How does the LTE 4G mobile Phone work?

We have been using mobile phones for a very long time now. I have been using smartphones for the last 8 years now, but I never cared to venture into the details of that how the phones actually work. Coming from an engineering background I already know some pieces but that knowledge is limited to “something to do with radio communication” 😀 😀

So this time I tried to take a little time and read about the details, also because 5G is knocking on our doors and people say amazing things about it. Some say it will revolutionize the world of communication(as if we are not already living a revolution) but yeah, they say it will help in remote surgery, better machine-to-machine communication, and self-driving cars and many more fields will evolve with it. It will be exciting to see.

In this blog, I am covering 4G or LTE (Long Term Evolution) and I will write another blog for 5G.

I made a strategy to give you a small description of the topic and then answer some key questions to give more clarity about the topic. So let’s get started.

The overall Architecture

In mostly all mobile communication here as well, we build the control session first and then the data session is formed. From the time you turn on your mobile phone till the time data connection starts, the whole communication happens in two phases, the first phase is control session buildup where the mobile subscriber’s identity, security parameters, user profiles etc are checked and validated. The second phase is data connection buildup where the mobile device is granted access to the internet or the call network, In the below diag control session, is marked with a blue dashed line and a black line indicates the data session.

The overall 4g network is divided into three parts, EUTRAN or radio network, EPC or evolved packet core, and then the internet..we have many different components involved in this architecture and we will see a brief intro of each of these components in the next sections of the blog.

Question-1 : How my mobile phone connects to the network?

We know mobile communication happens over the waves, precisely over radio waves. You might remember the electromagnetic waves spectrum, the whole spectrum is divided into ionizing and non-ionizing waves, ionizing waves are UV light, X-ray gamma-ray, etc and you should always avoid exposure to these waves. The non-ionizing band is chosen for the communications because it is known as harmless. you can send or receive these signals over the air without harming yourself or any other ecosystem…well… the latest researches claim this!!!

So your mobile operator picks one frequency in this nonionizing band and uses that between the mobile phone and the mobile tower for communication. Our mobile phones have an antenna which is also a transducer device used to generate as well as send radio signals. The frequency over which the mobile tower and phone talks is very high and can go in GHz range, we don’t have devices that can do this high rate or analog to digital processing(What is the fastest DSP today anyway??…something to look for!!). So our mobile phones convert these signals to moderate frequency signals also called baseband signals. And at the end, these signals get converted into digital signals for the applications to process the data. Something like the below gif.

Question-2 : How my mobile picks the right Operator?

Let’s take an example of the below situation where we have person blue and person orange sitting side by side. The blue person having a blue mobile wants to connect to the blue network operator and the orange person with orange mobile wants to connect with the orange person(what a colorful world!!). Now, we will have so many operators broadcasting the radio frequencies but how does the mobile phone distinguishes between the frequencies and identifies its own operator?

The answer to this question lies in the SIM card. The mobile phone selects the network to connect on the basis of three parameters.

  • If the signal strength is appropriate?
  • If the available network is defined as permitted network in the sim card.
  • If the network is providing services which mobile phone is asking for.

If all these conditions satisfy then the mobile starts the control connection with the mobile tower which is advertising the network.

Question-3: How does the provider identify my mobile phone?

SIM card has a minimal but very very important information stored in it. Some of that information is

  • (Integrated circuit card identifier) ICCID
  • International mobile subscriber identity (IMSI)
  • Authentication key (Ki)
  • Location area identity
  • SMS messages and contacts

When any mobile starts up it sends its IMSI to the operator. This value has 3 components, MCC(mobile country code) MNC (mobile network code ), and MSIN (mobile subscription identification number).

Now the Mobile operator has a device called HSS or home subscriber server that stores all the IMSIs and associated user data and every incoming connect request will be validated against the database in the HSS. If the subscriber’s IMSI is found in HSS then the further connection process starts.

Every IMSI has a user profile associated with it and it is stored at HSS. For example, User-1 has  voice and data subscription, User-2 has voice,data, and international roaming subscription, etc

Question-4 : How does the ISP track and bill me for my activities using phone?

Once the control session is built up the data connection starts, where the mobile tower also called EnodeB in 4G network directly connects with S-Gateway(Serving gateway) which in turn connects with P-gateway(PDN or packet gateway) that has the direct connection with the outside world.

Flow-based charging is done with the help of PCRF(Policy and Charging Rules Function). PCRF has an agent running on P–gateway which is called PCEF(Policy and Charging Enforcement Function ). PCEF is responsible for all flow-based charging, for example, for a SIM you sometimes when cross the allowed limit you will be charged more.PCRF helps to program PCEF to treat different traffic coming from mobile in accordance with the subscription that you have bought.

Now here you can guess if the government asks the service provider to tap or intercept somebody’s connection for lawful intercept then where it will do it since the P-gateway has the complete visibility of your data hence this is the place where it should be done.

Question-5 : How does the Roaming work? 

Roaming in the mobile world means that you have left your home network and went roaming to other networks, for example, an Australian person visits Asia in the below example…Roaming is done majorly in below two ways.

Home routed roaming :

In this method of roaming the control and data both the connections are steered to the home network and served by the home network directly.

Once the control session is established, all the data connections are also forwarded to the home P-gateway by the visited S-gateway(Serving gateway). This is called home routed roaming, you may experience delays or latency in this approach but it all depends upon the distance and the quality and volume of the channel between the providers.

Local Breakout Roaming:

The next roaming method is local breakout where the home network is only used for the control session phase as soon as we validate the user the data session is served by the visited network p-gateway itself… I like this method.


It was very interesting to know about mobile communications and it was even interesting to write about it. Now, the world is moving to 5G, some people on YouTube are also talking about 6G …can you believe it? But you would certainly appreciate my courage that I still took some time to learn and write about 4G, I am writing about an old tech!! Also, don’t get fooled by the operators who are providing 4G at the cost of 5G, believe me, there will be some ISPs like this. Also, you read me now please also hear me saying the same things I wrote. If you like the post or the video, please subscribe to me on YouTube.

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