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Diffstat (limited to 'doc/manuals')
| -rw-r--r-- | doc/manuals/chapters/handover.adoc | 101 |
1 files changed, 100 insertions, 1 deletions
diff --git a/doc/manuals/chapters/handover.adoc b/doc/manuals/chapters/handover.adoc index 8601dceb3..4de744b9e 100644 --- a/doc/manuals/chapters/handover.adoc +++ b/doc/manuals/chapters/handover.adoc @@ -1,4 +1,5 @@ -== Handover +[[cs_handover]] +== CS Handover Handover is the process of moving a continuously used channel (lchan) from one cell to another. Usually, that is an ongoing call, so that phones are able to @@ -144,6 +145,7 @@ more than one neighbor per given ARFCN+BSIC, these values can be re-used any number of times across a network, and even between cells managed by one and the same BSC. +[[config_neigh]] === Configuring Neighbors The most important step to enable handover in OsmoBSC is to configure each cell @@ -692,3 +694,100 @@ network 4G neighbor cells can be removed using the same command, just replacing `add` with `del`. + +[[ps_handover]] +== PS Handover + +Packet Switch Handover is mostly managed by the packet switching nodes such as +PCU, SGSN and GGSN. However, the PCU encounters a similar difficulty to that +explained in <<cs_handover>>: that is, it must find a way to translate +ARFCN+BSIC identifiers provided by the MS into the sort of identifiers +understood by the Core Network (SGSN). These identifiers are in this case +composed of RAC+CI (instead LAC+CI), or more specifically, CGI+RAC (named as +"Packet Switched CGI" or "CGI-PS" from now on for convenience). + +Hence, it feels natural extending the <<config_neigh>> storage in the BSC to +also provide Packet Switched related identifiers in order to provide the PCU the +required information to do the translations, instead of duplicating the whole +neighbor information in two different network nodes. + +This can be done, similarly to already presented CS related commands in +<<config_neigh>>, by using the `neighbor cgi-ps` VTY command, which allows for +specifying the extra identifier (RAC) required by PS related translations: + +.Example: configuring PS neighbors within the local BSS in osmo-bsc.cfg, identified by CGI-PS (CGI+RAC) +---- +network + bts 0 +neighbor cgi-ps <MCC> <MNC> <LAC> <RAC> <CI> arfcn <0-1023> bsic (<0-63>|any) +---- + +This information should already be present by default if one uses the `local BTS +number` to identify the network, as long as that BTS is properly configured to +have GPRS enabled and the RAC code is set by `gprs routing area <0-255>` +command: + +.Example: configuring PS neighbors within the local BSS in osmo-bsc.cfg, identified by local BTS number +---- +network + bts 0 + gprs mode gprs + gprs routing area 45 + neighbor bts 1 + ... + bts 1 + gprs mode egprs + gprs routing area 48 + neighbor bts 0 + ... +---- + +This PS information is solely used by the Neighbor Resolution Service, aimed at +nodes other than BSC itself, and described below. + +=== Neighbor Resolution Service CTRL interface + +This service is provided in order to provide the PCU a way to translate +ARCFCN+BSIC into CGI-PS in order to use RIM services and accomplish PS +Handovers. + +This interface is Osmocom specific, since the standard doesn't provide any +specifications on how to provide this kind of information to the PCU. + +Since the PCU can be either BSC-colocated or BTS-colocated (hence on a different +host than BSC), this must be a network-based interface. Since the service is +Osmocom-specific, it was decided to re-use the CTRL interface available in most +Osmocom processes (see <<control>>). + +Due to security concerns, the set of CTRL commands available in this +service is configured in a different IP address and port, since the service +needs to be reachable by all PCU under the BSC. This way the user can still +constrain the regular CTRL port (which may contains lots of configuration +related commands) listening in a more constrained address (eg. localhost). + +By default, this interface is disabled, and it is enabled by configuring the +desired IP address to bind to (and optionally a different port other than +well-known `4248`): + +.Example: Enable and configure Neighbor Resolution Service CTRL interface +---- +network + neighbor-resolution bind 127.0.0.1 5000 +---- + +osmo-pcu will then be able to connect to the BSC and query for resolution during eg. NACC requests from MS. + +The relevant commands are:: + +* neighbor_resolve_cgi_ps_from_lac_ci: +---- +GET neighbor_resolve_cgi_ps_from_lac_ci.<src_lac>.<src_cell_id>.<dst_arfcn>.<dst_bsic> +GET_REPLY <MCC>-<MNC>-<LAC>-<RAC>-<CI> +---- + +Where the `src_lac` and `src_ci` are provided by BTS/BSC over `PCUIF` interface +during startup (`INFO IND`), and `dst_arfcn` and `dst_bsic` are those of the +neighbor the PCU is willing to request the CGI-PS information. Hence, from the +`src` params the BSC is able to look up the BTS and afterwards apply the +translation of `dst` parameters based on the neighbor information available for +that BTS. |