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 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: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.
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: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 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???
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.
