3.4.4 Handover procedure

3GPP44.018GSM/EDGE Radio Resource Control (RRC) protocolMobile radio interface Layer 3 specificationRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

In dedicated mode, dual transfer mode or group transmit mode, an intercell or intracell change of channel(s) can be requested by the network RR sublayer. This change may be performed through the handover procedure. In case of intercell change in dual transfer mode, the DTM handover procedure can be used to change one dedicated channel together with one or more packet data channels (see sub-clause 3.7).

The network RR sublayer shall not request an intercell or intracell change of channel when a mobile station has temporarily entered the dedicated mode on a SDCCH, in order to initiate a priority uplink request.

For a talker in group transmit mode, the network RR sublayer shall delay the allocation of resources for handover of the talker to the voice group call channel of the target cell if the uplink of the target cell has already been granted to a subscriber with priority (this collision case may occur if the network supports uplink access option (i) as defined in 3GPP TS 43.068). If the allocation of the uplink to the new subscriber is confirmed, the network shall cancel the handover resource request and release the current talker. If the request for the uplink is rejected, the network proceeds with the handover of the current talker. Alternatively instead of delaying the handover, it may be continued to a different cell if possible.

NOTE: The decision to do a handover and the choice of the new cell is out of the scope of this technical specification.

The purpose of the handover procedure is to completely modify the channels allocated to the mobile station e.g. when the cell is changed. A change in the channel configuration nature is possible. This procedure is used only while in dedicated mode, dual transfer mode or group transmit mode.

The handover procedure shall not be used for changing between dependent configurations (see sub-clause 3.4.3).

The handover procedure includes:

– The suspension of normal operation except for RR management (layer 3).

– The disconnection of the main signalling link, and of the other links via local end release (layer 2), and the disconnection of the TCH(s) if any.

– The abortion of the packet resources (see 3GPP TS 44.060), if in class A mode of operation.

– The disconnection and the deactivation of previously assigned channels and their release (layer 1).

– The activation of the new channels, and their connection if applicable.

– The triggering of the establishment of data link connection for SAPI = 0 on the new channels.

The handover procedure is always initiated by the network.

3.4.4.1 Handover initiation

The network initiates the handover procedure by sending a HANDOVER COMMAND message to the mobile station on the main DCCH. It then starts timer T3103.

The network should not initiate a handover which may result in the transmission of a HANDOVER ACCESS message by the MS (see sub-clauses 3.4.4.2.1, 3.4.4.2.2, 3.4.4.2.3, 3.4.4.2.4) if the new traffic channel is in VAMOS mode. If it is determined from the HANDOVER COMMAND message that the assigned traffic channel is potentially in VAMOS mode (e.g. TSC Set 2 assigned, or VAMOS mode signaled), this shall not be considered as an error.

If the HANDOVER COMMAND message refers to a cell to which the mobile station is not synchronised to, this shall not be considered as an error (see 3GPP TS 45.008).

NOTE: The network should take into account limitations of certain mobile stations to understand formats used in the Frequency List IE, Frequency Short List IE, and Cell Channel Description IE used in the HANDOVER COMMAND message, see sub-clause 10.5.2.13, sub-clause 10.5.2.14, and sub-clause 10.5.2.1b.

When sending this message on the network side, and when receiving it on the mobile station side, all transmission of signalling layer messages except for those RR messages needed for this procedure and for abnormal cases, is suspended until resuming is indicated. These RR messages can be deduced from sub-clauses 3.4.3 and 8.5.1 "Radio Resource management".

Upon receipt of the HANDOVER COMMAND message, the mobile station initiates, as described in sub-clause 3.1.4, the release of link layer connections, disconnects the physical channels (including the packet resources, if in class A mode of operation), commands the switching to the assigned channels and initiates the establishment of lower layer connections (this includes the activation of the channels, their connection and the establishment of the data links).

The HANDOVER COMMAND message contains:

– The characteristics of the new channels, including for the multislot configuration and the TCH/H + TCH/H + ACCHs configuration the exact ACCHs to be used. The message may also contain definitions of the channel mode to be applied for one or several channel sets. If a previously undefined channel set is defined by the HANDOVER COMMAND message, a definition of the channel mode for the new channel set shall be included in the message.

– The characteristics of the new cell that are necessary to successfully communicate (e.g. frequency list in the case of slow frequency hopping), including the data that allows the mobile station to use the pre-knowledge about synchronization it acquires by the measurement process (i.e. BSIC + BCCH frequency).

– A power command (cf. 3GPP TS 45.008). The power level defined in this power command shall be used by the mobile station for the initial power on the new channel(s). It shall not affect the power used on the old channel(s).

– An indication of the physical channel establishment procedure to be used.

– A handover reference, used as specified in the following sub-clause. The choice of the handover reference by the network is out of the scope of this specification and left to the manufacturers.

– Optionally a timing advance to be used on the new cell.

– Optionally a cipher mode setting. In that case, this ciphering mode has to be applied on the new channel. If no such information is present, the ciphering mode is the same as on the previous channel. In either case the ciphering key shall not be changed as long as the key length remains unchanged. However, in case of a switch between ciphering algorithms requiring different key lengths, i.e. 64 or 128 bits, a change from the 64 bit key to the 128 bit key or vice versa must be performed. If the cipher mode setting IE indicates "start ciphering" the mobile station supporting Selective Ciphering of Downlink SACCH shall be able to decode both ciphered and not-ciphered SACCH blocks (see sub-clause 3.4.7a). In the case of GERAN A/Gb mode to GERAN A/Gb mode handover, if the HANDOVER COMMAND message contains a cipher mode setting IE that indicates "start ciphering" and neither an RR Ciphering Mode Setting procedure (see sub-clause 3.4.7) nor a UTRAN RRC Security Mode Control procedure (see 3GPP TS 25.331) has been successfully completed prior to the handover for this CS connection the HANDOVER COMMAND message shall be regarded as erroneous, a HANDOVER FAILURE message with cause "Protocol error unspecified" shall be returned immediately, and no further action taken. In the case of UTRAN to GERAN A/Gb mode handover or GERAN Iu mode to GERAN A/Gb mode handover or E-UTRAN to GERAN A/Gb mode SRVCC handover (see 3GPP TS 23.216 [94], 3GPP TS 29.280 [95]), the HANDOVER COMMAND message, which is sent transparently via RNC/BSC/eNB from BSS to the mobile station, shall always contain the cipher mode setting IE to indicate the ciphering mode to be used in GERAN A/Gb mode. If the cipher mode setting IE indicates "start ciphering" and neither an RR Ciphering Mode Setting procedure (see sub-clause 3.4.7) nor a UTRAN RRC Security Mode Control procedure (see 3GPP TS 25.331) has been successfully completed prior to the handover for this CS connection, the HANDOVER COMMAND message shall be regarded as erroneous and the applicable procedure as specified in 3GPP TS 25.331 for the case of an Invalid HANDOVER FROM UTRAN COMMAND message shall be followed; a HANDOVER FAILURE message with cause "Protocol error unspecified" shall be included in the HANDOVER FROM UTRAN FAILURE message. In the case of CDMA2000 to GERAN A/Gb mode handover, the HANDOVER COMMAND message, which is sent transparently via RNC from BSS to the mobile station, shall always contain the cipher mode setting IE.

– Optionally, in a voice group call, a VGCS target mode information element defining which RR mode is to be used on the new channel (i.e. dedicated mode or group transmit mode). If this information element is not present, the mode shall be assumed to be the same as on the previous channel. When the RR mode on the new channel is group transmit mode, the VGCS target mode information element shall also indicate the group cipher key number for the group cipher key to be used on the new channel or if the new channel is non ciphered. Additionally, when the RR mode is group transmit mode and the group cipher key number is non zero, then the VGCS Ciphering Parameters information element shall contain the CELL_GLOBAL_COUNT and optionally contain the VSTK_RAND, the target cell identity, the cell’s location area. If the VGCS target mode information element is not present, the ciphering mode and ciphering key shall be assumed to be the same as on the previous channel. If any of the following parameters are not included in the VGCS Ciphering Parameters information element, the parameter shall be assumed to be the same as on the previous channel: VSTK_RAND, the target cell identity, or the cell’s location area. The network shall include the VSTK_RAND (within the VGCS Ciphering Parameters IE) if the group call is ciphered, the new RR mode is group transmit mode and the old RR mode is dedicated mode or is not known. Mobile stations not supporting VGCS talking shall ignore the HANDOVER COMMAND message if the VGCS target mode information element or VGCS Ciphering Parameters information element is included in the message and shall send an RR STATUS message to the network with cause #96. If the HANDOVER COMMAND message contains a cipher mode setting information element together with either a VGCS target mode information element indicating a RR mode of group transmit mode, or a VGCS Ciphering Parameters information element, , then a mobile station supporting VGCS talking shall regard the message as erroneous, an HANDOVER FAILURE message with cause "Protocol error unspecified" shall be returned immediately, and no further action taken.

– Optionally, in a voice group call, if the RR mode of the new channel is dedicated mode, a cipher mode setting IE. In that case, this ciphering mode shall be applied on the new channel. If no such information is present, the ciphering mode is the same as on the previous channel, provided that the previous channel was also a dedicated channel. If no such information is present and the previous channel had RR mode group transmit mode, the new ciphering mode is "no ciphering". In either case the ciphering key to be used on the dedicated channel is the individual GSM ciphering key. The HANDOVER COMMAND message shall not contain a cipher mode setting IE that indicates "start ciphering", unless a CIPHERING MODE COMMAND message has been transmitted earlier in the RR connection or a group cipher key number different from zero has been transmitted for this voice group call. If a HANDOVER COMMAND message is received that contains a cipher mode setting IE indicating "start ciphering" and the mobile station has received neither a CIPHERING MODE COMMAND message earlier in the RR connection nor a group cipher key number different from zero for this voice group call, via any channel, then the HANDOVER COMMAND message shall be considered as erroneous, the mobile station shall send an HANDOVER FAILURE with cause "Protocol error unspecified", and no further action taken.

– Optionally, when the channel mode indicates that a multi-rate speech codec must be applied, the MultiRateconfiguration to be used in the new cell. The MultiRate Configuration IE defines the set of codec mode and related information to use after the handover. When accessing the new channel, the mobile station shall use for the Initial Codec Mode the mode specified in the MultiRate Configuration IE, if present, or apply by default the implicit rule defined in 3GPP TS 45.009.

– Optionally, if the network supports dedicated mode MBMS notification and the mobile previously completed the service information sending to the network, an indication of whether the service information sending should be completed on the main DCCH of the new cell.

– Optionally, if the target network and the MS support network sharing and DTM, the PLMN Index corresponding to the selected PLMN.

In addition, a HANDOVER COMMAND message may indicate a frequency change in progress, with a starting time and possibly alternative channel descriptions.

In the case of the reception of a HANDOVER COMMAND message which contains only the description of a channel to be used after the starting time, the mobile station shall wait up to the starting time before accessing the channel. If the starting time has already elapsed, the mobile shall access the channel as an immediate reaction to the reception of the message (see 3GPP TS 45.010 for the timing constraints).

In the case of a handover towards a GERAN cell to which the mobile station is not synchronised to and in the case of an intersystem handover to GERAN, at the reception of a HANDOVER COMMAND message which contains only the description of a channel to be used after the starting time, the mobile station shall wait up to the starting time before accessing the new channel. If the starting time has already elapsed, the mobile shall access the new channel as an immediate reaction to the reception of the message (see 3GPP TS 45.010 for the timing constraints). Between the reception of the HANDOVER COMMAND and the starting time there is no requirement for the mobile station to receive or transmit on the old channel.

NOTE: This case may result to a long interruption and should not be used.

If the message contains both the description of a channel to be used after the indicated time and of a channel to be used before, the mobile station accesses a channel as an immediate reaction to the reception of the message. If the moment the mobile station is ready to access is before the indicated time, the mobile station accesses the channels described for before the starting time. The mobile station then changes to the channel described for after the starting time at the indicated time. New parameters can be frequency list, MAIO and HSN. Other parameters describing the allocated channels must be identical to the parameters described for before the starting time. If the moment the mobile station is ready to access is after the starting time, the mobile station accesses the channel described for after the starting time.

In the case of a handover from a GERAN cell, if the channel mode indicates that a multi-rate speech codec must be applied, and the MultiRateConfiguration IE is not included in the HANDOVER COMMAND message, then the mobile station shall use on the new channel the AMR configuration it was using on the old channel when it received the HANDOVER COMMAND message. The MultiRate Configuration IE shall be included in the case of full rate channel to half rate channel handover or in the case of a change of multi-rate speech version.. If not included in thosecases, the mobile station shall behave as if the MultiRate Configuration IE was inconsistent (see sub-clause 3.4.4.4).

In the case of an intersystem handover to GERAN, if the channel mode indicates that a multi-rate speech codec must be applied, the MultiRateConfiguration IE shall be included in the HANDOVER COMMAND message. If not included the mobile station shall treat the HANDOVER COMMAND message as invalid and shall perform the corresponding RRC error handling, see 3GPP TS 25.331.

In the case of a VGCS talker that is handed over to a ciphered VGCS group channel the MS shall calculate the voice group ciphering keys from the following parameters, as described in 3GPP TS 43.020:

– VSTK_RAND;

– CGI (as supplied in the Handover Command);

– CELL_GLOBAL_COUNT (value of parameter in the target cell, as supplied in the Handover Command);

– Group Cipher Key Number (value obtained via the Handover Command);

– B22_COUNT – Bit 22 of COUNT ( as defined in 3GPP TS 43.020).

The Handover Command message shall provide the VGCS talker with the above parameters if the values have changed on handover or if the MS was not using VGCS/VBS ciphering prior to handover.

When the VGCS talker is handed over to a channel that is ciphered with VGCS ciphering, the talker adjust and maintain the CELL_GLOBAL_COUNT provided in the Handover Command message as described in sub-clause 3.3.3.1. Also, this talker shall fetch from the USIM the identity of its ciphering algorithm to use on the new resource, as described in 3GPP TS 43.020.

In the case of a VGCS talker that is handover a dedicated channel, the setting of the Cipher Mode IE in the Handover Command message shall indicate if the dedicated resource is ciphered. If the new resource is ciphered then the MS shall assume the following:

– the Cipher Mode Setting IE shall indicate the identity of the ciphering algorithm to use on the dedicated channel;. If this information is not present the MS shall use the algorithm that was last used when on a ciphered dedicated channel;

– the ciphering key sequence number shall be the same value as when the MS was last used on a ciphered dedicated channel.

3.4.4.2 Physical channel establishment

Four procedures are defined. The support of three of them is mandatory in the mobile station. The pseudo-synchronization case is optional in the mobile station. A pseudo-synchronized handover can be commanded only to a mobile station that can support it, as indicated in the classmark.

3.4.4.2.1 Finely synchronized cell case

If the mobile station knows that the timing advance with the new cell is not out of range, i.e. smaller than or equal to the maximum timing advance that can be coded as specified in 3GPP TS 44.004, or if the new cell does accept out of range timing advance as indicated in the HANDOVER COMMAND message, the mobile station proceeds as follows.

After having switched to the assigned channels, the mobile station sends four times the HANDOVER ACCESS message in four successive layer 1 frames on the main DCCH. This message is sent in an access burst. Its content is reduced to the handover reference information element. The transmission of these four messages is optional if so indicated by the network in the HANDOVER COMMAND message.

Before completion of the 4 access bursts on the DCCH, additional access bursts may also be sent on the SACCH.

In those cells that support extended TA values if TA value in new cell is greater than 63 and the HANDOVER COMMAND message indicates that the transmission of four HANDOVER ACCESS messages is optional the MS shall not transmit these four messages. MS shall not send additional bursts on the SACCH.

It then activates the channels in sending and receiving mode and connects the channels if need be.

If applicable, ciphering is immediately started . The access bursts are not ciphered, including when sent on the group channel.

3.4.4.2.2 Non synchronized cell case

After having switched to the assigned channels, the mobile station starts repeating the HANDOVER ACCESS message in successive layer 1 frames on the main DCCH and optionally on the SACCH. This message is sent in an access burst. Its content is reduced to the handover reference information element. The mobile station starts timer T3124 at the start point of the timeslot in which the HANDOVER ACCESS message is sent the first time on the main DCCH.

The mobile station then activates the channels in receiving mode and connects the channels if need be (only for reception).

If applicable, deciphering is then immediately started . The access bursts are not ciphered, including when sent on the group channel.

When the network has the RF characteristics that are necessary, it sends in unacknowledged mode a PHYSICAL INFORMATION message to the mobile station on the main DCCH. If applicable, ciphering and deciphering is immediately started (i.e., before even the reception of a correct access burst), and the message is sent enciphered.

The PHYSICAL INFORMATION message contains various physical layer related information, allowing a proper transmission by the mobile station.

When sending the PHYSICAL INFORMATION message, the network starts timer T3105. If this timer times out before the reception of a correctly decoded layer 2 frame in format A or B (see 3GPP TS 44.006), or a correctly decoded TCH frame from the mobile station, the network repeats the PHYSICAL INFORMATION message and restarts timer T3105. The maximum number of repetitions is Ny1.

The correct decoding of a frame means that the decoding algorithm and the error detection tests, if any, indicate no error.

When the mobile station receives a PHYSICAL INFORMATION message, it stops timer T3124, stops sending access bursts, activates the physical channels in sending and receiving mode and connects the channels if need be. If the allocated channel is an SDCCH (+ SACCH), performance of the mobile station must enable the mobile station to accept a correct PHYSICAL INFORMATION message sent by the network in any block while T3124 is running.

3.4.4.2.3 Pseudo-synchronized cell case

The details of the use of this procedure are described in 3GPP TS 45.010. The mobile station computes the timing advance to be used with the new cell from the real time difference value given in the HANDOVER COMMAND message. If the mobile station knows that the timing advance with the new cell is not out of range , i.e. smaller or equal to the maximum timing advance that can be coded as specified in 3GPP TS 44.004, or if the new cell accepts an out of range timing advance as indicated in the HANDOVER COMMAND message, the mobile station switches to the new channel and proceeds as follows.

After having switched to the assigned channels, the mobile station sends in four successive slots on the main DCCH a HANDOVER ACCESS message. This message is sent in random mode and thus does not follow the basic format. Its content is reduced to the handover reference information element. The transmission of these four messages is optional if so indicated by the network in the HANDOVER COMMAND message.

Before completion of the 4 access bursts on the DCCH, additional access bursts may also be sent on the SACCH.

The mobile station then activates the channels in sending and receiving mode and connects the channels if need be. The mobile station may activate the channels in receiving mode and connect the channels while sending access bursts.

If applicable, ciphering is then immediately started. The access bursts are not ciphered, including when sent on the group channel.

3.4.4.2.4 Pre-synchronized cell case

The details of the use of this procedure are described in 3GPP TS 45.010. The mobile station switches to the new channel and proceeds as follows.

After having switched to the assigned channels, the mobile station sends in four successive slots on the main DCCH a HANDOVER ACCESS message. This message is sent in an access burst and thus does not follow the basic format. Its content is reduced to the handover reference information element. The transmission of these four messages is optional if so indicated by the network in the HANDOVER COMMAND message.

Before completion of the 4 access bursts on the DCCH, additional access bursts may also be sent on the SACCH.

The mobile station then activates the channel in sending and receiving mode and connects the channels if need be. The timing advance value to be used with the new cell is:

– either the value contained in the HANDOVER COMMAND message if the timing advance information element is present; or

– the default value for pre-synchronized handover as defined in 3GPP TS 45.010, if the timing advance information element is not included in the HANDOVER COMMAND message. The MS may activate the channels in receiving mode and connect the channels while sending access bursts.

If applicable, ciphering is immediately started. The access bursts are not ciphered, including when sent on the group channel.

3.4.4.3 Handover completion

After lower layer connections are successfully established, the mobile station returns a HANDOVER COMPLETE message, specifying cause "normal event", to the network on the main DCCH.

The sending of this message on the mobile station side and its receipt on the network side allow the resumption of the transmission of signalling layer messages other than those for RR management.

When receiving the HANDOVER COMPLETE message, the network stops timer T3103 and releases the old channels.

If requested to do so in the HANDOVER COMMAND message, the mobile station includes the observed time difference it has measured when performing the handover, corrected by half the timing advance, in the HANDOVER COMPLETE message (detailed specifications are given in 3GPP TS 45.010).

If the new cell and the mobile station support DTM, the network shall send the DTM INFORMATION message on the main DCCH after the HANDOVER COMPLETE message has been received.

3.4.4.4 Abnormal cases

In the case of a synchronous or pseudo-synchronous handover, if the mobile station knows that the timing advance with the new cell is out of range, i.e. is bigger than the maximum timing advance that can be coded as specified in 3GPP TS 44.004, and if the new cell does not accept out of range timing advance as indicated in the HANDOVER COMMAND message, the mobile station sends a HANDOVER FAILURE message, cause "handover impossible, timing advance out of range", on the main signalling link and does not attempt that handover.

If the HANDOVER COMMAND message instructs the mobile station to use a Channel Description or Mode that it does not support, or if the Channel Mode to use is not defined for all channel sets, then the MS shall return a HANDOVER FAILURE message with cause "channel mode unacceptable", and the MS shall remain on the current channel(s) and uses the old Channel Description or Mode(s).

If the mobile station receives a HANDOVER COMMAND message containing an inconsistent MultiRateConfiguration IE, then the mobile station shall return a HANDOVER FAILURE message with cause "channel mode unacceptable", and the mobile station shall remain on the current channel(s) and uses the old Channel Description or Mode(s).

The MultiRate Configuration IE shall be considered as inconsistent by the MS if:

– the active set does not include any codec mode or the active set includes more than four codec modes; or

– one or more codec modes of the active codec set are not supported by the assigned channel; or

– the threshold and hysteresis values are not set according to requirements given in 3GPP TS 45.009.

If the HANDOVER COMMAND message instructs the mobile station to use a frequency that it is not capable of, then the mobile station shall return a HANDOVER FAILURE message with cause "frequency not implemented", and the mobile station shall remain on the current channel(s).

If the mobile station receives a HANDOVER COMMAND message with a Frequency List IE or Frequency Short List IE indicating frequencies that are not all in one band, then the mobile station shall stay on the current channel(s) and send a HANDOVER FAILURE message with cause "frequency not implemented". If the mobile station receives a HANDOVER COMMAND message with a Mobile Allocation IE indexing frequencies that are not all in one band, then the mobile station shall stay on the current channel(s) and send a HANDOVER FAILURE message with cause "frequency not implemented".

NOTE: A HANDOVER COMMAND message sent to a multi band mobile station shall not be considered invalid because it indicates target channel frequencies that are all in a different frequency band to that of the ARFCN in the Cell Description IE.

On the mobile station side, if timer T3124 times out (only in the non- synchronized case) or if a lower layer failure happens on the new channel before the HANDOVER COMPLETE message has been sent, the mobile station deactivates the new channels, reactivates the old channels, reconnects the TCHs if any and triggers the establishment of the main signalling link. It then sends a HANDOVER FAILURE message on the main signalling link and resumes normal operation as if no handover attempt had occurred. The operational parameters (e.g. ciphering mode) when returning on the old channel are those applied before the HANDOVER COMMAND message was received.

When the HANDOVER FAILURE message has been received, the network releases the new channels if they were dedicated channels and stops timers T3105 and stops T3103 in the non-synchronized case. If the new channels were VGCS channels, they shall be maintained.

If a DTM capable mobile station is in a cell supporting dual transfer mode and is in GMM READY state, it shall perform a Cell Update procedure immediately after sending the HANDOVER FAILURE message.

If a lower layer failure happens while attempting to connect back to the old channels, the standard rules are applied (cf. sub-clause 3.4.13.2 for dedicated mode and sub-clause 3.4.13.5 for group transmit mode).

On the network side, if timer T3103 elapses before either the HANDOVER COMPLETE message is received on the new channels, or a HANDOVER FAILURE message is received on the old channels, or the mobile station has re-established the call, the old channels are released if they were dedicated channels and all contexts related to the connections with that mobile station are cleared. If the old channel was a VGCS channel, it shall be maintained and the uplink shall be set free.

On the network side, if neither a correctly layer 2 frame in format A or B nor a correctly TCH frame have been received from the mobile station on the new channel, the newly allocated channels are released if they were dedicated channels. If the new channels were VGCS channels, they shall be maintained and the uplink shall be set free.

On the network side, lower layer failures occurring on the old channels after the sending of the HANDOVER COMMAND message are ignored. Lower layer failures occurring after the receipt of the SABM frame on the new main signalling link are treated following a general scheme (cf. sub-clause 3.4.13.2 for dedicated mode and sub-clause 3.4.13.5 for group transmit mode).