Data Communications Chapter 4: Digital Transmission

Transmission Modes

• Data transmission can be accomplished in two modes: parallel and serial
• In parallel mode, multiple bits are sent with each clock tick
• In serial mode, 1 bit is sent with each clock tick
• Serial transmission has three subclasses: asynchronous, synchronous, and isochronous

Parallel Transmission

• Advantage: speed
• Disadvantage: cost
• Requires n communication lines (wires) to transmit the data stream
• Limited to short distances due to high cost

Serial Transmission

• Advantage: cost-effective
• Reduces the cost of transmission over parallel by roughly a factor of n
• Requires conversion devices at the interface between the sender and the line (parallel-to-serial) and between the line and the receiver (serial-to-parallel)

Asynchronous Transmission

• Sends 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte
• May have a gap between each byte
• Efficiency calculation example: 20KB data transmission
• Cheap and effective, suitable for low-speed communication, e.g., keyboard to computer connection

Synchronous Transmission

• Bits are sent one after another without start or stop bits or gaps
• Receiver groups the bits
• Bits are usually sent as bytes, and many bytes are grouped in a frame
• Frame is identified with a start and an end byte
• Advantage: speed
• Suitable for high-speed applications, e.g., transmission of data from one computer to another
• Byte synchronization is accomplished in the data link layer
• Efficiency calculation example: 20KB data transmission

Isochronous Transmission

• Similar to synchronous transmission, but with almost zero time interval between blocks
• Collects synchronous and asynchronous data from several devices within a time slot (125 micro-second)
• Guarantees data arrival at a fixed rate
• Suitable for real-time audio and video applications, e.g., TV images broadcast

Advantages and Disadvantages of Isochronous Transmission

• Advantages: high transmission speed, no need to pause between each character, no start and stop bits required
• Disadvantages: requires primary storage device at the sender station, no error correction, and relatively expensive

PCM Decoder

• Recovers an analog signal from a digitized signal through:
  • Hold circuit that holds the amplitude value of a pulse till the next pulse arrives
  • Low-pass filter with a cutoff frequency equal to the highest frequency in the pre-sampled signal
• The higher the value of L, the less distorted the recovered signal is