Transcript Document
Testing
Permanent Link
• Definition – Horizontal cabling extending from the wall plate to the cross connect
WORK AREA
Wall Outlet Maximum Length 90 Meters Horizontal Cross-connect
TELECOMMUNICATIONS CLOSET
Channel
• Definition – The end-to-end transmission path connecting any two points at which application specific equipment is connected. Equipment and work area cables are included in the channel
Channel
Equipment Cable
WORK AREA
Wall Outlet Maximum Length 100 Meters Concentration Point Horizontal Cross-connect Patch Cable
TELECOMMUNICATIONS CLOSET
Permanent Link Test
• Configuration
WORK AREA
Wall Outlet Test Set Test Set Connection Cable (2 meters) Horizontal Cross-connect Maximum Length 94 meters Test Set
TELECOMMUNICATIONS CLOSET
Channel Test
• Configuration Test Set
WORK AREA
Equipment Cable Wall Outlet Cross-connect Horizontal Concentration Point Patch Cable Maximum Length 100 Meters Test Set
TELECOMMUNICATIONS CLOSET
Twisted Pair
• Before we look at each of the basic tests we need to review the – – Physical Electrical characteristics of a wire pair
Anatomy of a Wire Pair
• What we think of when we envision a wire pair.
Anatomy of a Wire Pair
• • However, to a signal it looks like: – – – Resistors Capacitors Inductors The cable is not made up of discrete components but has the characteristics of these components – – – Resistance Capacitance Inductance
Damaging the cable
• The characteristics are changed when a cable is: – – – Stretched (pulling / laying) Kinked (handling / storage / tight corners) Distorted (cable ties / stood upon / Heavy cable bundles)
Test Parameter
Wiremap Propagation Delay Delay Skew Cable Length Insertion Loss (IL) Return Loss (RL) Near-End Crosstalk (NEXT) Power Sum NEXT (PSNEXT) Equal-Level Far-End Crosstalk (ELFEXT) Power Sum ELFEXT (PSELFEXT) Attenuation-to-Crosstalk Ratio (ACR) Power sum ACR (PSACR) DC Loop Resistance
Physical Layer Tests
TIA-568-B
Pass/Fail Pass/Fail Pass/Fail Pass/Fail Pass/Fail Pass/Fail (except Cat3) Pass/Fail Pass/Fail Pass/Fail
ISO 11801:2002
Pass/Fail Pass/Fail Pass/Fail Information only Pass/Fail Pass/Fail Pass/Fail Pass/Fail Pass/Fail Pass/Fail Information only Information only Pass/Fail Pass/Fail (except Class C) Pass/Fail (except Class C) Pass/Fail
Wire Map
• Verification of the physical connection at each end of the cable • Checks for – Opens – – – – Shorts Crosses Reverses and any other misfiring Splits • Difficult to identify with a DC wire map tester • Must use an advanced tester • Will fail on crosstalk
Length
• Physical – Calculated based on the length markings on the cable • Maximum physical length of a permanent link is 90 meters • Maximum physical length of a channel is 100 meters
Length
• Electrical – Based on the propagation delay of a signal over the cable pair • Accomplished using a TDR (Time Domain Reflectometer) • Calculation base on the Nominal Velocity of Propagation (NVP) of the signal over the pair being tested
Attenuation
• • • • An analogy is friction Some of the signal is lost over distance Energy is dissipated in the form of heat Message may become to weak to be understood DATA DATA DATA DATA DATA DATA DATA DATA DATA
Decibels dB’s
•
Before we go any further we must talk a little about decibels (dB’s)
Decibels dB’s
• The deciBel is a convenient means of expressing the ratio of two values – it is commonly used for example to express the Gain or Attenuation of a signal path: Electrical/ Optical Signal Input
Signal Path
Electrical/ Optical Signal Output Gain or Attenuation = Output Signal ÷ Input Signal Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 17
Decibels dB’s
• deciBel expressions for the power and voltage level of a signal are slightly different: – Voltage Gain/Attenuation = 20
Log
10
V OUT V IN
– Power Gain/Attenuation = 10
Log
10
P OUT P IN
Note:
For Optical Fibre only the Power expression is relevant Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 18
Example 1
A NEXT reading is -40 dB for Pair 1 to Pair2. What percentage of the Pair 1 input voltage appears at the input of Pair 2 ?
ANSWER
20
Log
10
V NEXT
, 2
V IN
, 1 40
dB Log
10
V NEXT
, 2
V IN
, 1 40 20 2
V NEXT
, 2 10 2
V IN
, 1 0 .
01
or
1 % This result says that 1% of whatever is injected into Pair 1 will appear at the input of Pair 2 Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 19
Example 2
A FEXT reading is -50 dB from Pair 3 to Pair2. What percentage of the Pair 3 input voltage appears at the output of Pair 2 ?
ANSWER
20
Log
10
V FEXT
, 2
V IN
, 3 50
dB Log
10
V FEXT
, 2
V IN
, 3 50 20 2 .
5
V FEXT
, 2 10 2 .
5
V IN
, 3 0 .
003
or
0 .
3 % This result says that 0.3% of whatever is injected into Pair 3 will appear at the output of Pair 2 Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 20
Example 3
Pair 4 has an Attenuation reading of 20 dB. What percentage of the Pair 4 input voltage appears at the output of Pair 4 ?
ANSWER
20
Log
10
V OUT
, 4
V IN
, 4 20
dB Log
10
V OUT
, 4
V IN
, 4 20 20 1
V OUT
, 4
V IN
, 4 10 1 0 .
1
or
10 % This result says that 10% of whatever is injected into Pair 4 will appear at the output of Pair 4 Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 21
Back To Attenuation
• The loss of signal strength over the cable pair, measured in dB – Increases as the carrier frequency increases – Pass/fail based on the worst case attenuation of all pairs
Attenuation
Crosstalk
• Defined as the induction of a portion of the signal from one pair into the adjacent pairs – Measured in dB
Cross Talk Types
– NEXT • NEAR END CROSS TALK – ELFEXT • EQUAL LEVEL FAR END CROSS TALK – PSNEXT • POWER SUM NEAR END CROSS TALK – PSELFEXT • POWER SUM EQUAL LEVEL FAR END CROSS TALK
NEXT and ELFEXT
• A final point is that ELFEXT depends greatly on the length of cable being examined, whereas the NEXT is much less dependent
Crosstalk Components
• Inductive and Capacitive Relationships:
Cross Talk
• • • SPILL OVER ELECTRONS ARE LOST TO THE ADJACENT CABLE PAIRS THE MESSAGE IS CORRUPTED DATA DATA DDATA DATA DATA DATTA DATA DATA DATA DATA ATA DATA DATA DATA DAA DATA DATA
NEXT
• NEXT MEASURES THE CROSSTALK EFFECT ONE PAIR HAS UPON THE OTHER WITH RESPECT TO THE NEAR END OF THE CABLE
Near-End Crosstalk (NEXT)
Power Sum
• Use of the TIA Algorithm for Calculating Power Sum – – Calculation is derived from the conventional NEXT measurement.
Only one pair is energized at a time; formula “
sums
” the crosstalk effect of three energized pairs on the remaining fourth pair.
PSNEXT
• POWER SUM NEXT IS THE COMPUTED EFFECT OF THREE PAIRS UPON THE FOURTH WITH RESPECT TO THE NEAR END OF THE CABLE
ELFEXT
• ELFEXT MEASURES THE CROSSTALK EFFECT ONE PAIR HAS UPON THE OTHER WITH RESPECT TO THE FAR END OF THE CABLE
PSELFEXT
• POWER SUM ELFEXT IS THE COMPUTED EFFECT OF THREE PAIRS UPON THE FOURTH WITH RESPECT TO THE FAR END OF THE CABLE
NEXT and ELFEXT
• NEXT and ELFEXT (and crosstalk in general) depend very much on the frequency at which they are measured – they increase as the frequency increases – on average they may be described as follows:
NEXT
frequency
3 2
FEXT
frequency
2
NEXT and ELFEXT
• In practise this means as the Cat6 specification includes frequencies up to 250 MHz, that both the NEXT and ELFEXT measurements will increase dramatically – For example the NEXT at 250 MHz should be about 4 times higher than the NEXT at 100MHz for any given combination of pairs – Even worse the ELFEXT at 250 MHz should be over 6 times higher than the value at 100 MHz for any given combination of pairs – For these reasons (and more) cable that passes Cat 5/5e quite easily may still fail the Cat 6 standard
NEXT dB Calculations
CAT 5 to CAT 7 NEXT values at 100 MHz
SPECIFICATION CATEGORY 5 CATEGORY 5e CATEGORY 6 CATEGORY 7 NEXT 32 dB 35 dB 43 dB 71 dB Percentage 2.5% 1.8% 0.7% 0.03%
Copyright of the Applied Optoelectronics Centre DIT 19/08/99 Photocopying strictly prohibited 37
Attenuation to Crosstalk Ratio
• • Definition: – A ratio comparing the received signal with the near-end crosstalk on a cable It is not a direct test – – It is a comparison of the attenuation test and NEXT test Used to give an indication of how problem-free the cable line will be Attenuation-to-Crosstalk ratio (ACR) is the difference between the signal attenuation produced and NEXT and is measured in decibels (dB). The ACR indicates how much stronger the attenuated signal is than the crosstalk at the destination (receiving) end of a communications circuit. The ACR figure must be at least several decibels for proper performance. If the ACR is not large enough, errors will be frequent. In many cases, even a small improvement in ACR can cause a dramatic reduction in the bit error rate. Sometimes it may be necessary to switch from un-shielded twisted pair (UTP) cable to shielded twisted pair (STP) in order to increase the ACR.
Attenuation to Crosstalk Ratio (ACR)
Propagation Delay
• Propagation Delay – The time needed for the signal to travel from the transmitter to the receiver over a 100 Ohm 4-pair cable.
T 1
Delay
• THE TRANSMISSION TIME
T 2
Delay Skew
• THE DIFFERENCE IN TIME BETWEEN THE FASTEST AND SLOWEST PAIR
T 2 T 1
Delay Skew = Difference between
T 2 and T 3 T 3
Delay / Delay skew
• • DELAY – THE TRANSMISSION TIME DELAY SKEW (
X
) – THE DIFFERENCE IN TIME BETWEEN THE FASTEST AND SLOWEST PAIR
X
Propagation Delay
DC Loop Resistance
• DC Loop Resistance measures the total resistance through one wire pair looped at one end of the connection. This will increase with the length of the cable. DC resistance usually has less effect on a signal than insertion loss, but plays a major role if you require power over Ethernet. Also measured in ohms is the characteristic impedance of the cable, which is independent of the cable length
Return Loss
• • It measures the difference between test signal’s amplitude and the amplitude of signal reflections returned by the cable.
Information Provided: – Indicates how well the cable’s characteristic impedance matches its rated impedance Return Loss is the measurement (in dB) of the amount of signal that is reflected back toward the transmitter. The reflection of the signal is caused by the variations of impedance in the connectors and cable and is usually attributed to a poorly terminated wire. The greater the variation in impedance, the greater the return loss reading. If 3 pairs of wire pass by a substantial amount, but the 4 pair barely passes, it usually is an indication of a bad crimp or bad connection at the RJ45 plug.