Complete Description of Paging Procedure

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Complete Description of Paging Procedure
The paging procedure is performed for Mobile Terminating Calls (MTC) as well
as for SMS mobile terminated. The following message flow illustrates the pr
ocess:
    MS                         BTS             BSC            MSC
    ┃                          ┃              ┃UDT(paging)   ┃
    ┃                          ┃        PGCMD ┃<━━━━━━ ┃
    ┃pch:paging request type   ┃<━━━━━━ ┃              ┃
    ┃<━━━━━━━━━━     ┃              ┃              ┃
    ┃RACH:CHAN REQ             ┃              ┃              ┃
    ┃━━━━━━━━━━>     ┃CHNRD         ┃              ┃
    ┃                          ┃━━━━━━> ┃              ┃
    ┃                          ┃CHNAK         ┃              ┃
    ┃                          ┃━━━━━━> ┃              ┃
    ┃                          ┃IACMD(IMASS)  ┃              ┃
    ┃                          ┃<━━━━━   ┃              ┃
    ┃AGCH(IMASS)               ┃              ┃              ┃
    ┃<━━━━━━━━━━     ┃              ┃              ┃
    ┃SDCCH:SABM(PAGRES)        ┃              ┃              ┃
    ┃━━━━━━━━━━>     ┃              ┃              ┃
    ┃                          ┃ESTIN(PAGRES) ┃              ┃
    ┃                          ┃━━━━━━> ┃              ┃
    ┃SDCCH:UA                  ┃              ┃              ┃
    ┃<━━━━━━━━━━     ┃              ┃              ┃
    ┃                          ┃MRPCI         ┃              ┃
    ┃                          ┃<━━━━━   ┃              ┃
    ┃                          ┃              ┃CR(CL3IAGERS┃
    ┃                          ┃              ┃━━━━━━> ┃
    ┃                          ┃call continuation ┃          ┃
    ┃                          ┃………………  ┃              ┃
    ┃                          ┃              ┃              ┃
MSC
The MSC sends a PAGING message in SCCP connection-less mode.
This message contains
· the IMSI and optionally the TMSI of the paged MS
F If TMSI reallocation is activated the MSC will always transmit both the IM
SI and the TMSI in the first paging attempt. If no PAGING RESPONSE has been
received within an defined time period time the MSC repeats the paging proce
dure; in the second attempt, however, only the IMSI is transmitted in the PA
GING message. The timer timer for the supervision of the PAGING RESPONSE is
call T_PAG and can be administered with the SSS command
  MOD NETTIMER:BSSAPT=TPAG-<value>.
The paging procedure is repeated only once.
· the area in which the MS has to be paged.
F This information can be contained in different formats: either the MSC onl
y transmits the Location Area Identity (LAI) - which means that the BSC has
to determine the associated cells from its database itself - or the MSC may
transmit all Cell Identities. The most common format is to only transmit the
Location Area Identity and to leave the determination of the associated cel
ls to the BSC as this variant causes less load on the CCS7-link.
BSC
The BSC, having received the PAGING message from the MSC, determines the cel
ls associated to the Location Area Identity received and sends a
PAGING COMMAND message to all affected BTSEs via the LPDLR.
This message contains.
· the IMSI or the TMSI of the paged MS
F Whether the BSC uses the TMSI or the IMSI for the paging procedure depends
on the contents of the received PAGING message: if the PAGING message from
the MSC contains the TMSI in addition to the IMSI the BSC will always transm
it the TMSI in the PAGING COMMAND, otherwise it will transmit the IMSI.
· the paging group
F The paging group is used for ‘Discontinuous Reception (DRX)’ and is calc
ulated from the IMSI and the used CCCH configuration. The paging group deter
mines which CCCH blocks the MS shall monitor for incoming paging messages (b
y just monitoring a subset of the CCCH blocks for paging the MS can save bat
tery capacity).
Note: Another radio parameter relevant at this point is 'BS-PA-MFRMS' (datab
ase name NFRAMEPG = number of multiframes between paging). This parameter de
termines the distance (in multiframes) between two pagings of the same pagin
g group. It is broadcast in the SYS_INFO_TYPE3 in the IE 'Control Channel De
scription'. With this parameter the time needed for the paging procedure mig
ht be extended (in addition to the repetition time determined by the MSC).
· the channel number
F The channel number indicates the type of channel via which the infprmation
is to be transmitted (in this case the vale is 'Downlink CCCH (PCH + AGCH)'
).
BTS
The BTS, having received a number of PAGING COMMAND messages from the BSC as
sembles these pagings to the Um-message PAGING REQUEST TYPE 1, TYPE 2 or TYP
E 3 with which several MSs can be paged simultaneously. Which type of PAGING
REQUEST is used depends on the number and type of mobile identities that ar
e to be transmitted within the same PAGING REQUEST (see table below).
The BTS can buffer twice the number of CCCH blocks (1 CCCH block consists o
f 4 bursts) that are contained in one cycle of the selected CCCH configurati
on for transmission.
Examples:
a) MBCCHC:
3 CCCH blocks available ? the BTS can buffer 6 CCCH blocks
b) MAINBCCH: 9 blocks available ? the BTS can buffer 18 CCCH blocks
How many bursts resp. blocks are needed for a PAGING request message depends
on the actual length of the layer 3 message: A layer 3 message of 20 octets
length can be transmitted within one layer 2 message and thus within 1 CCCH
block. If the layer 3 message is longer, another complete block has to be u
sed even if not all the bursts of the block have to be used. The remaining p
art of the block is filled with filling bits. For the possible lengths of th
e PAGING REQUEST TYPE x please refer to GSM 04.08.
Conclusion: The number of CCCH blocks required for the paging procedure dyna
mically depends on the type and length of the layer 3 message (i.e. the PAGI
NG REQUEST TYPE x) which in turn depends on the number of mobile identities
that have to be paged at the same time.
MS
The MS monitors only a subset of the paging channels on the basis of the fol
lwing principle: The radio network parameter ?Number of multiframes between
paging‘ (BSC database parameter SET BTS PKGBTSC:NFRAMEPG, range 2..9) is br
oadcast on the BCCH (SYS_INFO_Type3) in the IE ‘Control Channel Description
’. It defines the number of multiframes (51 TDMA frames) between two transm
issions of the same paging message to mobiles of the same ‘paging group’ (
The ‘paging group’ is used for ‘Discontinuous Reception (DRX)’ and is ca
lculated from the IMSI and the used CCCH configuration). The paging group de
termines which CCCH blocks the MS shall monitor for incoming paging messages
. By just monitoring a subset of the CCCH blocks for paging the MS can save
battery capacity).
In case of paging channel overload the MS can also monitor paging channels o
f other paging CCCH blocks. For more details please refer to the document
’Overload Handling in BTS Paging queues’.
If the MS has recognized its identity (IMSI or TMSI) in one of the PAGING RE
QUEST messages transmitted in the cell it performs an access via the RACH (u
sing the establishment cause 'answer to paging'), receives the IMMEDIATE ASS
IGNMENT for the SDCCH via the AGCH and and transmits the PAGING RESPONSE tog
ether with the SABM (setup of the layer-2 connection on the Um). Ths leads t
o the setup of a dedicated SCCP connection between MSC and BSC and to call c
ompletion.