There are typically two ways to deploy ADSL. The traditional way is to Place the DSLAM (Digital Subscriber Line Access Multiplexer ) in the CO and using a splitter combined the POTS (Plane Old Telephone Service) and ADSL service on to a single twisted pair. An alternate method is to use a DLC (Digital Loop Carrier) that integrates the POTS and ADSL service over a common facility and delivers both services at RT (Remote Terminal) thereby bring both services closer to the customer.
ADSL Rate is decreased the longer the loop. ADSL Internet service is a layered service of IP over ATM and ATM over ADSL on the local loop ADSL service is aggregated at the DSLAM and customers share bandwidth on the ATM backbone of the DSLAM The DSLAM on its line side contains the ADSL modem (ATU-C) and on its network side a wideband ATM signal that is carried over T1/E1, multiple T1s/E1s, DS3/E3, SONET/SDH transport signals. Depending on the size of the ADSL concentration the user to network bandwidth ratio can very from 5:1 to 20:1 with 10:1 as a typical value. A Splitter is used at the CO to Both Combine the out going POTS and ADSL signals on the Local Loop and split the incoming POTS and ADSL signals from the customer to their respective network elements, The Micro Filters isolate the ADSL signal from interfering with the POTS and isolate the telephone sets from the ADSL Modem (ATU-R). Micro Filters do not completely isolate the POTS signal from the ADSL modem and relies on the Modem’s filter to perform the isolation. The Customer’s ADSL modem is connected to the customer’s computer via either Ethernet or USB (Universal Serial Buss). If Ethernet is used as the interface the customer’s computer could have this connection on a NIC (Network Interface Card).
A Splitter can isolate both the telephones and POTS from the ADSL modem and signal much better than Micro Filters. Also with a splitter the inside wiring configuration of the telephones isn’t of concern as it is with Micro Filters. The splitter at the customer’s location acts in the same ways as the splitter at the CO both combining and splitting the POTS and ADSL signals.
Using a DLC to Deploy ADSL allows Both the POTS and ADSL service to be derived over much shorter Local Loops than if deployed from the serving CO. When POTS and ADSL are intergraded using a DLC the RT line card includes and combines the three elements required to deliver the service, POTS circuitry, ADSL ATU-C modem and Splitter. POTS at the CO can be interfaced to the DLC COT (CO Terminal) using various standard based interfaces such as Analog ( Telcordia TR-57) or Digital (ITU V5.1 or V5.2, Telcordia GR303 or TR08).
Data Up Maximum rate of 640 Kbps Twisted pairs exhibit better immunity to cross-talk at lower frequencies, the up stream being n the lower end of frequency spectrum reduces cross-talk at CO were the highest concentration of ADSL signals occur. Data Down Maximum rate of 6.4 Mbps for standard ADSL Maximum rate of 1.5 Mbps for G.lite Rate affected by cable length: Longer cable length means lower rate Approximately 256 Kbps Down and 100 Kbps Up at the maximum Local Loop lengths and expected noise levels. Cross talk is less of an issue as there is less ADSL signal concentration in the distribution cable plant were the down stream ADSL signal is weakest and being received by the modems at the customer's locations.
DMT: Discrete Multi-ToneUp to 256 sub carriers. Each one is controlled by the ADSL protocol.
Voice Freq
1.1MHzTone#256
240kHzTone#56
160kHzTone#31
≈ 4kHz
4.3125kHz Spacing
20kHzTone
#1
“bit buckets” (DMT sub-carriers or “tones”)
DownstreamUpstream
4.3125kHz Spacing
69kHzPilot Tone
276kHzPilot Tone
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Presentation Notes
DMT Operation 256 carriers carry a maximum of 15 bits each for customer user. There are other bits added as overhead that are used by the DSLAM and customer’s modem. There can be a maximum of 200 carriers for the Downstream There can be a maximum of 25 carries for the Upstream If a data rate is provisioned that is less than what the local loop can handle the bits are evenly distributed amongst the available “bit buckets” rather than butting the maximum bits in fewer “buckets” Each Tone/Carrier is like an individual analog modem. The Pilot tones are the signals that start the synchronization process. Standard HDSL (2B1Q coded) can cause ADSL to be non operational by interfering with the Downstream Pilot Interfering signals can make some carries unusable but still allow service to be operational at a lower data rate ADSL signals within the same Binder group should be limited to 50% of the pairs As more ADSL customers are added a point can be reached where service is derogated for all users in the same binder group
How DMT Works: Not all Tones (Bit Buckets) need to be present in order to enable synchronization between the DSLAM and the Customer’s modem. There only needs to be more available data capacity within the Tones available than that the service is provisioned for. If there is noise and/or interferes on the local loop that remove some tones for service these tones would be skipped over. If the local loop is long where it starts to affect the higher frequency tones (typically over 12 K feet/3.6 km) so that that are not available the lower frequency tones will have more bits per tone to make up the difference.
ADSL2+ Vs. ADSL2 Vs. Standard ADSL: ADSL2 and Standard ADSL has the same number of “bit buckets” ADSL2+ has twice as many down stream buckets allowing up to twice the data rate to be delivered to the customer.
ADSL2+ Vs. ADSL2 On short loops (less than 3 kfeet/900 m) ADSL2+ can deliver 25 Mb/s. This would allow up to 5 high quality compress video channels to be delivered to a customer. This would also allow for one compressed HDTV channel to be delivered to a customer
Frequency Response to Cable Length: Twisted Pair telephone cable is highly capacitive and as such causes more signal loss at higher frequencies than at lower frequencies. This is why Load Coils, which combats capacitance, are used for POTS so that the higher frequencies of the human voice can be heard on long loop making the conversation and the talker more recognizable. As a cable becomes longer the loss at the higher frequencies becomes significantly greater making it impossible to use the higher ADSL DMT tones thereby reducing the down stream rate.
The affect of cable length: One of the characteristics of twisted pair cable is to attenuate higher frequency signals more than lower frequency signals as the cable gets longer. This is true for a good cable pair without any faults. But a cable with a fault would affect the signal even greater. This results in either the higher tones carrying fewer bits or not being available at all to carry any bits depending on the length of the cable pair. Once beyond 12 k feet/3.6 km the maximum ADSL data rate cannot be delivered.
ADSL Data Rate Vs. Loop Length & Noise: Figure 1: Cable length Vs. Down Stream Data Rate. As more noise is added to the cable pairs the data rate at the extreme length of the cable decrease For 24 gage with minimal noise (Blue line) the maximum data rate is approximately 1.8 Mb/s. For 24 gage with a high noise level (Yellow line) the maximum data rate is approximately 200 kb/s Also as more noise is added to the cable pairs the length at which the maximum ADSL data rate (6.4 Mb/s) can be obtained decreases. For 24 gage with minimal noise (Blue line) the maximum cable length that the maximum ADSL data rate can be obtained is 12 k feet/3.6 km. For 24 gage with a high noise level (Yellow line) the maximum cable length that the maximum ADSL data rate can be obtained is 7 k feet/2.1 km. Figure 2: Cable length Vs. Down Stream Data Rate With interferes: With the same noise and cable lengths as in Figure 1 the addition of 12 pairs with ADSL signals is added resulting in crosstalk (more noise) The maximum rate at the extreme length of the cable is further reduced The cable length at which the maximum ADSL rate can be obtained is also further reduced.
Verify Electrical outlets are correctly wired– Use Standard Hardware store outlet checker– If outlet isn’t OK recommend they get rewired
before service is installed. Verify that Phone service is still operational
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Customer Premises Issues: Flat drop wire having no twist on the Tip-Ring pair and in some cases no shield have very little immunity to ingress noise and does not maintain the same cable characteristics at high frequencies a does standard telephone cable pairs. Carbon Block protectors offer poor impedance characteristics to high frequencies that can degrade ADSL performance. Gas tube or solid state protectors must be used with solid state preferred as they offer much more protection. This applies to both CO and CPE protectors. Station Ground should be verified when installing ADSL as a good station ground will enhance ADSL performance by allowing a good return path for noise. Phone Service should be verified after ADSL installation to make sure the Micro-filters or splitters are operating properly.
Dynamic Adaptation: It would be ideal if all “Bit Buckets” would be received so that they would be able to carry the maximum 15 bits of data bits but various line conditions reduce the maximum data bits to be carried. Conditions that reduce the maximum data rate on a given local loop. Long Loop Length: reduces the higher frequency bit buckets ability to carry the Maximum number of data bits. Short bridge taps that are less than 1000 feet/300 m that are within 500 feet/150 m of the customer’s modem “notch out” bit buckets. AM radio that are less than 1100 kHz, T1 and HDSL interferes can also notch bit buckets Wideband noise the worst being other ADSL signals within the same binder group can affect all bit buckets The result is the the data rate is reduced but in most cases the ADSL service can still operate as the modem will adapt to these line conditions.
Maximum loop length– 18,000 ft over 24 gage– 14,000 ft over 26 gage
Remove Line Build-Out Capacitors
1300 Ohms maximum Tip to Ring Loop resistance
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Presentation Notes
General Loop Issues: Maximum loop length is dependent on cable gage Mix gage can reduce ADSL operational loop length Build out capacitors which typically deployed to make short POTS loop look longer to reduce signal levels must be removed as they can make ADSL inoperable. Load coils as a block to any signal above 4 kHz and therefore will block a ADSL signal Loops over 1300 ohms are electrically to long to support ADSL
Bridge Taps A bridge tap affects a ATU-C or ATU-R the closer it is to it Maximum allowable length of all bridge taps on a loop is 2,500 feet Maximum length of a singe bridge tap is 2,000 feet Maximum number of bridge taps allowed on a loop is 3 Bridge taps as short as 225 feet can affect ADSL performance The closer a Bridge tap is to a ADSL receiver the more it can interfere with the signal whether located near the CO or Customer’s location
No Bridge-Tap longer than 2,000 feet Total length of all Bridge-Taps less than 2,500 feet No more than 3 Bridge-Taps No Bridge-Taps within 500 ft of Customer ADSL Modem Bridge-Taps add to the total length of the Cable Pair
– If pair distance to customer = 10,000 feet/– And Bridge-Tap is = 2,000 feet– Cable distance to the customer is considered to be 12,000 feet
Short Bridge-Taps “notch” out Bit Bucket tones– 200ft to 1,000 feet of Bridge-Tap affect down stream ADSL– 1,500 ft to >2,000 feet of Bridge-Tap affect upstream ADSL
High level of CrosstalkBoth pairs are unusable for ADSL
SPLIT SPLIT“CORRECTION”
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Splits Causes pairs to be unbalanced and enable interfering noise and signals to impair ADSL data rate With a split two pairs are affected. A corrected split doesn’t correct the affect of the split All splits must be removed
DMT No double gage change e.g., 26 AWG to 22 AWG not allowed Every gage change results in an additional 1 dB loss
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Gage Changes No double gage change e.g., 26 AWG to 22 AWG not allowed Every gage change results in an additional 1 dB loss Going from a fine gage (26 gauge) to a coarse gage (24 gauge) and than back to a fine gage (26gauge) is not recommended as additional loss of up to 3 dB can occur.
Wet Cable SectionsCauses drastic impedance changes resulting in signal reflections
Causes insulation breakdown resulting in poor balance & foreign battery
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Wet Cable sections Causes drastic impedance changes resulting in signal reflections The impedance will change dynamically as a cable gets wetter or begins to dry out. Causes insulation breakdown resulting in poor balance & foreign battery This results in an increase in noise on the cable pairs. The ADSL signal will dynamically adjust but the service will degrade significantly as more and more moisture enters the cable. Foreign battery is cause the other shield to deteriorate further removing the cables ability to isolate the pairs from external noise sources.
Pairs in Same Binder Group (Cable Bundle) Other digital signals affect DTM performance
BRI ISDN, HDSL, E1 Carrier DMT ADSL signal in same Binder Group
Limit the number of ADSL customer within a binder group
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Cross Talk !n a 25 pair binder group a general rule is to not allow over 50% fill of like signal interferes A telephone cable’s twisted pair has less immunity to crosstalk at higher frequencies than at lower frequencies. Treat 2B1Q HDSL as a equal interferer to ADSL A 2B1Q HDSL Doublers can cause ADSL signals in the same binder group to be inoperable 2B1Q HDSL originating at a DLC and entering a binder group with ADSL originating from another location can cause the ADSL signal to be inoperable 2B1Q HDSL can affect both the up and down stream ADSL signals. BRI ISDN only can cause interference in the up stream ADSL signal, whereas T1 and E1 affect the down stream ADSL signal.
Improper Bonding & Grounding results in ingress interferesAC power influence
Increases foreign voltage and hazards to personnel & EquipmentLightning storm influence
Damage to Network & CP equipmentSpurious noise
Reduces ADSL rate & can cause modem resynchronise
X
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Bonding & Grounding: First and foremost Bonding and Grounding is for protection of those who work on cable pairs and the customer against high voltages and current that can be on a telephone line from either electrical storms or from a cross power line that comes in contact with the telephone cable. Good bonding and grounding allows for any external noise source induction to be removes from interfering with the twisted pair that carry the telephone and ADSL signal. Bonding and grounding is the first line of defense to block external noise from entering the cable pairs such as power line influence, AM radio signals and lightning induction.
This section is on basic copper measurements and how to use them to more effectively troubleshoot ADSL on the local loop. The intended results is a better understanding of: AC and DC current loadcoils Resistance Capacitance and opens Balance and “stress” Noise Resulting in helping better troubleshoot the most common causes of trouble for ADSL: loadcoils, bridged taps, and bad balance.
POTS local loop requirements: Span Voltage: Tip-to-Ring (T-R) voltage when measured without a termination at the customer’s location should measure at 52 volts. Tip-to-Ground (T-G) voltage should read less than 5 volts. Since the Tip is at the same potential as ground at the CO there shouldn’t be any difference except for foreign induced voltage. Ring-to-Tip (R-T) voltage when measured without a termination at the customer’s location should measure at -52 volts. Ringing Voltage: Measured at the customer’s location the ringing voltage (20 Hz) should not be less than 79 volts in order to be able to ring 5 standard ringer equivalent telephone station sets.
• Tip to Ring• Tip to Shield• Ring to Shield• Tip to all surrounding conductors• Ring to all surrounding conductors
– Capacitance Balance• Tip-Shield and Ring-Shield capacitance balance >98%
Ring to Shield
Tip to Shield
Tip to Ring
conductors insulation
A twisted pair hasCapacitance becauseit has 2 conductorsseparated by insulation
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POTS local loop requirements: Loop Capacitance All twisted pair telephone cable is primarily capacitive and resistive in nature having very little inductance. The capacitance of a cable is the primary cause of AC signal lost. The resistance of a cable is the cause for DC current lost. Capacitance Balance: When measuring T-G and R-G capacitance there shouldn’t be a difference between the measurements of more than 2%. If there is a difference grater than 2% there is either a left in tip or ring lead or water in the cable.
While we can use an opens or “kick” meter to convert capacitance to distance because the capacitance is CONSTANT between T – R, This is NOT TRUE between any other leads: the game really changes! If you do not set the meter or calculate with the right value per mile, an open meter will be WAY OFF. A meter still measures the capacitance, whether between T – Shield (ground), T1 – T2, or otherwise, it cannot give you an accurate distance without knowing the microfarads per mile, which must be pretty close to constant over the length of the pair. But cable is sometimes of mixed gage: Feeder, distribution and drop can vary. AND, the fill type (the insulation in this case) also varies for T-G and R-G measurements such as air core, pulp, jelly. So the value for capacitance can be way off in this environment. It’s critical to know the gage and fill type for most of the cable to get an accurate distance reading on one side to ground. You must enter the gage and fill type on your meter, if you know it in order to get the most accurate measurement. If you’re not sure, verify an open measurement with a TDR (Time Domain Refectometer) The TDR is good at measuring the distance to the end of the pair (an open). At least back up your estimates with the TDR when calculating distance on one side.
Capacitance Balance Tip to Ground C = Ring to Ground C +/-2% If Capacitance Balance not meet pair is
susceptible to ingress noise
shield
R – G(Shield)
R – G(Shield)
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Capacitance Balance: This is the comparison measurement of capacitance between T-G and R-G and should be 2% or less in value. Typically if Capacitance balance isn’t met a poor Longitudinal Balance measurement will also be made.
Loop Insulation Resistance– Tip to Ground = Ring to Ground
• If not equal pair is unbalanced• If > 4,000 ohms and < 3.5 M ohms a high resistance fault is present
• 3.5 Meg Ohms limit is based on Mechanical Meter movement– e.g., Brown Meter, Kick Meter, KS test set
• Newer “Electronic” meters will typically give a higher resistance value than a Mechanical meter
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POTS local loop requirements: In order have good telephone performance the insulation around the copper conductors of the tip and ring must have a minimum of resistance between all conductors. However, the greater the resistance the better the cable will perform especially when high frequency signals are on the cable such as ADSL. The traditional minimal insulation resistance value has been 3.5 Meg ohms based on the “Kick” meter (sometimes referred as a “Brown” meter or “KS” meter) but this meter has a relatively low ohms per volt sensitivity as compared to newer “digital” meters. As a result newer meters may read different resistance values than a “Kick” meter. All insulation measurements (T-R, T-G and R-G) should be relatively equal
Tip Resistance = Ring Resistance If Resistance Balance not meet pair is
susceptible to ingress noise Tip – Ground ohms = Ring – Ground ohms T – G Vs R – G difference < 3%
High Resistance Splice
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Resistive Balance: Risistive imbalance is caused when the tip or the ring have a greater resistance than its mating ring or tip. This is sometimes referred as High Resistance Open or a “HiRO” which is a high resistive serial fault. The typical causes of resistive imbalance is either a poor made connection (joint) or corrosion caused by moisture entering a connection (joint). Other locations where this type of fault occurs is a screw down terminals and telephone station set jacks that are corroded. The resultant affect of resistive imbalance is poor a Longitudinal Balance measurement causing noise to be heard on a POTS connection. For ADSL connection the result is intermittent connections and poor ADSL modem speeds. The best tool to find a high resistive serial fault is a TDR. However, because of the relatives resistance low resistance of the fault it is usually required to do the testing as close to the fault as possible.
Loop Current– Required to maintain proper Signal Volume & Touch Tone
level
– For Station Ground <25 ohms • Ring to Gnd current needs to be at least 1.33 times grater than Tip
to Ring loop current
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Loop Current: At the customer location’s drop termination the tip to ring loop current needs to be equal to or greater than 23 ma when terminated into 430 ohms. When making a Ring to Ground current measurement the current should be equal to 46 ma. The reason the required current is more than the tip to ring loop current is the ground resistance to the CO is required to be 25 ohms or less this is much less than the “tip ground resistance” that can be as high as 950 ohms.
Tone Levels: The maximum allowable loss on a local loop must be no greater than 8 dB at 1004 Hz. Gain Slop is a measurement of low (404 Hz) and high (2804 Hz) frequencies as compared to 1004 Hz. If these frequency levels are out of the specified range than either there is a fault within the POTS connection or the loop is to long.
Balance– A pairs ability to reject power line influence– Balance = PI – (Metallic Noise)
– For DSL lines Balance MUST be > 60dBrnC
P.I., Noise & Balance Limits
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POTS Local Loop Requirements: When noise metallic (tip-to-ring) is marginal or unacceptable and power influence (tip-to-ground & ring-to-ground) is acceptable Suspect a pair problem The pair is unbalanced either resistively (going open) or capacitively (one side open on a lateral (bridge-tap) beyond the working terminal, or crossed with a nonworking pair) In most cases a capacitive unbalance pair can be identified and isolated with an open meter. When both noise metallic and power influence are marginal ar unacceptable, suspect a grounding, bonding, or associated power company problem. Some typical power company problems can be an open capacitor bank, bad transformer, open neutral, etc.
AC volts tests:look for hazardous levelscheck ambient AC levels
3 factors effecting PI:1. Distance to source2. Strength of field3. Distance pairs to parallel power
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When most technicians troubleshoot a loop, they first measure AC volts: to check for hazardous voltages (note too the shield can have them) (We know that at 60Hz, only a small amperage can kill a person) To check for ambient, or stray AC voltages (Power Influence and possible hi-voltage crosses) Normally a few volts of AC is allowed: 3 to 10 (above 10 is unacceptable) The major source of ambient voltages comes from Power Influence This power influence creates noise This happens because (recall high school electricity experiments): we have a moving electromagnetic field (the “alternating” current in the power lines) A stationary conductor in this moving field= induced current on the conductor (like an electromagnet). Also like when conductors rotate inside magnets (on the front of everyone’s engine block in their car, the alternator). 3 factors determine how much current is induced: distance from the conductor to the source, the strength of the field, and most important, how far the lines parallel!
Let’s represent this induced current on a pair with arrows (this is in a perfect world, though I realize a few of your phone lines are a little less than perfect). Both conductors are influenced equally by the PI, so an equal current is induced. If we were to connect a butt set from R –G, we would hear a “roar” or a “hum”: this is the PI passing through to ground. But if we clip our buttset across T & R (mutual), we don’t hear the “hum”. the equal PI on T & R block each other and we do not hear it (and neither does the customer, a good thing). But what happens if one side, and not the other, has a ground?
Smaller PI on RING does not stop all of the PI on TIP
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The ground give some of the PI a path other than R, so not all of the induced current flows to the meter/telephone. � PI on one side is larger, and is not completely blocked by the PI of the other lead The excess PI crosses the termination as noise (your customer hears this if more than 20dB which is bad) The noise is actually caused by the PI on the good lead, flowing to ground (remember how AC behaves) So it’s important that the leads are the same so they counter noise, and this “sameness” between them is called: BALANCE Q: did the ground on R CAUSE the noise? NO the fault only provided a path for induced AC, which caused the noise Faults are passive, they do not actually cause noise. Lets see now a situation you may have seen where it’s important to understand balance (next slide)
Have you ever used a buttset on a pair and heard noise (mutual), but then checked balance and it was OK? (many have) Note that balance is fine in this situation. But yet the noise is too high? (20 is the max noise allowed across T – R, or mutual) Should we fix this pair (don’t answer yet)? Another critical, important lesson: SIDEKICKS READ JUST THE OPPOSITE OF THE DYNATEL AND HST! (memory aid: Little meters use little numbers, large meters use large numbers) If you forget everything else today, REMEMBER THIS.
Ever “stress” the line, and it’s OK, but you still hear noise on your butt set?
The balance is goodBut the PI is too High
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In this situation of two pairs: the PI, noise and balance are all fine On the second pair: the PI and noise are too high: IS THIS PAIR BROKEN? SHOULD WE FIX IT? (ask the audience) NO: the pair is NOT broken! The balance is fine, it is doing the job it was designed to do: balance of at least 60! YOU Refer this to the noise specialists or engineers!
Your “stress” test or Longitudinal Bal test is oneof the best pair quality tests, if not
THE BEST
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So what’s all this balance and noise stuff got to do with ADSL? Remember: what kind of signal is ADSL? DC or AC (AC) We have also already talked about bridged taps and balance, the degree to which the TIP ring is electrically the same as the RING lead. And we now know that good balance reduces the effect of power influence (and other noise too). AND NOISE IS CRITICAL TO DIGITAL SERVICES, such as ADSL, HDSL, T1, ISDN, etc.. The quality of the digital ADSL signal on the loop is affected by: signal to noise ratio (most important) loop resistance and faults (cause signal echoes and reflections) Bridged taps (increased capacitance and signal echoes) capacitance (higher ADSL frequencies attenuated more, where downstream data is carried). Some other disturbers: AM radio, fluorescent lights, electric motors, static electricity, cross talk from other services in the bundle (such as HDSL2 and E1) A balance test tells us so much about the quality of the loop, that it’s the best single test to tell you if the pair is good for ADSL (and other digital services)
Will this lateral/Bridge-Tap affect resistance measurement?
T
R
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To help us understand the differences, I’ll start by asking a question: (describe the loop: shorted, a bridged tap ½ way down the loop between our meter on the left end and the short on the far end; the lateral has no faults). CLICK mouse to display questions: Q1: will this lateral affect my resistance/ohmmeter? (NO) Q2: will this lateral affect my opens/capacitance meter? (YES) Why is this? Ohmmeters use DC current, which seaks a continuous path from one pole of the source back to the other pole of the source, like a car battery. It ignores other paths that don’t lead back. (it was key that the lateral had no faults, as some faults could provide a path back to the source). Opens meters use AC current, which seeks any path to ground, and will follow any path in search of ground. So the opens meter “sees” the bridged tap and lateral.
Will this lateral/Bridge-Tap affect Capacitance measurement? No faults
on lateral/Bridge-Tap
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To help us understand the differences, I’ll start by asking a question: (describe the loop: shorted, a bridged tap ½ way down the loop between our meter on the left end and the short on the far end; the lateral has no faults). CLICK mouse to display questions: Q1: will this lateral affect my resistance/ohmmeter? (NO) Q2: will this lateral affect my opens/capacitance meter? (YES) Why is this? Ohmmeters use DC current, which seaks a continuous path from one pole of the source back to the other pole of the source, like a car battery. It ignores other paths that don’t lead back. (it was key that the lateral had no faults, as some faults could provide a path back to the source). Opens meters use AC current, which seeks any path to ground, and will follow any path in search of ground. So the opens meter “sees” the bridged tap and lateral.
Let’s say we took our resistance reading, and converted it to feet: we came up with 1500 feet (457 m). Then we used our opens meter and came up with 2500 feet (762 m): What have we just discovered/found? A BRIDGED TAP! Remember, the ohmmeter uses DC current that bypasses the bridged tap An Opens or capacitance meter uses AC current that goes down all paths Can we locate our tap with resistance or opens? No So we need a TDR: the best use of a TDR is to look for a fault when you already know it is there! It is very difficult to use a TDR when you just clip it on and look at the all the bumps and squiggles (confirm this with a tech). Let’s talk a little more now about a capacitance/opens meter.
Testing with a TDR: A TDR (Time Domain Reflectometer) is a test tool that operates similar to RADAR and is sometimes referred to as a “Cable Radar Set”. A TDR sends out a electrical “pulse” of energy and depending on the type of line imperfection it encounters energy in the form of modified pulse is bounced back to the TDR. If a line had no imperfections and was terminated in the resistive or impedance characteristic of the line (typically 100 to 135 ohms for a telephone cable pair) no energy pulse would be returned to the TDR. TDR parameter adjustments A TDR calculates and displays the distance to an event and/or fault by knowing what the speed of the pulse it transmits on a specific cable pair type. By knowing the speed at which the pulse travels the TDR knows when a pulse is returned to it that half of the time it took to return to the TDR is equal to the distance to the event and/or fault. A telephone cable pair restricts electricity to travel at speed that is less than the speed of light mainly because of capacitance and resistance that is inherent in the cable pair. To adjust the TDR for different cable types a TDR usually allows the user to select the Cable type, such as Air Core, Jelly Filled, etc., to compensate for the speed at which a pulse will travel for a given cable type. If exact distance to event and/or fault location is required most TDRs allow the user to adjust the “Velocity of Propagation” that can be set to the actual speed that a pulse travels on a specific cable pair.
– When the Pulse encounters a fault it reflects energy back
– The reflected pulse energy returns and the distance is ½ the time
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Testing with a TDR: Depending on the range that a TDR is set to the will adjust the size of the pulse that is transmitted. The larger the pulse width the more energy that is transmitted. Short Range: A smaller pulse is transmitted because not very much energy is required to identify cable pair events when they are not far from the TDR All TDR pulses create a “blind” spot, so it is important to have the smallest pulse possible when looking for faults that are relatively at a short distance from the TDR so that they wont be “hidden” by the “blind” spot. Long Range: A larger pulse is used for a TDR’s longer ranges so that there is enough energy for the TDR to display faults that are ate a great distance from the TDR. Since a TDR’s pulse will lose energy with every “event” (splices/joints and or fault) it encounters as well as loss caused by the cable itself it needs more energy to reach the end of the cable. Finally the pulse not anly must deal with the cable loss as it travels down the cable pair but also it encounters the same loss on the return trip back to the TSR. How it works for an Open: When a TDR pulse encounters the end of the cable that is an un-terminated open (very high resistance) the pulse is returned as a positive pulse with a sharp leading edge. If the TDR pulse encountered a high resistance at the end of a cable pair the pulse that is returned would have a lesser value than a cable pair what was open.
– When the Pulse encounters a fault it reflects energy back
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Testing with a TDR How it works for a Short When a TDR pulse encounters a short (cross) the returned pulse will be inverted (negative) in value. Any event that is less in value that the characteristic impedance of the cable pair will be negative in value.
– When the Pulse encounters a fault it reflects energy back
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Testing with a TDR How it works for a Bridge-Tap When a TDR pulse encounters the location where a Bridge-Tap is attached to the main cable pair it appears as a lower impedance that the cable pair’s characteristic impedance and therefore returns a negative pulse. Additional pulse energy from the TDR continues to travel down the main cable pair and the Bridge-Tap cable pair. When the pulse energy reaches the end of the main cable pair and the Bridge-Tap cable pair end positive pulses are returned. Depending if the Bridge-Tap cable pair or the main cable pair is longer will depend on what pulse appears further away. The Bridge-Tap cable pair length can be measured if first of all the total length of the main cable pair is known thereby allowing the user to recognize which positive pulse is associated with the end of the main cable pair. The Bridge-Tap cable pair than can be measured from the leading edge of the negative returned pulse to the leading edge of the positive pulse associated with the end of the Bridge-Tap cable pair.
Cable Failures: Wet Section Using a TDR to locate a Wet Section
– Signature:• Downward dip at the point the Cable pair encounters water• TDR trace in the wet section usually is curved slightly • Wet section trace is also uneven• Upward rise @ end of the wet section where the pair leaves the water
– Affects:• After the entering the wet section the TDR trace is no longer valid
– Distance measurement is no longer accurate• Water attenuates the TDR signal
– Long length of a wet section may prevent the end of a wet section to be seen
– Temperature• Every degree Celsius of temperature change causes a cable to appear
0.00218 feet per degree Fahrenheit longer or shorter– Wire composition
• Copper, Aluminum, Mixed alloy– All affect the Electrical distance that will be different than the actual
distance• Adjustments must be made in test sets to account for the difference
FEET/OHM @ 68° F
GAUGE
124.2419
61.6522
35.5424
2426
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Presentation Notes
Resistance Bridge: Factors that affect Ohms measurements Since resistance for any material will change when temperature increases or decreases it is necessary to calibrate any measurement device to the temperature of the material that is to be measured. For Copper conductors as temperature increases its resistance increases and as temperature decreases its resistance decreases. Because the resistance of a copper conductor changes with temperature its “electrical” length changes with respect to the actual length of the copper wire. Wire Composition Different material used as conductors in telephone cables have difference resistance per unit length and their receptivity changes at a different rated than other materials and therefore need to be calibrated accordingly.
Setup and Operation– Identify the faulted conductor
• Use a High-Impedance meter to ID the faulted conductor– A low impedance meter cannot measure above 3.5
Megohms and will dry out faults.• If both Tip & Ring are faulted use the conductor with the
lowest resistive fault
– Test the good pair• Use a High-Impedance meter to verify the pair is good• The pair must test good in the Megohm range
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Resistive Bridge: Setup and Operation A High Impedance ohm meter requires less current to identify resistive fault and therefore has less chance to “dry” out the moisture that can cause a high resistance fault. By determining the location of a resistive fault on the conductor with the lowest resistance a more accurate distance to the fault will be measured as compared to trying to locate the fault using the conductor with the higher fault resistance. If both tip and ring are faulted nether can be used as a good pair as inaccurate measures will result. A “good” pair must have a resistance better than 3.5 meg ohms.�
– Connect one test lead & cable sheath section to ground– If Tip and Ring measure different Shorter distance is to open
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Open Meter: Unbalance Pair Measurements If either the tip or ring conductor is longer then the other conductor of the pair the distance to the open of the longer conductor is inaccurate. This caused by the capacitance of the longer conductor changing its value beyond the shorter conductors open. Always go to the shorter conductors open as it is the more accurate distance to its open.
– Open Meters are designed to be used on working pairs– If Tip & Ring measurements are shorter than the mutual
capacitance unit is non-working– 12 or more pairs in the same unit must be working or
grounded for accurate measurement – If a unit has fewer than 12 non-working pairs length
measurements will be shorter than actual length
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Open Meter: If a pair’s tip-to-ground and ring-to-ground capacitance measurements is shorter than the tip-to-ring mutual capacitance measurement the binder group is non working. There are two ways to measure to an open in a non working binder group: Calibrate the Open meter on a know good pair in the binder group to a know length Ground 12 pairs in the binder group this will make the binder group pairs appear as working pairs and provide an accurate open meter measurement.
Splits occur when conductors of pairs with different twist are spliced
– Causes noticeable cross-talk on POTS– 5 feet of split pairs will cause noticeable cross-talk– Detrimental to Digital service, i.e. ADSL
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Presentation Notes
Locating Splits: A Split Occurs when conductors of pairs with a twists are spliced together along a cable route. This is a man-made problem, and usually occurs in splice , however a split can also occur in cross boxes and access points. Because the twist control is destroyed the capacitance balance of the cable pair is affected The resultant affect is auditable crosstalk for POTS and no modem sync for ADSL. A split can look like a one side open fault if all the other pairs are in use in a binder group. Using an Open meter the pair will appear balanced from both ends
Locating Splits: If a far-end-to-split measurement is shorter than the distance-to-split , locate two access points whose separation matches the far-end-to-split measurement The split should be at each access location. If the far-end-to-split measurement is longer than the distance-to-split measurement, the cable should be opened at an access point near to the middle of the cable. One split will be seen in each direction
Using a TDR to find a Split & Corrected Split fault– Crosstalk Mode:
• Connect TDR Main and Reference leads to both split pairs
EndOf
cable pair
Splitlocation
Re-splitLocation
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Presentation Notes
Locating Splits with a TDR (Crosstalk Method): Connect both split pairs to the TEST and Reference leads of the TDR. For this test the TDR sends a pulse down the test pair and displays the pulse reflections on the reference pair. The display will be flat except where the crosstalk occurs at the split. At the crosstalk point a sharp spike will be present identifying the split location.. The spike may be positive or negative depending how the test leads are connected to the TDR.
Using a TDR to find a Split & Corrected Split fault– Comparison Mode:
• Connect TDR Main and Reference leads to both split pairs
Splitlocation
EndOf
cable pair
“Good”Pair
SplitPair
Split PairAppearsShorter
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Locating Splits with a TDR (Comparison method): Compares a Split pair to a known “Good” pair. The split pair will indicate a large reflection at the split location. If the open end of the cable pair can be seen by the TDR the distance for the split pair will measure shorter than the good pair that is in the same binder group. If the pair also contains a re-split, the best method for finding the re-split is to find the split first then locate the re-split using either the Crosstalk or Comparison methods.
– Less than 20 ma Unacceptable– 20 ma to 23 ma Marginal– 23 ma to 65 ma acceptable– Greater than 65 ma Unacceptable
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Quality of Service Parameters: Loop Current A typical 1500 ohm office is made up of the following 450 ohms CO Ballast Resistance for protection against shorts at the MDF A maximum loop resistance of 1300 ohms 430 allocated for inside wiring and the telephone station equipment Minimum Battery voltage of 44 Volts DC Using Ohms Law we get: (450 + 1300 + 430}= 2180 ohms Loop Current with Low CO Battery 44 Vdc ÷ 2180 Ω = 20 ma Loop Current with Typical Battery float Voltage 52 Vdc ÷ 2180 Ω = 24 ma
– Acceptable level equal or greater than 8 dB loss– Marginal level between 8 db & 10 dB loss– Unacceptable level less than 10 dB loss– 26 gauge 8 dB limit @ 14.6 k feet – 24 gauge 8 dB limit @ 18.5 k feet
Quality of Service Parameters: Circuit loss @ 68 F (20 C) 26 gauge loss @ 1000 Hz = 2.90 dB/Mile (1.80 dB/km) 8 dB loss @ 14.6 k ft (4.44 km) 24 gauge loss @ 1000 Hz = 2.28 dB/Mile (1.42 dB/km) 8 dB loss @ 18.5 k ft (5.63 km) 22 gauge loss @ 1000 Hz = 1.80 dB/Mile (1.12 dB/km) 8 dB loss @ 23.5 k ft (7.14 km) 1300 ohm Loop Resistance Limits @ 68 F (20 C) 26 gauge = 15.6 k ft (4.72 km) 24 gauge = 23.1 k ft (7.00 km) 22 gauge = 40.0 k ft (12.12 km)
• Assures protector operation in presents of Lightning & power faults– Measuring station ground
• First measure Tip-to-Ring loop current • Strap (short) Tip to Ring
• Remove strap and measure current to ground
• Results: Loop current should increase from T-R to R-G Measurement – > 150% for T-R > 60 ma: Example for 70 ma T-R, R-G > 150 ma– > 170% for T-R > 40 ma < 60 ma: Example for 50 ma T-R, R-G > 85 ma– > 180% for T-R > 23 ma < 40 ma: Example for 30 ma T-R, R-G > 54 ma
Exchange
TIP
RING
CurrentMeter Station
Ground
MeterTIPLead
MeterRINGLead
Exchange
TIP
RING
CurrentMeter Station
Ground
MeterTIPLead
MeterRINGLead
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Quality of Service Parameters: Station Ground The station ground should test to be 25 ohm or less to assure the station protector will fire when power or lightning is present on a circuit and therefore protect the customer and station equipment from damage.
