1. Multiplexing
3/9/2009

2. Multiplexing Multiple links on 1 physical line
Common on long-haul, high capacity, links
More cost effective when multiplexing is used
Include
frequency division multiplexing,
time division multiplexing,
code division multiplexing.

3. Multiplexing Techniques
FDM
TDM
CDM 3

4. Advantages and Disadvantages

5. Frequency Division Multiplexing

6. FDM Analog signaling is used to transmits the signals. Examples:
Broadcast radio and television,
cable television,
the AMPS cellular phone systems
The oldest multiplexing technique.
It involves analog signaling, it is more susceptible to noise.

7. FDM – Basic Operation
Analog
Signals
Summed
Summed
Subcarriers
Multiple signals can be modulated using different frequencies
Signals with different frequencies can be Multiplexed together
The multiplexed signal has a center frequency and bandwidth
Larger than total BW of all multiplexed signals
Shifted in
frequency No
overlap
B=Total BW

8. FDM – Example Transmitted TV signal
Total BW is 6 MHZ
Audio carrier operating at fca at f0+5.75
Color subcarrier operating at fcc at f MHz
Video subcarrier operating at fvc at f MHz
CATV has a bandwidth of about 500 MHZ
Many channels can be multiplexed together!

9. FDM – Multiplexing three voice signals
Voice signal has a range of KHz
Recall FM
Carrier
Freq.
Lower
Sideband
Upper
Sideband s1
Fc-Bw
Fc+Bw

10. Analog Carrier Systems
long-distance links use an FDM hierarchy
AT&T (USA) and ITU-T (International) variants
Group
12 voice channels (4kHz each) = 48kHz
in range 60kHz to 108kHz
Supergroup
FDM of 5 group signals supports 60 channels
on carriers between 420kHz and 612 kHz
Mastergroup
FDM of 10 supergroups supports 600 channels
so original signal can be modulated many times

11. Analog Carrier Systems
4KHz each
Range

12. Wavelength Division Multiplexing
FDM with multiple beams of light at different frequency
carried over optical fiber links
commercial systems with 160 channels of 10 Gbps
lab demo of 256 channels 39.8 Gbps
architecture similar to other FDM systems
multiplexer consolidates laser sources (1550nm) for transmission over single fiber
Optical amplifiers amplify all wavelengths
Demux separates channels at the destination
also have Dense Wavelength Division Multiplexing (DWDM)

13. Synchronous Time Division Multiplexing
The original time division multiplexing.
The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern.
T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing.

14. Synchronous Time Division Multiplexing - TDM

15. TDM – Basic Operation

16. TDM Link Control no headers and trailers
data link control protocols not needed
flow control
data rate of multiplexed line is fixed
if one channel receiver can not receive data, the others must carry on
corresponding source must be quenched
leaving empty slots
error control
errors detected & handled on individual channel

17. Example – Only one station is transmitting!

18. Another Example Byte interleaving Characters are shuffled!
The receiver reassembles each channel

19. Pulse Stuffing Reasons Pulse Stuffing a common solution
synchronizing data sources
having clocks in different sources drifting
having data rates from different sources not related by simple rational number
Pulse Stuffing a common solution
have outgoing data rate (excluding framing bits) higher than sum of incoming rates
stuff extra dummy bits or pulses into each incoming signal until it matches local clock
stuffed pulses inserted at fixed locations in frame and removed at demultiplexer

20. Example – TDM of analog and digital sources with different transmission rates1
Analog
Sampler
A/D
Converter

21. Example – TDM of analog and digital sources with different transmission rates
Conversing
Analog
To Digital 1
Analog
Sampler
A/D
Converter

22. Example – TDM of analog and digital sources with different transmission rates
Digital Signal
Pulse Stuffing
Digital

23. Note: Voice signal, hence minimum BW requirement is 4KHz per bit
Example – TDM of analog and digital sources with different transmission rates
16 Bits
32 Bits
2 Bit Each
Note: Voice signal, hence minimum BW requirement is 4KHz per bit

24. Digital Carrier Systems
long-distance links use an TDM hierarchy
AT&T (USA) and ITU-T variants
US system based on DS-1 format
can carry mixed voice and data signals
24 channels used for total data rate 1.544Mbps
each voice channel contains one word of digitized data (PCM, 8000 samples per sec)
same format for 56kbps digital data
can interleave DS-1 channels for higher rates
e.g., DS-2 is four DS-1 at 6.312Mbps

25. DS-1 Transmission Format
; Bit 8 indicated voice or data
(6x8KHz or 5x9.6KHz or 10x4.8 KHz) – The first bit is used to indicate the subrate

26. Statistical TDM in Synch TDM many slots are wasted
Statistical TDM allocates time slots dynamically based on demand
multiplexer scans input lines and collects data until frame full
line data rate lower than aggregate input line rates
may have problems during peak periods
must buffer inputs

27. Comparing Synchronous and Statistical TDM Frame Format
Each slot has channel ID and possibly message length information

28. Performance of Statistical TDM
Refer to your notes.

29. Asymmetrical Digital Subscriber Line (ADSL)
link between subscriber and network
uses currently installed twisted pair cable
is Asymmetric - bigger downstream than up
uses Frequency division multiplexing
has a range of up to 5.5km
Its underlying technology is Discrete Multitone (DMT)

30. High-bit-rate Digital Subscriber Line (HDSL) – Connecting the subscriber to the PSTN

31. Discrete Multitone (DMT)
A multiplexing technique commonly found in digital subscriber line (DSL) systems
The basic idea of DMT is to split the available bandwidth into a large number of subchannels
DMT then combines hundreds of different signals, or subchannels, into one stream
Each subchannel is quadrature amplitude modulated (recall - eight phase angles, four with double amplitudes)
Theoretically, 256 subchannels, each transmitting 60 kbps, yields Mbps. Unfortunately, there is noise.

32. Discrete Multitone (DMT)
Idea: If some subchannel can not carry any data, it can be turned off and the use of available bandwidth is optimized
High attenuation at higher frequencies

33. Discrete Multitone (DMT) – How…
On initialization, the DMT modem sends test signals to each subchannel to determine its S/N
Channels with higher S/N receive more bits
Each subchannel is quadrature amplitude modulated (recall - eight phase angles, four with double amplitudes)
Change alfa depending on the availability of the noise