START-
1. RADIO ACCESS PART
2. ESTABLISHMENT PART
3. AUTHENTICATION CHIPERING & TMSI
REALLOCATION PART
4. CALL SET UP
5. TCH SEIZED PART
6. CONVERSATION PART
7. RELEASE BY MS
8. STOP
1. RADIO ACCESS PART-
(1)
MS sends channel request msg to BTS using RACH. In
addition to message, it also contains random reference number to differentiate
between different MS who makes the request exactly at the same time.
(2)
BTS sends channel required msg to BSC along with first
timing advance estimate.
(3)
BSC sends channel activation message to BTS, before
sending message, BSC starts to search and allocate the SDCCH. Content of this
message is DTX control, CH. ID description and mobile allocation, ax
power level of MS and BTS, TA calculated by BSC.
(4)
BTS sends CH. Activation ACK Msg. to BSC, after this
BTS sends & receive SACCH.
(5)
Once BSC receives the above ACK, it sends immediate
assignment command to BTS and tells about SDCCH to be used.
(6)
BTS after receiving LAC , sends immediate assignment
message to MS. At this stage ,SDCCH ch. Is allocated to MS using AGCCH channel.
The contents of this message are, TA, description and allocated channel,
initial max transmission power & exact contents of channel request message.
2. ESTABLISHMENT
PART:-
(1) MS
sends CM service request message to BTS using SDCCH .in order to request
services for connection management, sub layer entities like circuit switched
connection established SS or SMS.
(2) After
BTS receives CM service request message, it sends same MSG to BSC with
established indication messages .it has two uses
(a) Tells
BSC that MS is in which channel link.
(b) BTS
identifies main signaling link and adds L3 information into this message
received from MS.
(3) Once
BSC receives above message, it sends connection request MSG to MSC.
(4) At
this stage, BTS sends UA (unnumbered ACK) MSG to MS. UA is L-2 level ACK. Used
in setting L-2 level link in LAPDM protocol.
(5) Once
MSC receives CM service request, it sends process access request MSG to VLR for
authentication, chipering, &TMSI, reallocation, procedures.
3. AUTHENTICATION,
CHIPERING & TMSI REALLOCATION:-
(1)AUTHENTICATION:-This procedure uses A3, A8 & 32 digit RAND , which are created in authentication centre & MS
SIM card, which is used for security purpose.
(a)VLR after receiving process access, request
message, VLR initiates authentication and sends it to MSC .this message
contains 32 digits RAN B.
(b) Once MSC receives authentication MSG,
it sends authentication request message to MS through different interface + 32
digit RAND .
(c) MS authentication procedure uses A3
& A8 algorithm, in sim cards A3 , A8, & 32digit RAND and same can be
found in authentication centre, then authentication went ok.
(2) CIPHERING: - It uses A5 algorithm & used for security.
(a) Ciphering means nobody can decode the
information b/w calling and called party, other than the called party, which
has ciphering key sequence.
(b) After authentication, VLR initiates
ciphering and sends it to MSC, after this, MSC sends ciphering mode commond MSg
to BSC, in order to start process b/w MS and BTS.
(c) Once BSC receives ciphering mode commond
Msg. it sends CMC msg along with encryption commond to BTS.
(d) After this BTS informed to MS
about beginning ciphering & start the recovering in ciphering mode & MS
once receives the CMC message, the MS enters the ciphering mode & sends
ciphering mode ACK MSG to VLR through different interface.
Ciphering process b/w MS and BTS
uses SDCCH.
(3) TMSI REALLOCATION:-
(a) After completion
of ciphering the VLR accepts the process access request MSG. and initiates TMSI
reallocation process and sends it to MSC, then MSC sends it to MS.
(b) The purpose of this process is to
providing the user identity confidently. This procedure is performed at least
at each change of location area.
TMSI reallocation commond message
contains new combination of TMSI & LA, allocated by network, usually, TMSI
reallocation commond message is sent to MS by using RR connection in ciphering
mode.
(c) When MS receives the TMSI
reallocation commond MSG, it stores the LAI in SIM, if received, identify IMSI
of MS it delete the previous stores TMSI.in both cases VLR is updated.
This procedure b/w BTS and MS uses SDCCH.
(4) CALL SET UP PART:-
(a) After authentication,
identification & ciphering the MS is in SDCCH & ready to start real
call set up signaling. The MS sends RIL-3RR set up message i.e. Sent to VLR.
This MSG. contains called party no. and description of required services and
some supplementary services.
(b) MSC receives the above message,
analyse the request and checks whether it can accepts it depending upon its
capacity to provide the service. If every thing is OK, then on one hand MSC
starts establish through network and on other hand send RIL-3RR call proceeding
msg. once MS receives this msg it enters mobile originated call proceeding (noc).
In call set up and call proceeding msg b/w MS & BTS uses SDCCH.
(c) After above completion, MSC
sends assignment request msg to BSC in order to start TCH allocation.
(d) once BSC
receives assignment request msg ,it
sends physical context msg to BTS ,by using msg BSC interfaces actual
timing advances from BTS in order to start the TCH allocation.
(e) after
this BSC sends channel activation msg to
BTS in order to activate TCH ,this msg contains information like channel no.,
activation type, channel mode, channel type, channel identity, encryption information,
power level of MS & BTS & finally TA.
(f) BTS sends back
channel activation ACK this msg. Contains
current TDMA frame no.
(g) BSC receives
ACK & sends assignment commond message to ms through BTS using SDCCH. At
this stage, TCH is allocated available to MS.
(h)after receiving
assignment commond msg the MS sends
SABH(set asynchronous balance mode) msg to BTS using FACH .this is 1-2 msg
& contains L3 information. the
contains of the msg are service request ,ciphering key set,ms ,identity.
(i) Once BTS receives
SABH, it sends estb. Indication msg. to BSC this msg has two uses.
(a) Description of
ch, MS is on FACCH.
(b) BTS
identifies main signaling link & add received information in L3.
(j) BTS after sending
ESTB indication msg. to BSC it sends UA (unnumbered ack) msg to MS . UA is L2
ACK msg used for L-2 link in LAPD protocol.
5. TCH SEIZED PART:-
(a) MS after receiving
UA message sends assignment complete msg to MSC, the MS informs to the n/w, it
is now on TCH & everything is ok.BSC
ack CH seizure to MSC.
(b) BSC sends channel release to msg. to BTS as
MS is on TCH & there is no need for signaling so SDCCH is released and ACK
is send back.
(c) On other side MSC
will receive information from external n/w report of the requested call established,
as seen by switch in-charge that the called party has been alerted of incoming
call.
(d) Once called
party lifts the phone, MSC sends connect msg to MS using FACCH at air interface.
After receiving msg MS enters to speech path.
(e) After that MS
sends ACK to MSC in FACCH , MS
6. CONVERSATION PART:-
During conversation ,MS will send measurement reports to BTS using SACCH
1/SEC or 2/sec at rate of 13 bps or 26 bps.the report contain information like,
RX level ,quality, TA, power control, present BCCH, serving cell, information
about neighbour. MS will have time to
decode neighboring cells when.
1. after receiving
and before transmission.
2. before
receiving and after transmission.
3. BTS sends
these measurement reports to BSC.
7. RELEASE BY MS: - (a)
after conversation part is over, MS sends disconnect msg to MSC using FACCH,
this msg will clear the end to end connection.
(b) when MSC
determines that has call no more reason to exist , it sends release msg to MS
,which answers back with an ACK to MSC only then lower connections
released & mobile returned to idle
state.
(c) Finally release
the RF channels.
8. STOP: - this
ends call processing.
No comments:
Post a Comment