Multiplexing and spreading
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Multiplexing and spreading techniques are used to combine multiple signals over a shared medium such as a communications channel. Frequency division multiplexing (FDM) combines signals by assigning each signal a unique, non-overlapping frequency band. Time division multiplexing (TDM) divides the available time on a channel into time slots that are allocated to individual signals in a cyclical, non-overlapping manner. Spread spectrum techniques like frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS) spread signals across a wider bandwidth to improve interference resistance and security.
Multiplexing and spreading
- 1. Multiplexing and Spreading Data Communication and Computer Networks Presentation By:- Pawan Singh Raikhola
- 2. 2 Sharing Medium M U X D E M U X A link is divided into channels n signals (input) n signals (output) 1 link, n channels Multiplexer Demultiplexer
- 3. 3 Frequency Division Multiplexing (FDM) Medium BW > Channel BW Each signal is modulated to a different carrier frequency E.g., broadcast radio Channel allocated even if no data An analog multiplexing technique to combine signals
- 4. 4 Conceptual View of FDM Channel 3 Channel 2 Channel 1 f1 f2 f3
- 7. 7 Guard Bands Strips of unused bandwidth to prevent signals from overlapping FDM 3 kHz f FDM (no guard band) FDM (with guard band) f 3 kHz 3 kHz 3 kHz f 3 kHz 3 kHz 3 kHz f 3 kHz f 3 kHz 3 kHz 3 kHz
- 8. 8 FDM: Example 1 Five voice channels, each with 3-kHz bandwidth, are to be multiplexed together. If there is a need for a guard band of 1.5 kHz, what is the minimum bandwidth of the link? f 3 kHz 3 kHz 3 kHz 3 kHz 3 kHz 1.5 kHz 1.5 kHz 1.5 kHz 1.5 kHz 21 kHz
- 9. 9 FDM: Example 2 Four digital-data channels, 1 Mbps each, use a satellite channel of 1 MHz. Design an appropriate configuration using FDM
- 10. 10 Analog Hierarchy F D M F D M F D M F D M 5 groups … 10 supergropus 6 master groups group supergroup master group Jumbo group 4 kHz 4 kHz 4 kHz … 12 voice channels Used by AT&T 48 kHz 12 voice channels 240 kHz 60 voice channels 2.52 MHz 600 voice channels 16.984 MHz 3600 voice channels
- 11. 11 The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously? Solution Each band is 25 MHz. If we divide 25 MHz by 30 kHz, we get 833.33. In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users. Example
- 12. 12 Multiplexer Demultiplexer Wavelength Division Multiplexing (WDM) WDM is a special case of FDM 1 2 3 1 2 3 1+2+3 Fiber-optic cable WDM WDM 1 2 3 1 2 3 1+2+3 An analog multiplexing technique to combine optical signals
- 13. 13 Medium Data Rate > Signal Data Rate Multiple digital signals interleaved in time Time slots are preassigned to sources and fixed are allocated even if no data do not have to be evenly distributed among sources Time Division Multiplexing (TDM) T D M A B C A B C A B C A B C Frame Time slot A digital multiplexing technique to combine data one unit
- 14. 14 Conceptual View of TDM Channel 3 Channel 2 Channel 1 Channel 3 Channel 2 Channel 1 Channel 3 Channel 2 Channel 1 2 3 1 M U X 1 2 3 2 3 1 2 3 1 1 2 3 D E M U X Data flow
- 15. 15 TDM Frames A frame consists of one complete cycle of time slots
- 16. 16 TDM: Example Four 1-Kbps connections are multiplexed together. A unit is 1 bit. Find: (1) the duration of 1 bit before multiplexing, (2) the transmission rate of the link, (3) the duration of a time slot, and (4) the duration of a frame?
- 17. 17 Empty Slot
- 18. 18 Synchronization B C A 1 … 1 0 1 0 … Synchronization pattern Multiplexer and demultiplexer must be synchronized Framing bits are used to provide synchronization B C A 0 B C A 1 B C A 0 framing bit
- 19. 19 Synchronization: Example Identify all the framing bits from this bit sequence output by a multiplexer 2 channels 1 unit = 2 bits 0 0 0 1 1 0 1 1 1 0 0 0 0 1 0 0 1 0 1 1
- 21. 21 TDM of Different Data Rates Data rate from one source may be faster than the others More than one time slot can be assigned to certain sources 4 kbps 4 kbps 8 kbps 16 kbps MUX A B C 1 frame B C A B Note: assume 1 unit = 1 bit 4 kbps 4 kbps 8 kbps 4 kbps 4 kbps 16 kbps MUX A B C
- 22. 22 TDM of Different Data Rates 5 kbps 3 kbps 8 kbps A B A B A A A A B B B B B A A A A A 1 frame A B A A A A B B MUX Note: assume 1 unit = 1 bit
- 23. 23 5 kbps 3 kbps A B 6 kbps Bit Padding Different data rates may not be integer multiples of each other Solution – Bit padding 9 kbps Bit Padding MUX A A B A A B B A A A B 3000 cycles/s Extra bit 1 ms Note: assume 1 unit = 1 bit A A B A A B B A A A A A A B B B
- 24. 24 Digital Signal (DS) Hierarchy T D M T D M T D M DS-1 DS-2 DS-3 DS-4 DS-0 T D M DS-0 … 24 channels 1.544 Mbps 24 DS-0 6.312 Mbps 4 DS-1 44.376 Mbps 7 DS-2 274.176 Mbps 6 DS-3 64 kbps 1 kbps = 1,000 bps 1 Mbps = 1,000 kbps
- 25. 25 DS Services and T Lines 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-4 274.176 4032 DS-0, DS-1, etc, are services T lines are used to implement these services
- 26. 26 T Lines and Analog Signals
- 28. 28 E Lines European's version of T lines Also used in Thailand E Line Rate (Mbps) Voice Channels E-1 2.048 30 E-2 8.448 120 E-3 34.368 480 E-4 139.264 1920
- 29. 29 Inverse Multiplexing Inverse DEMUX Inverse MUX high-speed line high-speed line low-speed lines
- 30. 30 Spread Spectrum Spread signal to use larger bandwidth To prevent eavesdropping To reduce effect from interference
- 31. 31 Frequency-Hopping SS "FHSS" – Frequency-Hopping Spread Spectrum Used in Bluetooth technology
- 32. 32 FHSS Cycles
- 33. 33 Direct-Sequence SS "DSSS" – Direct-Sequence Spread Spectrum Used in Wireless LANs
- 34. 34 DSSS and Interference Narrow Band Signal Frequency Amplitude Narrow Band Interference Spread Spectrum Signal
- 35. 35 DSSS Example