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Computer Network

Computer Network. Andrew S. Tanenbaum. Outline. The mobile telephone system Cable television Wireless LANS Broadband wireless Bluetooth Data Link layer switching Quality of service. Outline. The mobile telephone system Cable television Wireless LANS Broadband wireless Bluetooth

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Computer Network

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  1. Computer Network Andrew S. Tanenbaum

  2. Outline • The mobile telephone system • Cable television • Wireless LANS • Broadband wireless • Bluetooth • Data Link layer switching • Quality of service

  3. Outline • The mobile telephone system • Cable television • Wireless LANS • Broadband wireless • Bluetooth • Data Link layer switching • Quality of service

  4. Wireless telephones • Cordless phones -Never used for networking • Mobile phones – through three generations with different technologies: -Analog voice -Digital voice -Digital voice and data

  5. The mobile telephone system • First-generation mobile phones: analog voice -IMTS -AMPS • Second-generation mobile phones: digital voice -D-AMPS -GSM -CDMA • Third-generation mobile phones: digital voice and data -W-CDMA -CDMA2000 • 2.5G scheme -GPRS

  6. First-generation mobile phones: analog voice • 1946 push-to-talk system -A single channel for both sending and receiving • 1960 IMTS (Improved Mobile Telephone System) -High-powered transmitter -Two frequencies (sending/receiving) -23 channels spread out from 150 MHz to 450 MHz

  7. IMTS drawbacks • Due to the small number of channels, users often had to wait a long time before getting a dial tone. • Due to the lager power of the hilltop transmitter, adjacent systems had to be several hundred kilometers apart to avoid interference.

  8. AMTS (Advanced Mobile Phone System) • In all mobile phone systems, a geographic region is divided up into cells. • Key idea: -increases the system capacity (reuse of transmission frequencies) -less power is needed.

  9. The idea of frequency reuse

  10. MTSO (Mobile Telephone Switching Office) • Handoff - soft handoff -no loss of continuity -telephone needs to be able to tune to two frequencies at the same time - hard handoff

  11. AMTS Channels • The AMPS system uses 832 full-duplex channels, each consisting of a pair of simplex channels -832 simplex transmission channels from 824 to 849 MHz -832 simplex receive channels from 869 to 894 MHz • AMPS uses FDM to separate the channels

  12. AMPS call management • When a phone switch on • When a caller makes a call

  13. Second-generation mobile phones: digital voice • D-AMPS is fully digital • D-AMPS is designed to co-exist with AMPS • Upstream channels are in the 1850-1910 MHz • Downstream channels are in the 1930-1990 MHz

  14. D-AMPS • The voice signal is digitized and compressed • Users can share a single frequency pair using TDM

  15. A D-AMPS channel with users

  16. Difference between AMPS and D-AMPS • How handoff is handled

  17. GSM (The Global System for Mobile Communications) • GSM versus D-AMPS: -FDM is used with each mobile transmitting on one frequency receiving on a higher frequency -A single frequency pair is split by TDM into time slot shared by multiple mobiles -GSM has a much higher data rate per user than D-AMPS

  18. GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

  19. A partition of the GSM framing structure

  20. CDMA (Code Division Multiple Access) • D-AMPS , GSM use both FDM and TDM. • CDMA allows each station to transmit over the entire frequency spectrum all the time. • Multiple simultaneous transmissions are separated using coding theory.

  21. CDMA coding theory • Each bit time is subdivided into m short intervals called chips (There are 64 or 128 chips per bit). • Each station is assigned a unique m-bit code called a chip sequence. - To transmit 1 bit , a station sends its chip sequence - To transmit 0 bit , a station sends the one’s complement of its chip sequence

  22. Example (1/2)

  23. Example (2/2)

  24. Properties

  25. Third-generation mobile phones: digital voice and data • WCDMA (Wideband CDMA) -uses direct sequence spread spectrum -runs in a 5 MHz bandwidth -has been designed to interwork with GSM • CDMA2000 -not be designed to interwork with GSM -has the differences between WCDMA : chip rate, frame time, spectrum used, the way to do time synchronization

  26. 2.5 G schema GPRS (General Packet Radio Service) • Is an overlay packet network on top of D-AMPS or GSM. • Allows mobile stations to send and receive IP packets in a cell running a voice system

  27. Outline • The mobile telephone system • Cable television • Wireless LANS • Broadband wireless • Bluetooth • Data Link layer switching • Quality of service

  28. Cable television • Community antenna television • Internet over cable • Spectrum allocation • Cable modems • ADSL versus cable

  29. An early cable television system

  30. Part of a modern HFC system

  31. The fixed telephone system

  32. Spectrum allocation

  33. Cable modems • Internet access requires a cable modem • Cable modem is always on • Cable operators do not charge for connect time

  34. What happens when a cable modem is plugged in and powered up? (1/2) • The modem scans the downstream channels looking for a special packet periodically put out by the headend to provide system parameters to modems. • Modem announces its presence on one of the upstream channels • The headend responds by assigning the modem to its upstream and downstream channels

  35. What happens when a cable modem is plugged in and powered up? (2/2) • The modem determines its distance from the headend –ranging • During initialization, the headend also assigns each modem to a minislot to use for requesting upstream bandwidth • What happens when a computer wants to send a packet?

  36. Typical details of the upstream and downstream channels

  37. ADSL versus cable • Both use fiber in the backbone, but they differ on the edge • The increasing numbers have different effects on existing users on the two system • Availability and security and reliability are issues on which ADSL and cable differ

  38. Outline • The mobile telephone system • Cable television • Wireless LANS • Broadband wireless • Bluetooth • Data Link layer switching • Quality of service

  39. Wireless LANS • The 802.11 protocol stack • The 802.11 physical layer • The 802.11 MAC sublayer protocol • The 802.11 frame structure • services

  40. The 802.11 protocol stack • MAC sublayer determines how the channel is allocated, that is, who gets to transmit next • LLC sublayer hides the differences between the different 802 variants • 802.11 specifies three transmission techniques allowed in the physical layer -infrared method -short-range radio (FHSS/DSSS)

  41. Part of the 802.11 protocol stack

  42. The 802.11 physical layer(1/7) • Infrared option -uses diffused transmission at 0.85 or 0.95 microns -two speeds are permitted: 1 Mbps, 2Mbps -infrared signals can’t penetrate walls

  43. The 802.11 physical layer(2/7) • FHSS (Frequency Hopping Spread Spectrum) -uses 79 channels, each 1 MHz wide, starting at the low end of the 2.4GHz ISM band -A pseudorandom number generator is used to produce the sequence of frequencies hopped to

  44. The 802.11 physical layer(3/7) -The amount of time spent at each frequency—dwell time -advantages: 1.a fair way to allocate spectrum 2.security 3.good resistance to multipath fading 4.relatively insensitive to radio interference -disadvantage: low bandwidth

  45. The 802.11 physical layer(4/7) • DSSS (Direct Sequence Spread Spectrum) -restricts to 1 or 2 Mbps -has some similarities to the CDMA system -each bit is transmitted at 11 chips, using Barker sequence -uses phase shift modulation

  46. The 802.11 physical layer(5/7) • High-speed wireless LANs, 802.11a, uses OFDM (Orthogonal Frequency Division Multiplexing) -deliver up to 54 Mbps in the wider 5GHz ISM band -advantages: 1.good immunity to multipath fading 2.using noncontiguous bands (good spectrum efficiency)

  47. The 802.11 physical layer(6/7) • 802.11b uses HR-DSSS (High Rate Direct Sequence Spread Spectrum) -uses 11 million chips/sec to achieve 11Mbps in the 2.4GHz band -data rate 1,2 Mbps use phase shift modulation (compatibility with DSSS) -data rate 5.5,11Mbps use Walsh/Hadamard codes

  48. The 802.11 physical layer(7/7) • Although 802.11b is slower than 802.11a, its range is about 7 times greater. • 802.11g uses OFDM modulation of 802.11a, but operates in the narrow 2.4GHz ISM band along with 802.11b

  49. The 802.11 MAC sublayer protocol • The 802.11 MAC sublayer protocol is quite different from that of Ethernet due to the inherent complexity of the wireless environment compared to that of a wired system

  50. Two Problems

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