Configuration d'OpenBTS¶

Il y a trois fichiers de configuration pour OpenBTS qui sont localisés dans /etc/OpenBTS: OpenBTS.db, sipauthserve.db et smqueue.db.
Avec l'installation décrite précédemment, on peut utiliser directement les fichiers mis en pièce jointe (lecture avec sqlitebrowser).

Ces fichiers permettent aussi bien la configuration du logiciel sur le PC (port et adresse d'accès, emplacement de fichier,..), la configuration du réseau (autorisation d'appel, message d’accueil,taille des tables d'enregistrement,..) que la configuration de l'interface air du GSM (puissance d'émission, identité du réseau, choix de fréquence,..)

Configuration du logiciel et réseau¶

En utilisant les fichiers fournis, le réseau autorise tous portables à se connecter et à recevoir des appels. Aucun message d’accueil n'est envoyé.
Cela concerne tous les paramètres ne commencent pas par "GSM".

Configuration de l'interface air GSM¶

Cette configuration concerne toutes les entrées commençant par "GSM".
Ces paramètres ont été configuré pour faciliter la connexion du mobile à notre réseau et le forcer le plus possible a y rester dessus.

Une liste de ces paramètres avec leur description et effets est présentée:

GSM.RACH.TxInteger
OpenBTS: 0 -> 15
Value range: 3~12, 14, 16, 20, 25, 32, 50
Unit: Number of RACH timeslots (equivalent to one timeslot, 0.577ms)
Content: Extended Transmission Timeslots(T), used to calculate the number of timeslots in the interval between two channel requests sent by MS.
Recommendation: 25 (If the cell immediate assignment success rate is low, set the S with a larger value. Select 32 for satellite transmission, so as to reduce satellite transmission delay).
It is set to reduce the collisions on RACH. It mainly affects the execution efficiency of the immediate assignment process. The value of this parameter is related to CCCH configuration mode, both work together to determine parameter S. The number of timeslots (excluding the timeslot from where the message is sent) from MS starts the immediate assignment process to the first channel request message sent is a random value in the set {0, 1, …, MAX--1}. And the number of timeslots between any two adjacent channel request messages is a random value in the set {S, S+1, …, S+T-1}.
Generally, parameter S should be as small as possible (in order to shorten the access time of MS) when parameter T is fixed, but AGCH and SDCCH must not be overloaded. During the operation, a random value can be selected for parameter T for the cell with unknown traffic to ensure the minimum value of S. If AGCH or SDCCH of the cell is overloaded, then parameter T can be changed to make parameter S larger, until AGCH or SDCCH of the cell is not overloaded.
The range of value T can be specified according to the above principles (multiple values can be selected for parameter T corresponding to various values of parameter S). When the RACH collisions is serious, value T should be large. When the number of RACH collisions is small, value T should be as small as possible.

GSM.RADIO-LINK-TIMEOUT
GSM 04.08 chapter 10.5.2.3
OpenBTS : 0 -> 15
Value range: 4~64, the step size is 4
Unit: SACCH period (480ms)
Content: This parameter is used for MS to decide down-link disconnection in case of SACCH decoding failures.
Recommendation: 20~56
Once assigned with a dedicated channel, MS will start counter S. From then on, S will decrease by 1 when a SACCH message fails to be decoded, and will increase by 2 when decoded correctly. When S decreases to 0, there will be a radio link failure. This allows either re-establishment or release of the connection. If the value of this parameter is too small, the radio link will easily get failed which will result in call drops. If it is too large, MS will not release for a long time which will lower the availability of resources (this parameter functions for the downlink).
For area with little traffic (remote area), it is recommended to be between 52~62.
For area with light traffic and large coverage(suburb or countryside), it is recommended to be between 36~48.
For area with heavy traffic (urban), it is recommended to be between 20~32.

GSM.RACH.AC
0x400=1024 => NO emergency calls available
Different access classes of the network

GSM.MS.PowerMax/Min
min and max power emission of MS: 33 dBm -> 5 dBm (2 W -> 3.2 mW)

GSM.MS.TA.Damping & GSM.MS.Power.Damping
damping value for MS control loop.
damping => reduce oscillations to go back to equilibrium of the system
damping > 1 => return to equilibrium without oscillations, the bigger the slower.

GSM.Radio.PowerManager.TargetT3122
Target value in milliseconds for the hold-off time after an unsuccessful RACH request because no channels are available.

GSM.Radio.RSSITarget
Necessary RSSI on the USRP in order to receive signals without erros.
Run "noise" command in the OpenBTS CLI several times to get the worst (=biggest) value of noise RSSI for your setup. It should be a negative value, like -44dB. Add 6-8dB to the noise RSSI value to get the RSSI level we need to receive phone signal on the BTS without errors.

GSM.Radio.RxGain
Adjust "rxgain" to provide good reception at the edge of the base station reception area and at the same time to avoid saturation (and thus bad reception) close to the base station. The easiest way to estimate reception quality is to use "UPFER" value at the "chans" CLI command output. Zero "UPFER" means good reception, non-zero "UPFER" means errors on UL channel.

GSM.Radio.PowerManager.MaxAttenDB & GSM.Radio.PowerManager.MinAttenDB
Attenuation of the Tx power relative to the maximum base station power (which is 100 mW -> 20 dBm).It has two values ("max" and "min", but for a small base station it's easier to set them to the same value).
The OpenBTS implements a gradual power ramp up in order to reduce the the sudden load of subscription when turning on OpenBTS.
Place the test mobile phone at the edge of good reception area and increase "power" value until "DNBER" starts to increase. It should happen when "DNLEV" reaches around -105dB.

GSM.Timer.T3212
Value range: 0~255
Unit: 6 minutes
Content: It is Periodic Location Update Timer. It defines the period length of location updating.
Recommendation: 30 (for urban area), 20 (for suburban area)
MS will make location update when detecting the change of location, besides, MS will make periodic location update controlled by parameter T3212. Once MS read T3212 from system info., it will store it in SIM card. When the time exceeds T3212 value, the location update process will be triggered. The shorter the period the better the performance, but it will bring more signaling load for system. On setting of this parameter, the processing capabilities of MSC and BSC should be considered, also the flows of A interface, Abis interface, Um interface as well as those of HLR and VLR. Generally larger value for continuous covered urban area while smaller in the suburb, countryside or the place with poor coverage.
Large T3212 is recommended for the area with much traffic, and small T3212 (3 hours, 2 hours) for the area with a little traffic. For the area where the traffic exceeds the system capacity, it is recommended to set T3212 as 0 (no periodic location update). To set the value of T3212 properly, it’s necessary to conduct long-term measurement on the processing capability and flow of each entity in the system. If any overload occurs, increase the value T3212. Note that this value should be smaller than the period by which the network queries the IMSI attached subscriber. Otherwise, the following situation occurs: When MS has not done any operation in a certain time, and it is not yet the time for periodic location update, the network will set IMSI flag of MS as detach, because its query result shows that MS has not done any operation. Thus, the network will not process the paging of this MS. So, before MS initiates another round of periodic location update, once there is a call for the MS, the network will voice the calling subscriber that the called MS is out of service area or has been powered off. When MS reselects a different location area, MS will make a non-periodic location update and reset T3212 in the new cell. If it reselects in the same location area, then the timer value will be remainder of the original one divided by the new T3212.

GSM.MS.TA.Max
The unit is GSM symbols representing about 550 meters per symbol.
Can be used to control the range of the BTS.

GSM.CellSelection.CELL-RESELECT-HYSTERESIS
If the mobile is moving in a border area between location areas, it might repeatedly change between location areas. Each change requires location updating and cause heavy signaling load and risk paging message being lost. To prevent this, a cell reselect hysteresis parameter CRH is used.
The cell in a different location area will only be selected if the C1 of that cell is higher than the C1 of the current serving cell by the value of the Reselect Hysteresis. Since the Value of CRH may be different for each cell, the CRH used for comparison will be the one broadcast by the serving cell.
If the value is set very low then the mobile will Ping-Pong between location areas which will increase signaling load. If the value is set very high the mobile may camp in the wrong cell too long.

GSM.CellSelection.MS-TXPWR-MAX-CCH
Value range: 0~31
Unit: level
Content: This parameter determines the max. allowed output power of the MS when it begins to access a cell and has not yet received power command.
Recommendation: 5(900MHz), 0(1800MHz)
During MS’s communication with BTS, its transmitting power is controlled by the network via power command. This command is transmit in SACCH (There are two head bytes. One is power control byte, the other is time advance). MS must get the power control head from the downlink SACCH, and use its stipulated transmitting power as the output power. If power level of the MS cannot output that power, the transmitting power will use the nearest value.
Since SACCH is associated with signaling, it must be used after combining with other channels, such as SDCCH, TCH. Therefore, the network’s control of MS power begins after MS receives SACCH.While the power used by MS before it receives SACCH (power used when RACH is sent) is determined by the maximum power level of control channel.
List is the corresponding dBm values related to the 32 maximum transmitting power control levels for GSM900: {39, 39, 39, 37, 35, 33, 31, 29, 27, 25, 23, 21, 19, 17, 15, 13, 11, 9, 7, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5}
For GSM1800: {30, 28, 26, 24, 22, 20, 18, 16, 14, 12, 10, 8, 6, 4, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 36, 34, 32}.
This parameter will affect cell selection and cell reselection.
C1 = RLA_C - RXLEV_ACCESS_MIN- MAX, 0)
RLA_C: mean receiving level of MS
RXLEV_ACCESS_MIN: minimum receiving level of MS permitted to access
P: maximum physical transmitting power level of MS.

GSM.CellSelection.RXLEV-ACCESS-MIN
Range: 0 to 63
Unit: level
The minimum received signal level at the MS for which it is permitted to access the system.