Bandwidth Utilization Multiplexing and Spectrum Spreading

Bandwidth Utilization Multiplexing, Spectrum Spreading, Multiplexing: FDM, WDM, TDM and statistical TDM, Spread Spectrum, FHSS, DSSS

1. Chapter - 06 Bandwidth Utilization: Multiplexing and Spectrum Spreading Asst. Prof. Meenakshi Paul G . N. Khalsa College

2. OUTLINE 6.1 MULTIPLEXING 6.1.1 Frequency-Division Multiplexing 6.1.2 Wavelength-Division Multiplexing 6.1.3 Time-Division Multiplexing 6.2 SPREAD SPECTRUM 6.2.1 Frequency Hopping Spread Spectrum 6.2.2 Direct Sequence Spread Spectrum

3. 6.1 Multiplexing It is the process of combining multiple signals into one signal , over a shared medium . Communication is possible over the air (radio frequency), using a physical media (cable), and light (optical fiber). All mediums are capable of multiplexing. Multiplexing is achieved by using a device called Multiplexer ( MUX ) that combines n input lines to generate a single output line. i.e. many-to-one. Demultiplexing is the reverse process of multiplexing achieved by using a device called Demultiplexer ( DEMUX ) i.e. one-to-many. Multiplexing  was originally developed in the 1800s for telegraphy . Multiplexing  is widely  used  in many telecommunications applications, including telephony, internet communications, digital broadcasting and wireless telephony .

4. Process of Multiplexing The word channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many ( n) channels. The 'n' input lines are transmitted through a multiplexer and multiplexer combines the signals to form a composite signal . The composite signal is passed through a Demultiplexer and demultiplexer separates a signal to component signals and transfers them to their respective destinations . Advantages of Multiplexing :- More than one signal can be sent over a single medium . The bandwidth of a medium can be utilized effectively. Dividing a link into channels

5. Multiplexing Techniques Multiplexing techniques can be classified as: Analog Analog Digital

6. 6.1.1 Frequency-Division Multiplexing https://www.youtube.com/watch?v=f52bwNbuMDA

7. 6.1.1 Frequency-Division Multiplexinga FDM is an analog multiplexing technique that combines analog signals. When the carrier is frequency, FDM is used. FDM divides the spectrum or carrier bandwidth in logical channels and allocates one user to each channel. Each user can use the channel frequency independently and has exclusive access of it. All channels are divided in such a way that they do not overlap with each other. Channels are separated by guard bands . Guard band is a frequency which is not used by either channel.

8. 6. 8 Figure 6.3 Frequency-division multiplexing

9. A signal of a similar frequency range with the multiplexer, these similar signals modulate different carrier frequencies ( f1, f2, and f3). The resulting modulated signals are then combined into a single composite signal that is sent out over a media link that has enough bandwidth to accommodate it. 6. 9 FDM multiplexing process

10. The demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent component signals. The individual signals are then passed to a demodulator that separates them from their carriers and passes them to the output lines 6. 10 FDM demultiplexing example

11. 6.1.2. Wavelength Division Multiplexing WDM is an analog multiplexing technique. Working is same as FDM. In WDM different signals are optical or light signals that are transmitted through optical fiber. Various light waves from different sources are combined to form a composite light signal that is transmitted across the channel to the receiver. At the receiver side, this composite light signal is broken into different light waves by Demultiplexer. This Combining and the Splitting of light waves is done by using a PRISM. Prism bends beam of light based on the angle of incidence and the frequency of light wave.

12. 6.1.2. Wavelength Division Multiplexing Very narrow bands of light from different sources are combined to make a wider band of light. At the receiver, the signals are separated by the demultiplexer.

13. 6. 13 Figure Prisms in wavelength-division multiplexing and demultiplexing

14. 6.1.3. Time Division Multiplexing It is the digital multiplexing technique .  A multiplexing technique by which multiple data signals can be transmitted over a common communication channel in different time slots is known as  Time Division Multiplexing (TDM ) . It allows the division of the overall time domain into various fixed length time slots is known as time slot or slice. T he users get complete channel bandwidth to send signals but for a fixed time slot Here the time domain is divided into several recurrent slots of fixed length, and each signal is allotted a time slot on a round-robin basis .

15. https :// www.youtube.com/watch?v=aeJ55IySP_I https:// www.youtube.com/watch?v=pL3jnnue9Hc Types of TDM Synchronous TDM Asynchronous TDM 6. 15

16. 6.1.3.1 Synchronous Time-Division Multiplexing Each device is given same Time Slot to transmit the data over the link, whether the device has any data to transmit or not. Each device places its data onto the link when its Time Slot arrives, each device is given the possession of line turn by turn. If any device does not have data to send then its time slot remains empty . Time slots are organized into frames and each frame consists of one or more time slots. If there are ‘ n’ sending devices there will be ‘n ’ slots in frame.

17. Two fast-rotating switches, one for multiplexing and other for demultiplexing. The switches are synchronized and rotate at the same speed, but in opposite directions. On the multiplexing side, as the switch opens in front of a connection, that connection has the opportunity to send a unit onto the path. This process is called interleaving. On the demultiplexing side, as the switch opens in front of a connection, that connection has the opportunity to receive a unit from the path. 6. 17 Figure 6.15 Interleaving

18. 6.1.3.2 Empty Slots (Disadvantage) Synchronous TDM is not as efficient as it could be. If a source does not have data to send , the corresponding slot in the output frame is empty. Figure shows a case in which one of the input lines has no data to send and one slot in another input line has discontinuous data. The first output frame has three slots filled, the second frame has two slots filled, and the third frame has three slots filled. No frame is full.

19. 6.1.3.3 Data Rate Management (solution) One problem with TDM is how to handle a disparity in the input data rates. Multilevel Multiplexing Multiple-Slot Allocation Pulse Stuffing

20. 6.1.3.3.1 Multilevel multiplexing 6. 20

21. 6.1.3.3.2 Multiple-slot multiplexing 6. 21

22. 6.1.3.3.3 Pulse stuffing 6. 22

23. Frame Synchronizing Framing bits 6. 23

24. 6.1.3.2 Asynchronous TDM It is also known as Statistical Time Division multiplexing. Time slots are not fixed i.e. slots are Flexible. Total speed of the input lines can be greater than the capacity of the path. In ASTDM we have n input lines and m slots i.e. m less than n (m<n). Slots are not predefined rather slots are allocated to any of the device that has data to send.

25. TDM slot comparison 6. 25

26. Spread spectrum is a communication technique that spreads a narrowband communication signal over a wide range of frequencies for transmission then de-spreads it into the original data bandwidth at the receiver. Jamming and interference have less effect on Spread spectrum because it is: Resembles noise Hard to detect Hard to intercept Spread spectrum is designed to be used in wireless applications (LANs and WANs). 6.2 Spread Spectrum

27. Why Spread Spectrum? Spread spectrum signals are distributed over a wide range of frequencies and then collected back at the receiver These wideband signals are noise-like and hence difficult to detect or interfere with Initially adopted in military applications, for its resistance to jamming and difficulty of interception More recently, adopted in commercial wireless communications

28. Spread spectrum

29. SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming . To achieve these goals, spread spectrum techniques add redundancy. 6. 29 Frequency Hopping Spread Spectrum (FHSS ) Direct Sequence Spread Spectrum Synchronous (DSSS) Types of Spread Spectrum

30. 30 6.2.1 Frequency Hopping Spread Spectrum https ://www.youtube.com/watch?v=CkhA7s5GIGc Carrier changes frequency (HOPS) according to a pseudorandom Sequence. Pseudorandom sequence is a list of frequencies. The carrier hops through this lists of frequencies. The carrier then repeats this pattern. During Dwell Time the carrier remains at a certain frequency. During Hop Time the carrier hops to the next frequency. This signal is resistant but not immune to narrow band interference.

31. Frequency hopping spread spectrum (FHSS)

32. Frequency selection in FHSS

33. Figure 6.30 FHSS cycles

34. Bandwidth sharing

35. The original 802.11 FHSS standard supports 1 and 2 Mbps data rate. FHSS uses the 2.402 – 2.480 GHz frequency range in the ISM band. It splits the band into 79 non-overlapping channels with each channel 1 MHz wide. FHSS hops between channels at a minimum rate of 2.5 times per second. Each hop must cover at least 6 MHz The hopping channels for the US and Europe are shown below. 6.2.1 FHSS Contd..

36. 6.2.1 FHSS Contd.. Dwell Time The Dwell time per frequency is around 100 ms ( The FCC specifies a dwell time of 400 ms per carrier frequency in any 30 second time period) . Longer dwell time = greater throughput. Shorter dwell time = less throughput Hop Time Is measured in microseconds (us) and is generally around 200-300 us .

37. FHSS Disadvantages Not as fast as a wired LAN or the newer WLAN Standards Lower throughput due to interference. FHSS is subject to interference from other frequencies in the ISM band because it hops across the entire frequency spectrum. Adjacent FHSS access points can synchronize their hopping sequence to increase the number of co-located systems, however, it is prohibitively expensive.

38. https://www.youtube.com/watch?v=-1mxYWvfVWQ DSSS works by combining information bits (data signal) with higher data rate bit sequence ( pseudorandom number (PN) ). The PN is also called a Chipping Code ( eg ., the Barker chipping code) The bits resulting from combining the information bits with the chipping code are called chips - the result- which is then transmitted. The higher processing gain (more chips) increases the signal's resistance to interference by spreading it across a greater number of frequencies. IEEE has set their minimum processing gain to 11. The number of chips in the chipping code equates to the signal spreading ratio . Doubling the chipping speed doubles the signal spread and the required bandwidth. 6.2.2 Direct Sequence Spread Spectrum

39. DSSS 6. 39

40. 6. 40 DSSS example