1. CSC 335 Data Communications and Networking
Lecture 4a: Communication and Multiplexing
Dr. Cheer-Sun Yang

2. Motivation Next topics: transmission mode transmission standard
multiplexing- many signals of different carrier frequencies sharing a single medium to communication with many receivers
data compression

3. 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.

4. 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.

5. Serial vs. Parallel Communication

6. 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.

7. 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.

8. Asynchronous Data transmitted on character at a time
5 to 8 bits
Timing only needs maintaining within each character
Resync with each character

9. Asynchronous (diagram)

10. 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)

11. 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)

12. 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)

13. 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 patterns ending in
More efficient (lower overhead) than async

14. Synchronous (diagram)

16. Traditional Configurations