Here I will explain three different communication devices that can be used for worldwide communication. The three that I am going to explain are modems, satellites and mobile phones.
Modems (modulator-demodulator) – modems are used to allow computers to communicate with each other over long distances via telephone lines. Essentially their job is to convert a digital signal into an analogue signal and vice versa. This basically means that they convert the digital data (or binary) into modulated electrical signals in the voice frequency of a telephone line. A modem on the receiving end then demodulates these electrical signals into digital data.
A section of a telephone line is designated for the use of a broadband connection whilst the other part remains available for voice calls. These are technically the “bands” of a phone line; this is where broadband gets its name from. Broadband filters that are used separate these two bands so that they do not interfere with each other. Modems are generally classified by the amount of data they can send in a given amount of time; usually measured in bits per second.
Satellites – satellites are used to transmit a signal from one place to another. They are used to broadcast data all over the world. A satellite can send a signal from somewhere on Earth to a communications satellite in space which then bounces the signal back down to another satellite on Earth. This is done very quickly so the duration it takes to transfer data worldwide is instantaneous. The signal sent by a satellite is “line of sight” which basically means the signal can only travel in a straight line. If a signal was sent across the Atlantic Ocean from the UK to America it would not be able to reach the recipient satellite because the signal would either disappear into space or dissipate into the ocean. Over distances of about 25 miles microwave repeaters are installed to repeater and boost the signal. A geostationary communication satellite essentially acts as a repeater.
The diagram below shows how this works in order to be able to transmit signals around the world. Satellites in the sky are roughly 23300 miles from Earth; therefore the signals have to travel roughly 46600 miles from the sending satellite to the receiving one. These signals are sent via electromagnetic beams that travel at 186,000 miles per second which causes a latency of just 240 milliseconds.
Mobile phones – mobile phones are a very useful form of technology allowing voice communication all around the world which essentially work like radios. To get an understanding of how they work it is a good idea of how sophisticated they have become it is a good idea to compare them to walky-talkies. Walky-talkies are half-duplex which means two people communicating use the same frequency meaning only one person can speak at a time. A mobile phone is full-duplex which means it uses two frequencies; one for speaking and the other for listening. This means both people can speak at the same time.
Mobile phones operate within cells and are able to switch cells as they are moved from area to area; thus, not losing signal. The diagram below shows different cells within a large area; each cell typically serves around 10 square miles. Another thing you can see from the diagram is that the masts are positioned in a hexagon shaped pattern. This allows for as much area as possible to be covered by the masts to allow a signal wherever a mobile phone is used. This allows for flexible and mobile communication because as you move into a different cell area whilst using a mobile phone, it automatically switches to the mast in that area.
Signal Theory – P2
Signal processing is relating to the conversion between digital signals (binary) and analogue signals (electrical frequencies). This is usually done using a modem (modulator demodulator). A computer or transmission device sends bits (binary digits) to a modem which converts these to electrical pulses. A modem on the receiving end then coverts these electrical pulses back to 0’s and 1’s for another transmission device to be able to interpret the data that has been sent.
A bit is basically a string of 8 numbers that contains 0’s and 1’s. Each of these strings represents a number between 0 and 255 which is what the computer converts into useable data. To convert denary (standard number form) to binary you have to think of it as being similar to ones, tenths, hundredths and thousands (denary), but with 1 to 128 – doubling each time. You can have a number up to 255 because if you add all of the 8 numbers up you will get 255 (in binary this would be shown as “11111111” because it contains every number). I will show how numbers 1 – 7 are converted into binary below: