Non-Continuous Events In the Telephone Outside Plant TR30.3
Telecommunications Industry Association TR-30.3/99-04-620
(TIA)
San Jose, 12-13 April 1999
COMMITTEE CONTRIBUTION
Technical Committee TR-30 Meetings
SOURCE: AG Communication Systems/Lucent
CONTRIBUTOR: Randy Brown Phone: 602-581-4125
Fax: 602-582-7111
email:
TITLE: Non-Continuous Events in the Telephone Outside Plant
PROJECT: PN-4254/PN-4255
DISTRIBUTION: Members of TR30.3 and attendees at San Jose meeting.
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ABSTRACT
Some recent submissions1, 2 to T1E1.4 suggest that non-continuous events, such as power variances, can be detrimental to services shared within the binder. This contribution characterizes three ubiquitous, non-continuous Plain Old Telephone Service (POTS) events: ringing, supervision, and dial pulse in order to illustrate that intra-binder interference in DSL frequency bands has been present for decades in the Public Switched Telephone Network (PSTN). These normal events represent cross talk impairments of changing magnitude, which are more severe than the classes currently under consideration for the Spectral Management standard.
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1. INTRODUCTION
The discussion examines three common, non-continuous POTS events, with some supporting test measurements, which are known to couple into DSL frequencies on adjacent pairs.
2. DISCUSSION
2.1. Ringing
Ringing in North America is an AC voltage superimposed on a DC bias. The majority of installations in the US use non-sinusoidal 20 Hz ringing with a nominal rms. 90 volts at the ringing source. A significant number of telephone installations in North America use other frequencies that range from 16 2/3 to 66 2/3 with voltages from 85 to 1353. One ANSI standard sets the maximum voltage limit to 150V rms4 and notes cases where it can attain 175V rms.
Ringing is clearly a non-continuous disturber. At the beginning of each ringing burst there is a transition from -48-Volt battery feed to -48-Volt with superimposed AC ringing. Nominal interrupts are 2 seconds on and 4 seconds off. Custom ringing cadences with multiple ringing, such as triple cadences, are common. The ringing waveform is ideally a sine wave with its axis of symmetry shifted -48-Volts from zero. The ringing burst can be characterized in terms of 100's of milliseconds as shown in Figure 1. In this depiction, the sine wave starts and stops in unity with the DC bias and represents the best case relative to instantaneous power changes as a result of ring application and trip.
Figure 1. Standard Ringing Potential with Best Case Start/End
Elements of synchronization are related to the application of ringing in many applications, such as the use of a common ringing bus serving hundreds of lines. Central office implementations, in many cases, simultaneously ring multiple lines with concurrent cadence. As such, the application and withdraw of ringing is generally without regard to the phase angle of AC energy. The peak voltage when ringing is tripped can be the sum of the DC and greatest AC or approximately 170 volts as shown in Figure 2.
Figure 2. Standard Ringing Potential Worst Case Start/End
Ringing is not balanced within the pair as it is ordinarily applied on only the ring conductor of the telephone pair, while the tip conductor is held near earth ground. The unbalanced application of ringing increases the pair-to-pair coupling within the binder group.
In practice, multi-vibrators generate the wave shape of the ringing energy. To smooth the resulting wave edge, post filtering is applied to the generated waveform. The ringing interrupters switch from the AC and DC ringing potentials to a DC-only potential during the silent interval as illustrated in Figure 3. This switching is not synchronized with the ringing waveform.
Figure 3. Basic Ringing Generator
In its worst case, a generated ringing waveform is a trapezoidal shape, which means it has higher frequency components occurring at 25 mS intervals. Transient energies often result from gap switching in the ringing generator as shown in Figure 4.
Figure 4. Ringing Waveforms (Worst Case Generalization)
A square waveform is made of a fundamental frequency f and all the odd harmonics rising to infinity, that is, 3f, 5f, 7f and so on. As a ringing waveform nears a square or trapezoidal shape together with its instantaneous application on a line, it can be expected to produce fundamental harmonics across a wide array of frequencies. This was observed using a spectrum analyzer sampling distinctive ringing frequencies at 3kHz for approximately 10 minutes, on a copper pair with 135 Ohms termination impedance adjacent to a continually ringing line, within a binder of 10Kft 24AWG, for the upstream ADSL bandpass as shown in figure 5:
Figure 5. Ringing Impulse Noise within the ADSL Upstream Bandpass
With the noise floor shown of approximately -90dBm along with a peak ringing amplitude of about -58dBm, its clear that over 30dBm of ringing impulse noise within the frequencies of interest to the spectral management project is not unusual. This effect of ringing was also observed on various lengths of copper, with randomly selected pairs across a wide array of frequencies using ringing generators, line circuits and ringing cards in current commercial service as described in appendix A.
Various forms of ringing cadence exist as noted above such as "triple," "double," "long/short," "coded," and "teen ringing.6" For example, triple ringing bursts three times within 1800 mS as shown in Figure 6. These have the effect of increasing random, ring application and removal impulse effects as shown above. Plainly, one line alone ringing within a binder can represent a significant non-continuous disturber, which all DSL systems must accommodate.
Figure 6. Triple Ringing Interval
Telephone Switching systems typically have the capability of ringing as many as one-fourth of the connected lines. Accordingly, in the worst case, an average of 6 of the 25 pairs in a binder group could be in some phase of ringing application or removal.
2.2. Supervision (hook flash)
As shown in Figure 7, the DC potential is applied to the customer loop through a battery-feed device consisting of two inductive coils in series with tip and ring. An idle circuit is nominally 48 Volts with no current flowing.
Figure 7. Simple Battery Feed Arrangement
During service initiation, the customer closes the loop and a transient voltage migration occurs within the cable pair of greater than 40 volts, that is, it drops to 6 volts across the telephone set.
A sudden voltage change in the presence of distributed capacitance can couple as not all of it gets cancelled out. A wave front of the sudden change in loop voltage is unbounded and currently unrestricted. In a theoretical situation with a short customer loop and no capacative leak between tip and ring conductors, the instantaneous peak-to-peak value of these potentials can reach 2,000 volts due to the inductive reactance of the battery feed device. POTS filters for DSL are only on the pair connected to and adjacent pairs are susceptible to the type of inductive kick as described above. This exists throughout the network today.
In some cases, on longer loops, the voltage is boosted in order to achieve objective currents while the telephone is off hook. Therefore, 48 volts is the minimum of voltage change that may be encountered. This effect was observed on a spectrum analyzer lab setting as described above for ringing with the collector located at the premise side as shown in Figure 8.
Figure 8. Hook Switch Coupling
2.3. Dial Pulse
These are periodic transitions from on-hook to off-hook in order to convey numeric values typically at 10 pulses per second in North America. Usually, 40 ms make (close) versus 60 ms break (open) as there is less time required to build the magnetic flux versus lose it. As soon as the dial on the phone is turned, all of the resistance in the circuit (all the handset circuitry) is shunted. There is a solid short in the circuit in order to get ready to go to maximum current.
The shorter the loop the higher the current but the less the cross talk potential. This is just the opposite of longer loops. These phenomena exist on short and longer loops. The highest value is a zero (10 pulses). The random, reoccurring cross talk effects of dial pulse were observed on a spectrum analyzer with a lab setting as described above for supervision signaling with the collector located at the premise side as shown in Figure 9. In some jurisdictions, all telephone lines must support dial-pulse digit collection methods.
Figure 9. Dial Pulse Coupling
3. CONCLUSION
These conditions co-exist with ISDN today. Therefore, ringing, hook flash and dial pulse non-continuous characteristics exemplify tolerable interferers. Accordingly, services that are frequency bounded and current limited in order to conform to service classes within the spectral management project should be permitted independent of their temporal characteristics.
References:
[1] T1E1.4/99-103, "Stationarity and Time-Domain Specifications for Spectrum Management", Bellcore, March 11, 1999, Costa Mesa, California;
[2] J. Stiscia and D. Johnson, "Selected Measurements of Non-Stationary and Stationary Crosstalk Effects Upon FDM ADSL," T1E1.4/99-040, February 1, 1999.
[3] GTE Customer Handbook - 500, Issue 1, 1972
[4] T1.401-1993, "Interface Between Carriers and Customer Installations--Analog Voicegrade Switched Access Lines Using Loop-Start and Ground-Start Signaling."
[5] T1.413-1998, " Network and Customer Installation Interfaces - Asymmetric Digital Subscriber Line (ADSL) Metallic Interface."
[6] ANSI T1.401.02-1995, "Interface between Carriers and Customer Installations--Analog Voicegrade Switched Access Lines with Distinctive Alerting Features."
[7] Federal Communications Commission Rules and Regulations, Code of Federal Regulations Title 47: Part 68 - Connection of Telephone Equipment To The Network
Appendix A - Ringing Loop Simulation Setup & Testing Results
Setup
· Test method: Distinctive ringing on pair 1, read impulse noise on pair 2
· 3 kHz sampling frequency, 25kHz - 140kHz & 0-300kHz
· 135 Ohms termination impedance on pair 2
· 2524 (24 Gauge) SEALPIC-FSF SX08 98C cable
· 5REN load on pair 1 at premise during ringing tests
· 5 minutes collection for ringing, 1-2 minutes for dial pulse and supervision
· Commercial Line Card & Ringing Generator in use today in major ILEC market
· Measured at CO during ringing and premise during dial pulse and supervision
Results for 0-300kHz Ringing Sampling Frequency
Loop Length ft / Noise Floor dBm / Highest Peak dBm18,000 / -89.6 / -60.8
15,000 / -90.0 / -59.8
10,000 / -90.0 / -59.8
5,000 / -89.0 / -55.2
1,000 / -89.0 / -57.6
Coupling was noted across the frequencies of interest. 6dB variance was observed based on the pair selected.
AG Communication Systems/Lucent Page 8 of 1 04/13/99