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