Download - Chpt16a Communicating Over Phone Lines
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A+ Certification for PC Technician
Chapter 16Chapter 16Communicating Over Phone Lines
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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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Ver 1.0 10 Updated May 2001 Slide 13Chapter 16 Communicating Over Phone Line
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