1. 6. The Data Communication Interface

2. Contents • Asynchronous and Synchronous Transmission • Line Configuration • Interfacing

3. 1. Asynchronous and Synchronous Transmission • Problem of Transmission Sync • In order for the receiver to sample the incoming bits properly, • Must know the arrival time and duration of each bi that it receives. • Discordance between transmit clock and receive clock creates error • need proper treatment • Asynchronous Transmission • Avoid timing problem • by not sending long, uninterrupted stream of bits • Data are transmitted one character at a time • Advantage • simple and cheap • Disadvantage • high overhead (2 ~ 3 bits per character) • Problem on high-speed transmission

4. Remain idle or next start bit Odd,even, or unused Idle state of line Start bit 1-2 bit times 5 to 8 data bits 0 P bit stop 1 Character format 1. Asynchronous and Synchronous Transmission • Asynchronous transmission (continued) • Scheme • Start with START BIT • Followed by DATA BITS • Followed by PARITY BIT • End up with STOP BIT(S)

5. 0 100 200 300 400 500 600 700 800 start 0 93 186 279 372 465 558 651 744 Effect of timing error 1. Asynchronous and Synchronous Transmission • Asynchronous Transmission (continued) • Timing error condition example • assume 8bits unit, 7 percent clock discordance between transmitter and receiver • Error at 8-th bit • Framing error : bit arrangement break • bit 7 is STOP BIT condition & bit 8 is START BIT condition

6. 1. Asynchronous and Synchronous Transmission • Synchronous Transmission • Synchronize each side’s clock • Provide additional clock line(for short distance) • Embed the clock information into the data signal(e.g. Manchester) • Using carrier signal to synchronize (analog case) • Advantage • More efficient than asynchronous transmission (requires less overhead) • Disadvantage • Complex to systemize

7. 1. Asynchronous and Synchronous Transmission • Synchronous Transmission (continued) • Frame • Means larger size of bit stream including preamble, postamble, control information • Preamble & postamble : special bit pattern indicating start & end of frame • Synchronous transmission is far more efficient than asynchronous. why? 8-bit flag Control fields 8-bit flag Control fields Data field Synchronous frame format

8. 2. Line Configuration • Topology • Means physical arrangement of station • point-to-point • 1:1 I/O port connection • multipoint • 1:n I/O port connection(e.g. LAN configuration)

9. 2. Line Configuration • Full Duplex and Half Duplex • Half Duplex • Data can be transmitted only one way at a time • Two-way alternate • Full Duplex • Data can be sent and received simultaneously at the same time • Digital signaling (guided transmission) • Two separate transmission paths (Rx & Tx) : full duplex • One path : half duplex • Analog signaling • One frequency: half duplex (wireless), full duplex with two path (guided) • Two frequencies: full duplex (wireless), full duplex with one path (guided)

10. 3. Interfacing • Necessity of Interfacing • Simple digital signal is inadequate for direct link to transmission facility : Needs INTERFACING scheme • DTE(Data terminal equipment) : devices which using digital data such as NRZ-L signal (computers) • DCE(Data circuit-terminating equipment) : transform DTE signals to signals adequate to transmission line • Interfacing Standard must defined on following four scheme • Mechanical • Electrical • Functional • Procedural

11. 3. Interfacing • Typical system format

12. 3. Interfacing • V.24/EIA-232-F • Widely used interfacing format (ITU-T standard) • Used for connect DTE device to voice-grade modems • Mechanical Specification

13. 3. Interfacing • V.24/EIA-232-F (continued) • Electrical Specification • Defines the signaling between DTE and DCE • Voltage less than -3V : binary 1 • Voltage upper than +3V : binary 0 • 20Kbps, 15 meters • Functional Specification • Data Signals

14. 3. Interfacing • Control Signals

15. 3. Interfacing • Control Signals (continued) • Timing Signals • Ground

16. 3. Interfacing • V.24/EIA-232-E (continued) • Loopback scheme (Table 6.2)

17. DTE Receiving DTE Transmitting Request to send Transmit data to DCE Receive data time time DTE DTE DCE DCE Set Received-line signal detector Receive data from channel and send to DTE over Received data DCE ready Clear to send 3. Interfacing • V.24/EIA-232-E (continued) • Procedural Specification • Defines the sequence of circuit’s behavior • Example 1 : Asynchronous private line modem (limited-distance modem)

18. 3. Interfacing • V.24/EIA-232-E (continued) • Example 2: In case of using telephone network • Remote call procedure is needed • Ring Indicator (125) : Line active when remote host calls through line • DTE Ready (108.2) : Answer when DCE’s Ring Indicator activation

19. Dial Up Operation (1)

20. Dial Up Operation (2)

21. Dial Up Operation (3)

22. 3. Interfacing • V.24/EIA-232-E (continued) • Null modem • Used between devices close as not to create error • No DCE equipment required • Configure signal connection to make each DTE think themselves linked directly to the DCE(modem) DTE DTE

23. Physical connection for ISDN • Terminal equipment (TE) - DTE • Network-terminating equipment(NT) - DCE • Two pins are assigned for each direction • Both data and control infoma- tions are allotted same line