This article throws light upon the top five modes of communication available in India. The modes are: 1. Wireless Communications 2. Telephone Communications 3. Satellite Communications 4. Satellite Networks 5. Selecting a Means of Communication.
Mode # 1. Wireless Communications:
Cables, however, may not necessarily be the answer in all the cases of data communications. Other methods of data communications are also available and we shall next look at them. The use of cables almost always implies that they are servicing a Local Area Network over short distances, say in one building or in a few adjoining buildings.
However, if distances are greater, such as between two towns, or two cities far apart in the country or even two distant continents separated by sea, methods other than cables have to be thought of for data communications.
These generally can come in the form of wireless communications, communication through satellites and communications through telephones. Telephone systems at times may use satellite or wireless communications, but we shall treat each one independently.
Mode # 2. Telephone Communications:
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Telephone Systems provide the most widely used form of communications over distances of more than, say 100 metres. It is also the oldest form of distance communication that is still widely used.
Of course, the present-day telephone system bears very little resemblance with the original telephone system; as technology has developed, telephone systems have been appropriately upgraded to keep in line with this development in technology and they have often provided the necessary ideas for development of other forms of communication.
While the original purpose of the telephone system was to provide a convenient form of voice communication, they are still suitable for data communications between computers, particularly when recent technological improvements are applied to telephone technology. Also, they are publicly owned and in almost every country in the world, they are reasonably efficient.
As a result, they are often being used to connect computers that are close to each other, but are separated by public roads or land.
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Telephone systems, in spite of the present-day improvements, still have a tendency for high error rates and low bandwidth. However, their other advantages has made it necessary to look at ways to improve both their bandwidth and error rate so that they can be used for data communications.
Initially, telephone systems started by setting up a wire between two points; at the end of each point was a telephone instrument. The telephones and the connecting wires were the property of the two telephone users. Shortly, other users started to follow this and very soon the entire city became a jungle of wires which on occasions interfered with each other.
Clearly, this method of connecting telephones was not going to work. The first development was the concept of a switched telephone network. In this, all the telephone owners were connected to one exchange and at this exchange the caller was switched to the receiver’s connection. Pretty soon this single switching grew to multiple switching, which is the basic position today.
A variety of connections are used in the telephone system. These include twisted pairs, co-axial cables, micro-wave and Fibre Optic cables. While originally purely analogue signals were used in telephones, these are now being gradually replaced by digital signals. These have obvious advantages, cost less and are easier and cheaper to maintain.
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This is particularly useful in data transmission using computers. While long-distance transmission by telephone is gradually being converted to digital transmission, local telephones continue to use the old analogue transmission. However, for data transmission using computers, naturally digital transmission is required.
In order to do this, devices called modems have to be used for data transmission using computers. A modem, short for modulator/demodulator converts the analogue signal to digital, which can then be received by the computer and also converts the digital signal sent by the computer to analogue, which can be transmitted by the telephone lines.
Mode # 3. Satellite Communications:
Attempts at Satellite Communications were started in the 50s, but the initial attempts were not very successful because these attempts were to bounce signals off metal weather balloons.
The resultant signals were too weak to be of any practical use and progress had to wait until the idea of using the moon, instead of weather balloons to bounce signals off, was thought of. Further progress had to wait until artificial satellites were launched in the 60s.
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The advantage of using artificial satellites—instead of the moon—lies in the simple fact that in an artificial satellite, the incoming signal can be boosted before being rebroadcast. To that extent, an artificial satellite is like a repeater. Incoming signals are received by it through a transponder which boosts them and rebroadcasts them either in a narrow beam or in a more dispersed manner.
A transponder operates on a fixed portion of the spectrum and a useful satellite must have several of these transponders so that all its facilities can be usefully utilised. The rebroadcast is usually done on another frequency to avoid any interference with the incoming broadcast.
The basis of satellite communications is the use of geosynchronous satellites. A geosynchronous satellite, whose orbit is located at a distance of approximately 36000 km above the equator, matches its orbital speed with the rotational speed of the earth about its axis so that to an observer on the earth the satellite appears to be fixed in a particular place in the sky.
This gives a big advantage in as much as an antenna can permanently point to it. If the satellite was to move about from this fixed spot, the antenna would have to be constantly moved to track the satellite.
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There are also some disadvantages in using geosynchronous satellites for communication. If a communication has to be broadcast from some point in the satellite’s footprint to another point outside its footprint, the satellite cannot do so.
The height of the satellite above the equator is unchangeable and predetermined because as per the basic laws of Physics, the angular velocity of the satellite must be the same as that of a point on the earth and since the satellite is orbiting at a larger distance from the centre of rotation of the earth than the point on the surface of the earth, it must cover a greater amount of distance.
Now a satellite’s orbital period is guided by Kepler’s law according to which the orbital period varies in proportion to r1.5, where r is the orbital radius of the satellite. Therefore, we can conclude that the orbital velocity of the satellite is fixed and is greater than that of a point on the earth’s surface.
The orbital radius of the satellite (which we refer to as the height of the satellite above the earth’s surface), at which this geo-synchronisation occurs, is also fixed.
In order to avoid interference between satellites, it has been determined that an angular spread of at least 2 degrees must be maintained between two geosynchronous satellites.
This means that there can be a maximum of 360/2 or 180 such satellites in orbit at any one time, unless these satellites use different frequencies. International agreements have fixed that there are three satellite bands, which may be used. These three bands are “C“, “Ku” and “Ka” bands.
Of these, generally the “C” band is used for commercial traffic, but the “Ku” band and the “Ka” band are also used occasionally. Unfortunately, the “Ku” and the “Ka” bands are higher frequency bands and these tend to get absorbed by rain.
The frequency ranges and the frequencies allocated for up-linking and down-linking are given in Table 6.3. The bands used for government and military applications have, of course, been excluded from the table. The extended “C” band is used only in India. This has been done to meet government requirements.
A satellite transponder can typically encode 50 MHz; this could consist of a data stream of 50 Mbps or 800 voice channels each of 64 Kbps or combinations thereof. Typically, a satellite has up to 20 transponders, each capable of encoding 50 Mbps which may consist of combinations of voice and data.
VSAT:
VSATs or Very Small Aperture Terminals are a more recent development that permitted the development of low-cost micro stations. These low-cost micro stations are usually called VSAT stations.
These VSAT stations usually use 1 m antennae and output 1 watt of power typically, in the USA, while in India, the typical antennae being used are 1.8 m and the output is 2 watts. This is for earth stations for VSAT stations using TDM/TDMA technology.
For earth stations using SCPC technology, the antennae are 3.8 m in diameter and the power output is of the order of 5 watts. SCPC or Single Channel per Carrier uses either PAMA (Permanently Assigned Multiple Access) or DAMA (Demand Assigned Multiple Access). TDM/TDMA VSAT stations, because of this low amount of power, cannot talk to each other directly.
They usually have a large ground station reasonably close to the VSAT station where a large antenna is located. This station is usually called the hub. This large antenna is used to relay transmissions from these small VSAT stations to other VSAT stations via the satellite.
However, in case a hub is used, the transmission delay between two VSAT stations will be of the order of 0.5 seconds (since each round trip between a VSAT and the satellite through the hub will take approximately 250 ms and for transmission and this has to be done twice).
This propagation delay of 500 ms can be compared to the propagation delay of 3 µs per km in terrestrial microwave transmission and about 5 µs per km in cable transmission (copper as well as Fibre Optic).
VSATs, therefore, are cheap and small satellite earth stations, whose development has led to the reduction of cost of satellite earth stations and, as a result, greater usage of satellite transmission. In case of DAMA or PAMA, there is no hub and VSAT stations talk to each other directly through the satellite. The propagation delay, therefore, is halved.
Mode # 4. Satellite Networks:
Satellite-based WANs sometimes need satellite-based communications. It must be remembered that communications in WAN may use telephone connectivity and some of these telephone connectivity’s may be through satellite, but there may be other WANs which use direct satellite-based connectivity.
While multiple access channels are invariably present in LANs, there are also some WANs that are based on satellite-communication channels. They have their own special set of problems and in order to overcome them, several protocols have been introduced.
Communications satellites usually have about twelve transponders. Each transponder usually has a beam that covers some area of the earth below it.
This beam may be a wide beam covering a portion as much as 10,000 km wide or a spot beam covering a spot just 250 km wide. Satellite stations within this beam area can uplink frames to the satellite on the uplink frequency and the satellite can then rebroadcast them on the downlink frequency.
Naturally, the uplink and downlink frequencies are kept different so that the transponder does not go into oscillation. Satellites can do some on-board processing or merely rebroadcast the uplinked data. Satellites that do not do any on-board processing but merely rebroadcast the incoming data are usually named bent pipe satellites.
Each antenna can point to some specific area, transmit some frames and then point to some new area. While his “aiming of the antenna” is done electronically, it still takes some time in micro-seconds. The extent of time that the antenna is aimed at a particular area is called dwell time.
Naturally, this dwell time must be chosen very carefully because too much time may be wasted in aiming and aligning the antenna and too much dwell time will naturally waste a lot of time. Allocating the transponder channels may be an important design problem, but carrier sensing is impossible because there is a 270-msec propagation delay.
Therefore, when a station senses the state of a downlink channel, what it senses is what happened 270-msec ago. As a result of this, CSMA/CD protocols cannot be used.
Mode # 5. Selecting a Means of Communication:
Having discussed all the possibilities available for communications, one must look at the considerations to be taken into account by an organisation, for deciding on the optimum means of communication.
In other words, it could be constructive to go through an example of an industry where the issue is one of selecting a means of communication. We shall, therefore, discuss the data communication requirements for networks in India as relating to the organisation that we shall select for our example.
It must be remembered here that the figures of rates that have been assumed are purely indicative and are likely to keep on changing, depending or the state of the market, the status of developments in communications, the state of requirements for communications (including corporate requirement) and the government’s whims. Let us first talk of the scenario of an existing large company—TISCO.
The company has its headquarters in Jamshedpur, a medium-sized city in Bihar, close to its production facilities. Naturally, such a company will have its marketing headquarters in a large metropolitan city—in this case Kolkata. It also has marketing offices and stockyards in a large number of cities around the country. We will not attempt any corporate analysis of Information Technology requirements.
For the purpose of our discussions here we shall assume that there are requirements for high-volume data flow between the headquarters and the various branches. We shall also assume that each branch has its own local area network and that there is a requirement for connectivity between the server at headquarters and the clients at the branches. Admittedly, this assumed scenario is not complete.
Such questions, as the importance of the time factor in data transmission, need to be known and expressed in monetary terms. Also required will be the value chain between the marketing department of the steel company, its stockyards and its customers. This value chain analysis may not have been required a few years ago when the market was not competitive. But in today’s market situation it should be utilised.
However, it will be required when a total strategic analysis of the company’s Information Technology requirements are done. Here we are assuming that such connectivity is required and we are merely considering how to select the best form of connectivity.
For the purposes of this discussion, therefore, we shall assume that it is necessary for this company to have continuous connectivity between its headquarters and its branches. The issue to be decided is the kind of connectivity that should be utilised.
The considerations to be taken into account are:
1. Reliability of the method connectivity in relation to the level of reliability required
2. The volume of data be transmitted
3. The rate of data transmission
4. The likelihood and scope for future expansion
5. The limitations of the method of connectivity
6. The cost factor
The decision will obviously depend upon these considerations