1 Transmission of Digital Data : Interface and Modems

2 Background Background Digital Data Transmission Digital Data Transmission –Parallel –Serial DTE-DCE Interface DTE-DCE Interface Modems Modems

3 Background Encoded Information Data EncodingInformation Transmission Process Signal + Communication Link

4 Digital Data Transmission Primary concern in data transmission Primary concern in data transmission –Wiring –Data stream Send one bit Send one bit Send a group of bits Send a group of bits Two modes of sending bit streams Two modes of sending bit streams –Parallel mode –Serial mode

5 Digital Data Transmission Data Transmission Parallel Serial SynchronousAsynchronous

6 Parallel Transmission Groups of n bits Groups of n bits Send one group at one time Send one group at one time Use n wires to send n bits Use n wires to send n bits Advantage Advantage –Speed Disadvantage Disadvantage –Cost (n wires to transmit n bits) –Limited to short distances

7 Parallel Transmission

8 Serial Transmission One bit at one time One bit at one time Requires only one wire Requires only one wire Requires conversion devices at the interface between Requires conversion devices at the interface between –The sender and the line (parallel to serial) –The line and the receiver (serial to parallel) Advantage Advantage –Low cost

9 Serial Transmission

10 Serial Transmission (Asynchronous) Asynchronous Asynchronous –The timing of signal is unimportant –Information is retrieved and translated upon pattern –Patterns : grouping bit streams into bytes Each group (usually 8) is sent as a unit Each group (usually 8) is sent as a unit –The sending device sends each group without regard to a timer

11 Serial Transmission (Asynchronous)

12 Serial Transmission (Asynchronous) (cont.) Method Method –To alert the receiver : An extra bit (start bit -- 0) is added to the beginning of each byte An extra bit (start bit -- 0) is added to the beginning of each byte One or more extra bits are added to the end of the byte One or more extra bits are added to the end of the byte –Stop bits -- 1 A gap (idle channel/streams of stop bits) is added at the end of each byte A gap (idle channel/streams of stop bits) is added at the end of each byte Thus, start bit + stop bit + gap  alert the receiver the begin and end of each byte Thus, start bit + stop bit + gap  alert the receiver the begin and end of each byte

13 Serial Transmission (Asynchronous) (cont.) Advantage Advantage –Cheap –Effective Example: Example: –Low speed communication Terminal  computer Terminal  computer

14 Serial Transmission (Synchronous)

15 Serial Transmission (Synchronous) Synchronous Transmission Synchronous Transmission –Bit streams is combined into longer “frame” –A frame may consist of multiple bytes –No gap between each byte is added into a transmission link –The receiver has to separate the bit stream into bytes for decoding purpose –Timing is important in synchronous transmission –Byte synchronization is performed at the data link layer Advantage Advantage –Speed –  useful for high speed applications

16 DTE-DCE Interface DTE : Data Terminal Equipment DTE : Data Terminal Equipment DCE : Data Circuit-terminating Equipment DCE : Data Circuit-terminating Equipment DCE DTE DCE network

17 DTE-DCE Interface Sending End Sending End The DTE The DTE –generates the data and passes them to a DCE The DCE The DCE –converts the signal to a format appropriate to a transmission medium –Sends it onto the network Receiving End Receiving End –This process is reversed

18 DTE Includes any unit that functions as a source or a destination for binary data Includes any unit that functions as a source or a destination for binary data At the physical layer, it can be At the physical layer, it can be –A terminal –A computer –A printer –A fax machine, etc. DTEs do not communicate with other DTE directly DTEs do not communicate with other DTE directly

19 DCE Includes any functional units that transmits or receives analog/digital signal through a network Includes any functional units that transmits or receives analog/digital signal through a network At the physical layer, ex. Modems At the physical layer, ex. Modems Sending and receiving DCEs must use the modulating method (e.g. FSK) Sending and receiving DCEs must use the modulating method (e.g. FSK)

20 DTE-DCE Interface Standards DCE DTE DCE network DTE-DCE standards try to define the mechanical, electrical, and functional characteristics of the connection between the DTE and the DCE

21 EIA-232 Interface Previously called RS-232 Previously called RS-232 Defines the mechanical, electrical, and functional characteristics of the interface between a DTE and a DCE Defines the mechanical, electrical, and functional characteristics of the interface between a DTE and a DCE

22 EIA-232 Mechanical Specification Mechanical Specification –Interface 25-wire cable 25-wire cable Male and female DB-25 pin connector attached to either end Male and female DB-25 pin connector attached to either end The cable length not > 15 meters (50 feet) The cable length not > 15 meters (50 feet) Electrical Specification Electrical Specification –Defines the voltage levels and the type of signal to be transmitted between DTE & DCE

23 EIA-232 Sending the Data Sending the Data

24 EIA-232 Control and Timing Control and Timing –4 wires are used for data functions –21 wires are reserved for functions like, Control Control Timing Timing Grounding Grounding Testing Testing etc. etc.

25 EIA-232 Control and Timing A function is considered ON if it transmits a voltage of at least +3 and OFF if it transmits a voltage with a value < -3 volts

26 EIA-232 Functional Specification Functional Specification –DB-25 –DB-9

27 EIA-232 : DB-25 (male)

28 EIA-232 : DB-25

29 EIA-232 : DB-25 (Control Pins)

30 EIA-232 : DB-25 (Timing Pins)

31 EIA-232 : DB-25 (Other Pins)

32 EIA-232 : DB-9 Many of the pins in DB-25 implementation are not necessary Many of the pins in DB-25 implementation are not necessary A 9-pin version of EIA-232 (called DB-9) was developed A 9-pin version of EIA-232 (called DB-9) was developed

33 EIA-232 Functioning Example SynchronousFull-DuplexTransmission

34 EIA-232 Functioning Example

35 Null Modem Modems are not needed to connect two computers in the same room Modems are not needed to connect two computers in the same room Need : an interface to handle the exchange Need : an interface to handle the exchange –Readiness establishment –Data transfer –Data receipt – etc. A standard provided by the EIA to perform these tasks, called NULL MODEM A standard provided by the EIA to perform these tasks, called NULL MODEM

36 Null Modem

37 Crossing Connections

38 Other Interface Standards IEA-449 IEA-449 –DB-37/DB-9 RS-423/422 RS-423/422 EIA-530 EIA-530 Etc. Etc.

39 Modems Modulator/demodulator Modulator/demodulator Modulator Modulator –Converts a digital signal into an analog signal using ASK, FSK, PSK or QAM Demodulator Demodulator –Converts an analog signal into a digital signal

40 Modems