CSC 335 Data Communications

and Networking

Lecture 4a: Communication and Multiplexing

Dr. Cheer-Sun Yang

MotivationNext topics:• transmission mode• transmission standard• multiplexing- many signals of different carrier

frequencies sharing a single medium to communication with many receivers

• data compression

Transmission Mode

A transmission mode defines the way in which a bit group goes from one device to another. It also defines whether bits may travel to both directions simultaneously or whether devices must take turns sending and receiving. The former relates to the choice of grouping and the latter relates to the timing of transmission.

Serial vs. Parallel Communication

The first choice regarding transmission mode is how sender and receiver can choose to group data bits.

• Parallel transmission means that a group of bits is sent simultaneously by using a separate line (wire) for each bit. Usually this transmission mode is used between short-distance connections.

• Serial transmission means that a single wire is used to send a group of data bits one bit at a time. Serial communication is used mostly between distant connections.

Serial vs. Parallel Communication

Asynchronous Communication

There are two ways to provide serial communication: Synchronous and asynchronous transmission.

• In a broader sense, asynchronous transmission means that the sender and the receiver don’t have to synchronize before transmission. In an asynchronous system, the receiver must be ready to accept data whenever it arrives.

Asynchronous Communication

• In a more technical sense, communication hardware is classified as asynchronous if the electrical signal doesn’t contain information that the receiver can use to determine where individual bits begin and end.

Asynchronous

• Data transmitted on character at a time– 5 to 8 bits

• Timing only needs maintaining within each character

• Resync with each character

Asynchronous (diagram)

Asynchronous - Behavior

• In a steady stream, interval between characters is uniform (length of stop element)

• In idle state, receiver looks for transition 1 to 0

• Then samples next seven intervals (char length)

Asynchronous - Behavior

• Then looks for next 1 to 0 for next char

• Simple

• Cheap

• Overhead of 2 or 3 bits per char (~20%)

• Good for data with large gaps (keyboard)

Synchronous - Bit Level• Block of data transmitted without start or stop bits• Clocks must be synchronized• Can use separate clock line

– Good over short distances– Subject to impairments

• Embed clock signal in data– Manchester encoding– Carrier frequency (analog)

Synchronous - Block Level

• Need to indicate start and end of block

• Use preamble and postamble

– e.g. series of SYN (hex 16) characters

– e.g. block of 11111111 patterns ending in 11111110

• More efficient (lower overhead) than async

Synchronous (diagram)

Line Configuration

• Simplex

– One direction, e.g. Television

• Half duplex

– Either direction, but only one way at a time, e.g. police radio

• Full duplex

– Both directions at the same time, e.g. telephone

Traditional Configurations