Tài liệu Electrical Characteristics pptx

The total length of the cable pair, which allows for Both of these parameters combined are measured as Propagation Delay or the time delay between the sent and received signal.. The AirE

The AirES product range of cables are a true innovation in structured cabling In most evolutionary processes the gain in one attribute often sacrifices another With the AirES evolution all attributes, both physical and electrical, are improved to provide a "Win Win" situation for both the installer and customer

This white paper will focus on the electrical attribute advantages of AirES Herein, we will discuss the revolu-tionary development of the technology and the by-product effects on any and all electrical parameters A full glossary

of terms is included for further understanding

Electrical Characteristics

of the Evolution

Background

To fully understand the benefits of the TrueNet AirES

solution, one must first understand the fundamentals of

cabling and the hurdles overcome by this product

Good Dielectric Constant is key in producing high

quali-ty data communications cable The lower the Dielectric

Constant of the insulation material, the better the

resistance to breakdown when an electrical field is

applied Air, with a Dielectric Constant of 1.0, is the

best of all insulators and is the basis by which others

are measured ADC has understood this and has been

using air as an insulator in our connectivity products for

many years

Below is an example of different materials and their

Dielectric Constants Water, Glass and Air were added

to the list to give a better understanding as to

what constitutes a good Dielectric Constant It may

be relevant to note the Dielectric Constant of Glass

is higher than FEP insulation This results in fiber

optic cables having a lower Nominal Velocity of

Propagation (NVP) than UTP copper cables

The NVP is the speed a signal propagates through a

cable expressed as a percentage of the speed of light

in a vacuum (300 m/sec) and given the value of 1 The

NVP of a data communications cable can be

directly calculated from the dielectric constant of the

insulating material and differs with change in frequency

The speed of the signal over multi-pair data

communi-cations cable is critical for high speed networks This

can be attributed to two main factors

1 The speed at which the signal is traveling (NVP)

2 The total length of the cable pair, which allows for

Both of these parameters combined are measured as Propagation Delay or the time delay between the sent and received signal

One of the byproducts of using FEP as an insulation material over FRPE is an increase in the NVP due to its lower Dielectric Constant The typical NVP of SAME design cables, using FEP as an insulator over FRPE, would typically increase up to 4% in NVP, therefore making FEP a faster insulating material

Below is a table of typical NVP values for different cable categories:

Note the Type 1 cables of old had an advantage in NVP over current UTP designs, coming in at 78% Type 1 cable is able to achieve much higher NVP values through the foaming of the insulation materials This introduces air pockets within the dielectric Air has a much better Dielectric Constant than FEP, thus increas-ing the signal speed Type 1 cable was also shielded or PIMF (Pairs In Metal Foil) cable which allowed for crush resistance This may also occur on unshielded foamed insulation materials

The AirES Innovation:

ADC’s challenge was to develop a cabling insulation using air as an insulator, increasing NVP to the same levels as that of Type 1 cable, and at the same time, having a high level of crush resistance for UTP applica-tions The use of foamed insulation in UTP cables can prove to have an adverse effect on the integrity of the structure, as it leaves the cable susceptible to crushing

By placing solid ribs around the entire conductor, crush resistance has exceeded the requirements of UL444 by

Water

PVC

FEP Polyimide-Glass

Flame Retardant Polyethylene Air

78.5

4.3 3.6

2.5 2.1

1.0

√ ∑r

Cable Insulation Transmission Typical Type Material Type NVP

Polyethylene Ring

In the ADC AirES designed cables, AIR combined with

traditional FEP has been introduced as an insulating

material The result is a NVP that parallels Type 1 cable

and at the same time remains crush resistant

The total effect of using air combined with FEP as an

insulation material is a 31% reduction in Dielectric Loss

Here’s how it works

The equation for working out the Dielectric Loss due to

insulation type where E is the Dielectric constant of the

insulation material and Fp is the power factor of the

material is:

Within the original equation both the Dielectric Constant and the Power Factor of the material are reduced with the introduction of air

The effect is a 31% reduction in Loss due to Dielectric The obvious benefit to a reduced dielectric loss is a direct improvement to signal loss, i.e stronger signal strength This allows for a reduction in copper conduc-tor size without the sacrifice of performance on Attenuation, which has a greater impact on the mechanical attributes

Through the introduction of air pockets between the FEP and copper conductor the total Dielectric Constant

is reduced The capacitive effects are decreased* This is then brought back to the nominal 100? by reducing the outer diameter of the insulation The total effect is faster pair transmission on a smaller pair footprint

The effect of the faster NVP is low Propagation Delay Currently the allowable Delay for Cat5e and Cat 6 is 570ns between transmitter and receiver As mentioned before, the Propagation Delay is also a function of the length of the pair, including the twist The greater the twist rate the longer the pair The TrueNet AirES cable is able to reduce the amount of twist needed for each pair as well as increasing the NVP

*Impedance = the square root of the inductance (con-ductor effects) divided by the capacitance (insulation effects), or

Cable Insulation Transmission Typical

Type Material Type NVP

AirES Cat 5e FEP and Air 1000BaseT 78%

AirES Cat 6 FEP and Air 1000BaseT 78%

fep= 2.07

∑

fep= 1.80

∑

fep

Fp = 00030

fep

Fp = 00022

Or the Dielectric Constant

of FEP

Or the Dielectric Constant

of FEP and Air in AirES

Or the Power Factor of FEP

Or the Power Factor of FEP and Air in AirES

fep= 0039f Loss

fep= 0027f Loss

Dielectric Loss reduced by 31%

32% less cross sectional area

17% Reduced Distance

AirES AIR pockets as

an insulation material

Z=√C L

By reducing the capacitance, impedance is higher This

can be corrected by reducing insulation size, thus the

AirES invention

This results in a Propagation Delay of ≤475ns, 17%

bet-ter than the standard Allowing for a more equal time

delivery on Gigabit Ethernet This makes the work of

the electronics easier and gives more of a buffer for

error free transmission

Delay Skew:

Even more critical than the Propagation Delay is the

Delay Skew, the difference in time each signal takes to

arrive on all 4 pairs For 10/100BaseT transmission this

is not as critical since only 2 of the 4 pairs are being

used for transmission Delay Skew becomes important

only when we migrate to 1000BaseT (Gigabit)

tranmis-sion, as we are now transmitting on all 4 pairs at the

same time For optimal performance the signals should

arrive at the receiver as close to the same time as

possi-ble The standards allow for up to 45nS in delay

between the fastest and slowest pairs There are other

schools of thought that support a reduction to <25nS

The AirES cable, due to its fast NVP and reduced need

for twist lay variation operates at a <20nS Delay Skew

This is unparalleled by any other Category 5e and 6 UTP

cable on the market today To achieve the Near End

Cross Talk (NEXT) performance, all other manufacturers

must vary the twist lays greatly, increasing Delay Skew

As illustrated below, it is variation in twist lays which

allows for reduced NEXT within cable Often to increase

NEXT performance, Delay Skew must be compromised,

“robbing Peter to pay Paul”, so to speak With the

AirES innovation of introducing Air as an insulator the

AIR reducing the dielectric constant and capacitive cou-pling, there is less Crosstalk between pairs due to reduced noise In other words, noise doesn’t travel well through air!

The total effect of the cable construction is a smaller cable with better all around electrical performance In the example below the old version (industry standard design) on the left is compared with the new AirES design Once jacketed, the effect of having significantly smaller primary conductors carries through to the final overall cable outer diameter The result is a 28% tion in cross sectional area for Cat 5e and a 32% reduc-tion for Cat 6 This translates to greatly increased fill rate capacity and easier installation

Note: For more information regarding the mechanical advantages of AirES please see our "Mechanical Attributes" white paper

Quite often in our industry we struggle to understand the relationship between all parameters testing on UTP cables To break it down into simple terms we are inter-ested in Signal to Noise Ratios (ACR) How strong is the signal when it reaches the receiver and how much noise

is on the line Once again, typically increasing the per-formance of one reduces the perper-formance of the other

or the size of the cable must be increased, but not with AirES The Noise (Cross Talk) has been reduced, not by increased twist rate, but through the introduction of AIR This allows the twist rates in each pair to be less, resulting in a shorter length on each pair Ultimately decreasing the amount of Attenuation (Insertion Loss), thus supplying stronger signal strength

Propagation delay is the measured time of the signal

from the transmitter to the receiver for each pair

Delay skew is the difference in time between all

4 pairs for the signal to arrive at the receiver

Incident Signal

Attenuated Signal

Coupling

0.17” 0.20”

Glossary of Terms

Reproduced with Permission of Fluke Networks

Dielectric Constant:

The property of a dielectric which determines the

amount of electrostatic energy that can be stored by

the material when a given voltage is applied to it Also

called permativity

Length and NVP:

Length is defined as the physical or sheath length of the

cable It should correspond to the length derived from the

length markings commonly found on the outside jacket

of the cable Physical length is in contrast to electrical or

helical length, which is the length of the copper

conduc-tors Physical length will always be slightly less than

elec-trical length, due to the twisting of the conductors

To measure length, a test set first measures delay, then

uses the cable's nominal velocity of propagation to

cal-culate length Nominal Velocity of Propagation (NVP)

refers to the inherent speed of signal travel relative to

the speed of light in a vacuum (designated as a lower

case c) NVP is expressed as a percentage of c, for

example, 72%, or 0.72c All structured wiring cables

will have NVP values in the range of 0.6c to 0.9c

Similarly, if you know the physical length and the delay

of a cable you can calculate the NVP

In most instances, length is derived from the shortest

electrical length pair in the cable Because of delay

skew, the length of the four pairs often appears slightly

different This is normal and no cause for concern with

the exception of significant (over 10%) variances

Results Interpretation

The main concern when measuring length is that there

is not a lot of cable in any segment For horizontal

structured cabling this means 100 meters This is

because applications have been designed to support a

maximum signal propagation delay, and if the link is too

long, this delay could be exceeded Occasionally

installers may leave excess cable in the ceiling or wall in

anticipation of future needs While this is okay if it is

considered part of the overall run, tightly coiling excess

cable can lead to undesirable performance degradation

due to additional return loss and near end crosstalk

Troubleshooting Recommendations

One of the most common reasons for failing length on

a test is that the NVP is set incorrectly If you are not

careful and use the preset cable type it may not match

the NVP of the cable under test In this case, you can

have an NVP difference of 10% or more, which

trans-lates directly into a length error In the event the length

Assuming the NVP is correct, another cause of excess length is extra cabling looped in the ceiling or walls Does the link in question meet the anticipated plan? For example, in the case of an airline hanger or warehouse,

a remote station may be forced to be over 100 meters from the wiring closet If this has been planned for, and the intended application supports the excess length, then the link may fail structured wiring standards but still be approved for the application Some field testers allow customized autotests to be configured that per-mit variances from standard TIA and ISO/CENELEC requirements Such autotests are useful because they verify the installation meets requirements while allowing for planned variances

Propagation Delay:

Propagation delay, or delay, is a measure of the time required for a signal to propagate from one end of the circuit to the other Delay is measured in nanoseconds (nS) Typical delay for category 5e UTP is a bit less than

5 nS per meter (worst case allowed is 5.7 nS/m) A 100 meter cable might have delay as shown below

Delay is the principle reason for a length limitation in LAN cabling In many networking applications, such as those employing CSMA/CD, there is a maximum delay that can

be supported without losing control of communications Nominal Velocity of Propagation (NVP) on the other hand, is different NVP refers to the inherent speed of signal travel relative to the speed of light in a vacuum (designated as a lower case c) NVP is expressed as a percentage of c, for example, 72%, or 0.72c All struc-tured wiring cables will have NVP values in the range of 0.6c to 0.9c

Results Interpretation

Delay measurements are relatively straightforward Most structured wiring standards expect a maximum horizon-tal delay of 570 nS If design specifications allow, higher delay can be acceptable

Since each pair in the cable has its own unique twist ratio, the delay will vary in each pair This variance (delay skew, discussed in the next section) should not exceed 50 nS on any link segment up to 100 meters Standards require all pairs to meet the requirement It is possible to report just the worst case pair This will be the pair with the highest propagation delay

Troubleshooting Recommendations

Excessive propagation delay can have only one cause: the cable is too long If you fail propagation delay, check to ensure that the pass/fail criteria match the design specifications If so, the cable is too long In

Category 5) will still support most LAN applications.

However, the installation will fail most structured wiring

standards, such as those published by CENELEC,

ISO/IEC, and the TIA In some cases, if the customer

insists on the location of the terminal equipment, and

an excessive length cannot be avoided, you can verify

other cable parameters If they pass, you can provide

information that indicates the cable meets

frequency-dependent parameters but is non-compliant with

over-all standards due to excessive length This provides

pro-fessional results to the user while placing on them the

responsibility for non-compliant cabling

Propagation Delay Skew:

Propagation Delay Skew (skew) is the difference

between the propagation delay on the fastest and

slowest pairs in a UTP cable Some cable construction

employ different types of insulation materials on

differ-ent pairs This effect contributes to unique twist ratios

per pair and to skew

Skew is important because several high-speed

network-ing technologies, notably Gigabit Ethernet, use all four

pairs in the cable If the delay on one or more pairs is

significantly different from any other, then signals sent

at the same time from one end of the cable may arrive

at significantly different times at the receiver While

receivers are designed to accommodate some slight

variations in delay, a large skew will make it impossible

to recombine the original signal

Results Interpretation

Well-constructed and properly installed structured

cabling should have a skew less than 50 nanoseconds

(nSec) over a 100-meter link Lower skew is better

Anything under 25 nSec is excellent Skew between 45

and 50 nanoseconds is marginally acceptable

Troubleshooting Recommendations

If the skew is high, provided the intended application is

a 2-pair application such as 10Base-T or token ring, the

application should still perform If one pair is much

higher or lower in delay than the others, very high skew

may result Examine the delay results for each pair If

one pair exhibits uncharacteristically high or low delay,

re-examine the installation

Near End Crosstalk (NEXT):

When a current flows through a wire, an

electromag-netic field is created which can interfere with signals on

adjacent wires As frequency increases, this effect

becomes stronger Each pair is twisted because this

allows opposing fields in the wire pair to cancel each

cellation and the higher the data rate supported by the cable Maintaining this twist ratio is the single most important factor for a successful installation

If wires are not tightly twisted, the result is Near End Crosstalk (NEXT) Most of us have experienced a tele-phone call where we could hear another conversation faintly in the background This is crosstalk In fact, the name crosstalk derives from telephony applications where 'talk' came 'across' In LANs, NEXT occurs when

a strong signal on one pair of wires is picked up by an adjacent pair of wires NEXT is the portion of the trans-mitted signal that is electromagnetically coupled back into the received signal

Results Interpretation

Since NEXT is a measure of difference in signal strength between a disturbing pair and a disturbed pair, a larger number (less crosstalk) is more desirable than a smaller number (more crosstalk) Because NEXT varies signifi-cantly with frequency, it is important to measure it across a range of frequencies, typically 1 – 100 MHz If you look at the NEXT on a 50 meter segment of twisted pair cabling, it has a characteristic "roller coaster going uphill" shape That is, it varies up and down

significant-ly, while generally increasing in magnitude This is because twisted pair coupling becomes less effective for higher frequencies The field tester should compare suc-cessive readings across the frequency range against a typical pass/fail line, such as the Class D specification If the NEXT curve crosses the pass/fail line at any point, then the link does not meet the stated requirement Since NEXT characteristics are unique to each end of the link, six NEXT results should be obtained at each end

Troubleshooting Recommendations

In many cases, excessive crosstalk is due to poorly

twist-ed terminations at connection points All connections should be twisted to within 13 mm of the point of ter-mination according to ANSI/TIA/EIA 568B An additional note common to all standards is that the amount of untwist should be kept to a minimum Experience has shown that 13mm does not guarantee a PASS when field testing

Signal to Noise Ratio (ACR):

Attenuation to Crosstalk Ratio (ACR)

Attenuation to Crosstalk Ratio (ACR) is the difference between NEXT and the attenuation for the pair in the link under test Due to the effects of attenuation, sig-nals are at their weakest at the receiver end of the link But this is also where NEXT is the strongest Signals that survive attenuation must not get lost due to the effects

Using PSNEXT and attenuation, Power Sum ACR

(PSACR) can also be calculated PSACR is not required by

TIA/EIA 568-B Some field testers will report it anyway

However, if you desire PSACR you will need to specify

it's requirement in the statement of works document

During signal transmission over twisted pair cable, both

attenuation and crosstalk are active simultaneously The

combined effect of these two parameters is a very

good indicator of the real transmission quality of the

link This combined effect is characterized by the

Attenuation-to-Crosstalk Ratio (ACR) ACR is nearly

analogous to the definition of signal-to-noise ratio

(ACR excludes the effect of external noise that may

impact the signal transmission.)

Results Interpretation

ACR is an important figure of merit for twisted pair

links It provides a measure of how much 'headroom'

is available, or how much stronger the signal is than

the background noise Thus, the greater the ACR,

the better

Troubleshooting Recommendations

ACR is derived from NEXT and attenuation data Any steps taken to improve either NEXT or attenuation performance will improve ACR performance In practice, this usually means troubleshooting for NEXT because the only way to significantly improve attenuation is to shorten the length of the cable

Web Site: www.adc.com

From North America, Call Toll Free: 1-800-366-3891 • Outside of North America: +1-952-938-8080 Fax: +1-952-917-3237 • For a listing of ADC’s global sales office locations, please refer to our web site.

ADC Telecommunications, Inc., P.O Box 1101, Minneapolis, Minnesota USA 55440-1101 KRONE ® is a registered trademark of ADC Telecommunications, Inc.