Term
| 11.2 What are the four basic types of multiplexing? |
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Definition
There are four basic approaches to multiplexing that each have a set of variations
and implementations.
• Frequency Division Multiplexing (FDM)
• Wavelength Division Multiplexing (WDM)
• Time Division Multiplexing (TDM)
• Code Division Multiplexing (CDM) |
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Term
| 11.3 How does FDM use electromagnetic radiation? |
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Definition
In FDM, a set of radio stations can transmit electromagnetic signals simultaneously without interference
provided they each use a separate channel (i.e., carrier frequency) |
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Term
| 11.4 What is a guard band? |
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Definition
For proper communication without interference, we should choose a set of carrier frequencies with a gap
between them known as a guard band. The guard band reduces or eliminates the possible interference
between neighboring carrier signals. |
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Term
11.5 An FDM system may assign each channel a range of frequencies. Using a range is essential when
which type of modulation is used for each carrier? |
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Definition
A range of frequencies may be used in FDM for increasing data rate or increasing immunity against
noise/inference. |
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Term
| 11.6 Explain how a range of frequencies can be used to increase data rate. |
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Definition
To increase the overall data rate, a sender divides the frequency range of the channel into K carriers, and
sends 1 /K of the data over each carrier. |
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Term
| 11.8 What is the key mechanism used to combine or separate wavelengths of light in a WDM system? |
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Definition
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. |
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Term
| 11.9 Is a TDM system required to use round‐robin service? |
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Definition
Some TDM systems use a round-robin service while some others may not use round-robin, variations exist,
based on the need. |
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Term
| 11.10 Explain why framing and synchronization are important in a TDM system. |
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Definition
TDM requires that a demultiplexor stays synchronized with the multiplexor. Because a demultiplexor cannot
tell where a slot begins, a slight difference in the clocks used to time bits can cause a demultiplexor to
misinterpret the bit stream. To prevent misinterpretation, TDM used in the phone system includes an extra
framing channel as input. Instead of taking a complete slot, framing inserts a single bit in the stream on each
round. |
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Term
11.11 In a hierarchical TDM system, at what bit rate does the output of a given level need to operate?
(Express the answer in terms of the number and bit rate of inputs.) |
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Definition
| The bit rate can be calculated = Number of input lines x bit rate of individual line + framing bits |
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Term
11.12 Suppose N users compete using a statistical TDM system, and suppose the underlying physical
transport can send K bits per second. What is the minimum and maximum data rate that an individual
user can experience? |
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Definition
If we neglect the overhead generated by statistical TDM, a system will have two possibilities:
• Minimum: If all channels have equal data then the rate will be K/N bps
• Maximum: If only one channel active and the others are passive, then rate will be K bps |
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Term
11.13 Suppose an OC‐12 circuit is twenty percent the cost of an OC‐48 circuit. What multiplexing
technology can an ISP use to lower the cost of sending data at the OC‐48 rate. Explain. |
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Definition
An ISP may use inverse multiplexing, which can utilize 4 x OC-12, which will cost % 80 of a OC-48, assuming
no cost for equipment at the multiplexing/demultiplexing. |
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Term
| 11.15 Of the four basic multiplexing techniques, is CDM always the best? Explain. |
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Definition
In terms of resource utilization (speed, channel) CDM is the best, but in terms of complexity in
implementation and getting the system up and running may not be optimum solution. |
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