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

host computer terminal

one w ay only

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

host computer terminal

first one w ay....

terminal

...then the other

Half Duplex Illustration

Data Flow: Full Duplex

complete two-way simultaneous transmission

faster than half-duplex communication because no turnaround time is needed

host computer terminal

both w aysat the same time

Full Duplex Illustration

Transmission Modes

Asynchonous– 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– 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– 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– 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)