【文章內(nèi)容簡介】 of its original value – P2/P1 = 37/100 = .37 (37%/100%) – LOG10() = – 10*LOG10() = dB (negative, reflecting power reduction through attenuation) – In calculations, the Excel LOG10 function can be used 42 Figure 314: Decibels, Continued ? There are two useful approximations ? 3 dB loss is a reduction to very nearly 1/2 the original power – 6 dB loss is a decrease to 1/4 the original power – 9 dB loss is a decrease to 1/8 the original power – … ? 10 dB loss is a reduction to very nearly 1/10 the original power – 20 dB loss is a decrease to 1/100 the original power – … 43 Figure 313: Attenuation and Noise, Continued Power Distance Noise Floor Noise Noise Spike Signal Signal toNoise Ratio (SNR) Noise is random unwanted energy within the wire Its average is called the noise floor (噪聲基底 ) Random noise spikes (噪聲毛刺 ) cause errors A high signaltonoise ratio reduces noise error problems As a signal attenuates with distance, damaging noise spikes bee more mon Error 44 Limiting UTP Cord Length ? Limit UTP cord length to 100 meters – Limits attenuation to being a negligible problem – Limits noise problems being a negligible problem – Note that limiting cord lengths limits BOTH noise and attenuation problems 100 Meters Maximum Cord Length 45 ? Test Your Understanding ? P 157158 46 Figure 311: Unshielded Twisted Pair (UTP) Wiring, Continued ? Electromagic Interference (EMI) (Fig. 315) – Electromagic interference (電磁干擾 ) is electromagic energy from outside sources that adds to the signal ? From fluorescent lights, electrical motors, microwave ovens, etc. – The problem is that UTP cords are like long radio antennas. ? They pick up EMI energy nicely ? When they carry signals, they also send EMI energy out from themselves 47 Figure 315: Electromagic Interference (EMI) and Twisting Interference on the Two Halves of a Twist Cancels Out Twisted Wire Electromagic Interference (EMI) 48 Figure 316: Crosstalk Interference and Terminal Crosstalk Interference (交互干擾 ) Untwisted at Ends Signal Terminal Crosstalk Interference Crosstalk Interference Terminal crosstalk interference Normally is the biggest EMI problem for UTP 49 Figure 311: Unshielded Twisted Pair (UTP) Wiring, Continued ? Electromagic Interference (EMI) (Fig. 315) – Terminal crosstalk interference dominates interference in UTP – Terminal crosstalk interference is limited to an acceptable level by not untwisting wires more than a half inch ( cm) at each end of the cord to fit into the RJ45 connector – This reduces terminal crosstalk interference to a negligible level. cm or inches 50 UTP Limitations ? Limit cords to 100 meters – Limits BOTH noise AND attenuation problems to an acceptable level ? Do not untwist wires more than cm (a half inch) when placing them in RJ45 connectors – Limits terminal crosstalk interference to an acceptable level ? Neither pletely eliminates the problems but they usually reduce the problems to negligible levels 51 ? Test Your Understanding ? P 160 52 Figure 317: Serial Versus Parallel Transmission One Clock Cycle 1. Serial Transmission (1 bit per clock cycle) 2. Parallel Transmission (1 bit per clock cycle per wire pair) 4 bits per clock cycle on 4 pairs 1 bit 1 bit 1 bit 1 bit 1 bit Parallel transmission increases speed. But it is only workable over short distances. Parallel is not 4. It is more than one. 53 ? Test Your Understanding ? P 161 54 Figure 318: Wire Quality Standards ? Wiring Quality Standards – Rated by Category (Cat) Numbers ? Category Standards are Set by ANSI/TIA/EIA and ISO/IEC – In the United States, the TIA/EIA/ANSI568 governs UTP and optical fiber standards – In Europe and many other parts of the world, the standard is ISO/IEC 11801 – The two sets of standards are close but not identical 55 Figure 318: Wire Quality Standards ? UTP Categories 3 and 4 – Early data wiring, which could only handle Ether speeds up to 10 Mbps ? UTP Categories 5 and 5e – Most wiring installed today is Category 5e (enhanced) – Cat 5e and Cat 5 can handle Ether up to 1 Gbps – Most wiring sold today is Cat 5e 56 Figure 318: Wire Quality Standards ? UTP Category 6 – Relatively new – No better than Cat 5 or Cat 5e at 1 Gbps – Developed for higher Ether speeds of 10 Gbps ? But can only span 55 meters at that speed ? Book says cannot be used. This is an error. ? Category 6A (Augmented) – Able to carry Ether signals at 10 Gbps up 100 meters – The book said 55 meters, but this is an error Errors 57 Figure 318: Wire Quality Standards ? Category 7 STP – Shielded twisted pair (STP) rather than unshielded twisted pair (UTP) ? Metal foil shield around each pair to reduce crosstalk interference ? Metal mesh around all four pairs to reduce crosstalk from ot