1. CPEG514 Advanced Computer Networks
Week 6 Multiplexing Atef Abu Salim University of Nizwa Spring 2013/2014
CPEG512 Advanced Computer Networks 2

2. Multiplexing Categories
CPEG512 Advanced Computer Networks

3. Frequency Division Multiplexing
Analog signaling is used to transmit the signals.
Broadcast radio and television, cable television, and the AMPS cellular phone systems use frequency division multiplexing.
This technique is the oldest multiplexing technique.
Since it involves analog signaling, it is more susceptible to noise.
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4. Frequency Division Multiplexing
Frequency Division Multiplexing (FDM)
Primarily used for Analog data.
Bandwidth of medium equals or exceeds the sum of all channel bandwidths
Each signal is modulated to a different carrier frequency
Carrier frequencies separated so signals do not overlap (guard bands)
Can carry multiple telephone conversations over a single transmission channel.
Frequencies in each call are changed so they can be placed side-by-side in a wide-band channel and be transmitted as a group.
At the other end, the frequencies in each call are changed back to the original frequencies.
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5. Frequency Division Multiplexing
Frequency Division Multiplexing (FDM)
More efficient in terms of bandwidth than digital systems.
Downside is that noise is amplified along with the voice.
Has been replaced with Time Division Multiplexing (TDM).
e.g. broadcast radio
Channel allocated even if no data
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6. Frequency Division Multiplexing
FDM is an analog multiplexing technique that combines signals.
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7. Frequency Division Multiplexing
Frequency Division Multiplexing (FDM)
Assignment of non-overlapping frequency ranges to each “user” or signal on a medium.
A multiplexor accepts multiple analog inputs and assigns frequencies to each device. Modulation is used to move input signals into the assigned frequency ranges.
The multiplexor is attached to a high-speed communications line. A corresponding demultiplexor on the other end of the line separates the multiplexed signals.
FDM is only used with analog signals.
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8. FDM Process
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9. FDM Demultiplexing Process
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10. FDM Question #1 Question Solution
Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency domain without the use of guard bands.
Solution
Shift (modulate) each of the three voice channels to a different bandwidth, as shown on the next slide.
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11. Frequency Division Multiplexing of Three Voice Calls
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12. Frequency Division Multiplexing
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13. Frequency Division Multiplexing
Why stop there?
Group
12 Voice channels
Frequency range: 60 – 108 kHz
Super Group
5 Groups
60 Voice channels
Frequency range: 312 – 552 kHz
Master Group
10 Super Groups
600 Voice channels
Frequency range: 564 – 3084 kHz
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14. Frequency Division Multiplexing
Why stop there? (cont.)
Jumbo Group
6 Master Groups
3600 Voice channels
Frequency range: 564 – 17,548
Jumbo Group Multiplex
3 Jumbo Groups
10,800 Voice channels
Frequency range: 3124 – 60,556 kHz
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15. Frequency Division Multiplexing
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16. Frequency Division Multiplexing
Using FDM for data transmissions
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17. FDM Question #2 Question Solution
Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference?
Solution
For five channels, we need at least four guard bands. This means that the required bandwidth is at least:
(5 x 100) + (4 x 10) = 540 KHz
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18. FDM Question #2
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19. Time Division Multiplexing
Digital Technology.
Can carry multiple telephone conversations over a single transmission channel.
Analog speech signals are sampled and converted to pulses.
The samples are then coded by Pulse Code Modulation (PCM)
All the samples are transmitted in series over the same channel, one at a time.
At the other end, the signals are demodulated and each sample from each channel is routed to the proper channel.
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20. Time Division Multiplexing
Sharing of the signal is accomplished by dividing available transmission time on a medium among users.
Digital signaling is used exclusively.
Time division multiplexing comes in two basic forms:
Synchronous time division multiplexing, and
Statistical, or asynchronous time division multiplexing.
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21. Time Division Multiplexing
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22. Time Division Multiplexing
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23. TDM System Synchronous Time Division Multiplexing
The original time division multiplexing. Assigns a static fixed bandwidth (in bps) to each input device.
The multiplexor accepts input from attached devices in a round-robin fashion and transmits the data in a never ending pattern.
T1 and ISDN telephone lines are common examples of synchronous time division multiplexing.
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24. Synchronous Time Division Multiplexing
Data rate of medium equals or exceeds the sum of all the data rates of all the channels
Multiple digital signals interleaved in time
May be at bit level or block level
Time slots preassigned to sources and fixed
Time slots allocated even if no data
Time slots do not have to be evenly distributed amongst sources
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25. Synchronous Time Division Multiplexing
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26. Synchronous Time Division Multiplexing
If a device has nothing to transmit, the multiplexor will still insert a piece of data (typically just 0 bits) from that device into the multiplexed stream as a placeholder.
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27. Synchronous Time Division Multiplexing
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28. Synchronous Time Division Multiplexing
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29. Statistical Time Division Multiplexing
Sometimes called Asynchronous TDM
Transmits data only from active users
Does not transmit empty time slots
The multiplexor must create a more complex frame that contains data only from those input sources that have something to send.
Upon those devices wishing to transmit again, the statistical multiplexor must create a new frame containing data from all currently transmitting stations
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30. T-1 Multiplexor
The T-1 multiplexor stream is a continuous series of frames.
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31. Multiplexing: Sharing a Medium
T1 Frame Format
Each DS0 called a time slot
8000 frames/sec * 8 bits/slot = 64 Kbps
24 * = 193 bits/frame
8000 frames/sec * 193 bits/frame = Mbps
8000 Framing bits sent per second
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32. Transmission Technology
Framing Bits are used for:
Allow receiver to find the start-of-frame (frame synchronization).
Group sets of 12 frames into superframes.
Indicate which frames contain signaling bits.
Provide error checking (CRC).
Provide Facilities Data Link channel to transmit network management messages.
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33. Framing Bits
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34. Multiplexing Voice Calls on a T1
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35. T1 Frame Structure
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36. DS and T line rates Service Line Rate (Mbps) Voice Channels DS-1 T-1
1.544 24
DS-2
T-2
6.312 96
DS-3
T-3
44.736
672
DS-4
T-4
4032
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37. DS hierarchy
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38. Transmission Technology
D4 Framing – Superframe T-1 (1970)
Framing pattern is a repeating 12-bit sequence.
F-bit pattern:
Odd frames:
Even frames:
One Signaling Bit is “robbed” in the 6th and 12th frames for each time slot.
D4 Frame Format
Frames 1-5, 7-11:
Frames 6, 12:
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39. Transmission Technology
D5 Framing – Extended Superframe T1 (1983)
F-bit pattern marks 24-frame extended superframes
F-bit pattern:
Odd frames: Facilities Data Link
Even frames alternating:
Every 4th frame: (Framing pattern)
Every 4th frame: CRC for previous ESF
ESF Frame Advantages
Facilities Data Link
Network diagnostics and management messages sent between carrier equipment.
Cyclic Redundancy Check (CRC)
Allows error detection on T1 lines.
Carrier can offer Automatic Protection Switching service to customer (switches to another T1 line if errors are detected.)
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40. ISDN User Network Interface
ISDN allows multiplexing of devices over single ISDN line
Two interfaces
Basic ISDN Interface
Primary ISDN Interface
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41. Basic ISDN Interface
Digital data exchanged between subscriber and NTE - Full Duplex
Separate physical line for each direction
Pseudoternary coding scheme
1=no voltage
0=positive or negative 750mV +/-10%
Data rate 192kbps
Basic access is two 64kbps B channels and one 16kbps D channel
This gives 144kbps multiplexed over 192kbps
Remaining capacity used for framing and sync
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42. Basic ISDN Interface B channel is basic user channel Data PCM voice
Separate logical 64kbps connections o different destinations
D channel used for control or data
LAPD frames
Each frame 48 bits long
One frame every 250s
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43. Asymmetrical Digital Subscriber Line
ADSL
Link between subscriber and network
Local loop
Uses currently installed twisted pair cable
Can carry broader spectrum
1 MHz or more
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44. Digital Subscriber Line
xDSL Architecture
Central Office Architecture
PSTN was originally designed to carry voice traffic in 4kHz channels.
DSL service lines bypass the circuit-switching infrastructure of the PSTN and terminate at DSL Access Multiplexer’s, called DSLAM’s, at the CO.
The DSLAM routes data to your Internet Service Provider (ISP).
The DSLAM strips off voice traffic and routes it to the PSTN for normal processing.
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45. Digital Subscriber Line
xDSL Architecture (cont.)
ISP
DSLAM
PSTN
Switch
Internet
HUB
DSL CPE
Splitter
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46. Digital Subscriber Line
xDSL Coding
Discreet Multi-tone Modulation (DMT)
Transmits simultaneously in parallel on multiple narrowband carriers.
Each channel is modulated independently of each other on a carrier frequency located in the center of the frequency band being used by that channel.
The American National Standards Institute (ANSI) selected DMT with the use of 256 sub-carriers, each with a bandwidth of kHz.
Each sub-carrier can be modulated with a maximum of 15 bits per hertz, totaling 60 kbps.
DMT Frequency Spectrum
0 kHz – kHz = Voice (0 – 20 kHz is available).
4 kHz – 20 kHz = Separation.
20 kHz – 130 kHz = Upstream channel.
130 kHz – 140 kHz = Separation.
140 kHz – 1MHz = Downstream channel.
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47. Wavelength Division Multiplexing
Wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line.
Different wavelength lasers (called lambdas) transmit the multiple signals.
Each signal carried on the fiber can be transmitted at a different rate from the other signals.
Dense WDM – High number of lambdas (up to 80)
Coarse WDM – A few lambdas (5-6)
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48. Wavelength-Division Multiplexing
MUX
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49. WDM WDM is an analog multiplexing technique to combine optical signal.
Very narrow bands of light from different source are combined to make a wider band of light. At the receiver, the signal are separated by the demultiplexer.
Combine Multiple light source into one single light.
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50. WDM cont. Application of WDM the SONET network.
DWDM is a new method can multiplex a very large number of channels very close to one other. It achieves even greater efficiency.
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51. WDM cont.
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52. Code Division Multiple Access
The full bandwidth, i.e. 125MHz is shared by all users.
Each user has a unique code which he uses to decode his information bits.
Allows for sharing of resources instead of dedication, better utilization.
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