2. Data Transmission n Keep errors to an acceptable low probability n Bit-serial transmission n Parallel transmission

3. Communication Modes n Simplex n Half Duplex n Full Duplex

4. Data Flow: Simplex n only transmit in one direction n rarely used in data communications (WebTV ?) n e.g., receiving signals from the radio station or CATV n the sending station has only one transmitter the receiving station has only one receiver

5. Simplex Illustration

6. Data Flow: Half Duplex n data may travel in both directions, but only in one direction at a time n provides non-simultaneous two-way communication n computers use control signals to negotiate when to send and when to receive n the time it takes to switch between sending and receiving is called turnaround time

7. Half Duplex Illustration

8. Data Flow: Full Duplex n complete two-way simultaneous transmission n faster than half-duplex communication because no turnaround time is needed

9. Full Duplex Illustration

10. Transmission Modes n Asynchonous – start bit, data bits, stop bit n Synchronous – Block (or frame) & Transmit n Start of frame byte n Frame n End of frame byte n (fig 3.8)

11. Error Detection n Forward Error Control – along with each character or frame additional information is sent – if error is detected, correct info is inferred from received data n Feedback Error Control – check/detect error & request retry n Parity n Block Sum Check

12. Data Link Protocols n Echo Checking – relies on terminal operator – slow n Automatic Repeat Request (ARQ) – small control message or frame to acknowledge correct receipt – fig 4.1

13. ACK Methods n Continuous RQ – Use of buffer storage (fig4.7) – Selective retransmission (fig 4.10 - bad I- Frame, fig 4.11 Bad ACK) n Go Back N (fig 4.12)