The following drawings serve to illustrate the principles of the invention.
This disclosure provides for a power spectral characterization and the identification of available upstream frequency regions which would support communications. The present invention enables an automatic determination of how much RF power is available in a network for addition of additional services, and ingress power before a predetermined soft failure occurs. A soft failure is a degradation in signal quality which causes pre equalized errors to occur, but are within available limits of error correction, the intent being that there will be no noticeable impairment to the live services on a network. The test in the invention generally involves demodulation of a specified test QAM carrier and measurement of its signal quality to determine impact caused by stressing the network.
The methodology described in this invention instructs two DOCSIS terminal devices (cable modems or MTAs) to transmit simultaneously and measures the affects on a third communications channel, such as the MER (mean error ratio), BER (bit error rate), and PER (packet error rate). Subsequently, power is increased for the two DOCSIS terminal devices until, an impact on the communicating channel is detected. That is, it monitors the affects of increasing power in the return-path of the cable network on an active communications signal and logs the total power added when said power begins to impact the performance of the communications channel. The approach detailed in this disclosure requires that the three DOCSIS terminal devices reside on the same optical node. A methodology for isolating devices which reside on the same optical node is provided in a commonly assigned disclosure Attorney Docket No. BCS04122, entitled METHOD AND APPARATUS FOR GROUPING TERMINAL NETWORK DEVICES filed on Sep. 5, 2006 and assigned U.S. Ser. No. 11/470,034. Preferably, the power margin test should not occur in conjunction with other changes in the network, such as changing of optical routing, ingress level switching or any other routine or event that will likely cause RF levels to be unstable.
Adequate margin should also preferably be available in the network to allow the addition of 2 DOCSIS channels. This margin may be determined by first estimating the total power of the current upstream loading via FFT measurement, then adding a test channel at the same level of the cable modem channel and rerunning the FFT. If total power increase is less than 3 dB with cable modem and test channel loading combined then the system is still functioning in linear region and power addition from test channel is acceptable. Otherwise the optical link may be overdriven. The margin test should be repeated by adding the second test signal. The FFT should also be run with both test signals transmitting at the same time during the second test.
Preferably, an active Return Path is providing services at the time that the operator desires to associate (group) network elements according to common optical nodes. Also, this test picks test frequency locations based upon avoiding interference of 2nd order intermods on active data services. We are assuming adequate margin is available such that 3rd order products are not a problem for the active services. Also, the approach preferably uses DOCSIS cable modems to generate test signals. Therefore test signals will be one of the available DOCSIS bandwidths (200 kHz, 400 kHz, 800 kHz, 1600 kHz, 3200 kHz, 6400 kHz). Preferably, the test will use 800 kHz bandwidth due to narrow bandwidths minimize the amount of clean spectrum required within the return path, and because many modems have problems with the 400 and 200 kHz widths.
A RF transceiver (transmitter/receiver) 20 preferably provides bi-directional communication with a plurality of network elements 8 through optical transceivers 16, nodes 12 and a plurality of network taps (not shown). Those of skill in the art will appreciate that CMTS 10 may contain a plurality of RF transceivers, e.g. 8 RF transceivers and a spare RF transceiver. Each RF transceiver may support over 100 network elements. RF transceiver 20, such as a Broadcom 3140 receiver (transceiver), is preferably used to acquire equalizer values and burst mean error ratio (MER) measurements, packet error rate (PER) and bit error rate (BER). RF transceiver 20 may also include FFT module 308 to support power measurements. The communication characteristics of each receiver 20 may be stored on ROM 104 or RAM 106, or may be provided from an external source, such as headend 14. RAM 104 and/or ROM 106 may also carry instructions for microprocessor 102.
Upon receiving a downstream communication signal from a network element, via CMTS 10, CPU 30 preferably provides instructions to modulate one of the laser transmitters 312 to transmit the communication signal to nodes 12. Optical receivers 316 are preferably configured to monitor the optical signal transmitted by nodes 12, such as by receiving a portion of the signal. Optical receiver 316 preferably provides the monitored portion to the FFT module 308 where intermods may be determined and power monitor unit 310 where the power level in a specific frequency (such as the test frequency) may be measured or the total power of the signal may be measured.
An exemplary process for automatically determining the power margin available in the system on an optical node is illustrated in
Ideally, we want to find two frequencies that network elements NE1 and NE2 could transmit on which would not produce a 2nd order intermod at a third frequency to which network element NE3 may be assigned. Each of the three frequencies are preferably within the 5-42 MHz spectrum. The possible frequencies may be identified by a plurality of techniques, such as by empirically determining usable frequency regions for QPSK (quadrature phase shift keying, also referred to as four QAM) transmission from a survey process. The communication frequencies (f1 and f2) are preferably selected such that f1±f2 does not fall on f3 and each of f1, f2 and f3 lies between 5-42 MHz. The three frequencies are also preferably selected such that second order products from these frequencies do not fall on desired traffic in the network, if possible. Preferably, frequencies f1 and f2 can be activated as DOCSIS upstream channels with default upstream CMTS receive levels without causing any significant harm to any other active services.
As illustrated in
As illustrated in step S4 of
As illustrated in step S6 of
PL1 and PL2 may be the same power level and may be at level L which was assigned as the nominal power level. In this step, network elements 1 and 2 are preferably instructed to perform a station maintenance (SM) burst at exactly the same time. Those of skill in the art will appreciate that this may be done by lining up the minislots in the MAPS for the two upstream channels associated with network elements A and B. Those of skill in the art will also appreciate that the MAP or MAPS data provide a schedule of time slots which allocates different network elements specific time intervals in which they are allowed to transmit data to the CMTS. From a CMTS software perspective, this should not be a complicated problem as the IM broadcast intervals are already aligned across ALL channels within a single spectrum group. The FFT processor should also be configured to trigger samples based upon the MAP minislot interval when the two SM bursts from the network elements will align. The combined power (Pc) and the power of f3 (Pf3) are measured, as illustrated in step S10. It may be desirable to perform steps S8 and S10 several times to eliminate the possibility that a coincidental ingress happened at the exact same instance as the SM bursts.
The CMTS spare receiver may be used to make the error rate and power measurements to avoid impacting service provided to customers,. Alternatively, another receiver could be used to make the measurements by being taken “off line” or by adjusting for the impact caused by normal service.
If the simultaneous transmission has not increased the power level in the FFT cell at the test frequency (f3) to significantly impact the test signal, step S12, NO, then in step S18, then the power level of network element 1 or 2 or both is increased and the process in steps S8 and beyond is repeated. If the test signal from network element 3 is impacted, step S12 YES, the power addition and power margin are calculated, step S14 and logged in step S16.
The MER, PER and/or BER is measured at each incremental increase in power level and signals are increased until degradation in MER and more importantly a significant increase in PER is noted. The cause of this impairment is loading (compression) of the RF devices (most likely the return laser transmitter) in the system from the power created by the transmissions of network elements 1 and 2.
The processes in
The invention enables the technician or engineer to remotely characterize upstream total power margin quickly at a central location, such as the headened such as by using the Motorola BSR64000, rather than having to use external test equipment, such as the vector signal analyzer and deploying technicians to various locations within the cable plant without impacting active services. It also allows the MSO to plan for future offerings and schedule needed maintenance by allowing him/her to periodically monitor this power margin. All measurements may be made through the use of the existing terminal devices (specifically, DOCSIS terminal devices such as MTAs and cable modems) as well as headend equipment (specifically a DOCSIS CMTS).
Those of skill in the art will appreciate that the techniques of this invention allows an operator to determine available power margin on a network without the need for placing test instrumentation remotely in the cable plant. In addition, the technique discloses in the invention does not require an operator or technician to be dispatched to remote locations in the HFC network. All measurements may be made through the use of the existing terminal devices (specifically, DOCSIS terminal devices such as MTAs and cable modems) as well as headend equipment (specifically a DOCSIS CMTS). Accurate knowledge of available power margin will enable an operation to utilize the available resources of their network more efficiently, such as by adding additional network elements to portions of the network with a large power margin and shifting network elements away from portions with a small power margin to improve signal quality and network speed.
This application claims the benefit of U.S. Provisional Application No. 60/785,646 filed on Mar. 24, 2006, titled “Total Power Margin Test”, herein incorporated by reference in its entirety.
Number | Date | Country | |
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60785646 | Mar 2006 | US |