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Telecommunications Concepts

Explore the use of Telephone network for data transfer, including leased lines, local loop multiplexing, XDSL, PSTN data transfer, analog modems, ISDN, GSM, GPRS, WAP, UMTS, and more. Learn about various concepts and technologies involved in leveraging the telephone network for efficient data transmission.

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Telecommunications Concepts

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  1. Chapter 2.2 The Telephone Network used for Data Transmission TelecommunicationsConcepts

  2. Using the Telephone network for data Leased lines Local loop multiplexing : XDSL PSTN data transfer and analog modems ISDN data transfer GSM, GPRS, WAP and UMTS. Contents

  3. Using the Telephone network for data Leased lines Local loop multiplexing : XDSL PSTN data transfer and analog modems ISDN data transfer GSM, GPRS, WAP and UMTS. Contents

  4. SW PABX PABX Leased Lines from a TELephone COmpany SW SW SW SW SW SW

  5. Modem Modem Local Analog Leased Lines Both endpoints need to be wired to the same exchange 4 wires = 2 twisted pairs max throughput function of length

  6. Digital Leased Lines Via the SDH infrastructure SDH MUX twisted pairs or optical fibers High Speed modem High Speed modem n * 64 Kb/s or 56 Kb/s n * 2048 Kb/s or 1544 Kb/s

  7. Using the Telephone network for data Leased lines Local loop multiplexing : XDSL PSTN data transfer and analog modems ISDN data transfer GSM, GPRS, WAP and UMTS. Contents

  8. Data Rate = 0 - 6 Mb/s Energy Data Voice Frequency 300 3300 5000 1MHz X D S LFrequency Domain Multiplexing of Voice and Data over the local loop The bandwidth of the analog local loop is much larger than what is needed for analog or digital telephony

  9. ADSL ADSL ROUTER Co-located equipment SW A D S Lfor residential Internet access 600 Kb/s 6 Mb/s Subscriber's line Analog Voice Local telephone exchange

  10. ADSL ADSL Video Server Co-located equipment A D S Lfor Video On Demand A few Kb/s A few Mb/s Subscriber's line Analog Voice SW Local telephone exchange

  11. Using the Telephone network for data Leased lines Local loop multiplexing : XDSL PSTN data transfer and analog modems ISDN data transfer GSM, GPRS, WAP and UMTS. Contents

  12. 1 2 3 4 5 6 7 8 9 1 2 3 * 0 # 4 5 6 7 8 9 * 0 # Tone dialingDTMF data transmission 697 Hz 770 Hz 852 Hz 941 Hz In - band signaling : audio tones 1209 Hz 1336 Hz 1477 Hz

  13. DTMF over the PSTN PSTN Voice interface Signaling Prerecorded voice messages Numerical answers

  14. Modem with acoustic coupler (1985) PSTN Modems Transmit data by means of signals compatible with the analog telephone network

  15. DCE-DCE PSTN Modem Modem DTE DCE DCE DTE Modem Protocols DTE-DCE DCE-DTE

  16. V21 Asyn FD 0 - 300 b/s for mechanical teletypes V22 Syn FD 1200 b/s general purpose 1980-85 V22b Syn FD 2400 b/s general purpose 1985-90 V23 Asyn FD 75/1200 b/s Minitel V29 Syn HD 9600 b/s FAX V32 Syn FD 9600 b/s general purpose 1990-95 V34 Syn FD  33600 b/s general purpose 1995- ??? ( V-fast and V-last ) V90 Syn FD  56 Kb/sSpecial purpose modem with restricted applicability on the PSTN, for Internet access DCE to DCE ProtocolsThe V series Modems

  17. V21 Modem0-300 b/s - Full Duplex - FSK Used for electromechanical terminals and acoustic coupled modems Energy A>B B>A Frequency 980 1650 1850 2100 1180 A = Communication initiator

  18. V23 Modem0-75/0-1200 b/s - Full Duplex - FSK Used for Minitel in France Energy A>B B>A Frequency 390 450 1300 2100 A = Communication initiator

  19. A>B B>A 1200 1800 2400 V22 Modem1200 b/s, 600 Bd, Full Duplex through FDM General Purpose Modem (1980-1985) Energy Frequency A = Communication initiator

  20. A>B B>A 1200 1800 2400 V22bis Modem2400 b/s, 600 Bd, Full Duplex through FDM General Purpose Modem (1985-1990) Energy Frequency A = Communication initiator

  21. V29 Modem9600 b/s, 2400 Bd, Half Duplex Modem for FAX transmission Energy Frequency 1800

  22. V32 Modem9600 b/s, 2400 Bd, Full Duplex General Purpose Modem (1990-1995) Energy Frequency 1800

  23. Problem : insufficient bandwidth for FDM Simple Solution : Superpose incoming and outgoing signal Incoming signal = total signal - outgoing signal Outgoing signal is known ! Practical difficulties : Outgoing signal >> incoming signal Outgoing signal can be reflected at several points along the transmission channel EchoCancellation

  24. Echo Cancellation Correlation between transmitted and received signals without cancellation Tx Correlator Rx t 20 mS 10 0

  25. Complete solution : Minimize correlation between outgoing signal and signal at the input of the receiver section after subtraction of delayed and attenuated copies of outgoing signal. Necessary requirement : Outgoing and incoming signals uncorrelated Fulfilled by means of scramblers EchoCancellation

  26. Echo Cancellation Tx Delay lines Correlator - Rx

  27. Full duplex modemsover single pair telephone links • Frequency domain multiplexing (V21,V22,V23,..) • Divides the available bandwidth • Reserved for low speed modems • Time domain multiplexing (ping-pong) • Mainly used for half-duplex applications (such as FAX). • Echo Cancellation (V32, V34, ..) • Uses the full bandwidth • Full duplex transmission does not excessively degrade the S/N ratio.

  28. Instead of 16, 32 distinct states are transmitted Four data bits are encoded in 5 bit patterns. Each pattern is function of the 4 data bits, the previous patterns, and the modulation scheme. As each pattern encodes only 4 bits, not all sequences of patterns can be generated. Each possible erroneous sequence is closest to one and only one correct sequence Replacing any erroneous sequence by the closest correct one improves the error rate by a factor 10 to 100. Trellis encoding

  29. V34 Modem<= 33600 b/s, Full Duplex General Purpose Modem (1995 and thereafter – “V last” : Shannon’s limit) "Best effort modem" : measures bandwidth and signal/noise during initialization Power ? Frequency

  30. Data rate selection Maximum baud rate = f(bandwidth) Data rate = standardized to multiples of 2400 b/s Maximum data rate = function of acceptable error rate (= 10-5 .. 10-6) baud rate signal/noise ratio trellis coding scheme Data rate  baud rate * integer number V34 Modem<= 33600 b/s, Full Duplex

  31. V42 : Error correction. Data grouped in blocks Error correction by detection and retransmission Improves apparent error rate on poor links Variable transmission delay !!! V42 bis : Data Compression. Apparent throughput increased by optimal choice of data encoding over transmission link. Similar proprietary protocols also available V42 Modem options

  32. DCE-DCE PSTN Modem Modem DTE DCE DCE DTE Modem Protocols EIA232 USB … EIA232 USB … DTE-DCE DCE-DTE

  33. Electronic Industries Association EIA 232 D Electrical Mechanical Functional Procedural RS232C ITU - CCITT V24 Functional Procedural V28, V10, V11, … Electrical Mechanical Modem Interfaces

  34. Some EIA232 control lines 25 Pin 1 2 3 4 5 6 7 8 15 17 20 22 24 9 Pin 3 2 7 5 4 Name Protective Ground Transmitted data Received data Request to send Clear to send Data set ready Signal Ground Data carrier detect Transmitter clock (DCE) Receiver clock Data terminal ready Ring detector Transmitter clock (DTE) V24 101 103 104 105 106 107 102 109 114 115 108 125 113 DTE-DCE > < > < < <<< > < >

  35. Null Modem To interconnect directly a Data Terminal Equipment with another Data Terminal Equipment 1 2 3 4 5 6 7 20 Protective Ground Transmitted data Received data Request To Send Clear To Send Data Set Ready Signal Ground Data Terminal Ready

  36. Using the Telephone network for data Leased lines Local loop multiplexing : XDSL PSTN data transfer and analog modems ISDN data transfer GSM, GPRS, WAP and UMTS. Contents

  37. ISDN : the S Bus 2 B Channels (64 Kb/s) + D Channel (16 Kb/s) S interface U interface Network Terminator Terminal Adapter 192 Kb/s Up to 8 terminals

  38. Belgacom TWIN S bus available for extensions 192 Kb/s Up to 8 terminals Terminal Adapter U interface Network Terminator Terminal Adapter NT12ab

  39. DTMF over the ISDN ISDN Voice interface Signaling via D channel Prerecorded voice messages (B) Numerical answers (B) Ad-hoc digital DTMF generator required in ISDN phones

  40. 64 Kb/s ISDN ra ra 1920 Kb/s Data over the ISDN Rate Adapter : appears to the user as a synchronous modem, the clock being generated in the network. n * 64 Kb/s possible but some timing problems need to be solved in the RA's (# delays over # channels !)

  41. ISDN m m m m m m Data over ISDN and the PSTN PSTN V110 PSTN ISDN ISDN PSTN

  42. Today's residential Internet access problem: Residential users most use analog lines (data rate <= 33600 b/s) don't want ISDN (64kb/s) because of cost almost entire PSTN digital at 64kb/s Internet Service Providers can afford ISDN A solution: Split the modem on the ISP side into two parts The AtoD and DtoA converter in the user’s local exchange (in fact, the normal ISDN codec) The (digital) modulator/ demodulator at the ISP site. Asymetric data rates : From user to ISP : <= 33.6 Kb/s (V34) From ISP to user : 56 Kb/s (there is no normal AtoD converter involved) Hybrid ISDN/PSTN modems

  43. Hybrid V90 modems ISDN m’ m m” PSTN • Data transfer from ISP to user (in 56 K V90 mode): • m’ is a rate adapter and encoder transmitting data at 56 Kb/s (8000 samples with 7 bit each). • m” contains a normal ISDN D to A converter • m decodes the analog 8000 baud signal into a 56 Kb/s digital stream • Data transfer from user to ISP : • m contains a V34 modem transmitting from user to ISP • m” contains a normal ISDN A to D converter • m’ contains a V34 modem accepting already digitized V34 analog signals. • m’ + m” together constitute in fact the other modem

  44. Using the Telephone network for data Leased lines Local loop multiplexing : XDSL PSTN data transfer and analog modems ISDN data transfer GSM, GPRS, WAP and UMTS. Contents

  45. The Telephone NetworkSome details : GSM wireless network Digital switch Vocoder in phone GSM base station 13 Kb/s + signaling error rate  10-3

  46. The Telephone NetworkSome details : GSM wireless network Digital switch GSM base station VOCODER 13kb/s 64kb/s

  47. DTMF over the GSM GSM PSTN ISDN Voice interface V V DTMF Signaling Prerecorded voice messages (B) Numerical answers DTMF Numerical answers send to DTMF generator as short messages

  48. m m m m m ra ra Circuit Switched Data over GSM PSTN GSM v v V110 PSTN GSM GSM ISDN GSM

  49. V110 for GSM Async/ sync Data MUX Sampling GSM 13 kb/s ECC Modem Control • Raw data rate = 13 kb/s • Net data rate = 9.6 kb/s • Difference used for • Modem control signals • Forward error correction

  50. The GSM switched circuit is slow and expensive for data transmission Packet switching is better for data Statistically many radio slots are unused, even when all available circuits are in use. Conclusion : Use the available radio slots for packet data !!! Charges are volume based or fixed Example : MMS GPRS Concepts(General Packet Radio Services)

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