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Access Networks- exercises 1,2

2008/09

Copper lines (symmetrical pairs) properties

Ľ. Maceková - KEMT – FEI – TU – Košice - SR

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Introduction

...

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Fig.1 The areas of measurement and diagnostics of xDSL transmission chain

User Interface

Aplication Interface

Digital part

Analogue part

User Interface

Aplication Interface

Digital part

Analogue part

xDSL Transceiver Transceiver xDSL

Line parameters

Dig.signal parameters on physical transmission layer

Transmission parameters of higher layers

Param. of transmission device

Transmiss.parameters of user interface

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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 - LF with load coils l. - 15 kHz - LF unloaded - 120 kHz (12 tel.channels, 1st order

PCM) - HF - 552 kHz (120 tel.chann., 2nd order 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 GHzFSO – Free Space Optics – LASER ray through free spaceWireless (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

Transmission media classification and frequency functionalities [2] see also tab. cable comparing

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Properties of symmetrical pairs

shortopen ZZZ 110

-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

[3]

Measurements of Z0:

CjG

LjRZ

0

-see also this

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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 density value), 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)

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 mainly vf parameters: line attenuation, crosstalks NEXT and FEXT, longitudinal balance (LCL), return loss, charakteristic impedance, propagation velocity

],,,WsK,[V ....4 -122

HzKfRTkU

effN k=1,38.10-23 WsK-1

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Obr. dole: Impedance of twisted pairs in dependence of frequency. TA- with air separation, TE-lines with plastic isolation.

Properties - continue 1

Impedance matching

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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 (attenuation of reflection)

0

0

ZZ

ZZr

L

L

[dB] , log201

log200

0

ZZ

ZZ

rRL

L

L

- if impedances match r = 0, RL

- the case of total reflection: r = 1, RL = 0

!

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jCjGLjR

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

Properties - continue

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Properties - continue 3:

Insertion Loss (attenuation)– a definition by means of powers:

1T

R

P

PA

PR – receiver power

PT – transmition power

-in [dB], A is less then 0 dB (negative) by definition above

-the more often definition – for positive value of attenuation:

A[dB] = 10 log (PT / PR) …… > 0 dB

= 10log(PT/1mW) – 10 log( PR /1mW )

A[dB] = P T [dBm] – P R [dBm]

[W]

[dBmW]

P[dBW]=10logP[W]

P[dBm]=10logP[mW]

transfer path (line)PTPR

definition of dBW and dBm

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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 = 20 log10 (Vcomm / Vdiff) [dB] > 0 dB

LB measurement principle:

Vdiff

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Symmetrical pairs properties – continue:

Crosstalks

- 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 Attenuations and parameter ACR (Attenuation-to-Cross talk Ratio)

Pair 1

Pair 2

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[5, 6]

Examples of twisted pairs cables (STP, UTP, S/STP, S/UTP=FTP)

screened/ shielded

twisted-pair quad

1 pair of quad

1 pair of quadneighbouring pairs

10 pairs subgroup

50 pairs group

near pairs

near pairs

neighbouring pairscable jacket

cable screening

distant pairs

-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)

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S/STP cable [6]

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disturbing pair

NEXT- and FEXT attenuations measurements [1]

[dB] log102

1

N

NNEXT P

PA

[dB] log102

1

FN

NFEXT P

PA

receiver

receiver

disturbing pair

disturbing pair

disturbed pair

disturbed pair

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then, k-th pair in multicore cable, where several pairs are used to communication, may be disturbed by other pairs as follows:

PSNEXT - Power Sum NEXT:

n

kii

kiAkNEXT

NEXTPS,1

),(1,0, 10log10 [dB]

where ANEXT,k ... near end crosstalk between disturbing pair i and influenced pair k

n ... number of pairs in cable

Link properties impairments [3]

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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 ANEXT are 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(lit. [1])

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.

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Fig. Principle: we can measure the time and shape of reflected impulse from impedance mismatch, caused by a fault on the pair

TDR - Time Domain Reflectometry = measurement method

Fig. Resistence and capacitance imperfection

- diagnostics and location of impairments (damages, defects or fails) of metalic twisted pairs

leakageinerruption

defect locality

defect locality

reflection due to resistance character impairment

reflection due to capacitance character of the fault

transmitted impulse

reflection

time

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TDR - continue

cvr

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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)

s] -, m/s, s; m/s,[m; 2

.

2 xxx tPVFc

tv

l

- velocity of propagation in solid media (in cable):

If we measure tx.... then we can locate the place of fault in the pair (the distance of fault from measurement device)

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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