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Data Communications

Transmission of Digital Data: Interfaces and Modems

     

  1. Because an interface links two devices not necessarily made by the same manufacturer, its characteristics must be defined and standards must be established. Characteristics of an interface include its mechanical specifications, its electrical specifications, and its functional specifications.

     

  2. Digital Data Transmission

     

    • Parallel transmission is where n wires are used to send n bits at a time. It can therefore increase the transfer speed by a factor of n over serial transmisson. Because this is expensive, parallel transmission is usually limited to short distances.

       

    • In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices.

       

      • Asynchronous transmission is so named because the timing of a signal is unimportant. Information is received and transmitted by agreed-upon patterns, which are based on grouping the bit stream into bytes. One start bit (0) is sent at the beginning and one or more stop bits (1) at the end of each byte. There may be a gap between each byte.

         

      • In synchronous transmssion the bit stream is combined into longer "frames" which may contain multiple bytes. It is the responsibility of the receiver to group the bits. Byte synchronization is accomplished in the data link layer.

     

  3. Data Terminal Equipment (DTE) and data circuit-terminating (DCE) are used at each end of the communication interface. DTEs include any unit that functions either as a source or or as a destination for binary digital data. DCEs includes any functional unit that transmits or receives data in the form of an analog or digital signal through a network.

     

    • The EIA-232-D Interface (DB-25 and DB-9) defines not only the type of connectors to be used but also the specific cable and plugs and the functionality of each pin.

       

    • A null modem provides the DTE-DTE interface without the DCEs. For transmission to occur, the wires must be crossed so that pin 2 of the first DTE connects to pin 3 of the second DTE and pin 2 of the second DTE connects to pin 3 of the first.

       

    • The mechanical specifications of EIA-449 define a combination of two connectors: one with 37 pins DB-37 and one with 9 pins DB-9.

       

    • EIA-530 is a version of EIA-449 that uses DB-25 pins.

       

    • X.21 is an interface standard designed by ITU-T to pave the way for all-digital communication. It eliminates most of the control circuits of the EIA standards and instead directs their traffic over the data circuits. Both the DTE and the DCE must have added circuit logic that enables them to transform the control codes into bit streams that cn be sent over the data line. X.21 is designed to work with balanced circuits at 64 Kbps. By adding a byte timing pulse (pins 7 and 14), X.21 improves the over-all synchronization of transmission.

     

  4. Modems are DCEs. A modulator converts a digital signal into an analog signal using ASK, FSK, PSK, or QAM. A demodulator converts an analog signal into a digital signal. While a demodulator resembles an analog-to-digital converter, it is not in fact a converter of any kind. It does not sample a signal to create a digital facsimile; it merely reverses the process of modulation by performing demodulation.

     

    • The data rate of a link depends on the type of encoding used and the medium bandwidth. Every line has an upper limit and a lower limit on the frequencies of the signals it can carry. The effective bandwidth of a telephone line being used for data transmission is 2400 Hz, covering the range from 600 Hz to 3000 Hz.

       

    • Modem speed

       

      • ASK - bandwidth = baud rate. Because the effective bandwidth of a telephone line is 2400 Hz, the maximum baud rate is also 2400.

         

      • FSK - bandwidth = baud rate + frequency shift. The maximum baud rate is therefore 2400 minus the frequency shift. In full duplex - half of the bandwidth is available in either direction.

         

      • PSK and QAM - minimum bandwidth is the same as for ASK, but the bit rate can be greater depending on the number of bits that can be represented by each signal unit.

       

    • Bell modems were the first commercial modems produced by Bell Telephone Company.

       

      • 103/113 Series (V.21) -- transmission is asynchronous, using FSK modulation. Data rate is 300 bps.

         

      • 202 Series (V.23) -- transmission is asynchronous, using FSK modulation. Half duplex at 1200-2400 Hz.

         

      • 212 Series (V.22) -- The slower speed, 300 bps, uses FSK modulation for asynchronous transmission, just like the 103/113 series. The higher speed, 1200 bps, can operate in either asynchronous or synchronous mode and uses 4-PSK modulation.

         

      • 201 Series (V.26) -- Transmission is synchronous using 4-PSK modulation. Data rate is 2400 bps (or 1200 baud) in both half and full-duplex mode.

         

      • 208 Series (V.27) -- Transmission is synchronous using 8-PSK modulation. Data rate is 4800 bps.

         

      • 209 Series (V.29) -- Transmission is synchronous using 16-QAM modulation. Data rate is 9600 bps.

         

    • ITU-T modem standards

       

      • V.21, V.22, V.23, V.26, V.27, and V.29 are equivalent to Bell series modems.

         

      • V.22bis operates at either 1200 or 2400 bps and uses 4-DPSK (differential phase shift keying). The bit pattern defines the phase change, not the current phase. In 2400 bps mode it uses 16-QAM.

         

      • V.32 uses trellis coded modulation which is QAM plus a redundant bit. The value of the extra bit is calculated from the values of the data bits. V.32 calls for 32-QAM with a baud rate of 2400 or a data rate of 9600 bps.

         

      • V.32bis supports 14,400 bps and uses 64-QAM transmission.

         

      • V.32terbo is an enhanced version of V.32bis and uses 256-QAM to provide a bit rate of 19,200 bps.

         

      • V.33 is based on V.32 and uses trellis-coded modulation based on 128-QAM at 2400 baud or a data rate of 6 x 2400 = 14,400 bps.

         

      • V.34 is sometimes called V.fast and provides a bit rate of 28,800 or 33,600 bps. It also provides data compression.

         

      • V.42 uses link access procedure for modems (LAPM), which is a version of HDLC.

         

      • V.42bis includes all of the features of V.42 but also adds the Lempel-Ziv-Welch compression method.

         

       

    • 56K modems - V.90

       

      • Traditional modems have a limitation on the data rate (maximum of 33.6 Kbps, as determined by the Shannon formula). These modems may be used only if one party is using digital signaling. They are asymmetrical in that the downloading is a maximum of 56 Kbps, while uploading can be a maximum of 33.6 Kbps.

         

      • In the direction of Internet provider to user modem there is no quantization of data using PCM.

         

      • The maximum download speed is 56 Kbps because switching stations use PCM and inverse PCM, sampling at 8000 samples per second with 128 different levels (7 bits per sample). This results in 56 Kbps (8000 x 7 = 56,000) data rate at the switching station.

         

  5. Cable modems use a coxial cable that has a bandwidth up to 750 MHz. This bandwidth is normally divided into 6 MHz bands using frequency division multiplexing. Each band provides a TV channel. Two bands can be set aside to allow a user to download and upload information from the Internet.