程序代写代做代考 Multiplexing: “Resource Sharing” – cscodehelp代写
Multiplexing: “Resource Sharing”
EE450: Introduction to Computer Networks Professor A. E450, USC, Zahid 1
Multiplexing
Multiplexing is a resource sharing process allowing information from several information sources to be aggregated onto a single, high-speed link
Aggregate link
n Channels
Channel-side
n Channels
Aggregate-side
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WAN Link 2
Categories of Multiplexing
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Synchronous TDM Statistical TDM
Frequency Division Multiplexing
Useful bandwidth of medium exceeds required bandwidth of channel
In FDM, each signal is modulated to a different carrier frequency
Carrier frequencies separated so signals do not overlap (guard bands), example: Broadcast Radio
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FDM Process
Transmitter
Receiver
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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 is idle
Time slots do not have to be evenly distributed amongst sources
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Synchronous TDM (Cont.)
In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter.
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Synchronous TDM (Cont.)
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Empty Time Slots
Empty TS can’t be used by other Sources Waste of Bandwidth
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Multilevel Multiplexing
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Multiple-Slot Multiplexing
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Statistical (Asynchronous) TDM
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 (address)
Multiplexer scans input lines and collects data until frame full
Data rate on line lower than aggregate rates of input lines
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Synchronous vs. Statistical
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Performance of Statistical TDM
# of Inputs = 10
Rate of each input (active) = 1000 bps % of time a source is active = 50%
Case 1: Multiplexer capacity = 5000 bps Case 2: Multiplexer capacity = 7000 bps
Input
6
9
3
7
2
2
2
3
4
6
1
10
7
5
8
Capacity = 5000 bps
Output
5
5
5
5
5
4
2
3
4
5
2
5
5
5
5
Backlog
350
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1
5
3
5
2
0
0
0
0
1
0
5
7
7
10
8
Capacity = 7000 bps
Output
6
7
5
7
2
2
2
3
4
6
1
7
7
7
7
5
Backlog
0
2
0
0
0
0
0
0
0
0
0
3
3
1
2
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 delay
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Wave-Length Division Multiplexing
Prisms in wavelength-division multiplexing and de-multiplexing
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Cable vs. ADSL Bandwidth Allocations
U/L Channels 2 MHz each
D/L TV Broadcasting 6 MHz each
54 550
D/L Data & VoD 6 MHz each
600
5
42
750 MHz
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Two-Way Cable Access
Internet
D/B
PSTN
S
R G
Ethernet Switch
Mod
O/E
National Feed
Local News/ Commercials
Combiner
E/O
FN
SPLITTER
CABLE MODEM
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