VL2 POTS Splitters and Microfilters Introduction The purpose of this paper is to discuss the characteristics of splitters and microfilters for VL2 applications. Both CPE configurations are simplified into an equivalent loop representation in order to more easily compare the characteristics that will most significantly affect VL2 performance, specifically impedance changes due to the presence of bridge taps and loading of the LINE in the L frequency band. Splitters and Microfilters Overview In typical L deployments, Voice and L services are provide over the same line of twisted-pair copper wires. The copper wires carry signals with frequencies up to 30 MHz (VL2), enabling customer data rates of more than 100 MBps. The figure below illustrates an example of a VL2 band-plan: 4 khz POTS 0 1 1 2 2 3 3 0.025 0.138 3.75 5.2 8.5 12 23 30 MHz Figure 1: VL2 Band Plan Distribution ( Upstream, Downstream) As seen above, the line at the customer s premises contains voice signals (POTS- lower frequency range up to 4 khz) for the telephone as well as data signals (higher frequency range 25 khz 30 MHz) for L services. Signals intended for L service disrupt the service of the voice calls. More importantly, phone transient events disrupt L service making it is essential to isolate the low frequency POTS signal from L signals. At the, isolation between POTS signals and L signals is most commonly achieved by installing either a single CPE splitter or multiple microfilters (also known as in-line filters). www.aflglobal.com 1
VL2 POTS Splitters and Microfilters Microfilters A microfilter is a Low-Pass (LPF) that isolates the low frequency signals on the line from the high frequency (L) signals and prevents interference between the two. Microfilters have Line and POTS ports and are typically installed in-line with telephones, fax machines and/or answering machines, and can be easily installed by the customer. Figure 2 shows a typical household loop configuration with microfilters. The loop is defined by main segment L1 from the central office to the Line Port at the customer premises, L5 segment from the Line Port at the home to the modem, and additional line segments (bridge taps) L2, L3, and L4 which connect the line port with 3 in-line filters. (Note: These segments may represent one of three connections: 1) an in-line filter terminated with a telephonic device; 2) an unterminated in line-filter; or, 3) unterminated telephone wiring in the home.). It should be noted that in-line filters are connected in parallel with the line to the modem; a parallel connection with the line has a very high possibility of significantly changing the impedance of the line. Typical in-line filter configuration L2 L3 L4 L1 L5 Simplified loop of in-line filter configuration L2 L3 L1 L5 L4 Figure 2: Typical loop configuration with multiple in-line filter installation 2
VL2 POTS Splitters and Microfilters CPE Splitter A CPE splitter has 3 connections: Line, L and POTS. It provides low pass filtering between the Line Port and POTS port and a HPF (high pass filter) or all pass filter between Line and L ports. A typical Loop configuration of CPE splitter installation is shown in Figure 3. In this situation, the loop for L signal is defined by main segment L1 between the central office and the Line Port at the home, and short segment L5 between Line Port and modem. It should be noted that segments L2, L3, L4 do not represent bridge taps to L1 and L5 because they are isolated by the CPE splitter. Line Port L1 Phone Port L Modem Port L2 L3 L4 L5 AFL AL2/VL2 Splitter Figure 3: Typical loop configuration in a CPE splitter installation 3
VL2 POTS Splitters and Microfilters CPE Splitter vs. s Comparing the installation configurations of a CPE Splitter (Figure 3) and multiple in-line filters (Figure 2), two main differences are noticed: 1. A multiple filter installation has an equivalent loop with bridge taps, whereas a CPE Splitter installation has an equivalent straight line loop that eliminates bridge taps within the home. 2. Multiple filters are connected in parallel with line, while only one CPE splitter is connected to the line; therefore the resultant loading effect on the line is greater with in-line filters than with a CPE splitter. Bridge (Bridged) Tap A bridge tap is an unterminated branch in a cable, or any branch on a pair that is not a direct connection between the Line and the subscriber. For the purpose of this paper, we will refer to any T connection in the copper pairs as a bridge tap, be it unterminated or terminated (with microfilter and/or telephone equipment). Any signal transmitted on the main copper pair will also travel down the bridge tap. Open end of the bridge tap (or high impedance when filter is installed) will result of the signal being reflected back towards the main copper pair. In the voice band this reflection will result in echo; in the L band it will lower attainable data rates. In the case of longer bridge taps (20- ) very drastic changes in data rates will occur due to significant changes in insertion loss and impedance. Impedance comparison for the two installation options is shown in Figure 4. Ideal input impedance to the modem is 100 Ohms. 1400 1200 Impedance [Ohms ] 1000 800 600 400 200 0 frequency [khz] 3,000 6,000 9,000 12,000 15,000 18,000 21,000 24,000 27,000 Modulus of impedance seen by modem for configuration H.1 Modulus of impedance seen by modem for configuration A and B Figure 4: Difference of impedance to the modem for loops with in-line filters with in line filters vs CPE splitter with configuration found in Appendix C 4
VL2 POTS Splitters and Microfilters Calculations show that with an in-line filter configuration, bride tap segments of just in length can change the impedance seen by the modem more than 50%. This change in impedance will result in high reflection and a smaller amount of power transferred to the modem; results will be significantly decreased VL2 data rates. Data rates will not be similarly affected with a CPE splitter due to the fact that the CPE splitter eliminates bridge taps, separating L and POTS at the input of home network. In a configuration using in-line filters such as shown in the diagram below. A bridge tap of would create notches (highest loss of the signal) around 4 MHz, 12 MHz, 20 MHz and 29 MHz. This would lead to downstream and/or upstream degradation depending upon what bands are impacted by the notches. 40 feet To Telco 1000 feet 0 Insertion Loss [db] -20-40 -60 0 5 10 6 1 10 7 1.5 10 7 2 10 7 2.5 10 7 3 10 7 Frequency [Hz] Figure 5: Insertion loss notches corresponding with a 40 foot bridge tap 5
VL2 POTS Splitters and Microfilters Data Rate Comparison between Configurations using CPE Splitter vs. Microfilter Using a 2000 foot loop from the Telco Office to the : In-line filter configuration with a 3 foot bridge TAP between modem and phone line in-line filter as shown below. Measured 25% degradation in downstream data rate compared to configuration using a CPE Splitter. 3 feet To Telco 2000 feet In-line filter configuration with a 20 foot bridge TAP between modem and phone line in-line filter as shown below. Measured 45% degradation in downstream data rate compared to configuration using a CPE Splitter. 20 feet To Telco 2000 feet 6
VL2 POTS Splitters and Microfilters Measured Data Rates using Typical Home Wiring Configurations The following tables compare measured VL2 training rates (17a profile) with CPE splitters and various in-line filter configurations. Refer to Appendices A, B and C for diagrams of the in-line filter installations. VL2 Data Rates (AFL Splitter versus various Daisy Chain Configurations) 100 90 80 70 60 Mbits 50 40 30 20 10 0 Baseline AFL AL2/VL2 Splitter Microfilter Config A Microfilter Config B Microfilter Config C Microfilter Config D AFL AL2/VL2 SPLITTER CONFIG A CONFIG B CONFIG C CONFIG D AL2/VL2 splitter placed at beginning of chain. Chain extends from POTS port of AL2/VL2. L Modem is home run from L port. Daisy chain in-line filters apart. Last jack has wall mount L filter with modem. Daisy chain in-line filters apart. First jack of chain has wall mount L filter with modem. Daisy chain of POTS jacks apart. First jack of chain has wall mount L filter with modem. Daisy chain in-line filters apart. Fourth jack of chain has wall mount L filter with modem. * See Appendix A Daisy Chain Configuration for details on Configurations 7
VL2 POTS Splitters and Microfilters 100 100 90 VL2 Data Rates (AFL Splitter versus Star configuration of various lengths with length to of ) VL2 Data Rates (AFL Splitter versus Star configuration of various lengths with length to of ) 90 80 80 70 70 60 Mbits 60 50 Mbits 50 40 40 30 30 20 20 10 100 0 Baseline AFL AL2/VL2 Splitter Microfilter Config F.1 Microfilter Config F.2 Microfilter Config F.3 Baseline AFL AL2/VL2 Splitter Microfilter Config F.1 Microfilter Config F.2 Microfilter Config F.3 AFL AL2/VL2 SPLITTER CONFIG F NID-01V splitter placed at beginning of star TAP home run configuration. TAPs extend from POTS port of NID-01V. L modem is home run from L port. 5 TAP home run configuration with in-line filters on each end. length taps. * See Appendix B Star 20 foot phone lines for details on Configurations 100 VL2 Data Rates (AFL Splitter versus Star configuration of various lengths with length to of ) VL2 Data Rates (AFL Splitter versus Star configuration of various lengths with length to of ) 100 90 90 80 80 70 70 60 Mbits Mbits 60 50 50 40 40 30 30 20 20 10 U S U S 100 0 Baseline AFL AL2/VL2 Splitter Microfilter Config H.1 Microfilter Config H.2 Microfilter Config H.3 Baseline AFL AL2/VL2 Splitter Microfilter Config H.1 Microfilter Config H.2 Microfilter Config H.3 AFL AL2/VL2 SPLITTER CONFIG H AL2/VL2 splitter placed at beginning of star TAP home run configuration. TAPs extend from POTS port of NID-01V. L modem is home run from L port. 5 TAP home run configuration with in-line filters on each end. length taps. * See Appendix C Star 40 foot phone lines for details on Configurations 8
VL2 POTS Splitters and Microfilters Appendix A Daisy Chain Configuration Baseline AFL Splitter Phone Port L Modem Port Config A Config B L L Config C Config D L L 9
VL2 POTS Splitters and Microfilters Appendix B Star 20-foot Phone Lines Baseline AFL Splitter Phone Port L L Modem Port Config F.1 Config F.2 L AFLNID L Config F.3 L AFLNID 10
VL2 POTS Splitters and Microfilters Appendix C Star 40-foot Phone Lines Baseline AFL Splitter Phone Port L L Modem Port Config H.1 Config H.2 L L Config H.3 L 11
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