Учебное пособие состоит из двух частей и англо-русского словаря. Материалом для пособия послужило американское издание для изучающих компьютер P. Norton "Introduction to Computers"


UNIT 7. Networks and the Internet



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UNIT 7. Networks and the Internet
Networking at Home and Around the World


Data Communications Over Standard Telephone Lines


Networks (especially the Internet and large WANs, with nodes spread over a large geographic area) commonly transmit data across telephone lines. Although telephone companies are offering more digital lines (which are better suited to data transmission), most homes and businesses are still served by analog telephone lines. To transfer digital data over analog telephone lines, computers must use modems. When a computer sends data, its modem translates digital data into analog signals for transmission over standard telephone lines. At the receiving end, the computer's modem converts the analog signals back into digital data. The most important factors to consider when choosing a modem are internal versus external, transmission speed, data compression, and error correction. Modem transmission speeds are measured in bits per second (bps). Currently, the preferred standards for modems are 56.6 Kbps and higher.
Using digital connections, business networks and homes can transmit data many times faster than is possible over standard telephone lines. In areas where digital connection is not possible, homes and businesses are connected with standard analog lines, but high-speed digital lines are run between the telephone company's switching stations. The most popular digital telephone services are integrated services digital network (ISDN), T1, T3, and DSL. They offer faster data transfer rates and higher bandwidths than standard telephone lines.

Networks in the Home


New technologies enable homeowners to set up home networks to connect multiple computers. Home networks typically operate on existing media such as the home's telephone lines or cable wiring.


ISDN, T1, and T3

Many different kinds of digital services are offered by the phone companies. Some of the best known are called ISDN, T1, and T3. Of these, ISDN (integrated-service digital network) received the most attention in the past few years because it was the most affordable and the one most likely to make its way into homes and small businesses. ISDN, which stands for integrated services digital network, is a system that replaces all analog services with digital services.
When most people talk about ISDN, they are referring to a particular level of service called BRI (basic rate ISDN). BRI provides three communication channels on one line – two 64 or 56 Kbps data channels and one 19 Kbps channel that is used to set up and control calls. The two data channels can carry voice or data and they can be used simultaneously, so you can transmit data and carry on a conversation at the same time on the same line. Also, the channels can be combined so that BRI service can be used to transmit data at rates as high as 128 Kbps without compression.
Some telephone companies now offer BRI service in some locations – especially in large metropolitan areas. Installation can be expensive, but the cost of service is slowly coming down to compete with the basic rates offered for analog lines.
A higher level of service for ISDN is called primary rate ISDN, or PRI. In the United States, PRI provides twenty-four channels at 64 Kbps each, a total bandwidth of 1.544 Mbps. This level of bandwidth is also known as T1 service. In Europe, PRI service provides thirty-one data channels.
Although it is not specified by the ISDN standard, it is also possible to purchase lines from telephone companies that offer even more bandwidth. For example, a T3 line offers 672 channels of 64 Kbps each (plus control lines) for a total of 44.736 Mbps. Many telephone companies also offer services between the levels of BRI and PRI. Different businesses have all kinds of different needs for bandwidth, so telephone companies try to be as flexible as possible in their offerings.


DSL Technologies

One of the latest developments in connectivity is digital subscriber line (DSL). DSL is rapidly outpacing ISDN in areas where DSL is available because it is typically less expensive in terms of hardware, setup, and monthly costs. In fact, many local telephone companies are opting to develop DSL in their markets and are foregoing ISDN altogether.
Two key points that make DSL so attractive are its speed and its medium. DSL can achieve theoretical speeds up to 52 Mbps, a huge speed advantage over the fastest dial-up modems or even some digital connections. The second advantage is that DSL can use POTS lines, the standard copper wire used for telephone communications in most homes and businesses today. The typical home computer user can connect to the Internet or a private network through DSL at high transmission speeds, often for a cost that is competitive with standard dial-up connection. There are several types of DSL available in different markets, each offering different capabilities and rates:
Asymmetrical DSL (ADSL) uses discrete multitone (DMT) or carrierless amplitude phase (CAP) modulation.
Rate adaptive DSL (RADSL) adjusts the speed based on signal quality.
High-bit-rate DSL (HDSL) allows the telephone company to provide T1 speeds at a lower cost than T1 but requires two wire pairs.
ISDN DSL (IDSL) uses existing ISDN facilities.
Symmetric DSL (SDSL), a version of HDSL, uses a single pair of wires and provides slower transfer rates than HDSL.
Very-high-bit-rate DSL (VDSL) provides a high bandwidth with a commensurate cost and is geared primarily toward LAN and WAN connectivity.
The actual performance you can achieve with DSL depends on the type of DSL and the distance between the DSL modem and the telephone company's switch.


Avoiding a Tangled Mess

You may think that setting up a network is simple once you have selected the type of network, server, operating system, and other options you need. However, that is only the beginning.
Two of the most time-consuming and costly chores in networking involve choosing the type of wiring that will connect the nodes and servers, and deciding on the most efficient means of running the cable. These issues can be so confusing and costly that many companies have been created to deal with network cabling.
To show how troublesome network cabling issues can become, suppose that you have a warehouse that is 100 feet long and you want to install a network of eleven computers, all lined against one wall. This arrangement places the PCs 10 feet apart, which is the perfect spacing for cubicles.
If you want to use a bus topology, a terminator is placed on the first and last PCs, and ten 10-foot cables are run from one computer to the next. This method means that 100 feet of cable must be purchased and installed.
If you choose a ring or star topology for your network, however, your cabling requirements may increase dramatically. But the additional cabling and equipment (such as a hub) may make your network easier to manage. For instance, it can be easier to add new users to a star network than to a bus network. In such cases, the additional cabling requirements may well be worth the time and expense. Depending on the specific implementation you choose (that is, where you place the hub within the room), your star network may require two or three times more cable than a basic bus network.
There are additional factors to consider. Here are just a few questions networking experts must answer:
Is the network being installed in an existing building, or is it being added to a building under construction? It is always easier to work within a building under construction than to have to drill through walls and run cable in an existing building. For example, many historical buildings such as courthouses have brick walls internally and restrictions (by historical societies) on work that can be done. In such cases, creative alternatives have to be considered.
Can cabling run through the walls or must it run through ceilings? If the cabling will be run through the ceiling, the price of the material triples. Why? Fire codes prevent running ordinary networking cable in places where it can catch fire without being readily seen because of the noxious fumes given off during the burning.
Will conduit be required to encase the cabling? Will you need to use wall plates, wiring closets, and the like?
How much cabling will be exposed? Are there danger zones where people may trip over or drop items on the cabling or connectors?
What if the company wants to be able to rearrange itself on short notice, moving offices, desks, and computers at will? This option can immediately rule out star and bus topologies because the network cannot be brought down every time someone changes an office.
The picture is further complicated when the network must span multiple buildings or several floors of a building.




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