Listening to MS Paging Blocks | GPRS for Mobile Internet

5.4 Listening to MS Paging Blocks

If it needs to send GPRS data toward a given MS, the network analyzes the MS GMM state in order to decide whether it may or not initiate a paging procedure. When the MS is in GMM STANDBY state, the network does not know accurately the GPRS MS location. In fact, if the mobile camps on a new cell in (packet) idle mode within the same RA, it does not notify the network of a cell change in GMM STANDBY state. The paging procedure allows for the location of an MS within a whole RA by sending a paging message in all cells belonging to this MS's RA. When the MS detects a paging that is intended for it while it is in GMM STANDBY state, it answers by sending any LLC frame and changes into GMM READY state. Upon receipt of an LLC frame sent as a paging response, the network is able to send a resource assignment command for a downlink TBF.

Note

The network does not need to initiate a paging procedure if the MS is in GMM READY state because in this state the MS notifies the network of each cell change in a given RA. Therefore, the network is able to send directly a resource assignment command for a downlink TBF toward an MS in GMM READY state.

5.4.1 Network Operating Modes

Table 5.9 summarizes the MS behavior for the reception of paging messages for circuit and GPRS services as a function of the network-operating mode, MS class, RR operating mode (idle mode, dedicated mode, transfer mode), and presence or absence of PCCCH channels.

Table 5.9: MS Behavior for Receipt of Paging Message
	Class A	Class B	Class C
Mode I with PCCCH channels	(Packet) idle mode: decoding of PCCCH channels for CS and GPRS incoming calls. Dedicated mode: decoding of PCCCH channels to detect GPRS incoming calls. Packet transfer mode: decoding of PACCH to detect CS incoming calls.	(Packet) idle mode: decoding of PCCCH channels for CS and GPRS incoming calls. Dedicated mode: no decoding of PCCCH channels to detect GPRS incoming calls. Packet transfer mode: decoding of PACCH to detect CS incoming calls.	Class C configured in CS mode: decoding of CCCH channels to detect CS incoming calls during idle mode. Class C configured in GPRS mode: decoding of PCCCH channels to detect GPRS incoming calls during packet idle mode.
Mode I without PCCCH channels	(Packet) idle mode: decoding of CCCH channels for CS and GPRS incoming calls. Dedicated mode: decoding of CCCH channels to detect GPRS incoming calls. Packet transfer mode: decoding of PACCH to detect CS incoming calls.	(Packet) idle mode: decoding of CCCH channels for CS and GPRS incoming calls. Dedicated mode: no decoding of CCCH channels to detect GPRS incoming calls. Packet transfer mode: decoding of PACCH to detect CS incoming calls.	Class C configured in CS mode: decoding of CCCH channels to detect CS incoming calls during idle mode. Class C configured in GPRS mode: decoding of CCCH channels to detect GPRS incoming calls during packet idle mode.
Mode II	(Packet) idle mode: decoding of CCCH channels for CS and GPRS incoming calls. Dedicated mode: decoding of CCCH channels to detect GPRS incoming calls. Packet transfer mode: decoding of CCCH to detect CS incoming calls.	(Packet) idle mode: decoding of CCCH channels for CS and GPRS incoming calls. Dedicated mode: no decoding of CCCH channels to detect GPRS incoming calls. Packet transfer mode: no obligation to decode CCCH for CS incoming-call detection.	Class C configured in CS mode: decoding of CCCH channels to detect CS incoming calls during idle mode. Class C configured in GPRS mode: decoding of CCCH channels to detect GPRS incoming calls during packet idle mode.
Mode III with PCCCH channels	(Packet) idle mode: decoding of PCCCH channels for GPRS incoming calls and decoding of CCCH channels for CS incoming calls. Dedicated mode: decoding of PCCCH channels to detect GPRS incoming calls. Packet transfer mode: decoding of CCCH channels to detect CS incoming calls.	(Packet) idle mode: decoding of PCCCH channels for GPRS incoming calls and decoding of CCCH channels for CS incoming calls. Dedicated mode: no decoding of PCCCH channels to detect GPRS incoming calls. Packet transfer mode: no obligation to decode CCCH for CS incoming-call detection.	Class C configured in CS mode: decoding of CCCH channels to detect CS incoming calls during idle mode. Class C configured in GPRS mode: decoding of PCCCH channels to detect GPRS incoming calls during packet idle mode.
Mode III without PCCCH channels	(Packet) idle mode: decoding of CCCH channels for CS and GPRS incoming calls. Dedicated mode: decoding of CCCH channels to detect GPRS incoming calls. Packet transfer mode: decoding of CCCH channels to detect CS incoming calls.	(Packet) idle mode: decoding of CCCH channels for CS and GPRS incoming calls. Dedicated mode: no decoding of CCCH channels to detect GPRS incoming calls. Packet transfer mode: no obligation to decode CCCH for CS incoming-call detection.	Class C configured in CS mode: decoding of CCCH channels to detect CS incoming calls during idle mode. Class C configured in GPRS mode: decoding of CCCH channels to detect GPRS incoming calls during packet idle mode.

Note that in a network mode III that supports PCCCH channels, a class B MS will ensure a double idle mode operation; actually, the MS must monitor CCCH and PCCCH channels. As this behavior has an impact on the MS battery autonomy, some mobiles will be configured in class C in a network-operating mode III.

5.4.2 DRX Mode

In order to minimize the power consumption, the MS in idle mode is not required to listen continuously to the amount of information provided by the network. The (P)PCH logical channels have been split into several paging subchannels ; all paging messages addressed to an MS with a given IMSI are sent on the same paging subchannel. This feature is called DRX.

Note that the DRX feature allows for an increase in battery lifetime, but at the expense of a small increase in time delay for the establishment of circuit and GPRS incoming calls.

The method whereby paging subchannels are accessed is determined by either the network, in the case of PCH channels, or by the MS, in the case of PPCH channels.

5.4.2.1 (P)CCCH_GROUP Definition

As there may be multiple CCCH or PCCCH logical channels on different time slots of the same carrier or on different carriers in the same cell, the MS will first determine the right carrier and the right time slot for paging-channel decoding. Thus a CCCH_GROUP or PCCCH_GROUP addresses a specific CCCH or PCCCH for paging-message listening but also random accesses on RACH or PRACH.

The CCCH channel is found on time slot 0 of the BCCH carrier, but may also be found on time slots 2, 4, and 6. The parameter BS_CC_CHANS broadcast in the BCCH defines the number of CCCHs. The MS is able to define the right time slot CCCH channel according to the parameter CCCH_GROUP. CCCH_GROUP is a number between 0 and BS_CC_CHANS -1. The lowest-numbered CCCH_GROUP is mapped on the lowest -numbered time slot carrying CCCH, the next-higher-numbered CCCH_GROUP is mapped on the next -higher-numbered time slot carrying the CCCH, and so on. The CCCH_GROUP is defined by the following formula:

(5.19)

with N = number of paging blocks "available" in a 51- multiframe on one CCCH — number of 51-multiframes between transmission of paging messages to MSs.

Note that the CCCH_GROUP is used by MSs in circuit mode or GPRS mode for all network-operating modes if PBCCH/PCCCHs are not present in the cell. The MS determines the N value from parameters broadcast on BCCH.

The PCCCH channel may be found on several carriers and on several time slots per carrier. The parameter BS_PCC_CHANS (maximum value: 16) defines the number of PDCHs carrying the PCCCHs. The MS is able to identify the specific PCCCH according to the PCCCH_GROUP parameter. The PCCCH_GROUP is numbered from 0 to BS_PCC_CHANS - 1. The network broadcasts to the MS the organization of PCCCH by stating the list of used carriers. The mapping between the PCCCH_GROUP and the physical channel follows the PCCCH description broadcast on PBCCH. The lowest-numbered PCCCH_GROUP is mapped on the lowest-numbered time slot carrying PCCCH on the first PCCCH carrier. The next-higher-numbered PCCCH_GROUP is mapped on the next-higher-numbered time slot carrying the same carrier, and so on. When all time slots of the first PCCCH carrier are used, the next-higher-numbered PCCCH_GROUP is mapped on the lowest-numbered time slot carrying PCCCH on the next PDCH that carries, and so on. The PCCCH_GROUP is defined by the following formula:

(5.20)

Note that the formula given above is applied with respect to GPRS-attached mobiles if PBCCH/PCCCHs are present in the cell. Nevertheless, if the PBCCH/PCCCHs are not present in the cell, the GPRS mobile may choose to apply the PCCCH_GROUP definition instead of the CCCH_GROUP definition. In this case, the formula given above is slightly modified. The choice between these two different procedures is negotiated during the attachment.

5.4.2.2 PAGING_GROUP Definition

As there are several paging subchannels in the same physical channel carrying the CCCH identified by a CCCH_GROUP or on the same PDCH carrying the PCCCH identified by a PCCCH_GROUP, the MS will determine the right paging subchannel for paging blocks decoding according to the parameter PAGING_GROUP. This paging subchannel is defined as a set of multi- frames carrying the CCCH or the PCCCH. This set of multiframes defines the periodicity at which the MS is required to decode paging messages. This periodicity of listening is configured either by the network, when paging occurs on the physical channel carrying the CCCH, or by the MS, when paging occurs on the PDCH carrying the PCCCH or when paging occurs on the physical channel carrying the CCCH and the mobile applies the PCCCH_GROUP definition.

On CCCH Logical Channels

The BS_PA_MFRMS parameter defines the number of 51-multiframes for the periodicity of PCH subchannel paging decoding. The value is broadcast on BCCH and may range from 2 to 9. For instance, if the value is equal to 9, the MS will decode its paging subchannel every nine 51-multiframes. The number of paging subchannels N on one physical channel carrying the CCCH is equal to the number of paging blocks in one 51-multiframe multiplied by the periodicity of subchannel paging decoding (BS_PA_MFRMS value). The PAGING_GROUP is numbered from 0 to N - 1. The PAGING_GROUP is defined by the following formula:

(5.21)

with N = number of paging blocks "available" on one CCCH (value deduced from parameters broadcast on BCCH).

In order to find the required 51-multiframe containing the paging subchannel among the cycle of BS_PA_MFRMS 51-multiframes, the following equality will be checked:

(5.22)

with FN = frame number.

The required paging subchannel is identified in the required 51-multiframe by the following formula:

(5.23)

Figure 5.3 shows an example of the use of CCCH_GROUP and PAGING_GROUP concepts for the required paging subchannel PCH decoding.

Figure 5.3: Example of the use of CCCH_GROUP and PAGING_GROUP concepts for the required paging subchannel PCH decoding.

On PCCCH Logical Channels

The SPLIT_PG_CYCLE parameter defines the occurrence of paging blocks on the PDCH carrying the PCCCH belonging to the MS in DRX mode by specifying a number of subcycles used for PPCH subchannel paging decoding within a cycle of 64 52-multiframes. The SPLIT_PG_CYCLE value is fixed by the MS and is sent by the MS to the network during the GPRS attach procedure or during the RA update procedure.

The number of paging subchannels M used on every 64 52-multiframe is equal to a number of paging blocks in one 52-multiframe multiplied by 64, given by the following formula:

(5.24)

The PAGING_ GROUP is numbered from 0 to N - 1. The PAGING_GROUP is defined by the following formula:

(5.25)

with n : subblock number between 0 and MIN[ M , SPLIT_PG_CYCLE].

The term MIN[ M , SPLIT_PG_CYCLE] makes it possible to prevent more subperiods than paging blocks if SPLIT_PG_CYCLE > M.

The term Max (( n — N ) div SPLIT_PG_CYCLE), n ) makes it possible to verify that there are more subperiods than paging blocks and to have, for each new value of n, a value of the term MAX which is different from the ones previously calculated, ensuring that all the PAGING_GROUP values for each subcycle are different from one another.

Note that:

The PAGING_GROUP formula for PCCCH gives a number of values, each value being associated with a subcycle. The number of the PAGING_GROUP values is equal to the value MIN[M,SPLIT_PG_CYCLE]. The mobile will thus have to monitor the PCH a number of times equal to the value of the SPLIT_PG_CYCLE every 64 52-multiframes.
The PAGING_GROUP formula given above is slightly modified if a GPRS MS decides to apply SPLIT_PG_CYCLE on CCCH.
The term 64/SPLIT_PG_CYCLE is equivalent to the BS_PA_MFRMS.
The lower the BS_PA_MFRMS value (or the higher the SPLIT_PG_CYCLE value), the higher the recurrence of paging decoding and the lower the MS's autonomy in packet idle mode.

In order to find the required 52-multiframe containing the paging subchannel among the cycle of 64 52-multiframes, the following equality will be checked:

(5.26)

with FN = frame number.

The required paging subchannel is identified in the required 52-multiframe by the following formula:

(5.27)

Note that the same concept is used for the required paging subchannel decoding between PCH and PPCH. The main difference is bound by the number of PAGING_GROUP values depending on whether or not PCCCH logical channels are present. For one PAGING_GROUP value, the required paging subchannel PPCH is identified in the same way as the required PCH (see Figure 5.3).

5.4.3 Non-DRX Mode

After a TBF release or a measurement report in idle mode the mobile reverts to the non-DRX mode, in which it must decode all CCCH or PCCCH blocks, independently of its DRX period. If the network needs to initiate a new downlink TBF, it sends the downlink TBF allocation on any (P)CCCH related to the (P)CCCH_GROUP of the MS. The downlink TBF establishment is quicker for a MS in non-DRX mode because the network does not need to wait for the MS paging subchannel in order to send the paging message. After a TBF release, the non-DRX period is equal to the minimum value of two values. One value is set by the network (DRX_TIMER_MAX parameter) and given on broadcast channels, and the other one is set by the MS (NON_DRX_TIMER parameter) during the GPRS-attach procedure. At the end of a measurement report in idle mode, the non-DRX period is equal to a value set by the network (NC_NON_DRX_PERIOD parameter) and given on broadcast channels.

Note that:

There is a high probability that a new downlink TBF needs to be established a short time after the end of an uplink TBF or a downlink TBF. In the case of client-server relation, a downlink TBF may occur shortly after an uplink TBF. For instance, in the case where the uplink TBF is used to send a request, a downlink TBF may be established for the transfer of the response. There may be a latency period between two consecutive IP packets sent to a given MS. This means in some cases that several downlink TBFs are needed for the sending of consecutive IP packets toward the MS.
The non-DRX mode has an impact on MS autonomy in idle mode because in this period the MS must continuously decode the CCCH or the PCCCH blocks. The MS performs a tradeoff between autonomy and downlink TBF establishment time when setting the NON_DRX_TIMER value.

Moreover, the MS is in non-DRX mode during the GPRS attach and RA update procedures because the BSS might not know the IMSI of this MS. Without knowing the IMSI, the BSS is not able to compute the MS PAGING_GROUP and thus is not able to forward paging toward the right paging subchannel. This case may occur when IMSI is unknown at the HLR level.

5.4.4 Paging Modes

The paging configuration may change in time according to partial traffic congestion in a given cell. Paging mode information is given by the network to the MSs in order to control possible additional requirements on paging decoding. Three paging modes are defined:

Normal paging. The MS is required to decode all paging messages in the paging subchannel belonging to its paging group; this is the nominal case.
Extended paging or "next but one." The MS is required to decode all paging messages in the paging subchannel belonging to its paging group and also in another paging subchannel. This mode is used when there is a temporary overload on some of the subchannels. If PBCCH and PCCCH are present in the cell, the MS decodes an additional subchannel that is located on the third PPCH after the PPCH corresponding to the MS paging group. If PBCCH and PCCCH are not present in the cell, the MS decodes an additional paging subchannel, which is identified as the next PCH after the paging subchannel corresponding to its paging group.
Paging reorganization. The MS is required to decode all paging messages on (P)CCCH independently of its paging group. This mode is used by the network when it needs to change the logical channel organization between the (P)AGCH and (P)PCH.

5.4.5 Downlink Signaling Failure

A downlink signaling failure mechanism is implemented in GPRS mode as in GSM mode (see Figure 5.4). It is based on a downlink signaling failure counter (DSC), and it checks that the MS is able to successfully decode a message in its paging subchannel. The counter value is initialized with a DSC maximum value deduced from the paging period when the MS camps on a new cell or leaves the packet transfer mode. The DSC value is incremented by one for each paging message successfully decoded, and decremented by four for each paging message unsuccessfully decoded. There is a downlink signaling failure if the DSC value is lower than zero. A cell reselection is triggered on downlink signaling failure.

Figure 5.4: Downlink signaling failure mechanism.