I cannot make a call – why?!


I cannot make a call – why?!

Not only once you had a situation when, although you have paid a charge, (or pre-paid card) making a voice call ended in a fiasco. Why is this happening? It would seem that this is a lack of reception – not always.

Considerations were conducted for the 3G technology. The best example of the impossibility of making a voice call, in spite of all reception bars, is a concert or a mass event. It can be many reasons, but the most common are:

1) very high distortion in the uplink in the process of compiling a voice call,
2) very large number of active Internet connections (packet) in 3G technology,
3) depletion of the capacity of the cell phone.

In the first case, everyone wants to share own impressions of the event and begins to make voice calls at the same time. Each terminal begins to emit an electromagnetic field. More generally, in 3G technology during normal use, the transmitted signal is slightly below the noise level. Each signal is encoded in a different sequence. The decoder located in the fixture of the base station, pulls and amplifies the received signal and decodes it with the same sequence as the phone code. This procedure is called spread spectrum.

Spread spectrum – consist in transmitting a signal in a wide frequency bandwidth (much greater than the minimum bandwidth needed for its transmission), the result is dissipation of energy of the transmitted signal in greater bandwidth and lower levels in ether.

Very large interferences are generated when a great number of terminals start sending a voice call request at the same time. Terminals have the opportunity to regulate power with which they are transmitting. If phone does not get a response from the network, it increases power until the connection occurs. The power is increased to a certain maximum value (for the 3G technology, the maximum value is 24 dBm). Consequently, when all the phones “spread” already at maximum power, no terminal can get through to the network, because its signal is lost in noise level resulting from interference. There is a small chance that the phone, which is closest to the station, breaks through and gets a call.

Interferences – in telecommunication, it means interference of signal (either up (UL) and down (DL)), which may reduce the capacity of cell phones, prevent the proper functioning of the mobile network and mobile satellites.

Below a graphical representation of the above-described case.

Uplink signal levels on a typical number of units making a voice call

Uplink signal levels on a number of units making a voice call during mass events

In the second case, , the event participants share their impressions through photos, videos shared in social, film and other networks. They are sending large amounts of data to the network with packet transmission still enabled. The problem doesn’t occur if most or all of them use for this purpose the Internet in LTE technology. This case looks worse if there is a high packet traffic over 3G technology, because the cell resources are shared between voice calls and Internet.

Sending photos and videos by a crowd of several thousand fans generates very large packet traffic up. The principle is the same as for the voice calls. The mobile satellites, when getting rejections of packets, increase the transmission power, trying to send the files to the network at the same time. Contrary to the voice connection, in data transmission, files are subjected to fragmentation, and the lost fragments are retransmitted. As a result of the maximum power of terminals and continuous packet retransmission, uplink clogs due to interference.

In addition, for a given phone, transmission rate is allocated dynamically. Voice calls over 3G technology have a higher priority than packet data connections. For this reason, resources that were previously allocated for packet traffic call are “taken” for a voice, resulting in a decrease in throughput and an increase in the process of sending (upload) photos, etc. by network users.

In the third case, despite displaying the full reception, it is not possible to make a phone call or connect to the Internet and is not due to uplink interferences. This effect may be due to depletion of code tree, or simpler, due to exhaustion of the cell phone capacity. Exhaustion consists in allocation of all the available codes to current users. Each new user, when try to make a voice call or packet transmission, gets refusal from network due to the lack of available codes (lack of resources in the cell phone). Resources are allocated dynamically as a result of requesting (receiving) a voice call or packet data transmission (turning on the data transmission does not cause the resources reservation itself).

In summary, described cases do not happen only at concerts or mass events. They are found in areas with heavy traffic in the mobile network. It should be noted, that this three above-mentioned reasons for which we cannot call, can occur separately, but can also occur simultaneously in the same place and time.

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