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.

• Used when the bandwidth of the medium is greater than the required bandwidth of the devices connected to it.

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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.

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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.

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.

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.

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• 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.

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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

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DS Service and T lines

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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.

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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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