A. Zahid, EE450 1 Multiplexing Technologies Professor A. Zahid EE450: Computer Networks
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A. Zahid, EE450 1
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
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