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Access Networks - exercise s 1 ,2

Access Networks - exercise s 1 ,2. 200 8 /0 9. Copper lines (symmetrical pairs) properties. Ľ. Maceková - KEMT – FEI – TU – Košice - SR. Introduction. Transceiver. Transceiver. xDSL. xDSL. Apli cation Interface. Digit al part. Digit al part. Apli cation Interface. Line parameters.

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Access Networks - exercise s 1 ,2

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  1. Access Networks- exercises 1,2 2008/09 Copper lines (symmetrical pairs) properties Ľ. Maceková- KEMT – FEI – TU – Košice - SR

  2. Introduction • ...

  3. Transceiver Transceiver xDSL xDSL Aplication Interface Digital part Digital part Aplication Interface Line parameters User Interface Analogue part Analogue part User Interface Dig.signal parameters on physical transmission layer Transmission parameters of higher layers Param. of transmission device Transmiss.parameters of user interface Fig.1 The areas of measurement and diagnostics of xDSL transmission chain

  4. Transmission media classification and frequency functionalities [2] see also tab. cable comparing Wire lines – in outdoor (external, open air) – telephone wires above ground – 150 kHz - telephone l.with extended band - 1300 kHz (240 teleph.chann.) - vhv in energetics - 700 kHz - symmetrical l.cables - LFwith load coils l. - 15 kHz - LF unloaded - 120 kHz (12 tel.channels, 1st order PCM) - HF - 552 kHz (120 tel.chann., 2ndorder of PCM) - asymmetrical cables - microcoaxial c. 0,8/2,7 mm - 18 MHz (1440 tf. chann.) - little coax.cable 1,2/4,4 mm - 139 MHz (4th order of PCM) - middle coax.c. 2,6/9,5 mm - hundreds of MHz - waveguides - Φ 50 mm - 110 GHz (50 000 tf. chann., 40 TV) Light guide l. (optical),0,85 - 1,55.10-6m - singlemode f. - 10 GHz - multimode - step index fibers - 100 MHz - graded index fibers - 1 GHz FSO – Free Space Optics – LASER ray through free space Wireless (radiowave) connection - microwave (RR connections) - 14 GHz,necessary line of sight (2700 tf. chann., 1 TV) - troposphere connec. - 80 GHz - satellite conn. - 80 GHz - stratosphere conn. (HAP – High Altitude Platforms) – in development

  5. Properties of symmetrical pairs [3] -see also this -symmetrical pair – twisted pair~ long line (uniformly distributed line) = circuit with spread parameters equivalent circuit – with parameters R,G,L,C (with lumped-elements) - L, C suppress transmitted band (LP) Line characteristic impedance Measurements of Z0:

  6. Properties - continue 1 Noises and other disturbances (see also [7]) • internal system disturbance – mostly, it is the white noise(AWGN – additive whte Gausse noise with low power level about -140 dBm/Hz, which is power spectral densityvalue), i.e. thermal noise in all real resistance part of lines and at the input of receivers: • High frequency disturbance (RFI = Radio Frequency Interference) – interference in all pairs in the cable in the whole freq.band with various intensity • impulsive interference from various sources  the terms ingress, and in oposite side, there is egress(disturbing emission of radiation from line) k=1,38.10-23 WsK-1 The other properies of symmetrical pairs -their values must match the eur. standards EN 20288 for transmission cables for analogue and digital systems - There are evaluated (measured) DC and LF parameters like loop resistance, operational capacity, capacitance inbalance,capacitancy unbalance; and mainlyvf parameters: line attenuation, crosstalks NEXT and FEXT, longitudinal balance (LCL), return loss, charakteristicimpedance, propagation velocity

  7. Properties - continue 1 Impedance matching Obr. dole: Impedance of twisted pairs in dependence of frequency. TA- with air separation, TE-lineswith plastic isolation.

  8. Symmetrical pairs properties - continue Impedance matching • condition for max. exploitation of power: • Zsource= Z0 a ZLoad = Z0 • in other case: ... unmatching , reflections, more attenuation or completely loss of signal ! Reflection coefficient, Return Loss = RL (attenuationof reflection) - if impedances match r = 0, RL   - the case of total reflection: r = 1, RL = 0

  9. Properties - continue Specific measure of ...: α ...specific attenuation[Np/km], 1Np=8,686 dB β …specific phase shift [rad/km] Line Attenuation (overall attenuation) : A = α .l .... l... line length

  10. Properties - continue 3: Insertion Loss (attenuation)– a definition by means of powers: PR PT transfer path (line) PR – receiver power PT – transmition power • in[dB],A is less then 0 dB (negative) by definitionabove • the more often definition – for positive value of attenuation: • A[dB] = 10 log (PT / PR) …… > 0 dB [W] definition of dBW and dBm P[dBW]=10logP[W] = 10log(PT/1mW) – 10 log( PR /1mW) P[dBm]=10logP[mW] A[dB] = P T[dBm] – P R[dBm] [dBmW]

  11. Symmetrical pairs properties – continue: Longitudinal Balance = LB(It tells about unbalanced impedances Za, Zb of wires in pair.) LB = 20 (log10|Za+ Zb| / |Za– Zb|) [dB] ... > 0 dB LB measurement principle: Vdiff LB = 20 log10(Vcomm / Vdiff) [dB] > 0 dB

  12. Symmetrical pairs properties – continue: Crosstalks Pair 1 Pair 2 • NEXT - Near End CrossTalk – influencing of the 2nd pair by 1st pair on same end of cable • FEXT - Far End CrossTalk - … • the values of NEXT and FEXT depend on their position one-to-another (see the fig. on the next slide) – elimination of these influences (iterferences) is gained by multiplying of twisting of both wires in all pairs (we must realise also, that the less is radiation of the pair, the less is its tendention to be influenced and vice versa...) • aggregate disturbing is a function of all fractional pairs disturbing (see 16th slide) • the important value is NEXT- and FEXT Attenuationsandparameter ACR (Attenuation-to-Cross talk Ratio)

  13. Examples of twisted pairs cables (STP, UTP, S/STP, S/UTP=FTP) screened/ shielded twisted-pair quad 1 pair of quad neighbouring pairs 1 pair of quad 10 pairs subgroup • insulation is plastic (PE), sometimes foam insul.(less specific capacitance), wires: 0,4; 0,6; 0,8 mm and others • = core + jacket (Pb,Al or PE+ steel armouring = mechanical shielding and screening) 50 pairs group distant pairs near pairs near pairs cable screening neighbouring pairs cable jacket [5, 6]

  14. S/STP cable[6]

  15. NEXT- and FEXT attenuations measurements [1] disturbing pair disturbed pair receiver disturbing pair disturbing pair disturbed pair receiver

  16. then,k-th pairin multicore cable, where several pairs are used to communication, may be disturbed by other pairs as follows: • PSNEXT - Power SumNEXT: [dB] whereANEXT,k ... near end crosstalk between disturbing pair iand influenced pairk n ... number of pairs in cable

  17. Link properties impairments[3]

  18. The examples of real telecommunications cables • For good operation the screening of each pair is needed at high frequences, for prevent from crosstalks • Cables can have pair or quad construction, with wire diameter Φ from 0.4 to 0.8 mm, with plastic insulation based on PE or PP • Reference length for other parameters (specific parameters) is mostly 100 m. • Conductor loop resistance (DC loop resistance) must not be more then 30 Ω / 100m. • Insulation resistance of 1 km line must not be less then 500 MΩ • Capacitance unbalance to earth must not be more then 1600 pF/ km • Velocity of propagation must be 0.6.c or more at frequency 1 MHz and 0.65.c or more at freq. 10 and 100 MHz (c is velocity of electromagnetic waves propagation in vacuum). Difference of propagation velocity between pairs of cable must not be more then 40ns/100m (?) • The limits for line attenuation (of length 100m) and for ANEXTare introduced in Tab.1 and Tab.2. These limits must not be overshot in the whole determined frequence range. Tab.1 ... for cables up to 100 MHz Tab.2 ... for cables up to 600 MHz

  19. other parameters values: Input impedance must be nominally 100 Ω for unshielded cables, for shielded cables there are possible values 100, 120 and 150 Ω with allowance ± 15 Ω in the band from 1 to 100 MHz. (For cable up to 600 MHz is allowance ± 15Ω in the band up to 300 MHz and ± 25 Ω in the band from 300 to 600 MHz.) Return loss must be more then 23 dB in the band 10 – 100 MHz, in higher bands it can be less... etc. (lit. [1])

  20. TDR - Time Domain Reflectometry = measurement method - diagnostics and location of impairments (damages, defects or fails) of metalic twisted pairs defect locality reflection due to resistance character impairment time defect locality reflection transmitted impulse reflection due to capacitance character of the fault inerruption leakage Fig.Resistence and capacitance imperfection Fig. Principle: we can measure the time and shape of reflected impulse from impedance mismatch, caused by a fault on the pair

  21. TDR - continue - velocity of propagation in solid media (in cable): c = 2,9979.108 m/s εr = 1 for vacuum, 1,0167 for air, 2 – 5 for plastic Sometimes PVF (Propagation Velocity Factor)is mentioned, that is ratio between the velocity of propagation in cable to velocity of light in vacuum 0,6 or more) If we measure tx.... then we can locate the place of fault in the pair (the distance of fault from measurement device)

  22. References • [1] J. Vodrážka, M. Havlan: Přístupové přenosové systémy, Cvičení – Měření na na přípojkách xDSL. ČVUT, Praha,2003 • [2] V.Kapoun: Přístupové a transportní sítě. VUT v Brně, 1999. • [3]http://www.trendcomms.com/multimedia/training/broadband%20networks/web/main/Copper/CoverCopper.html • [4]V.Tarageľ: Meranie úč. vedení pre službu Magio – ADSL2+. Prezentácia, T-Com, 2007. • [5] http://en.wikipedia.org/wiki/Twisted_pair • [6] J. Vodrážka: Přenosové systémy v přístupové síti, ČVUT, Praha, 2003 • [7] L.Harte: Introduction to Dig.Subscr.Line (DSL) – Technologies, Operation and Systems, Althos, 2005

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