An expert mobile user who has good command over his Smartphone often lacks the knowledge of the underlying mobile technology which is helping him in making calls, connecting him to his favorite video or music streaming services, etc. 

Also sometimes I come across my friends who have good command over their mobiles, much more than me at least, and they even work for some mobile companies in a non-technical department, however their knowledge of the technology and the connectivity is quite weak.

When I tried to search internet to find a simple tutorial then I found many good articles on Wikipedia, etc but they were a bit more technical which forced me to contribute to this field, I hope some people will like it, in shaa Allah. 

In order to keep the article brief technical terms used in the tutorial are not explained in further details, the readers who are more interested in these may find Wikipedia and other internet resources more informative.

What is a Mobile Network composed of

A generic Telecom network is composed of different components supplied by a long list of vendors, these networks are organized in an interwoven topology based on the needs of inter-connectivity, Network Security and resilience.

However in order to understand the Telecom Network it can be segregated in a number of Building Blocks each with distinct capabilities and offering different and distinct services to other blocks, also these blocks have distinct interfaces to connect to other blocks with detailed specifications written and agreed through internationally accepted and agreed governing bodies like ITU, IEEE, etc.

Below is a list of generally known blocks and the service they offer and the interfaces they have:

RAN  (Radio Access Network)

RAN is comprised of the equipment that connects the user on Air Interface, in simple words it connects any mobile phone  or data device like tablets to the wireless network, further it aggregates the data from a large number of users and send it towards the core network where the user can get access to different services offered by the Network Operator.

Based on the Wireless Technology RAN equipment can have different names, however from a user's point of view all these components have the similar usability, i.e. to allow the users to use the network services.

RAN network looks very simple in the picture however it is very complex in nature as the connectivity of user to the network is on wireless media and it needs a lot of planning and optimization and thorough drive tests to ensure full network availability in coverage areas because any problem in RAN has its immediate effect on the end user experience.

Transmission

Transmission is an essential part of Telecom Network, it provides the backbone on which all other blocks connect to each other, in many cases it also provides the connectivity within then blocks as well.

Even though it is not separately mentioned in any of the pictures however the connecting lines in these picture which are within the blocks connecting small blocks and between the blocks may be passing through Transmission devices especially if the entities are not co-located geographically.

Transmission is comprised of many flavors; Physical Infrastructure wise from Coaxial Cables to Microwave Radio to Optical Fiber to VSAT, Technology wise from SDH to ATM to IP/MPLS, capacity wise from Fractional E1 to STM64 and from FE to 400GBPS.

Core Network

Core Network is the heart of the Network and it is most crucial as well because this is where many entities meet. Financially this is most vulnerable area because this is where all data is stored and where charging/billing takes place.

Each component of Core network has a specific thing to do, below we will briefly describe the function of each component:

MSC: Mobile Switching Center, this is where the calls are connected, however this connection is on Circuit Switch basis generally referred as the 2G/3G voice calls.

HLR/VLR: Even though it is not separately shown in the picture but the MSC is connected with two Database servers which hold the User Information, HLR has all the User which are local to the MSC while VLR has the data of the Users that are currently attached to the MSC even if they are not local but on roaming to this network.

HLR/VLR is for 2G and 3G users only, 4G users are provisioned in a different Database name HSS as mentioned below.

SGSN: Serving GPRS Support Node, this is where the data sessions are handled, browsing internet, making video calls is handled here, these sessions/calls are generally referred as Packet Switched as they are based on data packets not mere voice signals.

GGSN: Gateway GPRS Support Node, this is the equipment by which the users are connected to the Internet.

Charging is the component which keeps track of the billing of pre-paid and post paid customers.

ePC: Evolved Packet Core, this is the place where the 4G eNodeBs connect, there is no Packet or Circuit based segregation in ePC as all the users are handled in a data only manner, all voice, video and services are packet data based or generally speaking IP based.

One special ability of ePC is the offloading to the Wi-Fi network, if the LTE network is in congestion or the user is in a area of a better coverage of Wi-Fi or Femtocell then it can be smoothly offloaded to this network bringing ease to the precious LTE air interface to serve other customers.

HSS: This is almost the same Database like HLR/VLR and hold the User Information, the difference is that this Database is for 4G users, it is pertinent to note that a User SIM may be registered on any one or both of the Databases and based on it the services will be provided to him as per the operator's charging policies.

EIR: Equipment Identity Register is a database of the all active mobile handsets, this can be used to track the mobile theft and providing device based services.

AAA: Authentication, Authorization, Accounting server, this is the server where all user records are maintained, the users are authenticated, granted permission to use different resources.

Services/Application Network

These are different entities providing different services to the end users, i.e. the mobile user.

Whatever we do in our Smartphone, from making calls to browsing the internet, sending SMS to chatting on social network, making of video calls to streaming videos is called a service and each service is provided through the services network.

PSTN: Public Switched Telephone Network, this is the service which the user uses to make calls to landline network of the same company whose Mobile service is being used by him.

PLMN: Public Land Mobile Network, this is the interface from where you connect to other Mobile or Fixed Line operators, if you call to other networks then your call is connected through this entity.

Internet: As the name suggests this is from where you connect to the Internet

IMS: IP Multimedia Subsystem, this is your gateway to your operator's IP network, in the present day architecture all the IP services are being converged to an all IP network through IMS, this architecture will ease the way for managing and delivering new services to the end user.

Application Servers: This is the area where your mobile operators has put all the application servers and any user who is trying to access any service is connected to the Application Servers through the IP Multimedia Subsystem.

I hope this topic was useful for a general user to understand the Mobile network in a brief, below is the whole picture integrated together to fully understand end to end.

Picture Source: LTE Testing Brochure from Spirent www.spirent.com