week5 chap6
TRANSCRIPT
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Chapter 6. Bandwidth Utilization: Multiplexing and Spreading
1. Multiplexing
2. Spread Spectrum
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Bandwidth Utilization
• Bandwidth utilization is the wise use of available bandwidth to achieve specific goals.
• Two categories: multiplexing and spreading
• Efficiency can be achieved by multiplexing
• Privacy and anti-jamming can be achieved by spreading.
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Multiplexing
• Whenever the bandwidth of a medium linking two devices is greater Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. than the bandwidth needs of the devices, the link can be shared.
• Multiplexing is the set of techniques that allows the simultaneous Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.transmission of multiple signals across a single data link.
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Categories of Multiplexing
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Frequency Division Multiplexing
• FDM is an analog multiplexing technique that combines analog signals• Signals modulate different carrier frequencies• Modulated signals are combined into a composite signal• Channel - Bandwidth range to accommodate a modulated signal• Channels can be separated by strips of unused bandwidth (guard band) to
prevent overlapping
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FDM Process
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FDM Demultiplexing Example
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FDM: Example 1
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FDM: Example 2
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Analog Hierarchy
• Hierarchical system used by AT&T
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Wave Division Multiplexing
• Analog multiplexing technique to combine optical signals • Conceptually the same as FDM• Light signals transmitted through fiber optic channels• Combining different signals of different frequencies
(wavelengths)
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Prisms in WDM
• Combining and splitting of light sources are easily handled by a prism
• Prism bends a light beam based on the incidence angle and the frequency
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Time Division Multiplexing
• Digital multiplexing technique for combining several low-rate channels into one high-rate one
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TDM: Time Slots and Frames
• In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter
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TDM: Example 1
• Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find (a) the duration of 1 bit before multiplexing, (b) the transmission rate of the link, (c) the duration of a time slot, and (d) the duration of a frame?
a) The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).
b) The rate of the link is 4 Kbps.
c) The duration of each time slot 1/4 ms or 250 μs.
d) The duration of a frame 1 ms.
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Interleaving
• Interleaving can be done by bit, by byte, or by any other data unit
• The interleaved unit is of the same size in a given system
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Data Rate Management
• To handle a disparity in the input data rates
• Multilevel multiplexing, multiple-slot allocation and pulse stuffing
• Multilevel multiplexing
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Data Rate Management
• Multiple-slot allocation / Pulse stuffing
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Frame Synchronizing
• Synchronization between the multiplexing and demultiplexing is a major issue in TDM
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Digital Hierarchy
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DS and T Line Rates
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T-1 Line for Multiplexing Telephone Lines
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E Line Rates
• European use a version of T lines called E lines
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Empty Slots
• Synchronous TDM is not efficient in many cases
• Statistical TDM can improve the efficiency by removing the empty slot from the frame
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Statistical TDM
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Statistical TDM
• Addressing is required in Statistical TDM• Slot size: the ratio of the data size to address size must be
reasonable to make transmission efficient• No synchronization bit: no need for frame-level sync.• Bandwidth: normally less than the sum of the capacities of
each channel
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Spread Spectrum
• Combine signals from different sources to fit into a larger Combine signals from different sources to fit into a larger bandwidth to prevent eavesdropping and jamming by bandwidth to prevent eavesdropping and jamming by adding redundancyadding redundancy
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FHSS
• Frequency Hopping Spread Spectrum (FHSS)Frequency Hopping Spread Spectrum (FHSS)
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Frequency Selection in FHSS
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Frequency Cycles
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Bandwidth Sharing
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DSSS
• Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)• Replace each data bit with n bits using a spreading codeReplace each data bit with n bits using a spreading code• Each bit is assigned a code of n bits called chipsEach bit is assigned a code of n bits called chips
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DSSS Example