计算机网络第五版(英文版)ppt (2)

2.2.1 Twisted Pair (5)
• Field Testing of Twisted Pair Cabling System (3)
– Some related parameters (2)
• Equal level far-end crosstalk (ELFEXT) is a measure of the unwanted signal coupling from a transmitter at the near-end into a neighboring pair measured at the far-end relative to the received signal level measured on that same pair. • Power sum equal level far-end crosstalk (PSELFEXT) is a computation of the unwanted signal coupling from multiple transmitters at the near-end into a pair measured at the far-end relative to the received signal level on that same pair.
Max. data rate = B log2(1 + S/N) bits/sec
How fast signal can change How many levels can be
2.1.3 Maximum Data Rate of a Channel (2)
• A noise channel (2)
• A noiseless channel
– Nyquist’s theorem
Max. data rate = 2B log2V bits/sec
– Nyquist’s formula gives theoretical upper bound
• A noise channel (1)
– Shannon's theorem relates the data rate to the bandwidth (B) and signal strength (S) relative to the noise (N), assume random noise is present
2.2.1 Twisted Pair (3)
• The category of twisted pair in bandwidth • Link Terminology
– Full-duplex link
• Used for transmission in both directions at once. e.g., use different twisted pairs for each direction
– Baseband co-ax: 50Ω (Input Impedance) – Used for analog transissin co-ax, some time also called broadband co-ax, like TV cable: 75Ω
2.2.3 Power Lines
2.1.2 Bandwidth-Limited Signals (2)
• Baseband
– Signals that run from 0 up to a max. frequency
• Passband
– Signals that are shifted to a higher rang of frequencies
– Long distance ISP links, Fiber-to-the-Home (FTTH) – Light carried in very long, thin strand of glass
Light source (LED, laser)
Light trapped by total internal reflection
– SNR (Signal-to-Noise Ratio)
• The amount of thermal noise present is measured by the ratio of the signal power to the noise power • SNR is used to express the quantity 10log10(S/N) in dB (namely decibels)
– The specific length
ABCFra bibliotekDA+B+C+D≤ 100m
A+C+D≤ 10m
2.2.1 Twisted Pair (4)
• Field Testing of Twisted Pair Cabling System (2)
– Some related parameters (1)
an, bn weights of harmonics
2.1.2 Bandwidth-Limited Signals (1)
• Bandwidth
– All transmission facilities diminish different harmonics by different amounts, namely, signal distortion. – There is cutoff frequency fc, at which the received power has fallen by half, the amplitudes are transmitted mostly undiminished from 0 up to fc. – This width of the frequency rage is called the bandwidth. – (analog) bandwidth vs. (digital) bandwidth
2.2 Guided Transmission Media
2.2.1 Twisted Pair (1)
• Very common; used in LANs, telephone lines • Twists reduce radiated signal (interference) • The category of twisted pair in structure
The Physical Layer
Chapter 2
Communication system model
• To transmit digital signals over the analog channel, the converter employs a modem • To transmit analog signals over the digital channel, the converter employs a CODEC • Random noise vs. burst noise
– UTP (unshielded twisted pair) – STP (shielded twisted pair )
Category 5 UTP cable with four twisted pairs
2.2.1 Twisted Pair (2)
• The category of twisted pair in bandwidth
Photodetector
2.2.4 Fiber Cables (2)
• Light trapped by total internal reflection • The category of fiber cable
– Multimode fiber
• Light can bounce (50μm core) • Used with LEDs for cheaper, shorter distance links
• Household electrical wiring is another example of wires
– Convenient to use, but horrible for sending data
2.2.4 Fiber Cables (1)
• Common for high rates and long distances
– Shannon's formula gives theoretical upper bound of maximum data rate on the noise channel – The randomicity of random noise should be taken care of
Problems: 1, 2, 3, 4
– Half-duplex link
• Both directions, but not at the same time
– Simplex link
• Only one fixed direction at all times
2.2.1 Twisted Pair (3)
• Field Testing of Twisted Pair Cabling System (ANSI/TIA/EIA-TSB-67) (1)
– Single-mode fiber
• Core so narrow (10μm) light can propagate in a straight line • Used with lasers for long distances, e.g., 100km
2.1.2 Bandwidth-Limited Signals (3)
• Having less bandwidth (harmonics) degrades the signal