Focused Practice

I. Answer the following questions:

1. How great was the anticipation for 11g products?

2. When did vendors begin releasing 11g products?

3. Where do the new speeds allow 11g networks to go?

4. What do the new products also provide?

5. How do they do it?

6. What became a requirement for any new product seeking certification from the Wi-Fi Alliance industry group?

7. What are all the vendors planning?

8. Is Windows XP the only version of Windows that has built-in wireless support?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: WPA capability.

SECTION VII

Special technical decisions in tv, telephony, encrypton, nanotechnology

Unit 61

Grammar: The Participle. The Complex Sentence

Word List:

1. adaptive hybrid	адаптивный переходник
2. intermittent	чередующийся, прерывистый
3. adaptive echo cancellation	адаптивное уменьшение отраженного сигнала
4. echo signal	отраженный сигнал
5. speaker	абонент
6. burst	короткий сигнал, треск (помеха)
7. bump	бросок тока, всплеск
8. a 20-tap adaptive hybrid	адаптивное 20-контактное устройство для подключения абонентов
9. to restabilize	выходить из равновесия, возбуждаться
10. chirp	треск, чириканье (помеха)

Dynamics of an Adaptive Hybrid

A device called a “4:2 hybrid” is used in telephone systems to transform the four-wire long distance receive and transmit lines to and from a two-wire local line. An ideal hybrid would move all the incoming signals from the four-wire receive line to the two-wire local line, and simultaneously move the outgoing signals from the two-wire local line to the four-wire transmit line. In a real device, however, some of the energy in the incoming line will inevitably leak into the outgoing line. One solution, called adaptive echo cancellation, uses an adaptive filter to match, or identify, the dynamics of the leakage path. When the near end speaker is silent, and when the adaptive filter has matched the transfer function of the hybrid the echo of the far end speech is canceled. The adaptive solution to the hybrid problem is well established, and has been used successfully in a wide variety of situations.

In certain situations, however, an intermittent “bursting” or “chirping’ or “bumping” misbehavior arises. This was first brought to our attention by a series of laboratory tests at Tellabs Inc. which were designed to investigate the adaptive hybrid system when the received signal is significantly correlated with the near end signal within the time window of the adaptive filter. Such correlations become increasingly likely as the adaptive hybrid is used on communications lines of shorter length. The real time tests utilized a 20-tap adaptive hybrid at the near end of the line and a simple (nonadaptive) hybrid at the far end, giving 6-dB attenuation around the loop. Independent narrow-band modern signals were injected at each end. Long periods of close match between the output of the adaptive hybrid and the echo path were observed, and the hybrid appeared to be functioning well. Suddenly and with no warning, the system would begin “singing”, the signals degenerated into wild oscillation, and then the system would quickly restabilize. These bursts were intermittent and had no apparent cause.