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Technorealism -A phenomenon unleashed
S U N D A Y , O C T O B E R 2 1 , 2 0 0 7
Should missed calls be charged ?
Todays generation surprisingly believes more in the magic of radio
communication than in the old beliefs revolving around mental telepathy. It
is surely a byproduct of the freedom given by the wireless phenomenon both
on a technical as well as political front.
In India,the worlds largest democratic country, an intentionally short duration
unpicked "miss"call to a second party lasting not more than a second is not
charged by any of the service providers.
-This post emphasizes on highlighting the complexities involved in such an
originating GSM call.
Take for eg, Mr. X riding hastily on his bike on the Bombay-Pune express
highway has met with an accident. Mr. Y immediately comes on a rescue for
Mr. X. While rushing him to the hospital, he tries to contact Mr. Xs family
members, but he is unable to connect, he is consistently getting this
message: All the lines to this route are currently busy, please try after some
time
At the very same time, a Delhi based couple, geographically apart "recent shift
of the spouse to TCS Pune", is continuously playing missed call games @ 1
minute, hence awfully blocking the voice traffic lines, without actually using
it.
So, lets see what happens when this caller first tunes on his phone.
He needs a frequency to transmit. The frequency range normally lies between
800-1900 MHz . You can visualize this frequency, watching the fans running
in our houses at a frequency of 5-10 KHz . The
mobile checks a frequency list contained in the
SIM (most of you guys know this "thing" but for
those who don't, it's Subscriber Identity Module)
card, the removable memory chip in the handset.
With a SIM card, you can share your account from
mobile to mobile.
About Me
Pooja Gupta
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validate at least if the caller has sufficient money to make the call. Hence, the
need arises to check some databases on a server, often the UNIX workstations,
which could be half a country away. The Home Location Register (HLR), the
Visited Location Register (VLR), the Authentication Center (AuC) and the
Equipment Identity Register (EIR) are some of these databases. Also it is legal
to provide mechanisms by which the calls can be monitored by the security
personnel for safety reasons. While A and B are talking to each other, C can
join the call and listen silently.
The Home Location Register and the Visitor Location Register work together --
they permit both local operation and roaming outside the local service area.
You couldn't have used your mobile in Washington and then Bangalore
without these two electronic directories sharing information. The HLR stores
your international mobile equipment number or IMEI, the class of service you
have, your current city and your last known location area the place you last
used your mobile.
The VLR or visitor location registry contains roamer information, passing
through another carrier's system? Once the visited system detects your mobile,its VLR queries your assigned home location register. The VLR makes sure
you are a valid subscriber, then retrieves just enough information from the now
distant HLR to manage your call. It temporarily stores your last known location
area, the power your mobile uses, special services you subscribe to and so on.
Though traveling, the cellular network now knows where you are and can
direct calls to you.
The AuC is the Authentication Center, a secured database handling
authentication and encryption keys. Authentication verifies a mobile customer
with a complex challenge and reply routine. The network sends a randomly
generated number to the mobile. The mobile then performs a calculation
Mindbox
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against it with a number it has stored in its SIM and sends the result back. Only
if the switch gets the number it expects, does the call proceed. The AuC stores
all data needed to authenticate a call and later encrypt both voice traffic and
signaling messages.
EIR is a standard GSM network element that allows a mobile network to check
the type and serial number of a mobile device and determine whether or not to
offer any service. It's first purpose is to deny stolen or defective mobile service.
Good mobiles are allowed on the network, of course, as is faulty but still a
serviceable equipment.
Coming back to the earlier discussion, now that the mobile account has been
authenticated, the MSC ( mobile switching centre, part of network sub-system
as shown above), initiates the ciphering of the mobile data being sent on the
traffic channel, which is required to protect the call from eavesdropping. After
the ciphering handshake is complete, the mobile sends the set up message to set
up a voice call. The message contains the dialed digits and other information
needed for call establishment.
The network informs the mobile that the call set-up is in progress. After this,the mobile starts displaying on the screen Calling X. A dedicated voice line
is allocated by the MSC between the two parties to take this call. The original
MSC ( say HUTCH MSC , near to South-ex, Delhi) routes this call to the
destination MSC ( say AIRTEL MSC near Aundh, Pune). The destination
MSC further informs the calling party that the called party has been alerted via
a ring.
What next!!!
Now that the whole call set up procedure has been properly set up, the caller
impulsively disconnects the phone, and all the allocations done for that
particular call while leaving other calls on hold, immediately goes forlorn, the
worst part is that the caller is not even charged a single penny for
this "amusement" of his.
This really pings the minds of many of us to awaken our thinking buds and
analyze if this is really the right way ??
I agree, at times we dont have sufficient balance to make calls, but how do we
keep a check. On a second thought, if it is paid, then what is the cheaper
alternative?
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Can SMS ( short message service) provided at more cheaper rates, is the
answer, so the people always remain emotionally connected,
without "blocking" other GSM traffic users to a certain level.
Well!! , that's still a question which you have to answer!
So, for readers who want to understand the sequence flow of GSM networks a
little bit more, following links would prove quite helpful:
http://www.ee.surrey.ac.uk/Personal/L.Wood/constellations/tables/gsm.html
http://www.EventHelix.com/RealtimeMantra/Telecom/GSM_network_example
.htm
To check our GSM concepts understanding, here is a small quiz.
1. Can a subscriber be present in more than one HLR/VLR at a time?
2. Can a mobile phone have more than one MSISDN?
3. Why are uplink frequencies lower than downlink frequencies?
4. What is the frequency band followed in Europe, Asia, Africa and America?
5. What is the inherent difference between an SMS and a MMS?
6. Is "listening" more expensive or "speaking"?
Note: The reader can also flood this post with his/her choice of questions.
While posting comments, it would be worthwhile mentioning a little
background about the reader's profession.
If the missed calls start getting charged worldwide, it wont remain logical to
assure our dear ones by saying that we are just a call away!!
Rather it would make more sense in saying that im just a call plus a few
dollars away!! Right???
35 comments:
Anonymous said...
i was just browsing through the internet got your blog ,Great work ! i
must say . i have one query ,though not related to artical ,say Mobile
is in its acting home PLMN (huch subscriber of south ex delhi in hutch
mumbai). i power off the mobile and go to a place in mumbai where
there is no hutch coverage,it will search for covarage.mobile should
first search for last registered PLMN,which is not available in this case
so mobile will search for Home PLMN which is also not available.now
mobile will search for some PLMN list which is there in SIM.say those
'
Posted by Pilot-Pooja at 1:10 AM
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Fifth, it might result in service providers intentionally breaking the
calls so as to earn that extra buck. It does happen.
October 23, 2007 9:50 PM
pilot-pooja said...
Echoing PLMN query:
Well first of all many thanks!! for the comment and appreciation of
my work.
Coming to your query, yes the mobile will go to the "limited service" if
it's unable to find any of the network service providers available in
that area..
The second point,if your mobile service is having a national roaming
which faciliates you to move from one mobile operator to the
other,then of course it will search for any other available networks in
that area.In case it's unable to find one then it will go onto limited
service.
It will continue to show limited service until it finds a mobile network
service to which it can "latch" and provide you the service.
It will search for the home PLMN (your last query) only if it comes into
the coverage area of your home PLMN.Once you are in your home
PLMN coverage area,the mobile can easily set up a connection with
the base station with help of the information stored in the SIM card.It
can get all the required info from the HLR.
I hope it will answer your doubt, will be happy to explain you more on
this if you have any other queries!!!
October 24, 2007 6:19 AM
Anonymous said...
Thanks for response! i did get my query answered however still have
some doubt ,i would be happy if you can throw some light on it.
1)How MS knows that ,it is in its HPMLN ? ( i know Network broadcastscertaion system info continuously ,based on these info MS decides
that it is in which PLMN ,so is it like mobile continuously monitors
these information and camps to a perticular PLMN ???
if so ,my question is what is time period for this monitoring???
)
2)Say MS is in visted PLMN, it camps to VPLMN.before returning to
HPLMN it is switched off.now MS reached to a place where there is
both coverage available(VPLMN as well as HPLMN).MS is powered-
on ,now MS will search for last registerd PLMN which is VPLMN So MS
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fact that HPLMN is also available.so again my question is after what
time it will camp to HPLMN (as in this case both PLMN available).
Vielen Dank !
October 24, 2007 8:45 AM
Anonymous said...
Hey ! could you please tell me How emergency calls work without SIM ?
and how without-SIM emergency calls are different from with-SIM
emergency calls ?
-thanks
October 24, 2007 8:53 AM
Nitya said...
Well!!commenting on sandeep!!
1st point:
Of course it will be a wrong judgement to the customer's intentions
but in order to avoid such situations technology is the answer.
All the calls can be monitored and technology can extricate the actual
call disconnect from the user for the "amusement"of his from the one
owing to the network problem.
2nd point:
SEE!!either you have balance or you dont but think of the situation
when you dont have balance or you have enough balance for only a
single call.Suppose the missed call werent charged and you start
giving missed call in anticipation of getting a call back,nothing much
happens and you are not able to convey this to the other party.
Now think of this situation where missed call is charged but SMS is
almost free (or too less),you can always send the info to other party
without the hassles of giving missed call and waiting for the call back!!
What do you say??
3rd Point:
Again here also technology can play a bigger role by making a
differentiation between the calling party and the called party!!
In any case free or cheaper SMS will be a better option for the caller
than getting hung up.
4th point:
The difference between a missed call and user availability can also be
done with help of the technology as stated above.
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does not disconnect the call but it gets disconnected automatically
and that autheticates the call from not being an "amusement"
5th Point:
Since everything is automated these days the service provider will not
be bothered to disconnect the call intentionally just to earn a few
extra bucks and also besides that it will be quite apprehensive of
losing the customer on account of enormous call drops rather than
condoning the customer ahead with the call and earning !!
Well its just a hypothetical situation and these were my comments on
the points raised above.Please feel free to annotate on this if you do
not reckon it cogent!!!
At last,in my view, free SMS can be the answer for this and inthis way
the customer also will be better served !! Afterall at the end of the
day its the customer who is getting benefited!!!!!!!
October 24, 2007 4:06 PM
Amit Mittal said...
Good Insight Story...
But charging for missed call doesn't make sense as it's like...either
give me oxygen mask for free or charge others for natural oxygen, as
they are probably taking my share as well......:)
But a good technical insight no doubt..
October 24, 2007 8:43 PM
Sandeep said...
Nitya,
Whether a user disconnects the call, or the call fails due to network
unavailability, it is the same technically. The providers do not
differentiate in such calls, as I understand.
When we keep pinging up, the other party also realizes that it might
be something urgent, and hence might call back. The SMS ringers are
generally not continual, and hence might delay in delivering theURGENT message.
I am not very sure about the load SMS will cause on the infrastructure
and what sort of congestion it might lead to, but it does have effects
too, like we see during the festive occasions.
Also, if the SMS are made free, it might lead to decrease in revenue
for the provider, as many people would switch the medium to
communicate. As we do when we get that free SMS bonanza ;) As a
result, the providers would not be much inclined towards it.
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many examples (real for sure) where the providers have been doing
such nuisance. The so called automated processes have been charging
different rates for calls to the same number. There are thousands of
customer care requests out there.
The calling rates are one of the lowest in India as compared to the
developed countries of the world. At many places even incoming calls
are charged. In Japan, DoCoMo is the only provider, still that is the
best provider in the world. They have the most advanced network and
are fully loaded. Consumers are happy in these places too. Can our
providers match it?
It is the human mindset to seek things for free, but are we just in
using such a system? Think about the abuse of free systems like E-
mails, Office phones and of course, Orkut! (No offences meant)
I guess the real solution would be to educate the consumers against
the ills of abusing the system (citing examples of festival occasions,
Mumbai blasts etc.). on provider's part to upgrade their infrastructure
to prevent dead cell zones, optimize their capacity, issue sane
number of connections, and not just race to trap market.
Feel free to opine.
October 25, 2007 3:48 AM
pilot-pooja said...
Echoing Emergency calls Query:
Emergency call with SIM signifies that you are calling a pre-defined
number(which varies country to country),while using the services
provided by your mobile service provider.
In India, any emergency call maps to 100( mainly Police Station), be
the call made from landline or mobile.
Emergency call without SIM signifies that though you are not allowed
to make any normal calls from your mobile(mainly because there is no
balance left in the SIM), still since you might have landed in some
difficulty and you need some assistance from Police, hence using yourcurrent SIM, you can still make an emergency call, as mentioned
before.
Important points.
1. No emergency call is possible if no SIM is inserted in the mobile.
2. No emergency call is possible if you are at a place having no
network coverage.
3. Emergency call can still be made even in absence of home network
coverage, it will be made through the other networks received by
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your mobile.
October 25, 2007 4:33 AM
Anonymous said...
@ pilot-pooja
1)according to 3GGP specification 3GPP TS 22.101 section10,emergency calls shall be supported by the UE without a
SIM/USIM/ISIM being present.
thats why i asked how it is different from with SIM emergency call.
2)also i doubt about your 2 point ""No emergency call is possible if you
are at a place having no network coverage.""
i was reading yesterday even if there is no PLMN to camp on.it will
serch any acceptable cell which satisfies the cell selection criteria
(C1>0).
but my question is what is emergency call flow.??
3)specification also says 112 and 911 are mendatory emergency
numbers ,and have to be supported by MS.
As i am also studying emergency call related stuff.i found that
without SIM case network doesnt know identity of the mobile (i.e.
IMSI),chances are this functionality can be used by users as to make
pranks.my ultimate task is to find a mechanism to identify emergency
calls without a SIM.
P.S.-i am a mobile communicaion student....
October 25, 2007 5:52 AM
Nitya said...
Thanks for your conceptions Sandeep!!
According to Q.931 standard all the call disconnect codes can be
monitored and detected and that can be a good source of information
for such circumstances.
I totally agree that SMS will have a load on the traffic but at the same
time it will save the nework connection load and also SMS is a little
impeded form of communication which require a little extra effort
from our "easy-to-use" frenzy people of India.
On the other hand the charge for the missed call can be the alternate
source of revenue for the providers.
It is not the human mindset to seek things for free but to get enough
liberty to make use of all the services provided at ease.I agree there
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India, having declared as the world's largest democracy and second
most populated country, has the leisure of getting the cheapest
calling rates and today mobile phone is a every houshold entity.
Obviously the load will be too high as comapared to what it is in
Japan but as long as there is a high demand from a constantly growing
number of customers,there will be a healthy competition among the
service providers and after a certaint time, they can match.
Its just a matter of time when there will be hoard of service providers
burgeoning in the country and a zero tolerance service will be a
regulation!!
You just have to wait & watch!!!
October 28, 2007 3:41 PM
Sandeep said...
Thanks for clarifying on the Q.931 codes, I had absolutely no
knowledge about them.
As I take the liberty of guessing here, the number of missed calls
would be far small than the number of SMS exchanged daily, and
hence they compensating as an alternate source of income, might not
please the money-hungry providers.
Japan has a mobile density of 78% (100 million subscribers, Jan 07
with a population of 127 million), whereas India has a density of 18%(208 million, Sep 07 with a population of 1129 million, July 07). The
load factor hence would be again more in Japan. But even if we take
the load factor to be high, should not the providers improve their
infrastructure to cope up?
The growth in India does indeed exceeds of Japan, and we are still to
witness the focus on quality. The vision of one of the largest providers
in India, "Dream of pushing a mobile phone to every household" has
only made it to monger for more connections and money by following
any tactics.
I, myself had been a victim of the so called billing errors. Their
service is pathetic to an extent when 4 months after disconnecting a
connection, after obtaining a FULL AND FINAL PAYMENT made
certificate, I was being troubled by their lawyers to cough up more.
The recent run to obtain licenses by builders, real estate agent,
bourses who don't listen to their existing customers in the existing
domain, would only make it tough.
PS: This is my own personal opinion and I wish not to offend anyone
while expressing it.
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October 29, 2007 3:52 AM
pilot-pooja said...
Continuing with the emergency call discussions:
Thanks to bring this info to my notice and also for sharing your
academic background. This helps to respond back accordingly.Coming
back to the question:
An emergency SIM card along with the regular SIM card is always
present inside a cellular phone.When an emergency call is initiated
over a network,for example, a cellular network, the cellular phone
switches from the regular-use SIM card to the emergency SIM card.
This switching may take between 30 and 90 seconds. The cellular
phone, which is registered with the network using the regular-use SIM
card, unregisters from the cellular network, and subsequently re-
registers using the emergency SIM card. This switching may cause
precious time to be lost.
To determine if a call is an emergency one, the microcontroller inside
the cellular phone compares the number dialled on the keypad to the
emergency telephone number stored in the memory.
When an emergency call is placed i.e. 911, the emergency SIM card
module transmits an identification key to the network using the
previously established communication link, which is then verified and
assigned a channel after authentication. In a normal call,the IMSI of
the SIM is compared with the already existing IMSI in the user's HLR,
whereas this is not applicable in an emergency call.
In the emergency mode of operation, the user dials 112/911, the
request goes to the emergency service dept. through the mobile
antenna using the available network.The server further routes the call
say to a police station, which then responds back to the user.The user
again detects these wireless signals through the antenna.
In case ,the normal SIM is unavailable or the user does not have
sufficient balance to make a normal call,there may be a pre-defined
period after which the emergency call can be disconnected as per the
service aggrement withthe service provider.
It is not easy to identify emergency calls without a SIM.Theoretically
it may be possible to trace the prank caller using the IMEI number,
but in few countries it is against the law owing to security reasons.
October 30, 2007 7:22 AM
pilot-pooja said...
Replying to query related to HPLMN:
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"""Say MS is in visted PLMN, it camps to VPLMN.before returning to
HPLMN it is switched off."""""
I am not sure though, but there is a SW check inside the mobile SW.
If it is inside the Home PLMN or enters a region where both the HPLMN
and VPLMN are present, it cannot switch to any other network unless
the radiated power of the home network goes below a defined
threshold.
The monitoring time must be in micro seconds or even less, as it is
continuously radiating its system information (SI), as rightly pointed
by you.
Please revert back in case you have some more information to share.
November 27, 2007 9:19 AM
Anonymous said...
Specs say and i quote "A)Automatic Network Selection Mode Procedure
The MS selects and attempts registration on other PLMNs, if available
and allowable, in all of its bands of operation in the following order:
i) HPLMN (if not previously selected);
ii) each PLMN in the "PLMN Selector" data field in the SIM (in priority
order);
iii) other PLMNs with received signal level above 85 dBm in random
order;
iv) all other PLMNs in order of decreasing signal strength.
however roaming case is different
1)search for last registered PLMN
2)serach for HPLMN
3)search for BA list stored in SIM
4)search for a prescribed frequency band
//////
December 3, 2007 9:42 PM
Anonymous said...
1. Can a subscriber be present in more than one HLR/VLR at a time?
Yes,villeges near to english channel.
2. Can a mobile phone have more than one MSISDN?
Yes,why not !! i use one for official purpose and one for personal use.
3. Why are uplink frequencies lower than downlink frequencies?
-uplink/downlink power issue ( i guess :) )
4. What is the frequency band followed in Europe, Asia, Africa and
America?
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5. What is the inherent difference between an SMS and a MMS?
-SMS sent over SDCCH or SACCH but i guess MMS is GPRS issue !!! not
sure though
6. Is "listening" more expensive or "speaking"?
-ratio is 4:6 (stats)
December 12, 2007 1:46 AM
pilot-pooja said...
Answers to Quiz:
Good attempt!
1. NO:
HLR is primarily a single database but can be maintained as separate
databases when the data to be stored is more than the capacity. The
VLR is a temporary database of the subscribers who have roamed into
the particular area which the VLR serves.
Each Base Station in the network is served by exactly one VLR. Also, a
subscriber cannot co-exist in HLR and VLR.
2. YES:
It is possible for a SIM to have more than 1 MSISDN (the subscriber's
phone number).
The main MSISDN is the number used for making and receiving voice
calls and sending SMS/MMS, but it is possible for a SIM to have other
secondary MSISDNs associated with it for fax and data calls.In normal scenarios, one mobile phone supports only one SIM at a
time, but in the case of quad band phones (supporting 4 different
frequency bands simultaneously 850/900/1800/1900 MHz), each band
has a unique MSISDN.
3. Uplink-Downlink:
Each GSM frequency band is subdivided into downlink and uplink
bands. The uplink band is used for Mobile Station (MS) transmission
and the downlink band is for Base Transceiver Station (BTS)
transmission.In general, the downlink bands are 935-960 MHz for GSM 900 and 1805-
1880 MHz for GSM 1800. The uplink bands are 890-915 MHz for GSM
900 and 1710-1785 MHz for GSM 1800. There is a 45 MHz difference
between the uplink and downlink bands.
The transmitted power is directly proportional to the frequency.
Logically, any MS should transmit at a power lower than the BTS. Also,
many mobiles transmit simultaneously towards one BTS. The health
hazards are less when the MS transmits at lower power, since MS is
always in our immediate vicinity.
4. Frequency Bands:
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Europe/Asia/Africa: 900 MHz/1800 MHz
America: 850 MHz/1900 MHz
5. SMS-MMS:
SMS messages are limited to a few bytes, an MMS message has no size
limit.
Unlike SMS which supports only text, MMS includes multimedia like
sound, images and video.
Consequently, more bandwidth is required to send MMS, primarily
designed to work with the 3G technology.
P.S. http://kerjaannyagumi.blogspot.com/2007/03/cdma-vs-gsm.html
This link highlights the important differences between GSM and CDMA.
6. Listening-Speaking:
RACH paging channel is used by the MS for placing channel request to
the BSS.
AGCH paging channel is used by the BSS for allocating channel to the
MS.
All the mobiles speak to BSS via RACH and listen to BSS via AGCH.
Therefore all the mobiles after placing their channel request, keep on
listening to BSS to check if they have been allocated some channel,
which makes listening more expensive than speaking.
To minimize this listening cost, some algorithms exist for mobile
phones to listen only to particular paging timeslots.
Please revert back in case of any issues.
A similar theme based quiz from your side is surely welcome!
December 14, 2007 8:03 AM
Anonymous said...
It is possible for MS to send, and receive,SMS when in connected mode.
Such short messages are then sent over the SACCH part of the
dedicated channel assigned to the MS.also measurement reports also
sent over SACCH once every multiframe (once every 120ms).4 such
SAACH msgs will form 1 measurement report (480 ms)
So i want to know,IF MS is in CALL state and moving ,and its sending
first SACCH (measurement report) now if SMS comes,what will
happen ??
January 2, 2008 2:39 AM
pilot-pooja said...
Yes it is possible for MS to send and receive SMS when on call.
SMS Rx path is independent of Tx path, thereby receiving SMS will
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For sending SMS while on call, the mobile should be a class A mobile.
These mobiles support SMS and voice call simultaneously.
If SMS has to be sent to a different person (other than on call), two
separate paths will be allocated with separate signalling.
One will be TCH and other will be SDCCH with their associated SACCH
for sending the signalling info.
Else if SMS has to be sent to the same person currently on call, there
is no need to establish two separate paths. As rightly pointed by you,
SMS are then sent over the SACCH part of the dedicated channel
assigned to the MS.
The measurement reports are also sent periodically on the same
SACCH channel. It must be noted that SMS is of lower priority than the
measurement reports, it will be stored in the buffer, and will be
further sent in between the measurement reports.
Please share your views on this topic.
January 3, 2008 9:01 AM
Anonymous said...
Probabaly i wasnt able to frame my question correctly.i wanted to ask
if measurement reports are sent over SACCH once in every
multiframe,and 4 such SACCH are required by network to decode
measurement report.and if it is continuous thn when SMS will be
sent.perhaps i am missing some basics -:(
January 4, 2008 12:27 AM
pilot-pooja said...
The upper layer distinguishes between SMS and measurement report,
sent on SACCH associated with TCH, via Service Access Point
Identifier, commonly known as SAPI.
For SMS multiframe: SAPI = 3
For measurement report multiframe: SAPI = 0
Two SAPI(0) frames cannot be transmitted continuously, if a SAPI(3)frame is awaitng for transmission.
Depending on the size of the SMS, it may take more than one SAPI(3)
frame to be transmitted.
Until all the SMS data has been transmitted, between every
measurement SAPI(0) frame, there will be one SMS SAPI(3) frame.
The higher entitiy on the other side will decode the measurement
report only after reception of 4 signalling SAPI(0) frames.
P.S:
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Class A mobile is not required for sending sms while on call.
That is required for availing GSM and GPRS services simultaneously.
But that mobile should be able to send sms while on call.
Please correct/update me if you find the above reply unsatisfactory.
January 7, 2008 9:11 AM
Anonymous said...
Helllo Pooja,
So if i understood correctly,there is no restriction that MS should send
Measurement report(4 SACCH msgs) every 480 ms(in this perticular
case where SMS SAPI waiting to be transmitted.)???
Thanks
January 9, 2008 4:04 AM
pilot-pooja said...
Hello there,
Sorry for the ambiguous statements in the reply above.
let me give some idea about the basic framework, for others to follow.
Since this SACCH is associated with TCH, 26 frame multiframing
occurs at an interval of 120 ms.
The measurement report contained in 1 SACCH message is coded into
456 bits, which is further interleaved over 4 bursts.
Here SACCH occurs in 12th or 25th frame of each 26-frame
multiframe.
Since a 26-frame multiframe requires 120 ms, SACCH message over 4-
multiframes requires 480 ms.
Pre-conditions:The measurements are sent over SACCH, at least once every 960ms.
Two SAPI(0) frames cannot be transmitted continuously, if a SAPI(3)
frame is awaitng for transmission.
One measurement report once sent has to complete its 4 bursts in one
go, to be decoded properly.
If MS is sending its first SACCH (measurement report), it will first
complete 4 bursts ( in 480 ms) in a multiframe, then the SMS will be
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channel and during dedicated mode in SACCH associated with the
traffic channel carrying voice call.
Br
Harsh
February 9, 2008 12:15 AM
HKR said...
And regarding the discussion on emergency call .let me share the
understanding what i had.
The MS can be under three state.
normal service (MS is able to avail all the services as per the
subscription of the user(including emergency calls).)
Limited service (MS is allowed to make only emergency calls)
No service (MS is not allowed to make any calls neither emergency
calls).
emergency calls also need to follow the basic gsm concepts so even
for making a emergency call we need to have radio coverage.
emergency calls can be made even if we are not camped to own
PLMN. For making emergency calls ME is required to camp on to
the "acceptate cell" if he is not able to camp on the "suitable cell" .
And the criteria for acceptable cell is it should fulfill C1 criteria and
also the cell should not be barred. (as per 3gpp spec 03.22 , 05.08).
And Mobile equipment itself can also make a emergency call without a
sim.
And as mentioned in the comments about the concept of emergencysim card in the phone. I completely disagree with the same. There is
no such concept of emergency sim card for making the emergency call
so the whole comment mentioned on based of that is totally
irrelevant. Or if such a concept exist then can you just let me know
the specs you had referred for the same.
Regarding the call flow for the emergency call, its also fairly the same
as the normal call except few changes. When the MS initiates a
emergency call then the MS sends RACH (Random access burst) with
access cause as Emergency call to the BTS. and then the channel is
assigned to the MS if available. But the emergency call can be madeto the cell only if that cell supports emergency call. And if SIM is
present then the MS is identified with IMSI or TMSI. but when SIM is
not present most probably in the identity response the IMEI is used for
MS identity but yet not sure fully need to refer the specs again.
But as per understanding i think to identify the emergency call is
made either with SIM or without SIM might be detected by how the MS
is sending the identity request to the network. As the IMEI should be
used for identity if the IMSI is not available. and if the SIM is inserted
we always have a valid IMSI and the MS is identified with it. But to
reconfirm this fact it has to be cross verified from the specs.
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Br
Harsh
February 9, 2008 1:06 AM
Sandy said...
Lots of stuff has been written in comments. I guess its time for you to
take stock and release an updated version. My 2 cents.
February 9, 2008 4:57 AM
pilot-pooja said...
Thanks Harsh for sharing your knowledge.
Sandeep, your idea is nice, but i have time constraints as i am
working on my next post.
Will surely understand Harsh's comments in more details and release
another explanatory comment.
February 9, 2008 8:06 AM
Anonymous said...
Hi,
Regarding MS identification using IMEI,as suggested by HKR,Nope !!
Because EIR(Equipment Identitity register) is compeletely optional
feature in GSM system.so if there is no EIR present in a network,even
thn emergency call should work.
As i was more into identification of MS,in a emergency call
scenario,there are some developement surely neither based on
IMSI/TMSI nor IMEI ?
But this is all about Emergency call (CS)..what about packet switched
emergency call ? -:)
-Raj
February 21, 2008 2:15 AM
pilot-pooja said...
Hello Raj,
As per my understanding, i too agree with you that absence of EIR
should not affect emergency call.
Will look for the exact specifications and let all know.
I have never heard about PS based emergency call.Will have to work
on this.
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Please feel free to share any information on your side.
Regards,
Pooja
February 22, 2008 1:17 AM
Anonymous said...
Ok here we go ..
1)Situations where emergency calls can also be made:
-When SIM/USIM not present
-ME is in no coverage area
-If the subscriber is in his proper network but SIM fails preventing
IMSI/TMSI data for call setup
-It is left to the national authorities to decide whether the network
accepts emergency calls without the SIM/USIM.Its not mendetory
3GPP requirement
Situations where emergency calls can not be made:
-RX/TX of ME is off.(flight mode,hospital mode)
-When ME doesnt find any acceptable cell,it enters a limited service
state (Criteria C6)
It can camp to any acceptable cell,ME ignores PLMN information when
selecting acceptable cell.but ofcourse Cell must not be barred.The
radio path loss between ME and BTS must be less than a threshold
level (which is i guess set by Operator)
if the subscriber is in his proper network but SIM fails, preventing the
use of TMSI data for call setup, it is possible that the person will try
to call a common emergency number if there is an emergency
situation.subscriber cannot be identified in the subscriber
identification procedure, or authentication procedure.
And propably there is no need of authenticating a person who is trying
to call police,hospital,or fire department.Man!!! Person is in
emrgency really.
As wikipedia says:"Routine and non-urgent calls as well as hoax or prank calls to
emergency services numbers waste the time of both dispatchers and
emergency responders and can endanger lives.
False reports of emergencies are often prosecuted as crimes."
So it should be worthwhile saying :"DON'T try it to see if it works !!"
So still unanswered question:
1)identification of MS ..Do we really need that ?? ya,to find out prank
emergency callrs.so what is the way ??
2)Specs say :Only CS CN emergency calls are supported, no PS CN
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emergency calls (but who will make understand these reserachers??
they want PS emergency call as well)
_Raj
February 22, 2008 5:19 AM
pilot-pooja said...
Quite unsuccessful till date in finding info on PS emergency call:
Got this link:
http://www.3gpp1.net/specs/WorkItem-info/WI--32113.htm
Feature: DELETE - PS domain and IMS support for IMS Emergency
sessions
It shall be possible to establish an emergency session via the PS
domain and the IM CN subsystem to meet the requirements defined in
TS 22.101. Emergency sessions shall be routed to an emergency
centre in accordance with national regulations. This may be based
upon one or more default addresses stored in the ME and/or USIM and
information about the origin of the session. It shall be allowed to
establish a PS emergency session without the need to dial a dedicated
number to avoid the mis-connection in roaming case, such as connect
by menu, or a linkage to a car air bag control. The WI shall take into
account requirements coming from fixed broadband access to IMS and
seek for maximum commonality of architectural solutions between
3GPP and fixed broadband access to IMS.
The objective of this WI is to study and specify the functionalities
required to meet the requirements as defined in TS 22.101 and other
relevant specifications for emergency session handling in the PS
domain and the IM CN subsystem. The objective is to specify the
functionalities for both the UICC-less case, and the case when the UE
has a valid UICC. With respect to fixed broadband access to IMS the
objective is to keep work on the IM CN subsystem specific aspects of
IMS emergency calls independent from the PS domain aspects as much
as possible so that work on each part can progress individually.
Service aspects:
As defined in TS 22.101 and other relevant specifications to include
but not limited to:
The main focus is on:
Support for SIP emergency sessions and related packet bearers
Points to be considered, too:
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Using the existing emergency numbering schemes
Compliant with FCC mandates, European and other regulatory
requirements
Possibility to initiate emergency sessions to different emergency
service centers, depending on the type of emergency
The function shall be supported when roaming
Relations with Location Services
This WID studies Emergency location information for I-WLAN and
fixed broadband access. The impact to 3GPP LCS architecture to
support this aspect should be studied as part of the (xxxx) Stage 2 LCS
for 3GPP Interworking WLAN
February 26, 2008 3:58 AM
HKR said...
Hi,
ya, I agree with your statement."Because EIR(Equipment Identitity
register) is compeletely optional feature in GSM system.so if there is
no EIR present in a network,even thn emergency call should work."
but there are some exceptions to this statement too.
First of all this Emergency call is not a mandatory feature for the
network. It depends on the operator.
If emergency call is supported or not is transmitted by the BTS in
System Information 1/2/3. It specifies if the emergency call isallowed toa all MS, or the emergency call can be used by the
subscriber of the prioritised access class 11-15. This is stated in 3gpp
TS 44018 chapter 10.5.2.29 under 'RACH control Parameter'.
having said that, let me tell you the call flow for the emergency call.
1) MS sends RACH to the network with Establishment cause
as 'emergency call'
2) BTS assigns a dedicated channel (SDCCH or TCH in signalling mode)
for signalling and MS verification for call setup.
3) MS sends its capability in Classmark change to the network.
(CLassMark Enquiry / CLassmark change , RR Messages)4) MS sends the CM Service request with service type IE as 'Emergency
call' and the network responds with CM service Accept/Reject
depending on the options.
5) network sends IDENTITY REQUEST message with identification type
with which the MS should respond. (MM message)
6) MS then responds with the IDENTITY RESPONSE with the desired
identification type as mentioned in the earlier message. (MM
message.)
...
For MS identification, any one of the following MS identity can be
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used for this.
i) TMSI
ii) IMSI
iii) IMEI
iv) IMEISV
The priority with which the identity is requested is specified in 3gpp
TS 24008 chapter 10.5.1.4 under 'MS identity'.
and regarding your query for if identification needed for emergency
call. The answer is YES. Its mandatory for the network to ask for the
MS identity(even for emergency calls.). And for the presentation of
IMEI (refer 3gpp TS 2.09).
And as you said if there is no EIR then also the emergency calls can be
supported, yes but in that case emergency calls with sim only can be
made if allowed in the network. If the IMEI identity is asked, and if
the network doesnt support then the network sends CM Service Reject
with rejection cause #5 'IMEI not accepted'. As MM connection is
mandatory for the call setup. Regarding the same is specified in 3gpp
TS 24008 chapter 4.5.1.5 under 'MM connection establishment for
emergency calls'.
Hope this would provide you a bit better picture for what you are
looking for. You can also go through the spec specified with the
clauses to get more into it.
And regarding the PS emergency calls, i am not aware of it. And to my
understanding its neither implemented on the Mobile side and neither
its specifed in the spec. And i think that the gain over the complexityfor the implementation for the same is not that high.(as emergency
calls are not mandatory) So by now no one had implemented the
same.
Br
Harsh
March 2, 2008 3:15 AM
Ankur said...
Yess!!! thats a good article. I was also thinking about it as misses callsalso take same bandwidth as calls but they r not charged by any
service provider. Another point of view that missed calls is of no use
to callers also ( most of the time ) so charging it or not is a debatable
issue.
July 27, 2008 8:31 AM
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