The international project:

Measurement of the signal delay time in international communications

Participants:

1. Protopopov Alex, 10^th year student; Suwon-City, Republic of Korea project leader;

2. Protopopov V. V., Suwon-City, Republic of Korea scientific advisor;

3. Ivushkin I. Y., Moscow, Russia project engineer.

Aim of the project:

Theoretical and experimental research on the sources and real values of the time delays in signal propagation through international phone lines.

Abstract:

Everyday experience tells us that there are noticeable delay times in signal propagation from one phone to another, especially in mobile systems. Sometimes those delays become really annoying, hampering real-time communication between people throughout the globe. We decided to clarify possible reasons for these delays and to measure their values. Our international team was composed of people situated on the different parts of the globe: in Europe (Moscow) and in Asia (Suwon). That large lateral separation on the globe made it possible to measure delay times accurately and easily, using very primitive equipment, available for everyone. However, despite the seeming simplicity of the experiments deep theoretical understanding of the nature of telephony turned out to be necessary to successfully accomplish the experiment. The results of our measurements were documented in photographs, thus leaving no doubts in the reality of the measurements.

1. Introduction

Speaking with people via mobile phones, you often mark annoying delays between what you have just said and the answer of your partner. Intuitively, you begin doing long pauses between clauses, but it does not save the situation: your companion becomes more and more irritated, so that finally good relations me be broken. Some people do not understand the nature of these delays, attributing them uniquely to your arrogance or poor knowledge of language. Most often this happens to Koreans who cannot wait for long, and quickly become impatient, asking: «Can you speak English?» or «Are you drunk?». Thus, the role of the delays in phone communications is very important. Why does it happen and what are the typical values of the delays? We shall answer these questions in our research.

2. The nature of the delays

First let us outline the basics of mobile communication. In greatly simplified scheme the mobile communication system works as it is shown in Fig.1.

Fig.1.

Initially, the voice signal enters the microphone of your phone, generating the electrical signal. This signal is being converted into the code, and the phone sends a radio signal to the nearest server. The server looks like a big computer, and it handles the great number of incoming and outgoing communications. Obviously, it is impossible to process all of them simultaneously. Therefore, there is a queue at the input of the processor which introduce the first substantial delay T_server. The first server determines the target phone number to which you want to connect, and direct your message to another server nearest to the target phone. This second server also introduces a delay. Also, between the two servers there is a free-space propagation path L. Thus, we can expect the overall time delay

where c = 300,000 km/s is the speed of light. What could be the value of the free-space propagation delay within a city? With L » 10 km we have:

Practically, this is an infinitesimal value, which can be disregarded.

The power of a single server transmitter cannot be high because of the safety reasons. It is only sufficient to cover the distances about several city districts. Therefore, to connect international consumers it is necessary to use more powerful stations, and to combine them in a chain, using radio or cable links with intermediate amplifiers (Fig.2). There are no platforms in the ocean, so that to cover the oceans people use cables. Today, those are even not cables but optical fibers with quantum amplifiers of an extremely complicated physics and design.

(a)

(b)

Fig.2. (a) Radio chain; (b) cable chain.

Each amplifier also introduces some delay T_amplifier, so that eventually the delay time in the international communications can be expected to be much larger than inside cities:

where N is the number of amplifiers. The free-space propagation delay in international communications is also orders of magnitude larger than within cities. For example, the distance between the Moscow and the Seoul is of about 6,500 km over the shortest arc of the globe. Thus,

3. How to measure the delays?

The simplest conceivable idea is portrayed in Fig.3. At the one end of the communication link the telephone of a handset is connected electrically to the oscilloscope so that the coming audio signal is displayed at the cathode-ray tube of the oscilloscope as a short burst. At the other end there is no need in action at all: just put the handset into some isolated bowl, making audio connection between the telephone and the microphone. Then, making loud tap or click at the microphone at the left end of the communication link, we generate the short electrical signal which makes first (left) mark at the screen of the oscilloscope. This pulse travels to the right end of the link. Having reached the second handset, the electrical signal first transforms to a sound, then it is reflected from the walls of a bowl, transforms to an electrical signal again, and comes back to the oscilloscope, producing short second (right) mark at the screen. Thus, we seem to be able to easily measure the time interval between the first click and the returned signal. This would be the delay time.

But alas! If someone thinks he can use this method in practice then he is doomed: it will not work. Why? Because the best people of the world worked on the telephone for almost a century, having developed it to a state of nearly a perfect device. This needs some additional explanation.

Did you ever think how is it possible to send and receive telephone signals through only one wire? Did you ever wonder why during the WWII soldiers used to put only single wire cable for telephone connections? The answer is simple: because the telephone and the microphone of a handset are connected to the same single wire. But if it were so then we would hear our own voice in the telephone while speaking. How it happens that we don’t? Look at the Fig.4. The microphone and the telephone are connected through the transformer (in the early versions of the handsets) which blocks coming the signal from the microphone to the telephone. And visa-versa: the audio signal from the telephone does not generate electrical signal, coming back to the line from the microphone. It means that the bowl in Fig.3 would not create any substantial electrical signal back into the line.

In order to overcome this obstacle we have to use four handsets as it is shown in Fig.5.

At the active end of the communication link (the right end in Fig.5) someone clicks at the microphone of the first handset, and the signals departs in its long way to the other end of the link at the other continent. At the idle end (the left one in Fig.5) the two handsets are tightened together microphone-to-telephone. Then their inner negative feedback does not affect the returning signal, and it comes safely to another handset at the active end of the link. Here this returned signal has to be inputted into the oscilloscope, creating the second mark at its screen. The space between these marks defines the delay time.

We need four telephones, an oscilloscope, and some simplest electronics to connect the telephone to the oscilloscope. This connection can be done either by disassembling the handset and soldering wires directly to the telephone, or by attaching additional microphone to the telephone of a handset. We chose the second variant since it would leave the handset intact, and we did have a spare microphone. The fait of this spare microphone is very instructive, being worth mentioning here.

This summer we spent at Jeju island that is a beautiful place for recreation. And everything was going well until a wave covered the mobile phone of our project leader’s mother. Remember forever as the instruction: if that happens immediately disconnect the battery. Regretfully, we were not wise enough to do so. Very quickly the main chains were dissolved by electrolyze of a salt water, and the mobile phone deceased. The only useful things that we have gotten were the microphone and two telephones. This microphone, the so-called electreth microphone, is really a masterpiece of the technology: compact and very efficient (Fig.6). It is shown together with the needle tag to better understand its dimensions. Below the round cap of the microphone a thermo-contracting plastic tubing is seen which protects gentle leads of the microphone from damage.

Initially we measured the delay time within the Suwon city only. The experimental set-up together with the resultant oscilloscope screen-shot is portrayed in Fig.7.

The read-out microphone attached to the mobile phone by a sticking tape is seen in the lower left corner of the photograph. It was connected to the oscilloscope through simple amplifier in order to enhance the overall sensitivity. According to the photograph, the delay time within the Suwon city was about 0.4 s. This value must not be considered as something constant because it depends on the provider (KTF in our case). Obviously, this delay is introduced mostly by digital processing since the propagation delay in this case is only several microseconds.

Finally we present the result obtained on the line between the Moscow and the Suwon (Fig.8). The parameters displayed in the screen-shot were explained above.

Thus, the delay time in this case was recorded equal to 1.1 s, i.e., roughly three times longer than within the city. Again the main part of this delay is composed of the processing time and the so-called group delay within the electronic circuits.

We performed a research on the possible sources of the delays in the telephone communication links, and have measured the real values of the delay times within a local city and on the international communication line between the two distant parts of the world.

The local delay time within the Suwon city (Republic of Korea) was measured to be 0.4 s . On the international line between the Moscow (Russia) and the Suwon it occurred to be roughly three times longer, i.e., 1.1 s.