Please enable JavaScript.

Coggle requires JavaScript to display documents.

Chapter 6: Bandwidth Utilization - Coggle Diagram

- - - - Analog technique that can be applied when the bandwidth o a link is greater than the combines bandwidths o the signals to be transmitted.
        
        Each signal modulates a different carrier frequency. The modulated carriers are combined to form a new signal that is then sent across the link.
        
        In fDM, MUXes modulates and combine signals while demultiplexers decompose and demodulates.
        
        Channels may be separated by strips of unused bandwidth to prevent signals from overlapping and intefering with one another.
      - fDM is an analog multiplexing technique that combines analog signals.
      - fDM Process
      - Questions
        
        Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.
        
        Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference?
    - - Is a digital process that allows several connections to share the high bandwidth o a link. Instead of sharing a portion of the bandwidth as in fDM, time is shared.
        
        Each connection occupies a portion of the time in the link.
        
        In TDM, digital signals from n devices are interleaved with one another, forming a frame of data (bits, bytes, etc.). A device usually transmits its data in its assigned time slot.
        
        TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one.
      - Types of TDM
        
        Synchronous TDM
        
        Each input connection has an allotment in the output even if it is not sending the data.
        
        In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter.
        
        Questions
        
        Figure 6.14 shows synchronous TDM with a data stream for each input and one data stream for the output. The unit of data is 1 bit. Find (a) the input bit duration, (b) the output bit duration, (c) the output bit rate, and (d) the output frame rate.
        
        a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μs.
        
        b. The output bit duration is one-fourth of the input bit duration, or ¼ μs.
        
        c. The output bit rate is the inverse of the output bit duration or 1/(4μs) or 4 Mbps. This can also be deduced from the fact that the output rate is 4 times as fast as any input rate; so the output rate = 4 × 1 Mbps = 4 Mbps.
        
        d. The frame rate is always the same as any input rate. So the frame rate is 1,000,000 frames per second. Because we are sending 4 bits in each frame, we can verify the result of the previous question by multiplying the frame rate by the number of bits per frame.
        
        Four channels are multiplexed using TDM. If each channel sends 100 bytes /s and we multiplex 1 byte per channel, show the frame traveling on the link, the size of the frame, the duration of a frame, the frame rate, and the bit rate for the link.
        
        Solution
        The multiplexer is shown in Figure 6.16. Each frame carries 1 byte from each channel; the size of each frame, therefore, is 4 bytes, or 32 bits. Because each channel is sending 100 bytes/s and a frame carries 1 byte from each channel, the frame rate must be 100 frames per second. The bit rate is 100 × 32, or 3200 bps.
        
        frame synchronization
        
        Synchronization b/w the MUX and DeMUX in TDM is a major issue. If they are out of sync a bit might go to the wrong channel.
        
        One or more synchronization bits are usually added to the beginning of each frame. These bits, called framing bits, follow a pattern, frame to frame, that allows the demux to synchronize with the incoming stream.
        Usually, 1 sync bit per frame, alternating between 0 and 1.
        
        Telephone companies implement TDM through a hierarchy of digital signals, called DS service.
        
        DS-0 service is a single digital channel of 64 Kbps.
        
        To implement those services, the telephone companies use T lines (T-1 to T-4)
        
        These are lines with capacities precisely matched to the data rates of the DS-1 to DS-4 services.
        
        1 more item...
        
        Statistical (Asynchronous) TDM
        
        In synchronous TDM, each input has a reserved slot in the output frame. This can be inefficient if some input lines have no data to send.
        
        In statistical TDM, only when an input line has a slot’s worth of data to send is given a slot in the output frame.
        
        In statistical TDM, the number of slots in each frame is less than the number of input lines. The multiplexer check each input line in round-robin fashion; it allocates a slot for an input line if the line has data to send; otherwise it skips the line and checks the next line.
        
        In statistical TDM, we need to include the address of the receiver (or destination/ output line) inside each slot.