentityCell Selection Parameters

RACH Parameters


Neighbor Cells Description



Location Area IdentityCell IdentityCell OptionsPLMN Permitted

SYSTEM INFORMATION TYPE 7/TYPE 8 (OPTIONAL)

System Information Types 7 and 8 contain Cell Reselectparameters. Their function is to supplement SystemInformation Type 4.


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GSM/DCS MS POWER CONTROL PROCESS


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GSM/DCS MS POWER CONTROL PROCESS

GSM/DCS MS power output is controlled in levels, each level is separated by

2dBm as shown in the table below. However, individual adjustments can be

made in 2,4 or 6dB steps. 6dB adjustments are only possible with power

increases. The levels are shown in the table below:When first accessing a cell on the RACH and before receiving the first power

command an MS adopts the power level defined by the M_TXPWR_MAX_CCH

parameter broadcast on the BCCH of the cell.

The MS then periodically measures the received power level (RXLEV) and

reports this back to the BTS in the form of a measurement report which is forwarded

to the BSC. It also monitors the RxLev on adjacent cells but only the BCH of thesecells. The BTS commands the power level changes at the MS using the SACCH


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Mobile-Terminated Call


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Abuzer Study

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