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Multiplexing TechnologiesProfessor A. Zahid

EE450: Computer Networks

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Multiplexing

Channel-side

Aggregate-side

Aggregate link

Point-to-PointWAN Link

n Channelsn Channels

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Multiplexing• Multiplexing is a resource sharing process

allowing information from several information sources to be aggregated onto a single, high-speed link

Channel-side

Aggregate-side

Aggregate link

Point-to-PointWAN Link

n Channelsn Channels

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Classifications of Multiplexing

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Frequency Division Multiplexing• FDM• Useful bandwidth of medium exceeds

required bandwidth of channel• Each signal is modulated to a different

carrier frequency• Carrier frequencies separated so signals do

not overlap (guard bands)• e.g. broadcast radio• Channel allocated even if no data

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FDM: Multiplexing

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FDM: De-Multiplexing

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Time Division Multiplexing

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Synchronous TDM

• Data rate of medium exceeds data rate of digital signal to be transmitted

• Multiple digital signals interleaved in time• May be at bit level or block of bits• Time slots pre-assigned to sources and fixed• Time slots allocated even if source has no data• Time slots do not have to be evenly distributed

amongst sources

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Synchronous TDM

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Example of Synchronous TDM

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Synchronous De-Multiplexing

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Example of TDM Analog and Digital Sources

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Digital Carrier System• Hierarchy of TDM• USA/Canada/Japan use one system• Europe/South America use a similar (but

different) system• US system based on DS-1 format• Multiplexes 24 channels• Each frame has 8 bits per channel plus one

framing bit ⇒ 193 bits/frame• Frame Duration is 125µseconds

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T1 Multiplexing

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T1 Illustration

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Digital Hierarchy

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• In Synchronous TDM many slots are wasted• Statistical TDM allocates time slots

dynamically, i.e. based on demand• Every Slot has to start with a header

identifying the device• Multiplexer scans input lines and collects data

until frame full• Data rate on line lower than aggregate rates of

input lines

Asynchronous/Statistical TDM

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Asynchronous/Statistical TDM

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Example of a an Asynchronous TDM

a. Only three lines sending data

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Synchronous TDM v.s. Statistical TDM

• TDM – Gray Slots are actually carrying dataSlot 1 Slot 2 Slot 4Slot 3 Slot 1 Slot 3Slot 2

STDM – Same time – More information sent.Slot 1 Slot 2 Slot 2Slot 1 Slot 3 Slot 3Slot 1

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Conclusions• TDM – Guarantees the User a bandwidth

but on the contrary wastes valuable carrier capacity. Suitable for streamy type traffic like voice (digitized)

• STDM – Utilizes unused time slots. Suitable for Bursty-type traffic such as data – More efficient use of capacity– When times are busy, user suffers