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QUESTION: Why does the GPRS attach disturb GSM signalization in some networks?
Posted by Arne Hess - on Monday, 01.09.03 - 15:52:31 CET under 09 - Thoughts - Viewed 10772x
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Okay - I have to admit that this is a super high level question and topic but I hope that some of the PPCW.Net visitors and readers can answer this.

According to my server logs I have daily visitors from Nokia Networks, Ericsson Networks, Nortel Networks, Alcatel as well as from many GSM carriers from all over the world so if you are a technician, in charge for GPRS and or signalization (keyword SS7 ;-)) please read the complete story below and share your knowlede with us.

Some users from different GSM/GPRS carriers reported in several web boards (like andrew.j here in the PPCW.Net Forum) that GPRS attach disturbs the basic GSM functionalities like receiving calls or SMS:

I have recently got set up with GPRS access with my local phone operator, Jersey Telecom, only to encounter after i had used GPRS to download email or view web pages. I was unable to receive any incoming calls or SMS messages, all my calls where going direct to my voice mail, although i had ended the GPRS connection and the small G had gone from the signal strength indicator. I had a full strength indication and the operator logo was present (JT GSM). After talking to my network operator they insisted that the GPRS and GSM signals where totally separate and could not possibly cancel each other out. I have since had the GPRS removed from my account and the phone works perfectly.

Okay - for me it looks like if GPRS is attached it screws up the signalization or marks the MSISDN/IMSI as not connected in the HLR/VLR. What I completely not understand how this can happens? Basically GSM and GPRS works on different network layers so how can one screw up the other?

The only solution - some Smartphone users found so far - was disabling GPRS from their account (like andrew.j) which is completely useless if you have a data enabled terminal. Others suggested the Tanager should get a new Radio Stack to enable/disable GPRS attach.

All these solutions can't be the last word on the subject and I still don't get it how a network initiated PDP Context can disturb signalization or screw up the HLR/VLR.

I've already checked the 05.07 and 07.07 as well as the ETSI TS 101 393 without getting any hints. :-(

So now it's up to you my valuable visitors from the vendors or carriers. What's the reason, what's the result, what's the solution? Please feel free to use the comments below. Anonymous commenting is possible so you don't need to tell us from which carrier or vendor you are but it would be great if you could share your knowledge!

Cheers ~ Arne


 

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Comments
Posted by Markus from a Carrier on 01.09.03 - 17:48:59

I can only comment from a handset point of view, as I'm not that much into networks and what they should and shouldn't do. However, this issue looks to be a handset issue, even though the network does things which it shouldn't do from a user perspective. Further it would be interesting to know if the handset really only make a GPRS attach or tries to activate a PDP-context!?


Anyway, there are three different GPRS classes A, B and C.

Class A handsets can do CS and PS in parallel (download something from the internet while on a CS call). Class B handsets can also do both, but one after the other (no Wap browsing while on a voice call, no voice call coming through while surfing the internet). And finally Class C devices, which are data devices only (such as the old (first) RIM Blackberry).


Equally, there are different ways to attach for handsets. IMSI attach (GSM only), GPRS attach (GPRS only) and combined attach (GSM and GPRS attach).


The behavior which you describe, sounds a bit as if the handset makes an "IMSI detach" parallel to the GPRS attach and hence all CS services (voice, SMS etc.) do not work anymore (or are rerouted to the voicemail system) as the CS part of the phone looks to the network as if powered off..... But again, this is only a rough guess of what can happen.

To really see what's going on, you need to make traces. I hope I haven't confused you completely now! ;-)


Any more detailed explanations, just let me know!

Posted by Ozgur on 05.09.03 - 09:51:29

As far as I know While you downloading or uploading data via GPRS it simply impossible to get SMS or Voice calls.  This behavior is by design. But only during active data connection. If you only attached to GPRS network -idle- it should not effect your other GSM services.

I ll try to find out the technical explanation from our GPRS operation guys of this behavior just give me a 1 or 2 days

Posted by Hendo from Australian Carrier on 06.09.03 - 02:01:44

GPRS is a relatively a low cost upgrade to carriers existing GSM Networks to acheive packet data (2.5G), compared with deploying a brand new 3G / UMTS network. GPRS Introduces 3 new nodes, which are the PCU (PAcket COntrol Unit), located in the BSC and the SGSN & GGSN. GPRS & GSM share the same Radio Resources (Time Slots) which are controlled by the following hardware, (BSS = BSC(Base Station Controller) + BTS(Base Station Transceiver)).

As Markus commented, most GPRS phones are Class B, which means you can do GSM & GPRS (Voice & Data), but not at the same time. What this means in reality is that you can initiate a GPRS call (Data Call) & send / receive a voice call without dropping the Data Call, but during the voice call your data session is "suspended" which means you cant send or receive any data during the duration of the Voice Call. When you end the voice call your GPRS session is "resumed" and data flow can continue. Class A mobiles are in the standards, but will be expensive due to the fact you need two sets of radio hardware in the phone.

When you send / receive data during a GPRS session you are allocated from the radio network a TBF (Temporary Block Flow) which consists of an allocation of timelots for a period of time. If in the time you have an active TBF and someone tries to call you, it will come up as busy or if you have voice mail on your service, the caller will be diverted to Voice Mail. If you dont have an active TBF, the GPRS session will be suspended and the call with get through.

It may be the case you have old firmware on the phone or there could even be a issue with the Carriers Network ?

Posted by Ozgur on 08.09.03 - 23:45:31

Hendo responded before me. Simply:

Class B phones should work with GSM/Voice/SMS at the same time.

Class A phones required update on GSM/GPRS infrastructre on operator side.

Posted by Arne Hess on 09.09.03 - 12:34:48

@Hendo: Yes, all this is true and I know. However what my question is about is why is voice dropped if you get GPRS attached (not PDP context started, simply the GPRS attach).

This doesn't makes sense and seems to be a failure in the network environment since this happens with several devices from different brands.

Example: You have a GSM/GPRS enabled SIM card/contract. In the moment you switch on your get your IMSI and GPRS attached. So far so good but the GPRS attach disabples voice and not voice only but SMS-MT also. Why?

@Ozgur: Thanks for your feedback but you can receive SMS while PDP context is established (GPRS connection is alive). Also that's not the point as well the classes aren't the point (see my one year old colum "SPEED: How fast is GPRS and what's the maximum today" here: http://.../)

Posted by Anonymous on 09.09.03 - 13:12:04

Hi,

To date, all GPRS handsets are Class B types and there are no Class A in the market at all. In 2G/2.5G, both voice and data (i.e. GPRS) are taking up the same traffic timeslot and there are no differences at all. Depending on individual telco's implementation, you may dedicate Timeslots for GPRS and if not, default Timeslots will be used. Now, Voice calls will always take precedence over data, meaning to say, if a user is in data session for some time (say downloading Mb of files), if there's an incoming call, your GPRS device will pick up the Circuit switch call and your data session will suspend until the time CS calls end (as simple as that and no complications like active TBF. Sorry but this is not true at all).

Arne, maybe you would like to let us know what SmartPhones are those? So that carriers here in this discussion can just do a simple test if they happen to have this device in their company?? The way I look it, this looks more like a network issue than anything else (technical details may be too difficult to understand and explain here) coz GPRS handsets manufacturers will definetly be looking for Telcos to test their device if they want to make it a success and launch it (like O2 XDA).

It's the commitment of the people in the telco industry to ensure that this should not happen in the network, solely for the cause of making GPRS a success. And it's good that many manufacturers are coming up with Smartphones in the market, to push the awareness of GPRS to a greater height. This is especially true for a sensitive mobile market group that we have in Singapore. Any slight issue will have an adverse impact to user's perception of the Telco.

It will be great if you can let me know of the brand and model and maybe I can check with my commercial guys if they receive any approach from this manufacturers.  smile

Posted by jpo2 on 02.10.03 - 09:37:31

I fully support the last comment.

Arne I am an O2 user from the start and as all class b phone you will suspend your data call for getting a voice call. For sms MT or MO you will handle them while connected in GPRS (its all data). Travelling around the world I have use it without any issue. It happens that when roaming and swinging from a network to an other in the visited country, you may have issue as the data roaming agreements will not be the same. But coming back to the original issue, its a poor network implementation mainly based on the fact that no timeslots available for your voice call even if you feel connected.

Posted by roke on 21.10.03 - 17:29:11

I checked a lot today with different providers and hs. I have used three hs (SPV E-100/SonyEricsson P800/ Ericsson T39m) and the three networks (yes, we only have three) in Belgium. I took a place in midst of the city and performed the tests on the real busy hours as well as on the less busy hours. This just to be sure that I tested it with a normal network use and with an overload network use.

The test I performed were simple: synchronising with an IMAP mailserver with 20 mails of 19 Kb on it, then calling the test phone while sending/receiving, and calling again when finished sending/receiving. As gprs is not that fast, and I setup the phones to download the entire mails, I had plenty of time doing this smile

purpose of this test was just to check the speed of gprs on those three networks, as I work for one of them and I was really curious about the results. btw, I only have two different gprs contracts, so for the third test, I used a friend's sim. I will not post the results, as some of those networks could be offended, but here are the relevant issues for this thread:

all three hs are class B, but both ericssons were a lot faster in the send/receive task than the "real" smartphone. With all three hs, I encountered the problem you describe above, but not on all networks. However, the problems were obviously related to the amount of cmrs calling on those networks, because at the same time, I was getting several "network busy" signals when trying to call the test hs. For those of you who don't know that signal: It is used to indicate the fact that not enough timeslots are available to make a connection, like jpo2 indicated.

My conclusion here is the following: As soon as the hs uses gprs, the voice attach remains active, but the timeslots that are used for it are released until the gprs action has finished (the send/receive command in this case). At that time, the phone reclaims the timeslots which were released because they were not needed for the gprs action anymore. as soon as it reclaims the timeslots for voice, the sms which had been sent will arrive, and calls will come through again. Off course, if the hs doesn't reclaim those timeslots, you have the problem described by andrew.j...

btw, the same timeslots are used for gprs and voice, so these are not really different systems. most networks will reserve timeslots for voice, and some even put a maximum of gprs users per gsm site (so if a lot of people use gprs, some won't be able to use it until the others stop), but my guess would be if a certain hs gets a denial for the timeslots it requests, it just stops requesting. I think this is the problem some of the users here are experiencing, but to be sure of this, you would have to check the software on certain rulings like: reclaim an amount of timeslots in... sec/min/h, release timeslots if..., and so on. Also, if it is a software issue, most of the time this can be solved with a software update. I know, "real" smartphones don't have an update that fast, but for instance Nokia an SonyEricsson make updates for their phones on a regular basis. Nokia is the fastest to make an update, sometimes even an update per week, but in my opinion that's normal when you deliver phones who don't act normal or even explode without a reason smile

One last thing though: I am a "simple" salesperson, and don't have access to certain network settings e.g. reservation of timeslots, restrictions for gprs, etc...

However, in my free time, this has been an occupation for almost ten years now. Basically, all I know comes from the Internet and is not knowledge I got from any network. Therefore, it can be that I am mistaking because none of the above restrictions etc have been officially confirmed by the corresponding networks. (after all: who would admit those limitations, right? wink )

Posted by Deepak. on 15.06.04 - 12:11:43

Hi,

It's almost been 8-9 months since this query had been posted. Found this on Google while searching for a similar issue. When my GPRS module is either ringing on an incoming call or is connected, then as soon as I attach GPRS(with AT+CGATT=1), the voice call is dropped. I guess it is similar to the issue above. Would be grateful if I can get some kind of reply whether this is an expected behaviour for Class B GPRS or not, and why this would be happening

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