How GSM networks work, brief communication basics. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering /Mobile telephony

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Almost everyone used a cell phone, but few people thought - how does it all work? In this literary opus, we will try to consider how communication takes place from the point of view of your telecom operator.

When you dial a number and start calling, well, or someone calls you, your device communicates via radio with one of the antennas of the nearest base station.

Each of the base stations contains from one to twelve receiving and transmitting antennas directed in different directions to provide subscribers with communication from all sides. Antennas are also called "sectors" in professional jargon. You yourself have probably seen them many times - large gray rectangular blocks.

From the antenna, the signal is transmitted via cable directly to the control unit of the base station. The combination of sectors and the control block is usually called - BS, Base Station. Several base stations, whose antennas serve any particular territory or area of ​​the city, are connected to a special unit - the so-called LAC, Local Area Controller, "local area controller", often referred to simply controller. Up to 15 base stations are usually connected to one controller.

In turn, the controllers, which can also be several, are connected to the most central "brain" unit - MSC, Mobile services Switching Center, commonly known as switch. The switch provides output (and input) to city telephone lines, to other cellular operators, and so on.

That is, in the end, the whole scheme looks something like this:

Small GSM networks use only one switch, larger networks serving more than a million subscribers can use two, three or more MSC, combined with each other.

Why such complexity? It would seem that you can simply connect the antennas to the switch - and that's it, there would be no problems ... But not everything is so simple. The point here is one simple English word - handovers. This term refers to handover in cellular networks. That is, when you walk down the street or drive a car (train, bicycle, roller skates, asphalt paver ...) and at the same time talk on the phone, then in order for the connection not to be interrupted (and it is not interrupted), you need to switch Your phone from one sector to another, from one BS to another, from one Local Area to another, and so on. Accordingly, if the sectors were directly connected to the switch, then all these switching would have to be controlled by the switch, which already has something to do. A multi-level network scheme makes it possible to evenly distribute the load, which reduces the likelihood of equipment failure and, as a result, loss of communication.

Example - if you and your phone move from the coverage area of ​​one sector to the coverage area of ​​another, then the BS control unit is engaged in transferring the phone, without affecting the "superior" devices - LAC и MSC. Accordingly, if the transition occurs between different BS, then it is controlled LAC and so on.

The operation of the switch should be considered in a little more detail. A switch in a cellular network performs almost the same functions as a PBX in wired telephone networks. It is he who determines where you are calling, who is calling you, is responsible for the operation of additional services, and, in the end - in general, determines whether it is possible to call or not.

Let's stop at the last point - what happens when you turn on your phone?

Here, turn on your phone. Your SIM card has a special number, the so-called IMSI - International Subscriber Identification Number, International Subscriber Identification Number. This number is unique for every SIM card in the world, and it is by this number that operators distinguish one subscriber from another. When the phone is turned on, it sends this code, the base station transmits it to LAC, LAC - on the switch, in turn. This is where two additional modules associated with the switch come into play - HLR, Home Location Register и VLR Visitor Location Register. Respectively, Register of Home Subscribers и Register of Guest Subscribers. In HLR are stored IMSI all subscribers who are connected to this operator. AT VLR in turn, contains data on all subscribers who are currently using the network of this operator. IMSI transferred to HLR (of course, in a highly encrypted form; we will not go into details of encryption in detail, we will only say that another block is responsible for this process - AuC, Authentication Center), HLR, in turn, checks whether it has such a subscriber, and, if so, whether it is blocked, for example, for non-payment. If everything is in order, then this subscriber is registered in VLR and from that moment can make calls. Large operators may have not one, but several in parallel HLR и VLR. And now let's try to display all of the above in the figure:

Here we briefly reviewed how the cellular network works. In fact, everything is much more complicated there, but if you describe everything as it is thoroughly, then this presentation may well exceed "War and Peace" in terms of volume.

Next, we will consider how (and most importantly, why!) The operator writes off money from our account. As you have probably heard, there are three different types of tariff plans - the so-called "credit", "advance" and "prepaid", from English Pre-paidi.e. prepaid. What is the difference? Consider how money can be written off during a conversation:

Let's say you made a phone call. It was fixed on the switch - such and such a subscriber called there, talked, let's say, forty-five seconds.

The first case - you have a credit or advance payment system. In this case, the following happens: data about your and not only your calls are accumulated in the switch and then, in the order of the general queue, are transferred to a special block called billing, from English to bill - to pay bills. Биллинг is responsible for all issues related to subscribers' money - calculates the cost of calls, debits the subscription fee, debits money for services, and so on.

Transfer rate of information from MSC в Биллинг depends on how much processing power billing, or, in other words, with what speed he manages to translate technical data about calls made into direct money. Accordingly, the more subscribers talk, or the more "brake" billing, the slower the queue will move, respectively, the greater the delay between the conversation itself and the actual debiting of money for this conversation. This fact is connected with the dissatisfaction often expressed by some subscribers - "They say they steal money! I didn't talk for two days - they wrote off a certain amount ...". But at the same time, it does not take into account at all what conversations that took place, for example, three days ago, the money was not written off right away ... People try not to notice good things ... And these days, for example, billing could simply not work - due to an accident, or due to the fact that it was somehow modernized.

In the opposite direction - from billing to MSC - there is another line in which billing informs the switch about the status of subscribers' accounts. Again, a fairly common case - the account debt can reach several tens of dollars, and you can still call on the phone - this is just because the "reverse" queue has not yet come up and the switchboard does not yet know that you are a malicious defaulter and You should have been banned for a long time.

Advance tariffs differ from credit tariffs only in the way of settlement with the subscriber - in the first case, a person deposits some amount into the account, and the money for calls is gradually deducted from this amount. This method is convenient in that it allows you to plan and limit your communication costs to some extent. The second option is credit, in which the total cost of all calls for any period ("billing cycle"), usually for a month, is issued in the form of an invoice that the subscriber must pay. The credit system is convenient in that it insures you against those cases when you urgently need to call, and the money in the account suddenly runs out and the phone is blocked.

Prepays are arranged in a completely different way:

In prepaid billing as such is commonly referred to as "Prepaid by platform".

Directly at the moment of the beginning of the telephone connection, a direct connection is established between switch и solder platform. No queues, data is transmitted in both directions directly during the conversation, in real time. In this regard, the following characteristic features are inherent in paydays - this is the absence of a subscription fee (since there is no such thing as billing period), a limited set of additional services (it is technically difficult to charge them in the "real time" mode), the impossibility of "going into the red" - the conversation will simply be interrupted as soon as the money in the account runs out. Clear dignity prepaid is the ability to accurately control the amount of money in the account, and, as a result, their expenses.

В soldered sometimes there is some funny phenomenon - if prepaid platform for some reason refuses to work, for example, due to overload, then, accordingly, for subscribers prepaid tariffs during this time all calls become absolutely free. What, in fact, their - subscribers - can not but rejoice.

But how is our money calculated when we talk, being in roaming? And how does the phone work in roaming? Well, let's try to answer these questions:

room IMSI consists of 15 digits, and the first 5 digits, the so-called SS - Country Code (3 digits) and NC - Network Code (5 digits) - clearly characterize the operator to which this subscriber is connected. These five numbers VLR finds a guest operator HLR home operator and looks in it - but, in fact, can this subscriber use roaming with this operator? If yes, then IMSI prescribed by VLR guest operator, and HLR home - link to the same guest VLRto know where to look for the caller.

With the write-off of money in billing, the situation is also not very simple. Due to the fact that calls are processed by the guest switch, but the money is counted by its own, "home" billing, large delays in debiting funds are quite possible - up to a month. Although there are systems, for example, "camel2", which also work in roaming on the principle of prepaid, that is, they write off money in real time.

Here another question arises - what is the money debited for? roaming? If "at home" everything is clear - there are clearly defined tariff plans, then with roaming the situation is different - a lot of money is written off and it is not clear why. Well, let's try to figure it out:

All phone calls in roaming are divided into 3 main categories:

Incoming calls - in this case, the cost of the call consists of:

The cost of an international call from home to the guest region
+
Cost of an incoming call from a guest operator
+
Some surcharge depending on the specific guest operator

Outgoing call home:

Cost of an international call from the guest region home
+
Cost of an outgoing call from a guest operator

Outgoing call by guest region:

Cost of an outgoing call from a guest operator
+
Some surcharge depending on the specific operator

As you can see, the cost of calls in roaming depends only on two things - on which operator the subscriber is connected to at home and which operator the subscriber uses at home. At the same time, one very important thing is revealed - the cost of a minute in roaming absolutely does not depend on the tariff plan chosen by the subscriber.

I would like to add one more note - if two phones of one operator are roaming together with another operator (well, for example, two friends went on vacation), then it will be very expensive to talk to each other - the caller pays as for outgoing home, and the receiving a call - as for entering from the house. This is one of the disadvantages of the GSM standard - that communication in this case goes through the house. Although it is technically quite possible to arrange a connection "directly", but which of the operators will go for it if you can leave everything as it is and make money?

Another question that has often been of interest to owners of more than one mobile phone lately is how much will a forwarded call from one phone to another cost? And the answer to this question is quite realistic:

Let's say call forwarding to phone C is set from phone B. Phone A calls to phone B - accordingly, the call is redirected to phone C. In this case, they pay:

Phone A - as for outgoing to phone B
(actually, this is logical - after all, he calls him)
Phone B - pays the call forwarding price
(usually a few cents per minute)
+
the cost of an international call from the region where B is registered to the region where C is registered
(if the phones are in the same region, then this component is equal to zero).
Phone C - pays as for incoming calls from phone A

At the end of the topics, I would like to mention one more subtle point - how much will call forwarding cost in roaming? And here the most interesting begins:

For example, the phone has forwarding on a busy condition to a home number. Then, with an incoming call, the so-called "roaming loop"- the call will go to the home phone through the guest switch, respectively, the cost of such a forwarded call for roamer will be equal to the sum of the costs of incoming and outgoing home calls, plus the cost of the call forwarding itself. And what is funny at the same time - the roamer may not even know that such a call took place, and subsequently be surprised when he sees the bill for communication.

This implies practical advice - when traveling, it is advisable to turn off all types of forwarding (you can leave only unconditional - in this case, the "roaming loop" does not work), especially forwarding to voice mail - otherwise you can later wonder for a long time - "Where did this money go, then, A?".

List of terms used in the text

AuC - Autentification Center, the Authentication Center, is responsible for encoding information when transmitted over the network and received from the network
Billing - Billing, cash accounting system for the operator
BS - Base Station, a base station, several transmitting and receiving antennas belonging to one control device.
camel2 - one of the Prepaid systems, which implements instant withdrawal of funds in roaming
CC - Country Code, country code in the GSM standard (for Russia - 250)
GSM - Global System for Mobile Communications, the world's most widespread cellular communication standard
Handover - handset control transfer from one antenna/base station/LAC to another
HLR - Home Location Register, a register of home subscribers, contains detailed information about all subscribers connected to this operator.
IMEI - International Mobile Equipment Identification, the international serial number of the equipment in the GSM standard, is unique for each device
IMSI - International Mobile Subscriber Identification, the international serial number of a subscriber to GSM services, is unique for each subscriber
LAC - Local Area Controller, Local Area Controller, a device that controls the operation of a certain number of base stations, whose antennas serve a certain territory.
Local area - Local zone, territory served by BSs that are part of one LAC
MSC - Mobile services Switching Center, Mobile Services Control Center, the switch is the central link of the GSM network.
NC - Network Code, Network Code, code of a specific operator in a given country in the GSM standard (for MTS - 01, BeeLine - 99).
Prepaid - Prepaid, prepayment - a billing system based on instant debiting of funds.
roaming - Roaming, using the network of another, "guest" operator.
SIMs. - Subscriber Identification Module, Subscriber Identification Module, SIM card - an electronic unit inserted into the phone on which the subscriber's IMSI is recorded.
VLR - Visitor Location Register, register of active subscribers - contains information about all subscribers who currently use the services of this operator.

Publication: cxem.net

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