Transmission of Digital Data Interface and Modems

Transmission of Digital Data : Interface and Modems 1

Transmission of Digital Data : Interface and Modems n n Background Digital Data Transmission – Parallel – Serial n n DTE-DCE Interface Modems 2

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

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

Digital Data Transmission Parallel Serial Synchronous Asynchronous 5

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

Parallel Transmission 7

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

Serial Transmission 9

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

Serial Transmission (Asynchronous) 11

Serial Transmission (Asynchronous) (cont. ) n Method – To alert the receiver : n An extra bit (start bit -- 0) is added to the beginning of each byte n One or more extra bits are added to the end of the byte – Stop bits 1 -n n 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 12

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

Serial Transmission (Synchronous) 14

Serial Transmission (Synchronous) n 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 n Advantage – Speed – useful for high speed applications 15

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

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

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

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

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

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

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

EIA-232 n Sending the Data 23

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

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 25

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

EIA-232 : DB-25 (male) 27

EIA-232 : DB-25 28

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

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

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

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

Syn chr ono us EIA-232 Functioning Example Full. Dupl ex Tran smis sion 33

EIA-232 Functioning Example 34

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

Null Modem 36

Crossing Connections 37

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

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

Modems 40