Integrated Services Digital Network (ISDN)

ISDN services can be carried over existing telephone network infrastructure to terminal adapters (TAs) in the client machine. A common ISDN interface standard has a digital communications line consisting of three independent channels: two Bearer (B) channels, each at 64Kbit/s, and one Data (D) channel at 16Kbit/s. The D channel is used to carry io signalling and supervisory information to the network, while the В channels carry the data and can be linked to provide a 128Kbit/s data channel.

Wireless connections is The wireless alternatives come in two forms: satellite and cellular. Satellite systems require the use of a modem to maintain the upload. Downstream bandwidth is provided via a dedicated го satellite dish, connector hardware and proprietary software.

Cellular systems use assigned radio frequencies and are based around a network of transmitters that are arranged in a cellular network, much like cellular mobile phone systems.

The cable alternative.

Cable companies can also offer affordable broadband services over copper coaxial or fibre infrastructure networks. Theconnection is shared by several customers on a branch, so actual connection rates are variable, unlike ISDN and DSL.

Digital Subscriber Line (DSL).

 DSL technology capitalises on the existing network of copper infrastructure, but allows digital signals to be carried rather than analogue. It allows the full bandwidth of the copper twisted-pair telephone cabling to be utilised.

With splitter-based services, the DSL signal is pulled out from the phone line as it enters your premises and is wired separately to a DSL modem. This involves additional hardware and installation by the service provider at the customer site. The shielded option involves no installation, but the telephone company’s equipment and some of your so equipment might need upgrading.

With Asymmetric Digital Subscriber Line (ADSL), most of the duplex bandwidth is devoted to the downstream direction, with only a small proportion of bandwidth being available for upstream. Much Internet traffic through the client’s connection, such as Web browsing, downloads and video streaming, needs high downstream bandwidth, but user во requests and responses are less significant and therefore require less on the upstream. In addition, a small proportion of the downstream bandwidth can be devoted to voice rather than data,

 allowing you to hold phone conversations without requiring a separate line.

DSL-based services are a very low-cost option when compared to other solutions offering similar bandwidth, so they can го be made available to the customer at extremely competitive prices.

Task  4. Mark the following statements as True or False:

1. ISDN can only operate over a special digital telephone lin.

2. Two ISDN channels can be combined to give the user double the bandwidth.

3. Computers connected to a satellite system do not need a modem.

4. Cellular networks work in a similar way to mobile phone systems.

5. DSL systems require a special digital telephone line.

6. DSL systems use analogue signals.

7. You need a separate line to hold normal phone conversations on an ADSL system.

LESSON 27

The Types of Memory

Task  1. Read and translate the text:

Задание 1. Прочитайте и переведите текст

In all types of computer systems, from a small dedicated microprocessor system to the largest full size computer, memory plays a very essential part. The memory section of the microcomputer system serves the purpose of holding either information, that the computer will need, or information, that the computer has already generated, which will be utilized in the future. In other words, the memory of a microcomputer is used for storing the program and the data. There are two basic types of memory, namely read/write memory whose contents can be altered by writing new information into it and read-only memory (ROM) whose contents are fixed. Read/write memory is usually exclusively referred to as random-access memory (RAM) for historical reasons, although strictly speaking most modern read-only memories can also be accessed in a random order and therefore qualify for such a description. Random means that any one of the different memory locations can be written into or read from with equal ease and that it takes the same amount of time to address any one of the different memory locations within the same device. The semiconductor RAM memory is of the volatile type. That is, when power is removed, all information previously written in memory is lost. A problem develops because memory needs to have both non-volatility and read/write capability. At the time of this writing, there is not one RAM chip that has both of these desirable functions. The memory chips are either of the RAM family and have the read/write capability but are volatile or of the ROM family and have only the read capability but are non-volatile. So the read/write feature of the RAM makes it a very versatile memory device. However, its volatility presents a special problem. Recently, technology has advanced to the stage where nonvolatile RAM's can be made. These do not lose their information when the power in them is interrupted. The next generation of semiconductor RAM memories has advanced to higher density (more memory cells/chip) and higher operating frequencies. The essential difference as far as the user is concerned is hat ROM retains the information in it even without power, even if a program error or other fault causes the system to try and overwrite the contents of the ROM. It is therefore used mainly for storing fixed programs and constants. As the term ROM implies, this type of memory can only be read from and not written into by the user. Since the ROM has the feature of nonvolatility, it lends itself to applications such as dedicated subroutines including mathematical packages, monitor1programs, debugprograms and any program that has a fixed structure where there is no need to make changes to the program that resides in the ROM.

Task  2. Answer the following questions:

Задание 2. Ответьте на следующие вопросы:

1. Describe the recent level of technology development.

2. What types of memory do you know?

3. Read the text “The Types of Memory” and make a list of different types of a microcomputer memory.

Task 3. Make statements about these predictions for the next five years. Use the certainty expressions above.

 For example: All school children in my country will have mobile phones. - I think it’s unlikely that all school children will have mobile phones but it’s probable that many of the older pupils will have them.

1. ATM machines will use iris recognition rather than PIN numbers. You will get access to your account by looking at the machine.

2. People will vote in elections online.

3. Taxis will be robot-controlled.

4. TV journalists will be able to transmit what they see by using sensors in their optic nerves.

5. There will be more robots than people in developed countries.

6. Most computers will be voice-controlled.

7. Mobile phones will replace computers as the commonest way to access the Internet.

8. English will no longer be the commonest language for websites.

9. Email will be replaced by a voice-based system.

10. Computers will become more powerful.

Task  4. The Global Positioning System Link each set of sentences to make one sentence. You may omit, change or add words as required. Then form your sentences into two paragraphs to make a description of how the GPS works and its uses.

1. The GPS was developed by the US military. It was designed to pinpoint locations. The locations could be anywhere in the world.

2. It consists of 24 earth-orbiting satellites. The satellites are 17,000 kms. above the earth.

3. Each satellite broadcasts a coded radio signal. The signal indicates the time and the satellite’s exact position.

4. The satellites have atomic clocks. The clocks are accurate to one second every 70,000 years.

5. A GPS receiver contains a microprocessor. The microprocessor compares signals. The signals are from at least three satellites. The microprocessor calculates the latitude, longitude and altitude of the receiver.

LESSON 28

The Storage Medium

Task  1. Read and translate the text: