by Asela Bandara

Introduction

As we all know and experienced SMS messaging, it has become one of the most frequently used communication methodology all over the world. Getting known about the underline SMS technology is somewhat more specific to a certain stage of the GSM technology. That is because it is a much minimized subset of the whole GSM communication. Thus before go into too deep in SMS applications, it is always better at least to have some understanding on how this entire cellular communication works.

Cellular Communication 1

Figure 1

The cellular telephony was experienced in cars at its’ primitive stages. In the radio-telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter, big enough to transmit 40 or 50 miles (about 70 km).  The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously.

The duplex of the communication channels and their behaviors are very important in providing a successful call.

·    Full-duplex vs. Half-duplex:

A cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at once.

Figure 2

The AMPS (Advanced Mobile Phone System) uses a range of frequencies between 824 MHz and 894 MHz for analog cell phones, assigning 832 frequencies per each carrier: 790 for voice and 42 for data. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers). Cells are normally thought of as hexagons on a big hexagonal grid, as displayed in Figure 2. The same frequencies used in one cell can be used in any other non-adjacent cell at the same time, thus providing many telephony users to simultaneously utilize the cellular network.

Figure 3

Cell-phone Channels

·  A cell-phone carrier typically gets 832 radio frequencies to use in a city.

· Each cell phone uses two frequencies per call, a full duplex channel, so there are typically 395 voice channels per carrier. (The other 42 frequencies are used for control channels)

Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people can be talking on their cell phone at one time. Analog cellular systems are considered first-generation mobile technology, or 1G. With digital transmission methods (2G), the number of available channels increases. For example, a TDMA-based digital system (more on TDMA later) can carry three times as many calls as an analog system, so each cell has about 168 channels available.

Cell-phone Codes

Figure 4

· Electronic Serial Number (ESN) - a unique 32-bit number programmed into the phone when it is manufactured

· Mobile Identification Number (MIN) - a 10-digit number derived from your phone's number

· System Identification Code (SID) - a unique 5-digit number that is assigned to each carrier by the FCC

While the ESN is considered a permanent part of the phone, both the MIN and SID codes are programmed into the phone when you purchase a service plan and have the phone activated.

GSM Infrastructure

Figure 5

The Mobile Station

The Mobile Station (MS) is the user equipment in GSM. The MS is what the user can see of the GSM system, the cellular phone itself.

The Base Transciever Station

The Base Transceiver Station (BTS) is the entity corresponding to one site communicating with the Mobile Stations. Usually, the BTS will have an antenna with several TRXs (radio transceivers) that each communicate on one radio frequency. The link-level signaling on the radio-channels is interpreted in the BTS.

The Base Station Controller

Each Base Station Controller (BSC) controls the magnitude of several hundred BTSs. The BSC takes care of a number of different procedures regarding call setup, location update and handover for each MS.

The Mobile Switching Centre

The Mobile Switching Centre is a normal ISDN-switch with extended functionality to handle mobile subscribers. The basic function of the MSC is to switch speech and data connections between BSCs, other MSCs, other GSM-networks and external non-mobile-networks.

The Location Registers

With each MSC, there is associated a Visitors Location Register (VLR). The VLR can be associated with one or several MSCs. The VLR stores data about all customers who are roaming withing the location area of that MSC. This data is updated with the location update procedure initiated from the MS through the MSC, or directly from the subscriber Home Location Register (HLR). The HLR is the home register of the subscriber.

The Equipment Identity Register  

The Equipment Identity Register (EIR) is an optional register. Its purpose is to register IMEIs of mobile stations in use. By implementing the EIR the network provider can blacklist malfunctioning MSs or even receive reports to the operations centre when stolen mobile stations are used to make calls.

Services

The services in GSM can be categorized in two main groups:

• Tele services

The tele-services group consists of the basic speech transmission, the point-to-point short message service and the broadcast short message service. The speech transmission resembles normal telephony. Speech is digitalized in the Mobile Station, coded and sent across the radio-channel.

• Bearer services

The bearer services are parted into nine groups of transparent and non-transparent data-transmission services.

Calling

Once it is started, it listens for an SID on the control channel. If the phone is unable to find any control channels (no service bearers’ control frequencies given in that area), it shows “No Service” message in the device display. When the service is available, these control signals will notify the MTSO (Mobile Telephone Switching Office) with the current cell and its’ functioning frequencies. Then, a receiving call will be directed to the particular phone by the MTSO via control channel, informing the device to setup its’ frequency pair either to change to the current cell figures or to continue with the existing values.

When the phone moves from one cell to another, the two respective base stations coordinate each other through the MTSO and decide which base station/tower to be served to the phone next, considering the phone signaling strength due to both the cells. This is called handover switches to the new cell.

In each case where a phone moves from a cell to another at the middle of a call, normally it doesn’t drop but handover to the other available service provider where the local carrier doesn’t support. In any case where the SID on the control channel does not match the SID programmed into your phone, then the roaming procedures will be take place. This Roaming can be:

• Within the country but between different carriers.
• Within the country but between different phone networks.
• Between different countries.
  • In this case the charges would be comparatively very high.

Handover

Handover procedures are defined for each of the following cases:

• Intra-cell handover. The connections are transferred to another channel on the same BTS.
• Intern inter-cell handover. The connection is transferred to another BTS on the same BSC.
• MSC intern handover. The connection is transferred between BTSs belonging to two different BSCs within one MSC.
• MSC extern handover. The connection is transferred to a BTS within another MSC.

References:

http://www.sciencemuseum.org.uk/images/object_images/535x535/10213586.jpg

http://learn-networking.com/wp-content/uploads/2008/01/full-half-duplex.jpg
www.google.com for the other images.

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