More on Propagation Module B

2 More on Propagation n Modulation – Modems translate between digital devices and analog transmission lines. We will look at the processes used to modulate digital signals n Multiplexing – An important way to reduce costs is to multiplex (mix) several signals onto a transmission line n Trunk Lines – Trunk lines link the switches of carriers.

Modulation

4 n Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line n There are several forms of modulation – Amplitude modulation – Frequency modulation – Phase modulation – Complex modulation

5 The Modulation Problem n Modem accepts a digital signal from the computer – Really, binary--ones and zeros – Two voltage levels n Modem converts into waves (analog) Digital Signal (1101) Modem Analog Signal

6 Waves n Frequency of a wave – The number of complete cycles per second – Called Hertz – kHz, MHz, GHz, THz Frequency (Hz) Cycles in One Second

7 Frequency Modulation (FM) Low Frequency (0) High Frequency (1) Frequency Modulation (1011) Wavelength

8 n Physical distance between similar points in adjacent cycles – Not independent of frequency – Frequency * wavelength = speed of light in medium – In a harp, for instance, long strings have low sounds Wavelength (meters)

9 Amplitude Modulation (AM) n Amplitude is the intensity of the signal – Loud or soft Amplitude (power)

10 Amplitude Modulation (AM) Low Amplitude (0) High Amplitude (1) Amplitude Modulation (1011) Amplitude (low) Amplitude (high)

11 Phase n Two signals can have the same frequency and amplitude but have different phases--be at different points in their cycles at a given moment Basic Signal 180 degrees out of phase

12 Phase Modulation (PM) In Phase (0) 180 degrees out of phase (1) Frequency Modulation (1011)

13 Phase Modulation (PM) n Human hearing is largely insensitive to phase – So harder to grasp than FM, AM n But equipment is very sensitive to phase changes – PM is used in all recent forms of modulation for telephone modems

14 Complex Modulation n Modern Modems Mix Phase and Amplitude High Amplitude Low Amplitude 90 Degrees Out of Phase, High Amplitude In Phase 180 Degrees Out of Phase

15 Complex Modulation n Baud rate: number of times the state can change per second – Usually 2,400 to 3,200 baud for telephone modems n Bits sent per possible state change depends on number of possible states – 2 b =s – b=bits per time period – s=number of possible states – In our example, 2 b =8 – So b must be 3 – 3 bits are sent per time period

16 Complex Modulation n Bit rate = baud rate * bits/time period – bits/time period = 3, as just shown – So if the baud rate = 2,400 – Then the bit rate = 2,400 * 3 = 7,200 bits/second

Multiplexing

18 Multiplexing n Multiplexing mixes the signals of different conversations over a single transmission line – To reduce costs n There are several forms of multiplexing – Time division multiplexing – Statistical time division multiplexing – Frequency division multiplexing – Multiplexing at multiple layers – Inverse multiplexing (bonding)

19 Why Multiplex? n Reason 1: Economies of Scale – 64 kbps lines carry a single 64 kbps signal – T1 lines can multiplex 24 such signals – Yet T1 lines cost only about 3-7* times as much as 64 kbps lines n Example: Suppose you have ten 64 kbps signals – This will require ten 64 kbps lines – But one T1 line will carry them for only 3 to 7* times the cost of a single 64 kbps line n *Textbook says is more realistic New

20 Why Multiplex? n Reason 2: Data transmission tends to be bursty – Uses capacity of a line only a small fraction of the time Signal A Signal B n Multiplexing allows several conversations to share a single trunk line, lowering the cost for each

21 Economics of Multiplexing n Cost Savings – Economies of scale in transmission lines – Multiplexing to lower costs for bursty traffic n Cost Increases – Multiplexing costs money for multiplexers/demultiplexers at the two ends n Net Savings – Usually is very high

22 Time Division Multiplexing n Time is divided into short periods – In each period, one frame is sent n Frame times are further divided – Each subdivision is a slot Slot Frame

23 Simple Time Division Multiplexing (TDM) n Several connections are multiplexed onto a line – In figure, two signals: A and B n Each connection is given one slot per frame – Guaranteed the slot – Slot is wasted if the connection does not use it – Wasteful but still brings economies of scale – Inexpensive to implement AB A Slot not Used

24 Statistical Time Division Multiplexing (STDM) n Still Frames and Slots n But slots are assigned as needed – Connections that need more slots get them – More efficient use of line – More expensive to implement – But STDM is now cost-effective – Multiplexers at both ends must follow the same STDM standard ABAA Frame

25 Frequency Division Multiplexing (FDM) n Signals are sent in different channels – Signals sent in different channels do not interfere – Brings economies of scale – Used in radio transmission A B Frequency Channel

26 Combining TDM and FDM n Use Simple TDM Within a Channel Frequency Channel AB

27 Spread Spectrum Transmission n Ways to mix signals in a channel statistically – Greater efficiency in the use of the channel – Described in Module C Frequency Channel ABA

Carrier Trunk Lines

29 Carrier Trunk Lines n Trunk lines are high-speed lines that connect the switches of carriers n There are several kinds of trunk lines – Optical fiber – Radio transmission – Microwave transmission – Satellite transmission n LEOs n VSATs

30 Optical Fiber n Thin Core of Glass – Surrounded by glass cladding – Inject light in on-off pattern for 1s and 0s – Total reflection at core-cladding boundary – Little loss with distance Light Source Cladding Core Reflection

31 Optical Fiber n Modes – Light entering at different angles will take different amounts of time to reach the other end – Different ways of traveling are called modes – Light modes from successive bits will begin to overlap given enough distance, making the bits unreadable Light Source Reflection

32 Single Mode Fiber n Single Mode Fiber is very thin – Only one mode will propagate even over fairly long distances – Expensive to produce – Expensive to install (fragile, precise alignments needed) – Used by carriers to link distant switches

33 Multimode Fiber n Core is thicker – Modes will appear even over fairly short distances – Must limit distances to a few hundred meters – Inexpensive to purchase and install – Dominates LANs

34 Graded Index Multimode Fiber n Index of fraction is not constant in core – Varies from center to edge – Reduces time delays between different modes – Signal can go farther than if core has only a single index of fraction (step index multimode fiber) – Dominates multimode fiber today

35 Multimode Optical Fiber and Frequency n Signal Frequency has Impact on Propagation Distance before Mode Problems Become Serious n Short Wavelength (high frequency) – Signals do not travel as far before mode problems occur – Uses the least expensive light sources – Good for LAN use within buildings n Long Wavelength (low frequency) – More expensive light sources and fiber quality – Within large buildings and between buildings

36 Wave Division Multiplexing n Use multiple light sources of different frequencies – Place a separate signal on each – Increases the capacity of the optical fiber

37 SONET/SDH n High speed optical fiber trunk system of carriers – Called SONET in the United States – Called SDH in Europe n Arranged in a Dual Ring – If a link is broken, ring is wrapped and still works – Important because broken lines are common because of construction Wrapped Original

38 Radio Transmission n Oscillating electron generates electromagnetic waves with the frequency of the oscillation n Many electrons must be excited in an antenna for a strong signal

39 Omnidirectional Antennas n Signal is transmitted as a sphere – No need to point at a receiver (or transmitter) – Attenuation with distance is very high – Used in mobile situations where dishes are impossible

40 Dish Antenna n Dish captures a (relatively) large amount of signal – Focuses it on a single point (which is the real antenna) – Can deal with weaker signals – You must know where to point the dish – Good in radio trunk lines, some satellite systems

41 Frequency Bands n Propagation Characteristics Depend on Frequency – At lower frequencies, signals bend around objects, pass through walls, and are not attenuated by rain – At higher frequencies, there is more bandwidth per major band

42 Major Bands n Frequency Spectrum is Divided into Major Bands n Ultra High Frequency (UHF) – Signals still bend around objects and pass through walls – Cellular telephony n Super High Frequency (SHF) – Needs line-of-sight view of receiver – Rain attenuation is strong, especially at the higher end – High channel capacity – Used in microwave, satellites

43 Microwave Transmission n Terrestrial (Earth-Bound) System – Limited to line-of-sight transmission – Repeaters can relay signals around obstacles Line-of-Sight Transmission Repeater

44 Satellite Transmission n Essentially, places repeaters in sky – Idea thought of by Sir Arthur C. Clarke n Satellite broadcasts to an area called its footprint n Uplink is to satellite; downlink is from satellite Uplink Downlink Footprint

45 Satellite Frequency Bands n SHF Major Band is Subdivided – Uplink/downlink frequency range (GHz) – Downlink range is always lower CKuKaQ/V Uplink61430Higher Downlink41220Higher UsageHigh GrowingNot Yet

46 GEOs n Satellite orbits at 36 km (22,300 miles) – Orbital period is 24 hours – Appears stationary in the sky – Easy to aim dishes – Very far for signals to travel – Used in trunking

47 LEOs n Low Earth Orbit satellites – Orbits are only 500 to 2,000 km (300 to 500 miles) – Short distance means less attenuation – But 90-minute orbit, so pointing is difficult – Fortunately, close enough for omnidirectional antenna

48 MEOs n Medium Earth Orbit satellites – Orbits are 5,000 km to 15,000 km (3,000-9,000 miles) – Longer distance than LEOs means more attenuation – But longer orbit, so fewer hand-offs – Still close enough for omnidirectional antenna New

49 VSATs n Very Small Aperture Terminal satellite system – Small dishes for remote sites (0.25 to 1 meters) – Inexpensive for remote sites – Central hub is powerful and has large dish – Satellite is powerful – Used in direct broadcast for television – Companies use VSATs to bypass carrier networks

50 Satellite Limitations n Limited bandwidth, so expensive n Propagation delays for GEOs – Can be bad for data transmission n More expensive than fiber for high-capacity trunk needs

51 Specialized Satellite Usage n Thin Routes – Trunks of low volume – Company with several sites n Multipoint Transmission (One-to-Many) – Direct broadcast satellites for television – Distribute cable television channels to local systems – Distribute marketing information to remote sites n Mobile Systems – Cannot have a wire on a truck – Laptop Internet access