1. CS3505, the Internet -- physical layer

2. physical layer - purpose  To transmit bits, by encoding them onto signals; and to receive the signals, interpreting them as bits input : sequence of bit S, from DL or MAC layer output : sequence of bit S’, to DL or MAC layer ideally, S = S’

3. Physical layer definitions  signal 1. mechanism used to carry information over time OR distance 2. sign or gesture giving information 3. sequence of electrical or optical impulses or waves

4. Signals  examples:  physical gesture, wave, hand signal  flashes of light (eg, Morse code)  sound: vary tone, loudness or duration  flags  smoke  mirrors  electical voltages

5. transmission definition 1. the action of conveying electrical or optical signals from 1 point to 1 or more other points in space 2. the process of sending information from 1 point to another  things necessary for transmission systems  path for signal transfer (medium)  transform signal to appropriate from (code)  launch the signal (transmit)  remove, receive or detect the signal (receive)

6. signal - can be modeled as f(t) t --> f(t)... f(t) represents some physical quantity: voltage, amplitude, frequency, etc.

7. digital/ analog signals  digital signal 1. assumed to take on finite number of values, AND 2. has meaning only at discrete points in time.

8. digital/analog signals  analog signal: 1. a signal that is an analog of the quantity being represented; eg, signal voltage proportional to volume of sound 2. continuous in range of values 3. also continuous in time; always valued.

9. digital/analog signals  digital data : text, bits; discrete valued.  analog data : sound, vision; music, etc. continuous valued. Note: digital (analog) signals can transport both digital and analog data, so 4 combinations (DD,DA,AD,AA) possible

10. transmission media  transmission medium : the physical element through which signals must pass, from transmitter to receiver  examples: air, water, space, copper wires, optical fiber  two main categories: guided and unguided  propagation delays of signals in media

11. transmission terminology  spectrum - range of frequencies making up a signal  bandwidth  data rate

12. transmission terminology  bandwidth key factor in determining data rate;  however do not confuse bandwidth (hertz) and and data rate (bps)  obstacles to transmission--  attenuation  interference  cross talk

13. modems, codecs  modem ( modulator-demodulator )  translates a digital signal (bit) into an analog signal, for transmission as an analog signal; receives the corresponding analog signal, and translates back into digital (bit)  purpose: use analog medium for digital data/signals  example: PC modem, phone lines; TV cable modems

14. modems, codecs, bauds, bits  codec (codec/decoder)  converts analog data into digital form (bits), and the reverse.  main technique: PCM  PCM ( pulse code modulation )  absolute values, based on sampling theorem; (nearly) total information

15. channel capacity  channel - a path in the transmission medium through which signals/bits may pass  channel capacity - maximum number bits/sec the channel can support  factors which determine channel capacity  bandwidth  signalling technique  noise

16. transmission media  Guided Media  twisted pair (copper)  coaxial cable (copper)  optical fibers (silicon... plastic or glass)  Unguided Media  broadcast radio frequencies  terrestrial microwave  satellite microwave  infrared, FSO

17. transmission media : twisted pair  copper a good conductor of electricity  2 copper wires form circuit  twisting gives better electrical properties  backbone of the local telephone system  also heavily used in data comm., LANs  used for both digital, analog signals  basic telephone wire vs. "Cat 5"

18. transmission media : twisted pair  various quality levels: voice grade, “Cat 5”  data rates: 1-100 Mbps, depending on quality; voice grade at low end, Cat 5 top end.  higher quality are more tightly twisted  advantages  mature - well known technology  connections, splicing easy  production, installation techniques well known  relatively cheap, easy to install

19. transmission media : twisted pair  disadvantages  cost of copper  signal attenuation increases with frequency, starting at low frequencies  often needs shield to reduce noise pickup  suseptible to cross talk if lines close together  susceptible to lightning strikes  less bandwidth

20. transmission media : coaxial cable  thick cable, consisting of an inner copper core, insulator, surrounded by another conductor (braided shield), wrapped in a protective shield and outer cover  Properties (approx.)  bandwidth: ~500Mhz, analog  data rates: 500 Mbps or more  repeater spacing: 1-10 Km  Two basic types:  broadband (TV)  baseband (ethernet)

21. transmission media : coaxial cable  broadband: TV cable, analog signals  baseband: LANs, digital signals  Uses  long distance telephone  cable TV  LANs  Note: higher capacity than t.p., but also bulkier and difficult to work with in limited spaces

22. transmission media : coaxial cable  advantages  lower attenuation than t.p. at high frequencies  wider usable bandwidth  less susceptible to interference  easy to tap  disadvantages  physically bulky  limited bending radius  heavier

23. transmission media : optical fiber  OF a major milestone in communications; first fibers developed early1970s  since about 1988, majority of all U.S. long distance traffic over OF  due to OF, the networks have potential to be faster than the computer ---- a big flip flop

24. transmission media : optical fiber  A thin, flexible medium of extremely pure plastic/glass. Thickness about 2-125 microns. Core often 62.5 microns.  much higher data rates; 100M to several G.  prop. speed approximately 2/3 c, as with tp and coax; bits much smaller  repeater spacing: much higher... up to thousands of miles

25. transmission media : optical fiber  principle: each bit is transported by a tiny ray of light(darkness), guided by the medium.  requires extremely accurate transmitters, receivers; much finer degree of synchronization

26. transmission media : optical fiber  principle: total internal reflection  Two major types of fiber 1. multi-mode 2. single mode/monomode  limitations  modal dispersion (multimode)  material dispersion (single mode)  attenuation (single mode, at very high data rates)

27. transmission media : optical fiber  advantages  much higher bandwidth, real and potential  very low radiation, noise pickup; shielding not needed, crosstalk not a problem  very low attenuation, and little variation in 100-300 Mhz range  not susceptible to lightning, etc.  small physical size and weight  cost will decrease  very difficult to tap

28. transmission media : optical fiber  disadvantages  cost  technology less mature  splicing difficult and critical  installation more difficult  Key note: fiber has literally made the network faster than the computer. We have far to go before we reach the potential data rates of fiber....

29. unguided media : broadcast radio  lower frequency ranges: roughly 30MHz-1GHz  omnidirectional  data rates not as high as microwave, so less useful for data, but good for broadcast radio  better progagation characteristics; less attenuation, less interference from rain, etc.

30. unguided media  lower frequency ranges: broadcast radio 30-300 kHzMFAM radio 3-30 MHzHFshortwave radio, CB 30-300 MHzVHFFM radio, VHF TV  microwave frequency ranges: 2 to 40 GHz  infrared: just below visible light; frequency in hertz 10 11 - 10 14

31. unguided media : terrestrial microwave  focused beam, 1-2 degrees  high frequencies 3-20 GHz --> high data rates  paraboloid shaped antennas  better repeater spacing than cable  high data rates  more susceptible to rain, clouds, dust, etc. than others

32. unguided media : satellite microwave  high frequency; ( ~same as terr. uwave)  geosynchronous satellite --> repeater in sky  broadcast media  22,300 miles --> 35,000 Km  receives, xmits on diff. frequencies to avoid interference

33. unguided media : satellite microwave  need spacing of 4 deg. between satellites  significant prop delay ~ 250 ms  less difficulty with atmosphere  3 major differences with terr. microwave

34. unguided media : FSO, or free space optics  uses optical signals in open air  limited distances only  very high transmission rates possible  much faster to set up and take down then conventional wired networks  security less a problem than other wireless methods due to focused beam and limited distance  very useful in some cases

35. unguided media : infrared  uses infrared light to transmit bits  similiar characteristics as free space optical  also very useful in some cases

36. voice channel & telephone system  basic telephone network designed to deliver quality voice service  voice emits analog signal - sound waves - from 30 to 10,000 Hz. Human ears detect up to 20K Hz.  most energy in 200-3500 Hz range; Standard analog voice channel is 4000 Hz. This key number selected many years ago by phone company.  standard PCM digital voice channel is 64 Kbps.  most local telephone loops still analog  all long distance in US is digital; majority is fiber.

37. voice channel & telephone system  voice not very sensitive to most noise and distortion; for this and other reasons, local telco loops not well suited to modern data networks  However, the local telco networks are one of few comm. links between homes, businesses and rest of the world  Structure of U S Telephone networks /companies  local loops “last mile” and telcos  long distance networks and companies  network equipment

38. video channels and cable TV system  TV cable system established recent decades  switching equipment designed for broadcast TV  standard TV - 6 MHz per channel  coaxial cables capable carry many TV channels.  these have capacity to carry thousands of voice channels and/or high speed data -- but need appropriate switching equipment at home office, and in homes  AT&T attempted, failed to use coax cable system for Internet, voice -- probable a business error