How does a phone call work

This is how telephoning works with a cell phone

It's commonplace: you pick up your cell phone, dial the number you want, press the green button and a few seconds later the call is over. But while the call is being established, numerous things happen in the background that the customer does not even notice. We want to present these processes to you in detail using the o2 network. In the other cellular networks, however, the processes are largely identical.

We have already presented details on the technical structure of a cellular network, the so-called network architecture. This article therefore does not go into the structure of the network itself.

The call setup: is the cell phone allowed to do what it wants?

If you pick up your mobile phone and make a call to another customer of the same network provider, your call will be routed via the transmission masts and the corresponding controllers to the MSC responsible for you based on your location.

The first thing that is checked here is whether your SIM card is genuine and whether you are therefore authorized to make this call. The Authentication Center (AUC) supplies columns of numbers (so-called triples) with the help of which the exchange authenticates the subscriber. It is just a technical review.

Based on the number dialed, the exchange recognizes that the destination number remains within its own network. The Home Location Register (HLR) is then asked where the customer is. The HLR is something like the central customer database of every network provider. It links the customer's phone number with the SIM card and knows the customer's authorizations and any programmed call diversions.

So if a customer who is in Munich calls a customer who is currently in Berlin, the HLR knows that the customer is in the area of ​​the switching center responsible for Berlin and contacts them. The Berlin switching center then asks the Visitor Location Register (VLR) responsible for this MSC for data on the called party. The VLR knows in which region in Berlin the called party is, which services are supported and how the called mobile phone has to identify itself in the network.

The network must first search for the target cell phone

The telephone signal is sent to the end customer via radio antennae and transmitter masts.
Photo: As a result, the responsible Berlin switching center starts something like an exclamation in the region in which the person called is staying. These areas are called LAC. In the same way, there is a signal via all transmission masts that a call has been made for a certain cell phone, the cell phone should please report. There is therefore no direct addressing of an individual transmitter mast or the cell phone directly. The called mobile phone receives the call and reports to the exchange. This now knows which station the mobile phone is currently logged into. It is checked whether the SIM card is really the one that is expected. Only then does the Berlin exchange put the call through to the called party. All of these processes run very quickly, but anyone who calls from cell phone to cell phone notices that establishing a call can sometimes take a few seconds. This is mainly due to the exclamation of the called cell phone but also to the various database queries.

What happens if there is no reception?

If the person called has no reception, the cell phone has usually not logged off regularly from the network. The VLR still "thinks" that the customer can be reached in the region. The network usually only notices that this is not the case when there is a message for the customer. There is no answer from the cell phone to the exclamation broadcast over the entire LAC. As a result, the switching centers have no choice but to report back the unavailability. This information goes back to the customer's exchange.

On the basis of the HLR information, the exchange knows whether the customer has programmed a call diversion in the event of unavailability. This would then be handled like a phone call. If nothing is defined, the caller is informed that the subscriber is not available.

Every mobile phone user knows that it can take some time before this announcement is made. But everyone also knows that the announcement can sometimes come "immediately" after dialing the cell phone number. In this case, the person called has switched off his cell phone and logged off regularly from the cell phone network. The VLR passed this information on to the HLR and the HLR responds immediately to an exchange request. There is consequently no exclamation from the customer in the network.

What happens when you talk to other networks?

Several transmission elements are often mounted on the transmission masts.
Photo: If the customer does not call within his own network, but to another mobile network, to a landline or abroad, the call runs through a Gateway Mobile Switching Center, i.e. a gateway switching center. This serves as an intersection with other networks. For example, if the call goes from O2 to the telecommunications division of Telekom, the caller's MSC transfers the call to the GMSC. This then passes it on to Telekom. The identification of where the call has to be routed is made via the call number of the person called, the so-called MSISDN.

What happens with ported numbers?

Even if a 0171 number (actually Telekom) is now switched to O2, the call is initially routed to the Telekom's GMSC, since the call number (MSISDN) indicates that it is a Telekom number. At Telekom, the check is then carried out using a porting database, since the Telekom HLR cannot find any data on the number. Since the number is stored in the database with the note "CO2 network", the call is then passed on to CO2.

What happens with incoming calls?

Incoming calls arrive at the GMSC from other network operators. With the help of the HLR and VLR databases, the gateway switching center then forwards the calls to its own switching centers, which are currently supplying the called customers.

What happens if the cell phone is moved?

If a customer walks through the streets in a big city, different transmission masts are usually used after a few hundred meters than they were a short time before. However, the network does not have any information about the mast in which the cell phone is registered as long as the cell phone is not used. There is only a check every few hours to see whether the cell phone is still available. Only then would the information be given to the VLR and HLR that a cell phone can no longer be reached because it no longer has any reception. If a message arrives for the cell phone in the meantime, this information is then saved.

However: If the customer leaves the area of ​​a base station controller (BSC), there is a so-called location update. A BSC is usually responsible for a two-digit number of transmission towers. Naturally, you leave the area of ​​a BSC at some point, especially for longer distances. Theoretically, this can also be the case when the journey to work is only one kilometer away, if there is a BSC area of ​​responsibility between work and the office. Often the customer is then also in a new LAC. This new information is then stored again in the VLR and the network knows the new, rough location of the customer.

Booking after several days or abroad takes longer

If UMTS and GSM are in one location, there are usually two BTS cabinets next to each other.
Photo: If a cell phone is switched off for several days or if it logs on to the network for the first time (for example with a new contract or vacation abroad), a location update is not enough. A location register update must be carried out here.

In the case of vacation abroad, this means that a completely new data record for the guest customer must be created in the VLR of the foreign network. The VLR requests all relevant information from the HLR of the customer's network operator. This location register explains why it takes a relatively long time to switch on the mobile phone for the first time after landing abroad, while the mobile phone's registration in the network the next morning does not take longer than in Germany. The VLR still knows the customer the next morning and does not have to create any new data records, just update them.

Hand-over during a conversation is not always easy

If the mobile phone moves during a data or voice connection, the handover behaves slightly differently. After all, the new transmission tower has to know that it should take over a call. A distinction must be made between how many network levels the call transfer affects. The simplest variant is that only another sector of the same transmission mast is used.

So it's the same mast, just a different antenna that sends in a different direction on a different frequency. The base station regulates this handover itself. If the call has to be transferred to another transmission tower, the BSC will handle the transfer. However, if the new base station is in a different BSC area, the two BSCs must use the higher-level exchange (MSC) to regulate the handover. In the "worst" case, the BSC responsible for the new transmission mast is also subordinate to another exchange, so that these network elements first have to negotiate the handover with one another.

In the event of a handover, it must first be checked at the new transmitter whether there is any capacity for a further call. Otherwise the handover would fail. Ideally, if the transmission masts in inner-city locations are only occasionally overloaded, calls are transferred from an overloaded transmission mast to a less congested, slightly poorer-to-receive transmission mast, so that other calls do not have to be shifted in the event of an incoming call transfer. The BSC take on this task independently.

However, if all transmission masts are full or if the station density is so low that relocation is not possible - for example in rural areas - a planned handover will lead to the call being broken off. Theoretically, this can also happen if the change is too fast, for example when traveling on the ICE, if the new transmission mast is available too late with good quality.

A handover is initiated by communicating with the cell phone. This continuously controls the transmission and reception strength of the base station and at the same time receives the signals from other transmission masts. The network knows these neighborhood cells and then regulates the handover of calls to a cell whose reception parameters are better. Ideally, the user will not notice any of this.

Billing: differences between prepaid and postpaid

Invoicing is very simple for contract customers: Since it was determined in the HLR that the customer is entitled to carry out the desired actions, after the end of the action all that remains is to determine how long the action lasted and the corresponding billing information is sent to the billing system. Here, the corresponding price is calculated based on the stored tariff information and is included in the invoice, unless the customer has booked a flat rate.

Prepaid customers, on the other hand, repeatedly receive virtual free customers from the billing system. In order not to generate too many queries in the accounting system, this information is sent approximately every two minutes. Only when the credit is no longer sufficient for the next call interval is the real length of the last time interval sent.

A lot of effort for simple phone calls

The numerous steps show how much is happening in the network of a mobile phone provider so that a simple phone call can be made. These processes also make it plausible why everything cannot run as perfectly as in the fixed network, where the switching centers are set up much more simply.

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