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Sci Tech
Callers to the U.S. benefit the most
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Perceivable latency is seen with Internet telephony. It is 250-700 milliseconds when transported via fibre and 700-750 when transmitted via satellite. This makes the quality below par to PSTN which has a latency less than 70 milliseconds, says R. Prasad.
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PHOTO.M.MOORTHY
NOT LONG ago Sunday mornings were solely reserved for one important event. No, it was not the Mahabharat serial on the TV. It was more important than that and every member of the family looked forward to that hour. Well you guessed it right. It was a call from the son or daughter from the U.S. It made more sense as it always worked out cheaper to make a call from, say, the U.S. to India than the other way round.
The advent of Internet and its e-mail and chat facility has made this waiting less agonising. But these two have their own drawbacks. One longed to hear the voice and these applications could not substitute voice and take its pride of place. No more.
The Internet has once again brought about biggest change in the way people especially Indian families communicated with their kith living in distant lands. But this time around it was voice and not data communication. Internet telephony has for the first time made it affordable to make calls to Europe and the U.S. Net result — Sunday mornings are slowly losing their charm. But are they really? Can the Internet telephony that came into being from April 1 bring about a reversal in the call originating trends? Quite unlikely according to R. Ramaraj, Managing Director, Satyam Infoway Limited (Sify). "Making a call from the U.S. would still be cheaper compared to a Net telephony call from India. Net telephony has nevertheless reduced the margin in the rates substantially," he said.
While dwelling on the cost of a call, it is pertinent to add that Net telephone rates would basically depend on to where the call is being made. One could expect the cheapest price if the call is made to the U.S. Brace yourself, if on the other hand the call is made to any of our neighbouring countries or to countries like Singapore or Hong Kong the rates would be higher compared to the U.S. Puzzled? Well the rates are not determined by just the distance from India to say, Pakistan vis-à-vis the U.S.
Most of the well-established International Net telephone service providers are located in the U.S. Hence all calls first reach the U.S. before reaching the country of destination. If the destination of the call is the U.S., then the presence of several players there brings about a competition to carry the call to the final destination within the country. "Thus when the call is handed over to the competitive local exchange carrier (CLEC) in the U.S. the prices will be far cheaper due to competition," said Rustom Irani, Chief Technology Officer, Satyam Infoway Limited.
Now compare this to a call made to Pakistan. If the service provider does not have its own infrastructure in place then it has to seek the service of another service provider who can carry the call to Pakistan. This does not come free. A fee has to be paid to the service provider for carrying the call from the U.S. to the final destination. And in case the country where the call is terminating has a monopoly player then it makes it all the more costly as the tariff fixed will be administered. Unlike the conventional telephone service (PSTN) where 64 Kbytes is dedicated to voice, the Net telephony digitises and sends it to the destination as packets via the Internet where they get reassembled at the destination. "There is no bandwidth dedication when it is sent over the Internet. Hence there is no guarantee of quality," Mr. Rustom said. The voice competes with data like e-mail, chat and the like for bandwidth and there is no priority accorded for voice. However, this is not the case when the service provider has a dedicated backbone network and do not depend on the Internet to carry voice. These are called `managed voice telephony.'
The biggest challenge to voice sent over Internet is the problem of latency. Latency is the amount of time taken to digitise, packetise and make them ready for transport, and the time taken for transporting the packets from one end to the other. This is not peculiar to voice alone but is an inherent feature of the Internet. However, the latency is little realised or pose little problem when data unlike voice is transferred. "Latency is not noticed when a person browses the net but will immediately realise it in the case of voice," said P. Badri Narayanan, Senior Manager (Technology) Sify.
Amount of latency is dependent on various factors. The time when the call is made (peak/ no peak hour) and the distance the voice has to travel before reaching the final destination. The distance becomes crucial, as there is no dedicated bandwidth available for voice on the Internet. However, the amount of bandwidth between the U.S. and the country of destination will also determine the amount of latency. For instance, the bandwidth between Singapore and the U.S. is large and hence would not have a telling effect on the latency while this may not be the case with other countries.
In the case of satellite link the amount of latency depends on the number of hops before reaching the destination. "We have kept the number of hops to absolute minimum of maybe one. We may send the voice from the satellite to the destination either through optic fibre or through another hop to a satellite," Mr. Rustom said. Sify has 13 satellite gateways. "Though the latency is higher with satellites, we will be able to better manage the service and engineer the traffic as we own the gateways," stressed Mr. Rustom.
Latency is typically in the range of 250 milliseconds but can go as high as 700 milliseconds when transported via fibre and 700-750 when transmitted via satellite. Latency is 100-200 milliseconds in the case of managed network. It is less than 70 milliseconds in the case of PSTN and thus escaping human perception. The Mean Opinion Score in the case of PSTN is above 4 (in a scale of 1-5) while it is 2.5-3 in the case of VOIP. But what frustrates the user is not the latency per se but continuous variation in latency. Typically this could lead to a situation where the voice quality does not measure up to the toll quality standards. Even at 150-400 milliseconds latency there could be a distortion perceptible by the listener. Apart from latency, Internet telephony faces another problem of `jitters'. Jitters result due to latency variation.
Connectionless scheme of Internet telephony does not guarantee all packets reaching the destination and in a proper sequence. Certain techniques are however employed to rearrange the packets in the correct sequence that adds up to the latency. `Out of sequence' phenomenon is seldom encountered in the case of data. This is by virtue of the Transmission Controlled Protocol (TCP) followed in the case of data. Here a connection is identified both at the server and client end and packets are sent only when acknowledgement of receipt of the previous packet is received. So the connection is tracked in the case of data and this ensures that no packet is received out of sequence or is lost.
"Load will be higher and delay will also creep in if packets are sent on receiving an acknowledgement. This will only increase the latency," Mr. Rustom explained. However, managed networks can work on connection-oriented transmission (as in the case of data), as it is a dedicated network for voice.
Mr. Rustom likens the Net telephony through managed network to an air-conditioned compartment and using the Internet to an unreserved compartment. "You pay a price for the quality of service," he concludes.
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