Communication 4 Instructor: Dr. Eng. Mohammed Hassan Categories of multiplexing FREQUENCY DIVISION MULTIPLEXING (FDM) Frequency division multiplexing can be used with analog signals. A number of signals are carried simultaneously on the same medium by allocating to each signal a different frequency band. FDM is possible when the useful bandwidth of the transmission medium exceeds the required bandwidth of signals to be transmitted. A number of signals can be carried simultaneously if each signal is modulated onto a different carrier frequency and the carrier frequencies are sufficiently separated that the bandwidths of the signals do not significantly overlap. FREQUENCY DIVISION MULTIPLEXING (FDM) To prevent interference, the channels are separated by guard bands, which are unused portions of the spectrum. The composite signal transmitted across the medium is analog. Note, however, that the input signals may be either digital or analog. In the case of digital input, the input signals must be passed through modems to be converted to analog. In either case, each input analog signal must then be modulated to move it to the appropriate frequency band. Frequency-division multiplexing Wavelength Division Multiplexing Wave-Division Multiplexing [WDM] is an analog multiplexing technique to combine optical signals Optical fiber data rate is higher than the data rate of metallic transmission cable. Using a fiber-optic cable for one single line wastes the available bandwidth. Wavelength Division Multiplexing Prisms form the basis of optical multiplexing and demultiplexing • a multiplexor accepts beams of light of various wavelengths and uses a prism to combine them into a single beam • a demultiplexor uses a prism to separate the wavelengths. A prism bends a beam of light based on the angle of incidence and the frequency Synchronous Time Division Multiplexing Synchronous time division multiplexing can be used with digital signals or analog signals carrying digital data. In this form of multiplexing, data from various sources are carried in repetitive frames. Each frame consists of a set of time slots, and each source is assigned one or more time slots per frame. The effect is to interleave bits of data from the various sources. The interleaving can be at the bit level or in blocks of bytes or larger quantities. Synchronous TDM is called synchronous not because synchronous transmission is used, but because the time slots are preassigned to sources and fixed. The time slots for each source are transmitted whether or not the source has data to send. Synchronous Time Division Multiplexing 1. In synchronous TDM, each input connection has an allotment in the output even if it is not sending data. 2. In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter. THE PROBLEM WITH SYNCHRONOUS TDM: UNFILLED SLOTS Synchronous TDM is not always efficient THE PROBLEM WITH SYNCHRONOUS TDM: UNFILLED SLOTS • Synchronous TDM works well if each source produces data at a uniform, fixed rate equal to 1/N of the capacity of the shared medium • Many sources generate data in bursts, with idle time between bursts • To understand why, consider the example in Figure sources on the left produce data items at random • the synchronous multiplexor leaves a slot unfilled • if the corresponding source has not produced an item by the time the slot must be sent • In practice, a slot cannot be empty because the underlying system must continue to transmit data • the slot is assigned a value (such as zero) • and an extra bit is set to indicate that the value is invalid Figure TDM slot comparison INEFFICIENT USE OF BANDWIDTH Sometimes an input link may have no data to transmit. When that happens, one or more slots on the output link will go unused. That is wasteful of bandwidth. DATA RATE MANAGEMENT Not all input links maybe have the same data rate. Some links maybe slower. There maybe several different input link speeds There are three strategies that can be used to overcome the data rate mismatch: multilevel, multislot and pulse stuffing DATA RATE MATCHING Multilevel: used when the data rate of the input links are multiples of each other. Multislot: used when there is a GCD between the data rates. The higher bit rate channels are allocated more slots per frame, and the output frame rate is a multiple of each input link. Pulse Stuffing: used when there is no GCD between the links. The slowest speed link will be brought up to the speed of the other links by bit insertion, this is called pulse stuffing. Multilevel multiplexing Multiple-slot multiplexing Pulse stuffing Example 1 Two channels, one with a bit rate of 100 kbps and another with a bit rate of 200 kbps, are to be multiplexed. How this can be achieved? What is the frame rate? What is the frame duration? What is the bit rate of the link? Solution We can allocate one slot to the first channel and two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second because it carries 1 bit from the first channel. The bit rate is 100,000 frames/s × 3 bits per frame, or 300 kbps. 6. 1 9 Example 2 Ten sources, six with a bit rate of 200 kbps and four with a bit rate of 400 kbps are to be combined using multilevel TDM with no synchronizing bits. Answer the following questions about the final stage of the multiplexing: I. What is the size of a frame in bits? II. What is the frame rate? III. What is the duration of a frame? IV. What is the data rate? Example 3 Two channels, one with a bit rate of 190 kbps and another with a bit rate of 180 kbps, are to be multiplexed using pulse stuffing TDM with no synchronization bits. Answer the following questions: a. What is the size of a frame in bits? b. What is the frame rate? c. What is the duration of a frame? d. What is the data rate? Example 4 Four channels, two with a bit rate of 200 kbps and two with a bit rate of 150 kbps, are to be multiplexed using multiple slot TDM with no synchronization bits. Answer the following questions: a. What is the size of a frame in bits? b. What is the frame rate? c. What is the duration of a frame? d. What is the data rate?
