Cellular Radio, Isdn Networks And Satellite Communications Essay, Research Paper
Introduction
In describing Cellular Radio, ISDN (Integrated Services Digital Network) and Satellite Communications we must first have a general understanding of the meaning of these forms of Telecommunications. After a brief explanation of the different forms, they will be described in more detail. Then the present and future roles in Irish Telecommunications will be looked at and some examples given. This will then lead to the conclusion.
1. What is Cellular Radio?
Cellular Radio is the use of Technology using low powered radio transmission for transmission and receiving voice or data to a telephone network. Users can be stationary or mobile. It means users are no longer constrained to place to place as it is a wireless system.
2. What is ISDN (Integrated Services Digital Network)?
ISDN is a system which provides digital capacity i.e. voice, video and data transmission in digital form, to be sent and received by a customer. Unlike stand alone services such as telephone, telex, fax and data communications, ISDN is a network which can offer all these services as a package in which a user can talk, exchange data, faxes and other documents at the same time, instead of having to use different networks for these services.
3. What is Satellite Communications?
This is the use of orbiting satellites to relay microwave transmissions from earth stations to the satellite an to re-transmit those signals back down to another earth station by means of microwave transmission.
Cellular Radio
At present there are two types of cellular radio systems in operation in Ireland. These are:
1. TACS
This system is an analogue system in which the voice is transmitted by radio from the telephone to the nearest base station in analogue form. Analogue systems are at present throughout the world but each Country would have different standards. So a user can not use the phone in a different country, also as the signal it transmits is in analogue form the calls can be monitored quite easily with the help of a scanner. America s version of analogue cellular radio is called AMPS (Advanced Mobile Phone System).
2. GSM (Global System for Mobiles)
GSM is a more recent technical standard for cellular mobile phone systems, which was developed in Europe. It is a digital system, which voice and other sounds or data are converted to digital form before they are transmitted between the base station and the mobile phone and then converted back to analogue form.
There are major advantages in using the GSM system as opposed to the TACS system:
a. More complex signaling systems can be used e.g. text messages, fax and e-mail.
b. GSM is an international standard unlike the different analogue systems. GSM is used in most of Europe and parts of Asia. This means a user traveling to another country still has use of their mobile phone.
c. Radio signals carrying digital information are much harder to intercept than radio signals with analogue information. GSM is a more secure transmission.
d. As GSM is a digital system it is more suited to transmit computer data as it s normally in digital form.
e. GSM has higher voice quality in areas where the radio strength is low.
For these reasons the old system (TACS) is slowly on its way out. The new technology is taking over. There are currently two GSM operators in Ireland, ESAT and Telecom, with Meteor starting operations in early 1999.
How does a cellular radio system work?
Mobile telephone networks use a system called the cellular mobile telephone system. The way it works is that the area of coverage, e.g. Ireland would be divided into areas called cells (cellular) each cell would have a low power radio transmitter and receiver. This is called a base station. There is normally one base station in a cell although the cell can be subdivided to give better coverage to a densely populated area, such as Dublin.
When a mobile phone is in a cell it transmits and receives radio signals from the base station in the cell. As the base station is a low power transmitter, the radio signal would be weak. This is so that the radio signals are not picked up if they are out side the area of the cell. Each cell is assigned a set of frequency channels. There is a low range of frequency channels advalibile but it is possible for different cells to use to use the same frequencies. This is as long as they are not attached to another cell with these frequencies or else there would be interference.
Also cells do not have to be the same size. The size of the cell can vary depending on the amount of users in a particular area, for example, in country areas the cells would be quite big whereas in towns they would be smaller. This means more users can make calls in that area at the same time. As an area gets more populated and there are more mobile phone users, the cell can also be split to provided more coverage for the users.
When a mobile phone user dials a number, the phone transmits a signal to the nearest base station. The base station then picks a radio frequency that is not in use at that time and transmits a signal to the phone to tell it the frequency to use. The phone then tunes into that frequency. The base station then passes the number which was dialed to the mobile switching center (MSC), via either a land link (fiber optic cable) or a microwave link. The phone is then connected, or through a series of connections to the dialed number, landline or mobile.
The base station monitors the strength of the radio signal during the call. If the signal becomes low i.e. if the mobile phone user is moving out of the cell the base station notifies the MSC. The MSC then sends a signal to the base station in the cell that the user is about to enter informing the station that the mobile phone is entering that cell. The new base station then sends a signal to the mobile phone with a new frequency. The mobile phone tunes into this frequency and the call continues uninterrupted. This process is known as a hand-off .
If there is no frequencies available in this new cell, of if the user goes into an area where there is no base station the call is cut off. This is known as a drop-out .
The mobile switching center, although it has its own telephone exchanges, must be also connected to fixed telephone networks as most calls made from mobile phones are made to land lines and land lines to mobile phones. It uses it own telephone exchange if the call is to another mobile phone user of transfers the call to a fixed network if the call is to a land line.
Integrated Services Digital Network
The advantages of ISDN over the stand-alone systems can be broken down into three areas. The business sector, suppliers and the telecom carriers.
Business Sector
a. The office is a single automated system.
b. Only one ISDN terminal is needed.
c. The costs are reduced.
d. More access to remote information sources.
e. Video conferencing. Reduced travel expense etc.
f. Access to new telecommunications technology.
g. More productivity through quicker access speeds.
Suppliers
a. New or upgraded equipment needed to use system.
b. Standardised system.
c. As it is an international standard, there will be more development of it and lower development costs.
d. Large market for system.
Telecom Carriers
a. Expansion will lead to higher revenue.
b. Easier to upgrade networks as it is a standard system.
c. Reduced maintenance and installation.
d. Improved network management.
e. Better customer service and satisfaction.
The ISDN network can use digital transmission over ordinary copper wire. Users would need an ISDN adapter, which they use instead of a modem to receive digital transmissions. The access provider would also need an adapter. There are two levels of ISDN service available. These are narrow band and broad band. They both consist of channels, B (bearer) channel and D (delta) channel. The B channel carries data, voice and other services. The D channel carries control and signal information. Narrow band ISDN transmits at speeds up to 64,000 b/sec for each channel. Broad band ISDN would be used by large corporations and financial institutions such as banks, stock exchanges, and has 30 channels transmitting up to 2mb/sec.
Satellite Communications
A satellite is a wireless receiver that is launched into space and placed in orbit around The Earth. Satellites are used for many purposes such as, weather, television broadcast, Internet communications and global positioning systems (GPS). There are three types of communications satellites. These are named after the orbit in which they occupy.
1. Geostationary Satellite
These orbit the earth over the equator at 22,000-mile altitude. As it takes exactly 24hours to go around the earth they appear stationary. This means it will remain in the same spot on the Earth s surface. These satellites can see approximately 40% of The Earth s surface. A satellite dish aimed at the spot where the satellite is orbited can access them. Examples of this would be weather and television satellite.
2. Elliptical Satellite
These satellites revolve around the Earth in elliptical orbits. They move fast when they are near the earth and slower when travelling away. Examples of their uses are amateur radio and government satellites. They are followed using directional antenna, as they don t have a fixed position.
2. Low Earth Orbit Satellites (LEO)
These satellites are the most recent of satellite technology. They are a series of satellites, which would orbit much closer to Earth. They would orbit around the geographic pole and revolve around the earth in a few hours. They form a ring around the Earth making it possible to transmit signals from one point on earth to another, by means of a mobile phone.
The advantages of using satellite communications are as follows:
a. Communications to large regions quickly.
b. Cheaper than laying large amounts of cable, over mountains and under oceans.
c. Communication satellites are reliable.