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

3. Communication Modes
Simplex
Half Duplex
Full Duplex

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

5. Simplex Illustration

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

7. Half Duplex Illustration

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

9. Full Duplex Illustration

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

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

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

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