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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.

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Pooja Gupta

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Page chnorealism -A phenomenon unleashed: Should missed calls be charged ?

11/27p://techybeams.blogspot.com/2007/10/should-missed-calls-be-charged.html


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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.




10/26

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




11/26

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




12/26

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.




13/26

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:




14/26

"""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?




15/26

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:




16/26

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




17/26

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:




18/26

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


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...

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


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.




22/26

Please feel free to share any information on your side.

Regards,

Pooja


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


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


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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