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Sharing a physical link

Sharing a physical link. How can we maximize the utilization of the bandwidth of a physical link?. Multiplexing. Multiplexing allows us to combine several channels of information into one channel, such as computers connected to a wireless access point.

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Sharing a physical link

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  1. Sharing a physical link How can we maximize the utilization of the bandwidth of a physical link?

  2. Multiplexing • Multiplexing allows us to combine several channels of information into one channel, such as computers connected to a wireless access point. • Used when the bandwidth of the medium is greater than the required bandwidth of the devices connected to it.

  3. Frequency Division Multiplexing • Bandwidth is simply a range of frequencies available for sending signals. • We can divide that range into discrete chunks. • Each chunk will have a carrier frequency associated with it (the frequency in the middle of the chunk). • We can send as many signals as we have chunks by modulating those signals using the carrier frequencies of the chunks.

  4. Applications of FDM • Telephone land lines. • Radio • AM reserves 530 to 1700kHz with each station needing 10kHz of bandwidth • FM reserves 88.7 to 107.9mHz, with each station needing 200kHz of bandwidth. • Analog TV: Each channel uses 6MHz of bandwidth. • Early cell phones used FDM with two 30kHz channels for each user.

  5. Wavelength Division (WDM) • Used specifically for fiber-optic cables. • At the speed of light, wavelength and frequency are equivalent information, so the concept is the same as FDM. • Whereas in FDM we were combining and splitting frequencies, here we are combining and splitting beams of light. • Usually handled using some sort of prism.

  6. Time-Division (TDM) • Analogous to how modern multiprocessors dole out processor time. • Rather than divide up a link by channels and give each device a channel, we divide it up by segments of time, and each device gets to use the entire link for its time segment.

  7. Synchronous TDM takes a certain chunk of data from each input connection at set intervals (every T s). • This input data is bundled together into a frame, which is what is sent out over the medium.

  8. Data rate management • What if the input lines have different data rates? • When they are multiples of each other: • Multilevel multiplexing • Multiple-slot allocation • When they’re not: • Pulse stuffing

  9. DS Service and T lines

  10. Complications with S-TDM • Empty slots: S-TDM requires data be sampled from every input in every time slot. If there is no data from an input in a given slot, we will waste bandwidth by sending only partially filled frames.

  11. Statistical TDM • How can we overcome the empty slot problem? • Dynamically allocate slots so that all frames are full. • Only if an input line has data to send is it allocated a slot in the output frame.

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