| == BSC level configuration |
| |
| === Hand-over |
| |
| ==== Hand-over in GSM |
| |
| Hand-over is the process of changing a MS with a currently active |
| dedicated channel from one BTS to another BTS. As opposed to idle mode, |
| where the MS autonomously performs cell re-selection, in dedicated mode |
| this happens under network control. |
| |
| In order to determine when to perform hand-over, and to which cells, the |
| network requests the MS to perform measurements on a list of neighbor |
| cell channels, which the MS then reports back to the network in the form |
| of GSM RR 'Measurement Result' messages. Those messages contain the |
| downlink measurements as determined by the MS. |
| |
| Furthermore, the BTS also performs measurements on the uplink, and |
| communicates those by means of RSL to the BSC. |
| |
| The hand-over decision is made by an algorithm that processes those |
| measurement results and determines when to perform the hand-over. |
| |
| ==== Configuration of hand-over in OsmoBSC |
| |
| OsmoBSC only support so-called intra-BSC hand-over, where the hand-over is |
| performed between two BTSs within the same BSC. |
| |
| Hand-over is enabled and configured by the use of a set of `handover` |
| commands. Using those, you can tune the key parameters of the hand-over |
| algorithm and adapt it to your specific environment. |
| |
| .Example handover configuration snippet |
| ---- |
| handover 1 <1> |
| handover window rxlev averaging 10 <2> |
| handover window rxqual averaging 1 <3> |
| handover window rxlev neighbor averaging 10 <4> |
| handover power budget interval 6 <5> |
| handover power budget hysteresis 3 <6> |
| handover maximum distance 9999 <7> |
| ---- |
| <1> Enable hand-over |
| <2> Set the RxLev averaging window for the serving cell to 10 measurements |
| <3> Set the RxQual averaging window for the serving cell to 1 |
| measurement (no window) |
| <4> Set the RxLev averaging for neighbor cells to 10 measurements |
| <5> Check for the conditions of a power budget hand-over every 6 SACCH |
| frames |
| <6> A neighbor cell must be at least 3 dB stronger than the serving cell |
| to be considered a candidate for hand-over |
| <7> Perform a maximum distance hand-over if TA is larger 9999 (i.e. never) |
| |
| //TODO: Move all to BSC node |
| |
| === Timer Configuration |
| |
| The GSM specification specifies a variety of timers both on the network |
| as well as on the mobile station side. |
| |
| Those timers can be configured using the `timer tXXXX` command. |
| |
| .Configurable Timers |
| |=== |
| |node|timer|default|description |
| |network|t3101|10|Timeout for 'Immediate Assignment' (sec) |
| |network|t3103|?|Timeout for Handover (sec) |
| |network|t3105|40|Repetition of 'Physical Information' (sec) |
| |network|t3107|?|? |
| |network|t3109|?|RSL SACCH deactivation timeout (sec) |
| |network|t3111|?|RSL timeout to wait before releasing the RF channel (sec) |
| |network|t3113|60|Time to try paging for a subscriber (sec) |
| |network|t3115|?|? |
| |network|t3117|?|? |
| |network|t3119|?|? |
| |network|t3122|10|Waiting time after 'Immediate Assignment Reject' |
| |network|t3141|?|? |
| |=== |
| |
| //TODO: split between BSC and MSC timers |
| |
| === Discontinuous Transmission (DTX) |
| |
| GSM provides a full-duplex voice call service. However, in any |
| civilized communication between human beings, only one of the |
| participants is speaking at any given point in time. This means that |
| most of the time, one of the two directions of the radio link is |
| transmitting so-called 'silence frames'. |
| |
| During such periods of quiescence in one of the two directions, it is |
| possible to suppress transmission of most of the radio bursts, as there |
| is no voice signal to transport. GSM calls this feature 'Discontinuous |
| Transmission'. It exists separately for uplink (DTXu) and downlink |
| (DTXd). |
| |
| Downlink DTX is only permitted on non-primary transceivers (!= TRX0), as |
| TRX0 must always transmit at constant output power to ensure it is |
| detected during cell selection. |
| |
| Uplink DTX is possible on any TRX, and serves primarily two uses: |
| |
| possible on any TRX, and serves primarily two uses: |
| |
| . reducing the MS battery consumption by transmitting at a lower duty cycle |
| . reducing the uplink interference caused in surrounding cells that |
| re-use the same ARFCN. |
| |
| DTS for both uplink and downlink is implemented in the BTS. Not all BTS |
| models support it. |
| |
| The Osmocom BSC component can instruct the BTS to enable or disable |
| uplink and/or downlink DTX by means of A-bis OML. |
| |
| //TODO: Test/implement, at least for uplink |
| //TODO: Move to BSC |