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CIT 307 Online Data Communications. Digital Transmission. Kevin Siminski, Instructor. Module 5. Linecoding. Characteristics Line Coding Schemes Other Schemes. Line Coding. Signal level versus data level. DC Component. Lack of synchronization. Why is timing important?.
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CIT 307 OnlineData Communications Digital Transmission Kevin Siminski, Instructor Module 5
Linecoding • Characteristics • Line Coding Schemes • Other Schemes
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 received1 extra bps • At 1 Mbps: • 1,000,000 bits sent 1,001,000 bits received1000 extra bps
Unipolar Encoding Unipolar encoding uses only one voltage level.
Polar Encoding Polar encoding uses two voltage levels (positive and negative).
Polar encoding In NRZ-L the level of the signal is dependent upon the state of the bit. In NRZ-L the level of the signal is dependent upon the state of the bit.
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
Asynchronous Transmission Asynchronous here means “asynchronous at the byte level,” but the bits are still synchronized; their durations are the same.
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.
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