1. Digital Interfaces An interface links two devices Interface Standards define: – mechanical specifications - how many wires & connector type –electrical specifications - frequency, amplitude and phase of signal –functional - what does each wire do? –Procedural – how & when to perform functions

2. upated 1/20022 Serial vs. Parallel In serial transmission one bit is sent with each clock pulse. Two types of serial transmission: –asynchronous –synchronous In parallel transmission multiple bits are sent with each clock pulse. Faster/more expensive

3. Serial Transmission 0 1 1 0 0 0 1 0

4. Parallel Transmission 0110001001100010 0110001001100010

5. Serial Transmission Asynchronous & Synchronous Concerned with timing issues How does the receiver know when the bit period begins and ends? Small timing difference become more significant over time if no synchronization takes place between sender and receiver

6. Synchronizations

7. Asynchronous Transmission Serial communication Data transmitted 1 character at a time Character format is 1 start & 1 or more stop bits, plus data of 5-8 bits Character may include parity bit Timing needed only within each character Resynchronization each start bit Uses simple, cheap technology Wastes 20-30% of bandwidth

8. Asynchronous Transmission STOP BIT START BIT 0 1 1 0 0 0 1 0 Gaps of indeterminate size

9. Synchronous Transmission Serial communication Large blocks of bits transmitted without start/stop codes Synchronized by clock signal or clocking data Data framed by preamble/post amble bit patterns More efficient than asynchronous Overhead typically below 5% Used at higher speeds than asynchronous

10. Synchronous Transmission 0 1 1 0 0 0 1 0 0 0 1 0

11. Synchronization Choices Low-speed terminals and PCs commonly use asynchronous transmission –inexpensive –“burst” tendency of communication reduces impact of inefficiency Large systems and networks commonly use synchronous transmission –overhead too expensive; efficiency necessary –error-checking more important

12. Generic Communications Interface Illustration DTE DCE DTE Generates Data Converts to transmission media/ converts to generated data Receives Data (Network) DCE Converts to transmission media/ converts to generated data

13. RS-232C (EIA 232C) EIA’s “Recommended Standard” (RS) Specifies mechanical, electrical, functional, and procedural aspects of the interface Used for connections between DTEs and voice-grade modems, and many other applications

14. Mechanical Specifications 25-pin connector with a specific arrangement of leads DTE devices usually have male DB25 connectors while DCE devices have female In practice, fewer than 25 wires are generally used in applications

15. DB-25 Female DB-25 Male RS-232 DB-25 Connectors

16. RS-232 DB-9 Connectors Limited RS-232

17. Electrical Specifications Specifies signaling between DTE and DCE Uses NRZ-L encoding –Voltage -15V to -3V = binary 1 –Voltage +15V to +3V = binary 0 Rated for < 20Kbps and < 15Mts –greater distances and rates are theoretically possible, but not necessarily wise

18. Functional Specification Specifies the role of the individual circuits Data circuits in both directions allow full- duplex communication Timing signals allow for synchronous transmission (although asynchronous transmission is more common)

19. Functional Specification - DB-25 Pinouts

20. Procedural Specification Multiple procedures are specified Provides means of attachment between computer and modem –Specifies method of transmitting data between devices –Specifies method of cooperation for exchange of data between devices –Handshaking between the two equipment

21. Null Modem Cable SG DTR DSR RTS CTS CD TD RD SG DTR DSR RTS CTS CD TD RD Allows DTE to DTE direct communication

22. upated 1/200222 Modems Modulator - converts digital signal to analog signal Demodulator - converts analog signal to digital signal

23. upated 1/200223 Data Rate Encoding Technique Physical properties of medium Can increase speed by increasing frequency but every line has upper and lower limits frequency range = bandwidth

24. upated 1/200224 Bit Rate Summary