"BICSI Conference Dubai 2008 Shield Effectiveness"
C f D b i BICSI Conference Dubai 2008 Shield Effectiveness Gautier Humbert, RCDD Sales Manager Gulf Countries Summary 1- Why Shielded ? 2 -Theory of Shield efficiency 3- Recommended Installation Methods 4- Reality of installed systems 5- 5 Testing 6- Reminder of the Facts 7- Recommendations Summary 1- Why Shielded 2 -Theory of Shield efficiency 3- Recommended Installation Methods 4- Reality of installed systems 5- 5 Testing 6- Reminder of the Facts 7- Recommendations Copper transmission • In copper transmission, some of the energy travels outside the conductor as electromagnetic waves. • And at the high frequencies we use in communications, the problem is worse: Because of the skin effect, these waves become more energetic d i di t d t f th and even more energy is radiated out of the conductor. Copper transmission •The same process can happen in reverse: electromagnetic waves crossing a copper conductor can cause electrons to flow. •Tight twisting helps create a balanced line which reduces the energy radiated out of pairs, and therefore the energy picked by adjacent pairs as cross-talk. Copper transmission Immunity of a pair (common mode rejection) N +V +V +N Tx Rx -V +V-(-V)= 2V -V +N V N ( V N) 2V +V+N-(-V+N)=2V Best results are obtained when : – The symmetry of the twisted pair is PERFECT – A HIGH TWIST COUNT is exposed to the interference field e conductors o pair are J C – The co ducto s of a pa a e ADJACENT • 100 Ohms impedance Copper transmission • Pairs are designed to minimize noise • The geometry of the cable is the most critical factor • To increase the bandwidth capacity of the cable, two solutions: • Increase the precision of the pair twist • Or use a shield. Copper transmission To reduce interference some cables rely only on the precision of the pair twist… …some combine tight pair twist with shield… …and others rely only on the shield. Summary y 1- Why shielded 2 -Theory of Shield efficiency 3 Recommended Installation Methods 3- 4- Reality of installed systems 5- Testing 6- Reminder of the Facts 7- Recommendations Theory of Shield Efficiency BICSI TDMM 11 The shield currents counteract EMI thanks to the connection to ground on both sides. Summary 1- Why Shielded 2 -Theory of Shield efficiency 3- Recommended Installation Methods 4- Reality of installed systems 5- 5 Testing 6- Reminder of the Facts 7- Recommendations Recommended Installation Methods 11801 ed.2 ISO 11801, ed 2 11.1 : “Work area, equipment cords and the equipment attachment should be screened d h ll id i i ” and if so, shall provide screen continuity.” Recommended Installation Methods B2 1 TIA/EIA 568 B2-1 Recommended Installation Methods TDMM11 BICSI TDMM11, Chapter 2 – Shield Effectiveness • “…the bl hi ld d t b d d t b th d th t th “ th cable shield needs to be grounded at both ends so that the shield currents can counteract the effects of EM unwanted signal induction from an external field. Unfortunately, this also creates the p y g g possibility of conducted unwanted signal due to ground loop p currents.” • “Grounding the shield at one end: 300kHz to – Breaks the ground loop. 3MHz ! – Is ineffective at high frequencies (i.e., length > λ/10). – Common-mode field coupling is still present.” Recommended Installation Methods Design of Manufacturer X “…The workstation end is not bonded to ground. ground ” “…the bonding… at the workstation end is accomplished by connecting a p shielded patch cord between the outlet and the equipment.” Summary y 1- Why Shielded ? 2 -Theory of Shield efficiency 3 Recommended Installation Methods 3- 4- Reality of installed systems 5- Testing 6- Reminder of the Facts 7- Recommendations Reality Many laptops do NOT have a ground wire. Reality Network Interface Cards Many NIC’s have a filter between the cage and the ground 75 Ohm Reality g PoE devices NEVER have a ground connection. (no electrical plug !) Once again…no ground ! Reality Design of Manufacturer Y “ An alternative practice is to use a UTP patch cord between the outlet and the workstation, effectively g g y grounding only one end...” “The advantage of this option is that the risk of establishing a substantial ground loop current is greatly reduced.” Summary 1- Why Shielded ? Th f Shield ffi i 2 -Theory of Shi ld efficiency 3- Recommended Installation Methods 4- Reality of installed systems 5 5- Testing 6- Reminder of the Facts 7- 7 Recommendations Testing Does the wiremap test guarantee performance ? Testing Of course not ! You need to test the performance on site. The certificate of the product ensures that it is manufactured as per standards, the field testing , g certifies that the installation meets requirements. Testing Did you ever check how the shield was tested in a ScTP system? y Wiremap !!! This gives you absolutely no information concerning the impedance of the shield at the high frequencies of 250 to 600 MHz. Testing You don’t trust the installer for the laying of the cable, and the connection of the wires in the outlet. But you trust him for the shield efficiency, y which is the hardest to verify. Summary 1- Why Shielded ? Th f Shield ffi i 2 -Theory of Shi ld efficiency 3- Recommended Installation Methods 4- Reality of installed systems 5 5- Testing 6- Reminder of the Facts 7- 7 Recommendations Facts Johnson, Dr Howard Johnson recognized as “Outstanding contributor” to IEEE P802.3z : • In high-speed digital applications, a low impedance connection between the shield and the equipment chassis at both ends is required in order for the shield to do its job. • The shield connection impedance must be low in the frequency range over which you propose p p pp g g p y, for the shield to operate. In low-speed applications involving high-impedance circuitry, where most of the near-field energy surrounding the conductors is in the electric field mode (as opposed to the magnetic field mode), shields need only be grounded at one end. In this case the shield acts as a Faraday cage surrounding the conductors, prevent the egress (or ingress) of electric fields. • high-speed low-impedance circuitry, near-field In high speed applications involving low impedance circuitry most of the near field energy surrounding the conductors is in the magnetic field mode, and for that problem, only a magnetic shield will work. That’s what the double-grounded shield provides. Grounding both ends of the shield permits high-frequency currents to circulate in the shield, which will counteract the currents flowing in the signal conductors. These counteracting currents create conductors magnetic fields that cancel the magnetic fields emanating from the signal conductors, providing a magnetic shielding effect. Facts In structured cabling: • The grounding at both ends is never verified since it depends on the equipment used. • The shield efficiency is never tested. y • Various manufacturers disagree on the best bonding and g g g grounding methods for shielded systems. Summary 1- Why Shielded ? Th f Shield ffi i 2 -Theory of Shi ld efficiency 3- Recommended Installation Methods 4- Reality of installed systems 5 5- Testing 6- Reminder of the Facts 7- Recommendations 7 R d ti Recommendations Does thi mean th t shielded systems d D this that hi ld d t do not work ? Of course not. Shielded t till the highest Shi ld d systems are still th hi h t performance copper solution…in a f t i t ll ti perfect installation. Recommendations We i l thought th t shielded W previously th ht that hi ld d systems were more reliable than UTP. With high speed digital applications, it’s not anymore. the case anymore performance, Shielded is still high performance but UTP is 100% tested and certified. Recommendations What about “Smart Buildings” The structured cabling in a Smart building must accept all IP technologies, including high data rate and PoE. Shielded systems are not 100% tested on site. Shielded systems don’t work with all computers. Shielded systems are not compatible with PoE. Shielded systems are NOT adapted for smart building. Comments and Questions Thank you Gautier Humbert, RCDD Ortronics Sales Manager Gulf Countries Gautier.email@example.com