Sunday, October 21, 2007

Should missed calls be charged ?

Today’s 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 world’s 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. X’s 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, let’s 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.The mobile communicates to a tower which cannot be seen in the line of sight and is normally quite far away. Now a days in this embedded world cell phones are manufactured in pint-size, if it is allowed to have towers nearby, like in Japan. But there are many constraints in placing the towers, for eg finding a safe rented place with network and electricity access.

The mobile becomes a receiver first, checking for a signal from any of the base station within its range. It measures the received level for each broadcast channel. The GSM system, not the handset, decides after this test which cell site should take the call from the mobile. That's usually the cell site delivering the highest signal strength to the mobile. Now, what's next?

The mobile now looks for the frequency control message containing the time and frequency corrections. This step is required so that transmissions from the mobile reach the base station at the precise time and with the correct frequency. GSM is a time based multiplexed system. Many calls run on the same radio frequency, each part of every call divided by time. Our new call must fit into the digital train somewhere. Once the mobile is assigned a place in this digital freight train, it can take and send a call.

But can we make a call? Not yet.
After connecting, a call must be authenticated. The GSM network must 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 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 don’t 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?

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:

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 won’t 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 i’m just a call plus a few dollars away!! Right???

Note: I express my gratitude to authors of  a many valuable GSM/ Radio access network links available free on net  for providing excellent tutoials, whose material i have used while writing this post.


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 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 mobile will search for some PLMN list which is there in SIM.say those PLMN's are too not what mobile will go to a limited service state ?? my question is after what time mobile will again search for home PLMN??

warning : might be a foolish question to ask ,still i am asking ..??? thanx ...

Anonymous said...

Hi, Wonderful piece of work done, truely its good to read the complete call flow in a simple language....Never thought of it in such a detail but thanks to you for this amzing work..

Sandeep said...

Quite informative! Here are a few downsides I can think of, if missed calls were charged

First, say someone is traveling by a train/bus. 86% highways and rail tracks in India are not covered by most of the providers, as against their claims. So let's say someone makes a call at Station X, and the train starts. Before the call could be completed, he enters a dead zone. Now, the consumer would be charged when he intends to make a call. This would be the wrong judgment of user's intentions and the problem of providers infrastructure.

Second, On your example of the accident scene on Bombay-Pune expressway. Let's say Mr. Y who came for help, does not have a mobile phone or sufficient balance to make any call. He picks up the phone of Mr. X which again in turn has very less balance to complete one complete call. Y searches for the home number, dials and cuts. Yes, indeed a missed call to save money. No response from the other party. He again tries, still fails to have a response. In a couple of tries, even that balance is over. Hence, Y can not make any call for help now. This would again be a NO-NO situation to be in.

Third, Mr. X goes on to collect some reports from the hospital. It is a no mobile usage area, so he turns silent. On the service desk, he has a call. He turns down the call for obvious reasons, takes his reports, come out in five minutes. During this time, he might receive another call which he needs to turn down. In such a scenario, Mr. X due to his personal obligations would end up making other people pay on his behalf.

Fourth, How would you distinguish between missed calls and user not available? Say, Mr. X makes a call to Mr. Y who is has left his place at home. Being something urgent, X keeps making call to Y ever 10 minutes or so for the next couple of hours. As the mobile is unattended it keeps buzzing without answering. X keeps getting charged when in fact the call never matured. How would you differentiate these calls, for the network usage is same in all cases.

Fifth, it might result in service providers intentionally breaking the calls so as to earn that extra buck. It does happen.

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!!!

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 broadcasts certaion 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 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 will camp to still mobile will be in roaming despite of the fact that HPLMN is also again my question is after what time it will camp to HPLMN (as in this case both PLMN available).

Vielen Dank !

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 ?


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.
It's quite apparent that if the called party is not available, the user 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!!!!!!!

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..

Sandeep said...


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 the URGENT 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.

Last, about the providers doing the intentional tango, I have quite 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.

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 your current 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 your mobile.

Anonymous said...

@ pilot-pooja

1)according to 3GGP specification 3GPP TS 22.101 section 10,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 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 ultimate task is to find a mechanism to identify emergency calls without a SIM.

P.S.-i am a mobile communicaion student....

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 shoud be a limit to it!!

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!!!

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.

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 with
the 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.

pilot-pooja said...

Replying to query related to HPLMN:

"""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.

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


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?
-no idea !!
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)

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:

Europe/Asia/Africa: 900 MHz/1800 MHz
America: 850 MHz/1900 MHz


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.
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!

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 ??

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 work normally.

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.

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 -:(

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.

One correction:
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.

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.)???


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.

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 transmitted in next 480 ms in the next multiframe, after that the measurement report has to be sent, irrespective of the fact that complete SMS has been transmitted or not.

4 SAPI(0) bursts: measurement report
4 SAPI(3) bursts: sms
4 SAPI(0) bursts: measurement report
4 SAPI(3) bursts: sms
...and so on.

Please respond to inform me if my understanding is not correct.


Anonymous said...

Thnaks pooja


HKR said...

good post !
though i wont participate on the viewpoint if it should be charged or not .. Technology is created and its upon us how to use it efficiently.

Talking on the techincal aspects what has been posted in the blog and the comments, some of the things i found that they not comply to the specification and so i thought of writing it just to give a correct picture of it as per my understanding on the topic.

Reg. the discussion that i read above on sending/receiving the sms during the call .... yes its possible and i think to very much extent the concepts had been explained above already. what i was not able to digest was regarding the sending of sms to the person on call or to the different person during the call. the sending procedure for sending the sms is not affected by the same. As per the specification when we are in decidated maode (Traffic call), we have been allocated either a full rate or half rate channel with its associated signalling and everything has to be transmitted in that only. When sending a sms we use SAPI 3 frames as mentioned. and the SMS is send to SM-SC using the SM-TP protocol, and its not required to differenate if the reciepient of the sms is the same as the person on call or not. And in no case the additional Radio resource is allocated to the MS for sending sms when we are already in dedicated mode.

And in idle mode the sms is send by the MS on SDCCH channel. and the same from the network side. During idle its send on SDCCH channel and during dedicated mode in SACCH associated with the traffic channel carrying voice call.


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 emergency sim 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 made to 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.


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.

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.

Anonymous said...

Regarding MS identification using IMEI,as suggested by HKR,Nope !!

Because EIR(Equipment Identitity register) is compeletely optional feature in GSM 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 ? -:)


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.

Please feel free to share any information on your side.


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 what is the way ??
2)Specs say :Only CS CN emergency calls are supported, no PS CN emergency calls (but who will make understand these reserachers?? they want PS emergency call as well)


pilot-pooja said...

Quite unsuccessful till date in finding info on PS emergency call:

Got this link:

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:

§ 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

HKR said...

ya, I agree with your statement."Because EIR(Equipment Identitity register) is compeletely optional feature in GSM 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 is allowed 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 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 used for this.
ii) IMSI
iii) IMEI

The priority with which the identity is requested is specified in 3gpp TS 24008 chapter 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 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 complexity for the implementation for the same is not that high.(as emergency calls are not mandatory) So by now no one had implemented the same.


Ankur said...

Yess!!! thats a good article. I was also thinking about it as misses calls also 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.

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Anonymous said...

Just really love you guys… what fantastic and well researched information. Thank you bunches.

Anonymous said...

When the subscriber roams into ohter PLMN will it get the servie,eventhough it is not the preferred ont?

Pilot-Pooja said...

As per his roaming settings, this issue alreay covered in comments.

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