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

Communication Modes Simplex Half Duplex Full Duplex

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

Simplex Illustration

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

Half Duplex Illustration

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

Full Duplex Illustration

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)

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

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

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