CIT 307 Online Data Communications Digital Transmission Module 5 Kevin Siminski, Instructor

Linecoding Characteristics Line Coding Schemes Other Schemes

Line Coding

Signal level versus data level

DC Component

Lack of synchronization

Why is timing important? In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps? How many if the data rate is 1 Mbps?

Why timing (continued)? At 1 Kbps: – 1000 bits sent  1001 bits received  1 extra bps At 1 Mbps: – 1,000,000 bits sent  1,001,000 bits received  1000 extra bps

Line Coding

Unipolar Encoding Unipolar encoding uses only one voltage level.

Unipolar Encoding

Polar Encoding Polar encoding uses two voltage levels (positive and negative).

Polar Encoding

Polar encoding In NRZ-L the level of the signal is dependent upon the state of the bit.

NRZ-L & NRZ-I encoding

RZ encoding

Synchronization A good encoded digital signal must contain a provision for synchronization. Timing/synchronization is one of the most important elements in Layer 1

Bi-polar Encoding In bipolar encoding, we use three levels: positive, zero, and negative. This is the most common encoding mechanism used in North America transmission systems

Bi-polar AMI encoding

Data transmission

Parallel Transmission

Serial Transmission

Asynchronous Transmission Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.

Asynchronous Transmission

Synchronous Transmission In synchronous transmission, we send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits.

Synchronous Transmission

Important point to consider Timing/Synchronization is the single most important element in a data communications network. Encoding transforms 0’s & 1’s into electronic pulses. The North American standard used today is Bi-Polar encoding