Modems

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Modems

• Chapter 17
• Mike Fuszner

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Modem

• The term modem is an abbreviation for
  modulator/demodulator.
• Modems are used in PCs to enable computers to
  communicate with each other via standard
  telephone lines.
• They convert analog signals into digital signals
  that can be understood by the PCs COM ports.
• Modems also take the digital signals from the
  PCs COM ports and convert them into analog
  signals for standard telephone lines.
• They transmit data as a series of
  individual 1s and 0sserial
  communication

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Serial and Parallel Communication

• Serial Communication
• Transmits data as a series of 1s and 0s.

• Parallel Communication
• Transmits data in discrete 8-bit chunks

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UART Chip

• Both internal and external modems convert data
  between analog and digital
• CPUs must receive data in 8-bit parallel chunks
• Serial bits are converted into 8-bit parallel by
  the Universal Asynchronous Receiver/ Transmitter
  (UART) chip

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UARTS on Modems
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Asynchronous Communication

• Asynchronous communication
• Asynchronous data transfer is the primary way in
  which two serial ports communicate.
• In this type of transfer, the data is chopped
  into 7 or 8-bit packets.
• Each packet begins with a start bit and ends with
  a stop bit.
• Asynchronous communication packets have a parity
  bit, which is used for error detection.

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Synchronous Communication

• Synchronous communication is used for high-speed
  connections.
• It enables two modems to connect, synchronize
  data rates, and perform data transfers.

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Synchronous Organization

• Before the data is sent, the modem sends a series
  of standardized bytes called SYNC bytes for
  synchronization.
• After the SYNC bytes, the sending modem adds a
  start-of-text marker.
• The data in a synchronous transmission is
  processed in packets or blocks of fixed length.
• The data packets end with an end-of-text (ETX)
  marker and error-checking characters (ECC) after
  the data has been transferred.

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File Transfer Protocols

• The commonly used file transfer protocols are
• ASCII no error checking
• XMODEM 128-byte blocks with error checking
• YMODEM 1,024-byte blocks with error checking
• ZMODEM features of both xmodem and ymodem plus
  crash recovery, automatic downloading, and a
  faster streaming file-transfer method
• KERMIT uploading and downloading
  from a mainframe

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Baud Rate

• Baud rate is the basic cycle of time that a modem
  uses as its carrier frequency.
• The modem speed is measured in bits per second
  (bps).
• A 2400 baud rate modem that transmits 2 bits per
  time cycle has a speed of 4800 bits per second

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Flow Control (Handshaking)

• Flow control, also known as handshaking, is the
  process by which two serial devices verify a
  conversation.
• Flow control is essential to take care of the
  local and end-to-end conversations that take
  place during a file transfer.
• PCs handle local flow control using hardware or
  software.

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Flow Control (Handshaking)

• Hardware flow control uses extra wires called
  ready to send (RTS) and clear to send (CTS).
• Called RTS/CTS
• When in doubt, use hardware flow control

• Software flow control uses special characters
  called XON and XOFF to signal the beginning and
  end of data transmission respectively.
• Called XON/XOFF
• Slower than hardware flow control and rarely used
  today

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Error Detection Data Compression

• End-to-end error detection may be software or
  hardware based
• Data compression is performed on-the-fly

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Communication Standards

• The first modem standard was introduced by Bell
  212A/103.
• Could send data at 300 or 1,200 baud
• Other modems emulated Bell 212A modems
• The Consultative Committee on International
  Telephony and Telegraphy (CCITT) introduced the V
  standards, which include modem speed, data
  compression, error correction, and fax.
• Now called the International Telecommunication
  Union Telecommunications Standardization Sector
  (ITU-T)
• The Microcom Company released a series of
  standards for error detection and data
  compression.
• Supported by most modems but superceded by ITU-T
  standards

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Transmission Standards
More standards
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Modem Commands

• Modem commands vary from manufacturer to
  manufacturer.
• Most manufacturers use the Advanced Technology
  (AT) command set.
• The AT commands are typed at the command prompt.

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AT Dialog Between User Modem
See website www.modemhelp.net
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AT Commands Example
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RS-232 Connector

• The RS-232 connector is used to make a physical
  connection through a serial port from an external
  modem to a PC.
• RS Recommended Standards
• An Electronic Industries Associations (EIA)
  standard
• RS-232 describes the signals not the connector
• Revision D specified a 25-pin D-type connector
• IBM developed a 9-pin
  connector

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RS-232 Pinouts
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Modem Commands

• The Data Communication Equipment (DCE) and the
  Data Terminal Equipment (DTE) are devices that
  send and receive data respectively.
• The connection between the DTE and DCE is called
  a handshake.

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Telephone Lines

• Telephone lines were designed for voice
  communication.
• The basic analog phone line is designed to handle
  speeds only up to 2,400 bps.
• They run at approximately 48 volts.
• If the voltage is lower, the phone company will
  condition the line (add some voltage)
• The phone company only guarantees up to 2400
  baud. So if you have excessive noise, complain
  that you have a high-speed modem but the highest
  baud rate you can get is 1200. Then they may
  check the line.

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UART Chips

• Universal Asynchronous Receiver/Transmitter
  (UART) chips take parallel information off data
  buses and convert them into serial information to
  be sent out to a modem or a serial device.
• They also receive serial information and convert
  them into parallel information to be put back on
  data buses.
• The UART chip provides control signals for flow
  control to serial devices.
• It is essential to know the type of UART used in
  COM ports.
• You need a 16550A or better. Check in the Phone
  and Modem Options applet in Win2000 or the More
  Info on the Diagnostics tab of the Modems applet
  in Control Panel in Win98.

See UART types on page 963 in the book.
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Fax Modems

• Fax modems are electromechanical devices that
  translate wire-based signals into marks on paper.
• Modems transmit facsimiles as single dots (bits
  of data), which are then converted into text.
• The PC environment has designed four different
  classes of fax standards.
• Group 1 (1974) and Group 2 (1976) analog
  standards for fax modems at 300-baud
• Group 3 (G3) up to 14400 bps for fax, 2400 for
  data
• V-classes exist with G3
• Make sure your fax-modem has a G3 fax-modem
  standard marked on the box

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Modem Negotiations

• Modem tones are a standardized series of queries
  and responses between modems to determine optimal
  speeds, data transmission, and error detection.
• Sometimes called the mating call between modems

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TAPI

• Telephony Application Programmers Interface
  (TAPI) is a set of dynamic link libraries (DLL)
  that enable Windows 3.x and Windows 9x to perform
  telephone functions.
• It configures the modem type, IRQ and I/O
  address, local phone number, long-distance
  dialing codes and any other information required
  by it to be able to use a modem.
• It provides the ability to handle faxing, call
  forwarding, caller ID, and paging.

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Installation and Troubleshooting
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Set Up and Verify the COM Ports

• COM ports and IRQ conflicts are the common causes
  of modem failures.
• Non-conflicting I/O addresses and interrupts must
  be installed to avoid such problems.
• The Device Manager can be used for checking the
  working of a modem.

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Verify the Modems BIOS

• Modern modems have onboard BIOS with the ability
  to handle different commands from modem drivers
  or other modems.
• The BIOS manifests itself as a flash ROM and can
  be easily upgraded through software after
  determining the right version.

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Set Up the Correct Command Set

• It is essential to ascertain if the communication
  software knows the type of modem in order to
  enable it to give correct AT commands.

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Fixing Modem Problems

• The steps to be followed for troubleshooting a
  modem problem are
• Use the Modem Doctor shareware program to ensure
  that the modem is using non-conflicting COM ports
  and IRQs.
• Reinstall the modem software and reconfigure the
  driver.
• Make sure that the modem is plugged into a good
  phone line.

www.modemdoctor.com
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