What is the relationship between cellular and mobile communication network?

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The power of a single base station is actually very small. Although they seem to be complex and huge, they can only cover a radius of a few hundred meters. In densely populated urban areas, a 4G base station with a frequency band of 1800MHz can only cover a radius of about 300 meters.

If you are walking while talking on the phone and unknowingly run from the coverage area of ​​one base station to the coverage area of ​​another base station, then your service must be seamlessly handed over between the two base stations to ensure that the call is uninterrupted.

Therefore, isolated base stations cannot provide good services alone. It is necessary for many base stations to unite, abide by the same rules, share resources and work together to meet mobile communication needs.

These large numbers of base stations, together with other transmission and control nodes, form a "network". The industry generally calls this network a " cellular mobile communication network ".

It is not difficult to see that the term "cellular mobile communication network" contains three concepts: " cellular ", " mobile " and " communication network ". The following will introduce them one by one.

What is honeycomb?

Speaking of beehives, the first thing that comes to mind is definitely a huge hornet's nest with ferocious hornets buzzing around it. Poking a hornet's nest was once a childhood joy or nightmare for many people.

Real honeycomb

In contrast, although bees also sting, they are much more docile than wasps. If we go deep into the inside of a beehive, we will find that they are made up of many perfect regular hexagons. These regular hexagons are seamlessly connected to form a large network, which will continue to expand as the bee colony grows.

Bees in the hive

Although the coverage of a single base station is very limited, if we make the coverage of each base station a perfect regular hexagon, and combine multiple base stations, wouldn’t we be able to achieve seamless coverage of a large area?

Cellular network

As shown in the figure above, multiple base stations are neatly arranged together. Each regular hexagonal "beehive" is called a " Cell " ( translated into Chinese as a community ). The system composed of multiple such cells is called a Cellular Network .

If you have an Apple phone, open the settings and you will see options related to "Cellular Network". After clicking in, you will also see options related to "Cellular Data".

Settings on iPhone

Although it seems so mysterious, in fact, all current operators' networks are cellular networks, and Apple's settings are nothing more than some settings about basic functions such as operators, voice, data, hotspots, etc.

The word "network" that will appear frequently below refers to the cellular network composed of multiple base stations.

How to move?

The biggest difference between a mobile phone, also known as a "mobile phone", and a landline phone is that a mobile phone can be carried with you and moved around.

This leads to a most critical problem that cellular networks must solve: how to enable these users who are on the move and don’t know where they are to make calls and access the Internet anytime and anywhere?

Step 1 : Cell selection and reselection

When a mobile phone is turned on, it will constantly detect which base station has the strongest signal, and sort and confirm the best service cell. Once selected, the mobile phone will be stationed in this cell, and the signal indicator will be displayed on the phone, and the number of signal bars can be clearly seen at a glance. This process is called "cell selection".

Cell Selection

Moreover, since mobile phones can be carried around, they cannot stay in one cell all the time. They must constantly scan the signal strength of neighboring cells. Once a better cell is found, they will abandon the original cell without hesitation and move to the new one. This process is called "cell reselection."

Cell Reselection

The second trick: location update

First, the network is divided into multiple "location areas" according to geographical areas. A location area includes the area covered by a group of base stations. Within this area, each base station continuously broadcasts its location area code to users.

Location area division

After the mobile phone is turned on, it detects the location area where it is and reports it to the network. This process is called "location update". In this way, the network knows the location of the mobile phone, and if there is a call in the future, it will go to this location area to remind the mobile phone to answer the call.

Location updates on boot

If I only report my location when I turn on the phone, the network will still worry: if the phone breaks down or runs out of power after a while, won’t I be disconnected again? Even worse, the phone must actively chirp every few minutes to tell me that it is still there, alive, and can be contacted!

Periodic location updates

If the phone does not report the situation, the network will assume that the phone has moved to a place with no signal and cannot be contacted. If someone calls the phone, it will say, "Sorry, the user you are calling is out of service."

Furthermore, if the mobile phone runs and finds itself in another location area, it must say hello to the network: I have arrived at a new location, please find me here in the future, there is no need to go back to the original place!

Location Update for Location Area Change

The third trick: switch

Imagine if you are holding a mobile phone, sitting in a car and talking on the phone, moving from one community to the border of another community, and seeing the signal getting weaker and weaker, and the call may be dropped at any time, the situation is very critical. What can you do?

Fortunately, the mobile phone has been measuring the signal strength of adjacent cells. Once the service cell finds that the signal in the adjacent cell is strong to a certain extent, it can judge that the mobile phone has reached the border and it is time to hand over the call to another cell. It will then quickly contact the new cell to make all preparations for the handover of services. Only after receiving a positive reply can the mobile phone be connected to the new cell and the call can continue.

Switching process

This process is called "switching".

Note: Cell selection, reselection and location update are all actions taken by the phone when it is idle, which are not that urgent; while handover is an action taken by the phone when it is in a call state, which must be completed quickly and accurately, otherwise the handover may fail and the call may be disconnected. Therefore, the handover success rate is a very critical assessment indicator.

Switching process diagram

Communication network architecture

Once multiple base stations are connected into a cellular network, the efficiency of autonomy relying on mutual communication between the base stations is too low, so it is necessary to introduce a central control point to uniformly manage multiple base stations.

In the 2G and 3G era, multiple base stations are managed by one controller, and multiple controllers are managed by the core network, forming a three-layer pyramid architecture.

2G/3G communication network architecture

The name of controller is very straightforward, it is used to control base stations. The 2G controller is called "Base Station Controller (BSC)", and the 3G controller is named "Radio Network Controller (RNC)" to show its class. The industry generally calls it by the English abbreviation BSC/RNC or simply calls it controller.

The core network also has the word "network" in its name, and it is also a very complex network. But we won't go into details here. Just imagine the core network as a powerful router that connects and manages all base stations and controllers, so that all services, whether making calls, sending text messages, multimedia messages, or surfing the Internet, can find their destinations smoothly.

In the 4G era, in order to reduce latency and simplify the architecture, the base station controller was removed and the base stations were directly managed by the core network. However, although this architecture is simple, it is not so convenient to use.

4G communication network architecture

This is because if the core network is left to manage everything, the load will be too heavy; if the base stations are delegated to coordinate resource scheduling, interference and other issues point-to-point, the efficiency will be low and the effect will be poor. As a result, 4G will struggle with this architecture throughout its life.

In the 5G era, after learning from the lessons of the 4G era, we have returned to the old path of the 2G and 3G eras, which is to split the base station into centralized units (CU) and distributed units ( DU). One CU manages multiple DUs, and the core network then manages a smaller number of CUs.

5G communication network architecture

However, 5G is very flexible. The CU and DU can be separated or not, as shown in the figure above. Whether to separate the DU and CU depends on the development stage of 5G and the latency requirements of specific services. This is also a manifestation of the spiral rise of the communication network architecture.

This article is reproduced from Wireless Deep Sea, thanks for sharing

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