Pau Espin Pedrol | 1db53c8 | 2020-07-20 13:06:38 +0200 | [diff] [blame] | 1 | == BSC level configuration |
| 2 | |
Pau Espin Pedrol | 1db53c8 | 2020-07-20 13:06:38 +0200 | [diff] [blame] | 3 | === Hand-over |
| 4 | |
| 5 | ==== Hand-over in GSM |
| 6 | |
| 7 | Hand-over is the process of changing a MS with a currently active |
| 8 | dedicated channel from one BTS to another BTS. As opposed to idle mode, |
| 9 | where the MS autonomously performs cell re-selection, in dedicated mode |
| 10 | this happens under network control. |
| 11 | |
| 12 | In order to determine when to perform hand-over, and to which cells, the |
| 13 | network requests the MS to perform measurements on a list of neighbor |
| 14 | cell channels, which the MS then reports back to the network in the form |
| 15 | of GSM RR 'Measurement Result' messages. Those messages contain the |
| 16 | downlink measurements as determined by the MS. |
| 17 | |
| 18 | Furthermore, the BTS also performs measurements on the uplink, and |
| 19 | communicates those by means of RSL to the BSC. |
| 20 | |
| 21 | The hand-over decision is made by an algorithm that processes those |
| 22 | measurement results and determines when to perform the hand-over. |
| 23 | |
Pau Espin Pedrol | 5487387 | 2020-07-20 13:11:04 +0200 | [diff] [blame] | 24 | ==== Configuration of hand-over in OsmoBSC |
Pau Espin Pedrol | 1db53c8 | 2020-07-20 13:06:38 +0200 | [diff] [blame] | 25 | |
Pau Espin Pedrol | 5487387 | 2020-07-20 13:11:04 +0200 | [diff] [blame] | 26 | OsmoBSC only support so-called intra-BSC hand-over, where the hand-over is |
| 27 | performed between two BTSs within the same BSC. |
Pau Espin Pedrol | 1db53c8 | 2020-07-20 13:06:38 +0200 | [diff] [blame] | 28 | |
| 29 | Hand-over is enabled and configured by the use of a set of `handover` |
| 30 | commands. Using those, you can tune the key parameters of the hand-over |
| 31 | algorithm and adapt it to your specific environment. |
| 32 | |
| 33 | .Example handover configuration snippet |
| 34 | ---- |
| 35 | handover 1 <1> |
| 36 | handover window rxlev averaging 10 <2> |
| 37 | handover window rxqual averaging 1 <3> |
| 38 | handover window rxlev neighbor averaging 10 <4> |
| 39 | handover power budget interval 6 <5> |
| 40 | handover power budget hysteresis 3 <6> |
| 41 | handover maximum distance 9999 <7> |
| 42 | ---- |
| 43 | <1> Enable hand-over |
| 44 | <2> Set the RxLev averaging window for the serving cell to 10 measurements |
| 45 | <3> Set the RxQual averaging window for the serving cell to 1 |
| 46 | measurement (no window) |
| 47 | <4> Set the RxLev averaging for neighbor cells to 10 measurements |
| 48 | <5> Check for the conditions of a power budget hand-over every 6 SACCH |
| 49 | frames |
| 50 | <6> A neighbor cell must be at least 3 dB stronger than the serving cell |
| 51 | to be considered a candidate for hand-over |
| 52 | <7> Perform a maximum distance hand-over if TA is larger 9999 (i.e. never) |
| 53 | |
| 54 | //TODO: Move all to BSC node |
| 55 | |
| 56 | === Timer Configuration |
| 57 | |
| 58 | The GSM specification specifies a variety of timers both on the network |
| 59 | as well as on the mobile station side. |
| 60 | |
| 61 | Those timers can be configured using the `timer tXXXX` command. |
| 62 | |
| 63 | .Configurable Timers |
| 64 | |=== |
| 65 | |node|timer|default|description |
| 66 | |network|t3101|10|Timeout for 'Immediate Assignment' (sec) |
| 67 | |network|t3103|?|Timeout for Handover (sec) |
| 68 | |network|t3105|40|Repetition of 'Physical Information' (sec) |
| 69 | |network|t3107|?|? |
| 70 | |network|t3109|?|RSL SACCH deactivation timeout (sec) |
| 71 | |network|t3111|?|RSL timeout to wait before releasing the RF channel (sec) |
| 72 | |network|t3113|60|Time to try paging for a subscriber (sec) |
| 73 | |network|t3115|?|? |
| 74 | |network|t3117|?|? |
| 75 | |network|t3119|?|? |
| 76 | |network|t3122|10|Waiting time after 'Immediate Assignment Reject' |
| 77 | |network|t3141|?|? |
| 78 | |=== |
| 79 | |
| 80 | //TODO: split between BSC and MSC timers |
| 81 | |
| 82 | === Discontinuous Transmission (DTX) |
| 83 | |
| 84 | GSM provides a full-duplex voice call service. However, in any |
| 85 | civilized communication between human beings, only one of the |
| 86 | participants is speaking at any given point in time. This means that |
| 87 | most of the time, one of the two directions of the radio link is |
| 88 | transmitting so-called 'silence frames'. |
| 89 | |
| 90 | During such periods of quiescence in one of the two directions, it is |
| 91 | possible to suppress transmission of most of the radio bursts, as there |
| 92 | is no voice signal to transport. GSM calls this feature 'Discontinuous |
| 93 | Transmission'. It exists separately for uplink (DTXu) and downlink |
| 94 | (DTXd). |
| 95 | |
| 96 | Downlink DTX is only permitted on non-primary transceivers (!= TRX0), as |
| 97 | TRX0 must always transmit at constant output power to ensure it is |
| 98 | detected during cell selection. |
| 99 | |
| 100 | Uplink DTX is possible on any TRX, and serves primarily two uses: |
| 101 | |
| 102 | possible on any TRX, and serves primarily two uses: |
| 103 | |
| 104 | . reducing the MS battery consumption by transmitting at a lower duty cycle |
| 105 | . reducing the uplink interference caused in surrounding cells that |
| 106 | re-use the same ARFCN. |
| 107 | |
| 108 | DTS for both uplink and downlink is implemented in the BTS. Not all BTS |
| 109 | models support it. |
| 110 | |
| 111 | The Osmocom BSC component can instruct the BTS to enable or disable |
| 112 | uplink and/or downlink DTX by means of A-bis OML. |
| 113 | |
| 114 | //TODO: Test/implement, at least for uplink |
| 115 | //TODO: Move to BSC |