chpt16a communicating over phone lines

8/9/2019 Chpt16a Communicating Over Phone Lines

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A+ Certification for PC Technician

Chapter 16Chapter 16Communicating Over Phone Lines


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Ver 1.0 10 Updated May 2001 Slide 2Chapter 16 Communicating Over Phone Line

ModemModem

What does modem stands for?

Modulator Demodulator

What is Modulation?

Sending modem converting digitalcomputer signals to analog signals thatcan be sent over a phone line.

What is Demodulation?

Receiving modem converting analogsignals back into digital data.

How 2 Modems Communicate?

CPU sent digital information, in theform of ones and zeros, to the modem.The modem then analyzes thisinformation and converts it to analogsignals, that can be sent over a phoneline. Another modem then receivesthese signals, converts them back intodigital data, and sends the data to thereceiving CPU.

Convert digital Signal

to Analog Signal and vice

versa

Analog SignalDigital Signal

Telephone Line Computer


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UARTUART

What is UART stand for?

Universal Asynchronous ReceiverTransmitter

What does UART do?

Handles asynchronous serialcommunication. Every computer contains

a UART to manage the serial ports.

Which UART is currently implementedin most PC?

16550 UART

What is the Maximum speed supported?

115kbps. If the speed is set too highfor the UART, communication will beunreliable and performance may beworse than at a lower speed.

How to check UART Speed?

1. Start/Settings/Control Panel2. Select 'Modems' in Control Panel

3. Select the Diagnostics tab

4. Select the COM port that yourModem is attached to

5. Press the 'More Info...' button


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Asynchronous and Synchronous Data TransferAsynchronous and Synchronous Data Transfer

What is Asynchronous and Synchronous

data transfer?Methods of sending data over a phoneline

How does Synchronous datatransmission works?Data is sent via a bit-stream. That is,as a group of characters in a singlestream

How is it done?Modems gather groups of charactersinto a buffer, where they are prepared

to be sent as such a stream. Beforesending data, modems synchronize witheach other by sending specialcharacters, called synchronization, orsyn, characters.

How does Asynchronous data

transmission works?Data is coded into a series of pulses,including a start bit and a stop bit. Astart bit is sent by the sending modemto inform the receiving modem that acharacter is to be sent. The data isthen sent, followed by a stop bitsignaling that the transfer is complete.

01 0 0 1 0

Startbit

Stop

bit

Asynchronous Data Transfer

0 0


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Transmission ModesTransmission Modes

What are the 3 data transmission

modes?Simplex, Half-duplex and Full-duplex

What is Simplex?

Data flows in one direction only

How does data flows in Half-duplex?

Data flows in both directions, but onlyin one direction at a time.

How does data flows in Full-duplex?

Allows simultaneous transmission inboth directions. Done by transmittingdata at one frequency in one directionand another frequency in the oppositedirection, by using 4-wire lines,

Modem uses which type oftransmission?

Most modems sold today are full-duplex.


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Modem SpeedModem Speed

What is the transmission speed

measured in?Kbps (kilo-bits-per-second)

How do 2 modem established thetransfer speed?

Handshaking. The 2 modem negotiate

the highest supported speed by the 2units.

What are the handshaking standards?

1. v.34 for speed up to 36kbps

2. K56flex and X2 for speed up to56kbps

3. v.90 for speed up to to 56kbps(replace both K56flex and X2,which are proprietary.


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Port SpeedPort Speed

What is Port Speed?

Speed between the PCs COM port andthe Modem. Each Port has a speed thatcan be set independently. The Windows95/98 default speed for all Serial Portsis just 9,600 bit per second.

What happen if the Port Speed is settoo High?

A busy CPU may not be able to emptythe Port's data buffer before new dataarrives there, and data will getsdamaged.

What happen if the Port Speed is settoo Low?

Whole data transfer process will slowdown

What is the recommended Port Speed?

9,600 modem: select 9,600 bps14,400 modem: select 19,200 bps19,200 modem: select 38,400 bps28,800 modem: select 57,600 bps33,600 modem: select 57,600 bps ortry 115,200 bpsfaster modem: select 115,200 bps or

230,400 bps

How to Change Port Speed?1. Click the My Computer icon on the

Desktop.2. Click the Dial-Up Networking icon.

3. Right click the icon for yourInternet connection and thenchoose properties.

4. In the General tab click Configure.5. Change the speed in the Maximum

speed listbox.


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Flow Control, Data CompressionFlow Control, Data Compression

Why modem requires Flow Control?

Often, one modem in a connection iscapable of sending data much fasterthan the other can receive. Flow controlallows the receiving modem to tell theother to pause while it catches up.

What are the 2 types of flow control?1. Software (XON/XOFF)

2. Hardware (RTS/CTS)

Which is preferred?

Hardware. Is faster and much morereliable than software flow control.

What is Data compression in data

transmission?Data compression allows modems toincrease throughput (more data to besend) without increasing thetransmission speed.

Which are the 2 compression protocols?

1. MNP-5, compression ratio of 2:1,2. v.42bis, compression ratio of 4:1

How modem determines whichcompression protocols to use?Compression can occur only when the

two communicating modems support thesame compression techniques. If bothsupports more than one type ofcompression, the modems will use themost powerful technique common toboth.


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ModemModem

What are the types of Modem?

1. External2. Internal

Where do you install a internal modem?Into the expansion slot. Currently,modem are PCI type.

Where do you install a external modem?Via a serial cable to either of the RS-232 port (com port).How is the modem connected to thetelephone line?RJ11 (registered jack 11)

What are the indicator lights on theexternal modem used for?Troubleshooting

What the signals provided?

HS (high speed) show modem at highest speed

AA (Auto answer) Automatically answer the phone

CD (Carrier detect) remote connection is in effect

OH (off hook) Modem is using the phone

RD (Receive data) Modem is receiving data

SD (Send data) Modem is sending data

TR (Terminal ready) PC is able to communicate

MR (Modem ready) Modem is ready to communicate


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Installing ModemInstalling Modem

What are the likely problem with

Internal modem?Conflict of IRQ.

How it happened?

Internal modem may set to COM 2. Inthis case, the COM 2 on the

motherboard BIOS would have to beDisabled.

How to test the modem after

installation? Control Panel Modem Applet Diagnostics Click on the Modem Click More info


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Common AT CommandsCommon AT Commands

Commands Functions

AT Your modem should respond with an OK. Your modem is plugged

in and turned on.

ATA Answer in coming call

ATL0 Set speak volume level. ATL0 is the lowest; ATL1 is the default;

ATL2 is medium and ATL3 is high

ATD Deal a number eg ATDT2222222.

T for Tone. You may include 9 for outside line.

ATH Hangs the modem

ATZ Reset the modem

AT&T1 Local analogue loop back test. Test modem and local port.

AT&T (6,7) Remote digital loop back test with remote modem.

What is the use of AT commands?

To control and configure a modem.

Where can you use the AT commands?

Terminal emulation software, eg Hyper

Terminal


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Testing ModemTesting Modem

How to test modem?

Loopback test. There are Analog andDigital Loopback test.

What is Analog Loopback test?

Modem self-test. Data from thekeyboard or an internal test pattern is

sent to the modem's transmitter,turned into analog form, looped back toits receiver and converted back intodigital form.

What is Digital Loopback test?

Checks the modem's RS-232 interfaceand the cable that connects thecomputer and the modem. The modemreceives data (in the form of digitalsignals) from the computer immediatelyreturns the data to the screen forverification.

Analog Loopback Test

Test Data

Test Data

Local Digital Loopback TestLocal PCRemote PC

Test Data

Remote Digital Loopback TestLocal PCRemote PC


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Troubleshooting ModemTroubleshooting Modem

Modem does not

response

Verify cable is connect correctly for external modem. For

internal modem, re-sit the cardVerify the COM Ports

IRQ is the main reason for Modem not working

Use Device Manager to check for ConflictSoftware says no

Modem found

Check modem is looking for the correct port

Check for conflictDevice manager should Modem detect and no conflict

Modem works

intermittently

Check whether you installed the correct driver for the model

Check telephone (complain)

Data is lost in

downloading

Enable hardware flow control, disable software flow control


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Quick FactsQuick Facts

Modem converts parallel to digital signals and vice versa

16550 UART performs the job of signal conversionMode of transfer: simplex, duplex and full-duplex

In Asynchronous data transfer, data is transferred intermittently

In Synchronous data transfer, data is transferred at regular interval

Modem speed (line speed) is measured by kbps (no of bits per second)

Port speed is the speed of data transfer between DTE (PC) and DCE (modem)

Controlling signal flow - Hardware flow control (RTS/CTS) is preferred to softwareXON/XOFF

Compress protocol, V.42bis (4:1) is preferred over MNP-5 (2:1)

Speed standard, V.90 is current standard implementation

AT Commands: ATDT dialing with tone, ATDH hanging up, ATDZ reset the modem

External modem is normally connected to COM 2 (IRQ 3)

Modem not Responding check modem card, check IRQData lost enable hardware flow control

chpt16a communicating over phone lines

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