You Can t Fake Fast. The Promise of 100 Mbps and Vectored Services Raises Bar on Network Quality. White Paper

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1 You Can t Fake Fast he Promise of 100 Mbps and Vectored Services aises Bar on Network Quality he next generation of VDSL2 technology call Bonding & Vectoring promises performances upwards of 100Mbs on your existing copper network (FN). his allows you to economically deliver significant performance boost to your customers without costly fiber investment. But can your copper plant handle the demands of bonding & vectoring? able of contents VDSL Bonding echnology VDSL Vectoring echnology FEX Noise and NEX Noise Speed and Bandwidth Preparing Copper Cable for 100MBs Article by John Graham and David ran, Fluke Networks

2 You Can t Fake Fast he Promise of 100 Mbps and Vectored Services aises Bar on Network Quality As a provider of VDSL2 based residential services (IPV, High Speed Internet, etc.) you have already made a significant investment in your copper network. But will it be fast enough to support customer demands over the next several years? Given that cable companies are already offering network speeds upwards of 100 Mbps, what are the challenges facing your network to provide a similar performance and beyond? How will you meet those challenges? he good news is that you don t have to make that huge investment in a fiber-to-the home network. Once again, next generation copper based technologies are coming to the rescue. By using a combination of new VDSL2 based copper bonding and vectoring technologies, you will be able to deliver fiber-like speeds (upwards of 100 Mbps) that rival those of the competition. hese are serious speed increases that will require a network free of impairments to reliably support these ultra-fast speeds. Now, more than ever is the time to truly condition your network to deliver this performance. You can t ignore impairments or marginal network conditions. Simply put, you can t fake fast. What is the best way to prepare the copper network to deliver on the promise of 100Mbs services to customers? ead on to get answers to these questions and more as we take a deep dive into what it takes to get the copper network ready for 100 Mbps speeds. VDSL Bonding echnology VDSL bonding technologies address the need for higher speeds by dealing with the DSL signal itself two or more DSL signals are better than one. Pair bonding is the process of combining the capacity of one or more pairs in such a way that the bonded pairs act like one BIG capacity pair (see Figure 1 VDSL2 2 Pair Bonding). he data carrying ability of each pair remains the same, but bonding two equally capable pairs together effectively doubles the total data carrying capacity. It is kind of like a super highway more lanes mean a BIGGE capacity to carry traffic. Fluke Networks 2

3 Un-bonded Pairs Bonded Pairs Primary Pair ~30 Mbps VDSL2 Modem Primary Pair ~50 Mbps VDSL2 Modem Secondary Pair ~20 Mbps VDSL2 Modem Secondary Pair Upper Layers of the CP/IP Stack Ethernet Packets Bonding Block Pair #1 Pair #2 Bonding Block Ethernet Packets Upper Layers of the CP/IP Stack Layer 3-5 Layer 2 Layer 1 Layer 2 Layer 3-5 Figure 1 : VDSL2 Pair Bonding VDSL Vectoring echnology DSL vectoring technologies address the need for higher speeds by dealing with speed killing noise. DSL receivers deliver high speeds when noise levels are low. herefore, it makes sense to mitigate noise at the receiver to keep speeds high. he total noise present at a DSL receiver is best thought of as a group of individual noise components that add together to create a total noise problem. Among those noise components that contribute significantly to total noise would be Near End Crosstalk noise (NEX) and Far End Crosstalk (FEX) noise. DSL Vectoring is a complex microprocessor intensive method of mitigating a specific kind of noise - FEX noise (see Figure 2 FEX Noise). oday, several DSLAM vendors are providing VDSL2 vectoring solutions. Fluke Networks 3

4 NEX Noise In the early days of DSL planning, it became clear that the best way to mitigate NEX noise was to keep the Downstream and Upstream transmission bands separate from one another. Such an approach would not add significantly to the cost of silicon. FEX Noise Dealing with FEX noise is more challenging. In those early days of DSL planning the computing power required to cancel FEX noise at the silicon level would not have been cost effective. However, today vendors are shipping DSLAMs with cost effective VDSL2 vectoring FNEX noise mitigation capabilities. he Downstream ransmitters of ALL your neighbor s DSLAM Modem Ports each make an FEX Noise contribution to the total noise signal received by your G Modem. Your Neighbors G Modems DSLAM Modem (Ports) Your G Modem he UpStream ransmitters of ALL your neighbor s G Modems each make an FEX Noise contribution to the total noise signal received by your DSLAM Modem Port. Your Neighbors G Modems he transmission of a DSL signal over copper is limited by just two things: 1) Noise Signal Power Level at the eceiver 2) DSL Signal Power Level at the eceiver Noise DSL Figure 2 : Far End Crosstalk (FEX Noise Fluke Networks 4

5 Speed Possibilities DSL speeds are measured in bits-per-second (bps). he 100 Mbps speed objective bandied about today came from the IU- G VDSL2 standard which defines a VDSL2 17a profile with a minimum bi-directional net data rate capability of 100 Mbps (the Shannon-Hartley theoretical limit of a 17 MHz bandwidth is closer to 246 Mbps). he standard also defines a VDSL2 30a profile with a minimum capability of 217 Mbps (the Shannon-Hartley theoretical limit of a 30 MHz bandwidth is closer to 417 Mbps). When pair bonding is used in conjunction with DSL technology, there is a significant jump in total speed. wo bonded pairs as used in VDSL2 deployments also introduce the possibility of a third phantom channel to further increase speed Mbps or higher may be possible. Bandwidth Anytime there is an increase in bandwidth, there will be a need to re-visit existing copper network qualification methods. Bandwidth is the available spectrum for transmission. POS service only uses 4 KHz of bandwidth. Qualifying copper pairs for POS is relatively easy. But then along came analog modems and fax machines that used frequencies within that 4 KHz bandwidth that had not been qualified. Do you remember all those new trouble tickets? Now imagine what happens when you upgrade from an 8 MHz bandwidth network (VDSL2 Profile 8d) to a 17 MHz network (VDSL2 Profile 17a). Sure, you have qualified the network for 8 MHz, but what about that part of the bandwidth from 8 to 17 MHz that wasn t qualified? How do you know it will be free of frequency dependent trouble? Over the years there has been a steady increase in the bandwidth requirements of DSL technologies: Bandwidth Spectrum equirement xdsl echnology ypical Speeds 1.1 MHz ADSL Up to 8 Mbps 2.2 MHz ADSL2+ Up to 20 Mbps 8 MHz VDSL2 Profiles 8a, 8b, 8c, & 8d Up to 30 Mbps 12 MHz VDSL2 Profiles 12a & 12b Up to 40 Mbps (when bonding is used) 17 MHz VDSL2 Profile 17a Up to 100 Mbps (when bonding & vectoring are used) at up to 2Kft (600m) his bandwidth spectrum profile is expected to be the most widely adopted due to excellent balance of rate (speed) and reach (loop length) 30 MHz VDSL2 Profile 30a Up to 217 Mbps (or even higher when bonding & vectoring are used). Note: this requires very short loops, typically less than 500ft (61m). Fluke Networks 5

6 he 17Mhz and 30Mhz are the new VDSl2 profiles being adopted to support bonding & vectoring to achieve 100Mbs speeds. he 17Mhz profile is expected to be the most widely adopted profile for bonding & vectoring due to an attractive combination of rate (supports up to 100Mbs with bonding& vectoring) and reach (loop length up to 2kft or 600m). Preparing the Copper Cable for 100 Mbps or Higher Speed Services Now that you have an understanding of the technologies, standards and speed possibilities, let s focus on preparing the copper network for 100 Mbps or even higher speed services. he addition of bonding and vectoring technologies to existing VDSL2 deployments will require accurate records and additional qualification tests. ecords Accuracy ecords verification is a process unto itself. elephone companies maintain database records (cable, pair, binding post, status, etc.) about copper pairs. hese records are changing to keep pace with advancing DSL technologies. hey need to be periodically refreshed to reflect the current state of pairs in the field. When a customer calls in to request high speed DSL service it is important that customer agents quickly respond with accurate information as to whether or not that service can be provided. Here are some of the new challenges in a VDSL2 bonded and vectored network related to records accuracy: here will be an increase of bonded pairs in the new network that will require a better understanding of total pair capacity. How many pairs do you have to work with? Pair selection for bonding will require a better understanding of the utilization of available pairs. How many pairs are going to each customer? How long are the pairs? Are there spare pairs in the serving terminal? VDSL2 vectoring can be done at the board or system level. Since vectored noise mitigation only works for common VDSL2 based services, how will you manage these vector groups? he DSLAM vectoring engine can only mitigate so much FNEX noise. How will you manage its resources? Pair Qualification esting A pair that is free of trouble will deliver the highest speeds. Here are some of the new qualification testing challenges for a VDSL2 bonded and vectored network that makes use of bandwidths out to 17 MHz or 30 MHz: he total speed of bonded pairs will be limited by pair quality. Pairs that are free of trouble and with the best longitudinal balance will deliver the highest speeds. Noise is a speed killer. Is there noise in the new bandwidth? Noise on one pair can crosstalk onto adjacent pairs. Is the total crosstalk noise at a DSLAM limiting the speeds for all? Will the high level of FNEX noise overwhelm the DSLAM vectoring engine that is attempting to mitigate this noise? Fluke Networks 6

7 Signal attenuation is a speed killer. Is there frequency dependent trouble in the new bandwidth that will attenuate the signal? Bridged-ap can reduce speeds. he new network will have to deal with the effects of even shorter bridge-tap lengths. Clearly it is still important to perform the same core physical layer testing required for VDSL2 qualification - tests that identify foreign voltage, insulation resistance breakdown, opens (capacitance), noise and bridged-tap. But the new network will need some additional tests that identify any frequency dependent trouble in the new untested bandwidth. he following describes the comprehensive set of qualification tests we recommend for VDSL2 bonding & vectoring to support 100Mbs service. First, the following core tests help find some common line faults : 1. Foreign AC and DC Voltage he only DC voltages allowed on a copper pair are those that have been designed to be there. Unfortunately, DC voltages from other pairs may appear if the cable has been compromised water, broken insulation, etc. he only AC voltages allowed on a copper pair are those that have been designed to be there. Unfortunately, AC voltages couple onto copper pairs from all those power lines that run parallel with telephone cables. 2. Insulation esistance here should be high insulation resistance between copper pairs (ip-to-ing, ip-to-ground, ing-to-ground). Unfortunately, over time cables begin to deteriorate resulting in a lower insulation resistance. Fluke Networks 7

8 3. Capacitance Copper cable is designed to have the same mutual capacitance between the wires that make up a pair (ip and ing). However, it is possible for this capacitance to change over time due to bad splices, water, bridged-tap, etc. Also, the ip-to-ground capacitance should be very close to the ing-to-ground capacitance. 4. Bridge ap Detection he International Engineering Consortium describes a bridge tap as any unterminated portion of a loop not in the direct talking path between the CO or DLC and the customer s premise equipment (telephone or modem). When a signal is reflected from the unterminated portion, the reflection can interfere with the original signal. Bridge taps, particularly short taps, are much more of a problem in VDSL than in ADSL due to the higher frequency of VDSL. Bridge taps cause what is referred to as a notch effect, which can adversely affect the carriers at these frequencies. In general, the length of bridge tap will determine the severity of the notch. Finding and removing bridge taps is more critical than ever before when it comes to providing reliable 100Mbs performance at 17Mhz and 30Mhz profiles. But as we now know, bonding and vectoring performance requires additional tests that you may not have been performing. hese additional tests are important especially out to 17Mhz and 30Mhz to find faults that could impact performance at these higher frequencies. 5. Bandwidth Noise (out to 30 MHz) Noise is a speed killer. Noise should be analyzed across the entire bandwidth being used. he noise on a tested pair must be broken down into its component parts such that proper mitigation techniques can be applied. VDSL2 Vectoring technology deals specifically with the cancelation of FEX component noise it will not be able to deal with F or impulse noise. Along with loss, this test provides key information related to bit rate estimations. he noise level at a receiver will limit transmission. Fluke Networks 8

9 6. Wideband Longitudinal Balance (out to 30 MHz) When you hear the term longitudinal, think ip-to-ground or ing-to-ground. When you hear the term balance, think equality between ip-to-ground and ing-to-ground. When you hear the term impedance, think frequency dependent resistance. A pair is longitudinally balanced IF the ip-to-ground impedance and ing-to-ground impedance is equal for each frequency in the transmission bandwidth. Why is it important for pairs to have good longitudinal balance? oday s telephony transmission methods use differential mode transmitters to send signals across twisted pairs to differential mode receivers capable of rejecting common mode noise. Since most noise is common mode, the receiver can effectively reject it. However, when a pair is unbalanced at a particular frequency, any noise at that frequency will appear as differential mode noise to the receiver, and the receiver will not be able to reject it: Output Signal Differential Mode VDSL ransmitter Differential Mode VDSL eceiver eceived Signal Good Signals (IPV) are differential mode signals that part of the signal on the ip wire is the exact opposite of that part of the signal on the ing wire. Most Bad Signals (Noise) are common mode signals that part of the signal on the ip wire is the same as that part of the signal on the ing wire. Fortunately, these noise signals are canceled out by differential mode receivers. A pair with Longitudinal Balance problems at particular frequencies will introduce Bad Signals (Noise) with differential mode characteristics. Unfortunately, these noise signals are not canceled out by differential mode receivers, causing them to add to the Good Signal. Figure 3 : Importance of Good Longitudinal Balance Since reducing noise is a critical aspect for vectored services, it is important that the line is longitudinally balance. In addition, since VDSL2 utilizes distinct sub-carriers are Khz spacing (or bins ) it is important that the longitudinal balance test be performed at each of this frequency across the entire 17Mhz or 30Mhz operating spectrum. Fluke Networks 9

10 Figure 4 : Single-end Wideband Longitudinal est to detect potential noise issues Eliminating a longitudinal balance problem has a double benefit a properly balanced pair will generate less disturber noise and will have greater immunity to disturber noise generated by adjacent pairs. Of course the ability to perform this Wideband Longitudinal test single-end (without requiring a far end device) would be best to reduce test and troubleshooting time. 7. Bandwidth Loss Measurement (out to 30 MHz) Having the ability to measure the loss of each frequency within a useable bandwidth (the attenuation curve) reveals important information about the overall transmission ability across a pair. Utilizing a single-ended technique to make this measurement eliminates the requirement to have a test set on the opposite end of the pair. Along with noise, this test provides key information related to bit rate estimations. he attenuation curve accurately represents the signal level at the receiver along with any loss due to physical faults (bridged-tap, bad splices, etc.). Fluke Networks 10

11 8. Bandwidth Bit ate Estimations Accurate DownStream and UpStream bit rate estimations provide key insight into the ability of a pair to deliver on the 100 Mbps performance target. A properly designed bit rate estimation algorithm knows how to utilize all sources of information (physical tests AND database inquiries) pertaining to the tested pair to provide an accurate estimate of expected DownStream / UpStream rates. Physical tests reveal important information related to attenuation and noise on the pair. Database inquiries reveal important information about the effects of other pairs in the binder on the tested pair. Bonding and vectoring introduce new complexities that must be accounted for by a good rate estimate algorithm: Impact of noise and attenuation across two pairs Changeable bandwidths (e.g. 17a versus 30a) Noise contributions of adjacent pairs Expected improvements when vectoring is turned on Expected improvements when bonded pairs are used instead of a single pair Lost performance due to bridged-tap Lost performance due to foreign noise Performance impact on neighboring pairs When done correctly, this estimation can serve as the sole pass or fail indicator of a tested pair. Fluke Networks 11

12 Conclusion In conclusion, you can see that your network s ability to support the latest VDSL2 bonding & vectoring technology to deliver up to 100Mbs requires that it be free of impairments. he need to identify problems such as 17Mhz noise and longitudinal balance are now more important than ever before. You are pushing your network more than ever, delivering more bandwidth than ever before. his requires the highest quality network, you cannot mask these impairments if you want to achieve upwards of 100Mbs. You cannot fake FAS. Fluke Networks P.O. Box 777, Everett, WA USA Fluke Networks operates in more than 50 countries worldwide. o find your local office contact details, go to Fluke Corporation. All rights reserved. Fluke Networks 12