Cellular Network Architecture: An overview

The working principle of the cellular network is a very interesting one and I am very happy to share it with everyone. The cellular network allows a user to connect wirelessly to the PSTN (Public Switched Telephone Network). To really appreciate what technology has done for us all in the evolution of Cellular networks, let’s take a brief look at what PSTN looked like initially.

credit: writagram
The PSTN has been on the speedy evolution since the first voice call was transmitted by Alexander Graham Bell in the year 1876. That first call was made using a ring down circuit which implies that the call was made without dialing any number, just two devices connected electrically. Also the call did not ring at the other end and the call was only on-way, hence only one person could talk, just like radio broadcasting services. The design was later improved to two way transmission which allowed each person access to the medium.

credit: Cisco Press

One major characteristic of this setup was that it required physical cable to each location. To reach any destination, there must be a physical cable connection to that very destination. For instance, if you want to reach four of your friends on the phone, you’ll need six cables to do this, hence this formula was generated to that effect;

N*(N-1)/2
Where N is the number of persons you want to reach.

credit: Cisco Press

It was really glaring that this set up cannot take us far, not just by looking at the cost of getting physical cable running but how cumbersome it will definitely get. A device called SWITCH was later introduced and the phones were connected to the switch as a central point but the switch was a manual one controlled by the operator who asks every caller where he/she wanted the call to be directed to and then manually connect the two voice channels.

The setup now looks like the image shown on the left. Bells were kept at each terminal which is electrically activated whenever a call comes in. This is just a tip of how inconveniencing legacy PSTN was. These operations of course is still present but now in a more convenient (for the users), more complicated (for the operators), and miniaturized. With this I will say we are ready to begin.

Why cellular?

The cell phone is nothing but a complex radio, the smart attached to its name is just a recognition of the fact that more functionality has been added to it but by default a cell phone is nothing but a complex radio. Back in the 1950’s, mobile phones were not that common, very big antennas were mounted in the cities that could transmit for a distant a bit above 50 miles! Such a long distance right?

But that came with a big price, it could only contain 25 channels, hence only about 25 people can make use of the channel at once. No longer sounding funny right. Also to be able to power such antenna with very high gain, you need a tremendous amount of power and you guessed it right, the transmitting frequency is also not healthy.

credit: Career ride

A cell is a pictorial representation of the area covered by Base Station (BS). A base station is that point where all our wireless phones are connected to on a carrier network (your service provider). More on the BS later. As stated above, the long range antennas were replaced with lower powered radios (about 0.3 to 6 watts), hence allowing for more frequency and now requires more antennas for the same area of coverage. The cell is hexadecimal in shape and in the middle of each cell is an antenna.

The area covered by these cells depends on the transmission power of the BS. Hence, as we move away from the BS, the strength of transmission reduces. In reality, the shape of the cell might even overlap or have big spaces in-between areas covered by two adjacent cells. The main aim of the cell is to maximize the available frequency channel and hence accommodate more users. To achieve this, two main techniques are applied and these are:

Frequency reuse and handover and

Introduction of medium access methods

Frequency reuse is applied not only in telecommunication. A very simple example is in FM radio transmission. When you are traveling like 5 states away from your current state, the radio station you are tuned to is likely to be another station in your destination state depending on the frequency planning of your country. Frequency reuse is a technique for making use of the same set of frequencies and or channels in the same communication environment with the sole aim of increasing bandwidth and capacity of the system.

credit: Cisco Press
Here, the radiating antenna (BS) within a cell is allowed to transmit at a given set of frequency/frequencies. Here in the cellular world, the reuse pattern is within cells and these cells are allowed to repeat in a cluster. Hence, the frequency set used in one cell can be found in another cell but the spacing between the cells must be in such a way that interference would not occur.

credit:boddunan

The process of handoff is used when users moves from cell to cell. A well planned cell clusters will have a seamless handoff, this is experienced when traffic is transferred from one BS to another without experiencing a distortion in call or data transmission process. Handoffs are high priority operations, in fact, initiation of handoff by a user is often attended to before a request to initiate a call by another user. As shown above, there are two types of handoff, the soft handoff and the hard handoff. The handoff process in which the resource available to a source cell, BS1, is retained and used in parallel with a destination cell, BS2 is termed soft handoff. In hard handoff, the resources of the source cell is not used within the target cell, hence the user experiences absence of signal during the handoff process and any process the user is carrying out is disrupted during the process. This type of handoff is predominant in my country, lol.

The channels available are further utilized using access methods like CDMA, FDMA, TDMA, and many other access methods. These are multiplexing methods which make available communication services to many users thereby maximizing the available medium (which are very costly, even for service providers).

Image credit: Taitradio

Code Division Multiple Access (CDMA) are predominantly used in our third generational (3G) cellular telecom system and has been around since the 1980s. It was invented by Qualcomm alongside two United States network operators; Ameritech and Nynex, and later on, AT&T and Motorola joined the development team. CDMA is characterized by the use of codes to increase the bandwidth of a channel.

This code is known as spreading codes and functions to spread data using series of codes and these codes are independent of the data. CDMA splits a time slot within the same frequency into codes and unlike other multi access modes, these slots are actually transmitted simultaneously as shown in the figure to the left. The only problem associated with this form of communication is that noise in the medium increases with an increase in the number of users.

Introduction of codes to the channel multiplexing technique also improved the level of security because the data being transmitted through a channel is done so using the already mentioned spreading codes. To be able to access the transmitted data, the receiving end must have a knowledge of the code used in the transmission of the data.

Image credit: plutosemi

Frequency Division Multiple Access (FDMA) is a channel multiplexing technique which functions by dividing the available frequency into several channels with each channels being able to transmit voice and other digital services. FDMA is often associated analog services like the AMPS (advanced mobile phone service) found predominantly in the North America.

In FDMA, the data to be transmitted are then splitted and each enclosed in a sub-carrier signal (modulation i.e, wrapping of signal with another signal with higher frequency) which are then transmitted after being linearly mixed together. One of the widely used FDMA technique is the WDMA (Wavelength Division Multiple Access) which are applied in fiber optics technology where a fiber optic cable which has very high bandwidth are subdivided into different light frequency (or better still wavelength) each capable of data transmission.

Time Division Multiple Access (TDMA) is also a multiplexing technique widely used in global system for mobile communication (GSM) and involves the use of available frequency with respect to time. Here, the users are assigned a separate time slot for transmission of data within a frequency, which is to say that the same frequency is used at different time. The introduction of TDMA is to increase the capacity of FDMA. Since FDMA can provide more frequency hence increasing the available channel, TDMA even increases the bandwidth by allowing more users into the medium by playing with time.

credit: Telecom Academy

In TDMA, interference can be disregarded since lower number of the available channel is used for transmission. Also the assigning of the time slot must be well synchronized among the users, hence every user is assigned a time slot for each frame and the whole bandwidth available to the channel are available for use for a giving period of time.

The hierarchical architecture

credit: Babansadik
The cellular network is a hierarchical network, well arranged and well-structured operational specification, all these are done with easy and organized management with efficient services in mind. The hierarchy is of three levels viz:

The Base system Subsystem (BSS),

The Network Switching Subsystem (NSS) and

The Network Management Subsystem.

The BSS (Base System Subsystem) comprises of the Mobile station (MS), the Base Transceiver Station (BTS) and the Base Control Station (BSC). A very good example of the MS is your mobile phone and performs various functions in the network including data and voice transmission, synchronization (both time and frequency), provision of location information, etc.

The MS is identified uniquely on the network using sets of numbers like the International Mobile Equipment Identity (IMEI), International Mobile Subscriber’s Identity (IMSI). The IMEI can be retrieve from your MS by typing the short code *#06#. IMSI is also a unique identity for the MS and is internationally recognized, this information is stored on SIM (Subscriber Identity Module), the Home Location Register and the Visitor Location Register (more on that soon).

The BTS is responsible for frequency hopping, random access detection (RAD), and above all, the transmission of the frequency and time synchronization signal. The BSC controls the various BTS connected to it and forms as the connection between the MSC and the BTS. It also performs handover between cells (as described above), frequency relocation and controls power within the station and the transceiver systems.

The Mobile Switching Center (MSC) just as the name implies performs all the switching functions in the cellular network. It is also the point of connection between the cellular network and other networks like the PSTN and the data networks. It also serves as the gateway for subscribers that are roaming. With this, it is obvious that all our calls must reach the MSC before they are redirected to their various destinations (call setup). Billing of all services is also done at the MSC and also the location of the various components of the network is done at the MSC.

Image credit: Washington University

To achieve some of these monstrous functions, the MSC maintains two basic databases called the HLR (Home Location Register) and VLR (Visitor Location Register). The HLR is the main database maintained by the MSC for storing customer’s information like authentication information, numbers, encryption values, subscriber status, customer’s associated VLR, etc. The VLR functions to reduce overhead on the HLR. The HLR updates the VLR on the entry of a subscriber to its area.

Just like the name implies, the Network Management Subsystem oversees and monitors various functions and parts of the network. Its major function can be grouped into fault, performance, and configuration management.

Reference

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