Networking at Home the Office and Globe

section 18

This lesson includes the following sections:

• Data Communication with Standard Telephone Lines and Modems

• Digital Telephone Lines

• Networks in the Home

• Networks commonly use dedicated media to transmit data. However, the public telephone system can also be used for data communications.

• Telephone system – called plain old telephone system (POTS)

• Standard phone lines transmit data much more slowly than network media, but devices such as modems make phone lines practical for data transmission over long distances.

• Many people and businesses use modems to exchange data, and to establish connections with office networks.

Data Communications over Standard Telephone Lines

• First modem for personal computers is developed by Hayes Microcomputer Products, Inc., in 1978.

• Most telephone lines attached to home and businesses are analog, not digital.

• Because PCs transmit and receive data in digital format, a device called a modem is needed to convert

digital data to analog format for transmission over phone lines.

Data Communications over Standard Telephone Lines - Modems

• When receiving data from another computer, the modem converts it from analog format to digital format.

• Modem (modulator-demodulator)•Modulation phase: turn the digital signals into analog signals•Demodulation phase: analog signals are converted into digital signals

Data Communications over Standard Telephone Lines - Modems (Cont.)

The analog signal(audible)

is sent throughtelephone lines.

MODEM MODEM 011010001011010001

Digital signal Digital signal

This modem convertsdigital to analog.

This modem convertsanalog to digital.

• Modem can be•A simple circuit board, i.e., an expansion card•An external device that plugs into a serial port•A PC Card (previously called PCMCIA Card)•Software model: replace most the hardware in a hardware modem with an application program.

•Easy upgrade

• Current model can make phone call while connecting to the internet

Data Communications over Standard Telephone Lines - Choosing a Modem

Internal Modem External

Modem

在這部外接數據機的背面，是將它連接到電腦、電話插座、與電話的接線。

內接數據機插入電腦的擴充槽之一

這部筆記型電腦配備有 PC卡型式的數據機。這數據機附有行動電話，所以使用者不需使用電話線就可以登入網路。

• Another type of modem that is increasing in use is the cable modem.

•It connects the computer to the internet through the cable TV system.

Data Communications over Standard Telephone Lines - Choosing a Modem

• Transmission speed - the speed at which the modem sends data - which is measured in bits per second.

• Data compression, the technology the modem uses to shrink data so it can be transmitted faster.

• Error correction, the method the modem uses to ensure data is sent and received without errors.

• Internal versus external, which describes whether or not the modem fits inside the PC case.

Data Communications over Standard Telephone Lines - Choosing a Modem

• Sending a file to another computer is called uploading.

• Receiving a file from another computer is called downloading.

Data Communications over Standard Telephone Lines - Uses for a Modem

Modems are primarily used for file transfer, or sending files to a remote computer

The files are transmitted by file transfer protocol (FTP)

• Full-duplex: data can be travel in both directions at the same time.

• Half-deplex: data can be sent in both directions but only one direction at a time.

Data Communications over Standard Telephone Lines - Uses for a Modem

Modem communication

大多數的現代數據機與網路連線是全雙工的。但偶爾也會使用半雙工傳輸來連接電腦。

• Telephone lines has throughput limitation.

• Thus, telephone companies are now installing digital telephone lines, which are dedicated to

transmitting data in digital format.

• Digital phone lines transmit data at much higher speeds than standard analog phone lines.

• Often, data travels across analog lines and digital lines. In such cases, data may need to be converted from one format to another multiple times before reaching its destination. (As shown in Next Figure)

Using Digital Telephone Lines

傳送資料時，電腦傳送數位訊號到數據機，然後它將類比訊號傳送到轉換站機，然後它又傳送數位訊號到另一個轉換站。

然後這程序顛倒，直到它抵達接收的電腦為止。

• The transformation from analog to digital lines will affect most users in:

•Need a different phone – a digital one

•Does not need a modem to send data. Instead, you need an adapter

•Can send data much more quickly

Using Digital Telephone Lines (Cont.)

利用地區數位電話線，由傳送電腦到接收電腦的資料可保持為數位格式。

• ISDN, T1, and T3

• DSL

• ATM

• Cable Modem

The most commonly used digital telephone services are:

Using Digital Telephone Lines - Common Digital Services

Using Digital Telephone Lines – ISDN, T1, and T3

• Integrated Services Digital Network (ISDN) is a system that replaces analog phone services with digital services.

• Basic rate ISDN (BRI) offers three channels on one phone line: two 64 or 56 Kbps channels for data and one 19Kbps channel for control. BRI transmits data up to 128 Kbps.

• Primary rate ISDN (PRI) offers 24 channels at 64 Kbps, (In the U.S.) a total bandwidth of 1.544 Mbps. This is T1 service. In Europe, PRI service provides 31 data channels.

Using Digital Telephone Lines – ISDN, T1, and T3 (Cont.)

• Using even more channels, T3 service offers up to 672 channels of 64 Kbps each for a total of 44.736 Mbps.

• Digital Subscriber Line (DSL) service is outpacing ISDN services since it is less expensive in terms of hardware, setup, and monthly costs.

• Two key points that make DSL attractive are its speed and medium.

•Standard DSL offers speeds of 52 Mbps theoretically.•DSL can use POTS lines, the standard copper wire used for telephone communications.

Using Digital Telephone Lines - DSL Technologies

• Several types of DSL service are available, reaching transmission speeds up to 51.84 Mbps.

•Asymmetric DSL (ADSL)

•Rate adaptive DSL (RADSL)

•High-bit-rate DSL (HDSL)

•ISDN DSL (IDSL)

•Symmetric DSL (SDSL)

•Very-high-bit-rate DSL (VDSL)

Using Digital Telephone Lines - DSL Technologies (Cont.)

• Asynchronous transfer mode (ATM) digital service is offered as a high-bandwidth, efficient means for transferring multimedia content, data, and voice over phone lines.

• Initially, ATM is proposed for different kinds of data (voice, data, video) on the telephone system. However, it can also implemented on computer LANs and WANs.

• Some types of ATM service can reach transmission speeds of 10,000 Mbps.

Using Digital Telephone Lines - ATM

• Cable modems allow users to connect their PCs to the Internet via the local cable television system.

• Cable companies offer Internet service by combining television and data signals and distributing them over the cable system.

• Cable modem service can achieve speeds of 27 Mbps.

Using Digital Telephone Lines - Cable Modem Connections

纜線數據機系統結合一般的有線電視網路，與連接到網際網路的廣域網路

• Because more homes now have multiple computers, home networks are gaining in popularity. Home networks offer the same advantages to home users as to a business.

• Home networks are typically based on existing telephone or wireless technologies.

• Popular PC operating systems, such as Windows and the Mac OS, provide simple networking tools that are adequate for running a home network.

Networks in the Home

有數百萬的家庭目前有多部 PCs。新的科技讓家庭可以在家中使用現有的媒體 (例如電話線 )與現成的作業系統 (例如視

窗 )而建立網路

• Explain how computer data travels over telephone lines.

• Explain a modem's function.

• List four features you should consider when evaluating modems.

• Differentiate four types of digital telephone services.

• Describe one potential use for a home network.

.

Section 7b review