- [Presenter] For most of us, a mobile phone is a part of our lives. But I'm sure you're curious minds have always been struck by such questions as how a mobile phone makes a call, and why there are different generations of mobile communications? (upbeat music) Let's explore the technologybehind mobile communications. When you speak on your phone, your voice is picked up byyour phone's microphone. The microphone turns yourvoice into a digital signal with the help of MEMS sensor and IC. The digital signal contains your voice in the form of zeros and ones. An antenna inside the phonereceives these zeros and ones and transmits them in theform of electromagnetic waves. Electromagnetic wavestransmit the zeros and ones by altering the wave characteristics, such as the amplitude, frequency, phase, or combinations of these. For example, in the case of frequency, zero and one are transmitted by using low and highfrequencies respectively. So, if you could find a way to transmit these electromagnetic waves to your friend's phone, you would be able to establish a call. However, electromagnetic waves are incapable of traveling long distances. They lose their strengthdue to the presence of physical objects, electrical equipment, and some environmental factors. In fact, if there were no such issues, even then, electromagnetic waveswould not carry on forever, due to the Earth's curved structure. To overcome these issues,cell towers were introduced, using the concept of cellular technology. In cellular technology, a geographic area isdivided into hexagonal cells with each cell having itsown tower and frequency slot. Generally, these cell towersare connected through wires, or more specifically,optical fiber cables. These optical fiber cablesare laid under the ground or the ocean, to provide national orinternational connectivity. The electromagnetic wavesproduced by your phone are picked up by the tower in your cell and convert them into highfrequency light pulses. These light pulses are carriedto the base transceiver box, located at the base of the tower for further signal processing, After processing, yourvoice signal is routed towards the destination tower. Upon receiving the pulses, the destination tower radiates it outwards in the form of electromagnetic waves, and your friend's phonethen receives the signal. This signal undergoes a reverse process, and your friend hears your voice. So, it's true that mobile communications are not entirely wireless, they do use a wired medium too. This is how mobilecommunications are carried out. However, there was a big issue that we intentionally left unanswered. Mobile communication is only successful when your tower transfers thesignal to your friends tower. But how does your tower know in which cell tower areayour friend is located? Well, for this process,the cell tower gets help from something called amobile switching center. The MSC is the central pointof a group of cell towers. Before moving further, let's explain moreinformation about the MSC. When you purchase a SIM card, all the subscription information is registered in a specified MSC. This MSC will be your home MSC. The home MSC stores informationsuch as service plans, your current location,and your activity status. If you move outside therange of your home MSC, the new MSC, which serves you instead, is known as a foreign MSC. As you enter a foreign MSC region, it communicates with your home MSC. In short, your home MSC always knows which MSC area you are in. To understand in which celllocation the subscriber is within the MSE area, the MSC uses a few techniques. One way is to updatethe subscriber location after a certain period. When the phone crosses apredefined number of towers, the location update is again done. The last one of these iswhen the phone is turned on. Let's try to understandall of these procedures with an example. Suppose, Emma wants to call John. When Emma dials John's number, the call request arrivesat Emma's home MSC. Upon receiving John's number, the request will beforwarded to John's home MSC. Now, John's MSC checksfor his current MSC. If John is in his home MSC, the call requests will be immediately sent to his current cell location, and it checks whether Johnis engaged on another call, or if his mobile is switched off. If everything is positive,John's phone rings, and the call will be connected. However, if John is not in his home MSC, John's home MSC simplyforwards the call request to the foreign MSC. The foreign MSC will follow the previouslyexplained procedure to locate John's phone, and will then establish the call. Now, let's discuss whythe frequency spectrum is quite important inmobile phone communications. To transfer zeros and onesin digital communication, each subscriber isallocated a frequency range. However, the frequency spectrum available for cellular communicationsis quite limited, and there are billions of subscribers. This issue is solved withthe help of two technologies, one frequency slot distribution, and two, multiple access technique. In the first technique, different frequency slotsare carefully allocated to different cell towers. In the multiple access technique, this frequency slot isefficiently distributed amongst all the activeusers in the cell area. Now, the big question. Why are there different generations of mobile phone technologies? 1G originally allowedusers, for the first time, to carry a phone withouta cable attached to it. But 1G suffered from two major problems. The first problem was thatthe wireless transmission was in an analog format. Analog signals that are easilyaltered by external sources. So, it provided poor voicequality and poor security. The second problem was that it used the frequency divisionmultiple access technique, which used the availablespectrum in an inefficient way. These factors paved the wayfor the second generation of mobile communications, 2G used digital multiple accesstechnologies, namely TDMA, or CDMA technology. The second generation also introduced a revolutionarydata service, SMS, and internet browsing. 3G technology was focused on giving a higher data transfer speed. It used a WCD multiple access technique, along with an increase in bandwidth. To achieve this, the 3G speed of two Mbps allowed the transfer of data for uses such as GPS, videos,voice calls, et cetera. 3G was a huge step in the transformation of the basic phone to a smartphone. Next came 4G, which achievedspeeds of 20 to 100 Mbps. This was suitable for highresolution movies and television. This higher speed was made possible due to the OFD multiple access technology, and MIMO technology. MIMO uses multipletransmitter receiver antennas inside both the mobilephone and the towers. The next generation ofmobile communication, 5G, to be rolled out soon, will use enhanced MIMOtechnology and millimeter waves. It will provide seamless connectivity to support the internet of things, such as driverless cars and smart homes. Would you like to learnhow a touchscreen works? Please check out this video. Please don't forget tosubscribe to Learn Engineering, and your support atpatreon.com is invaluable. Thank you.