Important Concepts at the Physical Layer2-*

Transmission Media2- *

Various CharacteristicsGuided media v.s. unguided mediaE.g., twisted pair v.s. airPoint-to-point v.s. multipoint linkE.g., A cross-over Ethernet cable connecting two PCs v.s. an Ethernet coaxial cable connecting multiple PCsFull-duplex v.s. half-duplex v.s. simplexE.g., Ethernet v.s. Ethernet v.s. ?

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Transmission Signal2- *

A Signal Is Made of Many Frequencies2- *

A Signal Can Be Expressed in the Frequency Domainf3f4/[sin(2ft) + (1/3)sin(2(3f)t)]2- *

Frequency, Spectrum, Bandwidth The spectrum of a signalThe range of frequencies that it contains. (can be infinite.)The absolute bandwidth of a signalThe width of the spectrumThe (effective) bandwidth of a signalThe band of frequencies where most of the energy of the signal is contained.BW = 3f f = 2f3ff2- *

For a given data rate (bits/sec), if we are willing to use more bandwidth to send a signal, its quality at the receiver will be better, and the receiver can thus more correctly interpret the transmitted bits.There is a trade-off between BW and BER.2- *

Link Bandwidth v.s. Signal BandwidthWhen people say that the bandwidth of a link is 100 Mbps, what they mean is that the transmission links characteristics can only allow frequencies that are below 100Mbps to effectively propagate. Do not be confused with the bandwidth of a signal.2- *

Transmission Impairments Limit a Links BandwidthAttenuation and attenuation distortionDifferent frequency components are attenuated at different factors.2- *

Delay distortionDifferent frequency components are delayed at different factors.2- *

NoiseThermal, impulse, crosstalk, etc.2- *

Multipath interference (wireless broadcast) The same signal may be reflected and propagated along multiple different paths. These signals may take different times to arrive at the receiver. (delay distortion) When they arrive at the receiver, their signal strengths may vary a lot. (attenuation distortion)2- *

Channel CapacityData rate, bandwidth, noise, and error rate are closely related.Nyquist bandwidthC = B log2(M), M is the number of discrete signals or voltage levels.Shannon capacityC = B log2( 1 + SNR), SNR is signal to noise ratio(SNR)db = 10 log10 (signal power/noise power)2- *

Some FactsThe higher the data rate of a signal, the greater is its effective bandwidth.The higher the bandwidth of signal, the higher link bandwidth is required to correctly receive and interpret the signal.The higher the date rate of a signal, the greater is its BER (bit error rate).2- *

Data Encoding2- *

Digital data -> digital signalE.g. Ethernet (our focus)Digital data -> analog signalE.g., ModemAnalog data -> digital signalVoice/Video over IPAnalog data -> analog signalE.g. AM/FMThe Four Different Applications Are All Possible 2- *

Metrics for Data Encoding SchemesRequired bandwidthA lack of high-frequency components means that less bandwidth is required for transmission.ClockingThe transmitter and receivers clocks need to be precisely synchronized.Error detectionIs it easy to detect an error?Noise immunityIs the code robust to errors?Cost and complexityIs the code easy to implement?2- *

802.3 Ethernet802.5 token ring2- *

No way to know when a string of bits has started orended if a string of 0s is transmitted.2- *

The Advantages and Disadvantages of Manchester Encoding Advantages:Synchronization is embedded in the signalNo DC componentNo physical attachment is needed. Only AC coupling is needed. Provide better electrical isolation.Easier error detection If there is no transition in a bit time, there is an error.Higher noise immunityTo invert a bit is harder. You need to precisely invert the first and second half of a bit signal.Disadvantage:Require higher bandwidthIn every bit time, there is a transition. Therefore, 100 Mbps Fast Ethernet and Gigabit Ethernet do not use this scheme.

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Digital to Analog ModulationSchemes2- *

Analog to Analog Modulation SchemesWhy do we use a modulation scheme to transmit an analog signal?2- *

Propagation Delay2- *

Propagation Delay Cannot be ImprovedPropagation delay is the time a signal takes to travel from one end to the other end of a transmission link. Propagation delay cannot be shortened.Unless you can find something that goes faster than the light!In contrast, the link bandwidth has been improved (increased) a lot to 10^12 bit/sec.Propagation delay thus is the performance bottleneck of some distributed systems and control mechanisms (e.g. congestion control or shower temperature control)2- *

Propagation Delay Has Nothing to Do With BandwidthNo matter whether we use 10 Mbps Ethernet, 100 Mbps Fast Ethernet, or 1000 Mbps Gigabit Ethernet, the propagation delay from Taiwan to the U.S. are all the same.In 1000 Mbps Gigabit Ethernet, bits are transmitted denser than in 10 Mbps Ethernet. However, they do not arrive at the receiver quicker!2- *

Data Transmission Time + Signal Propagation DelayFor a piece of data to arrive at the receiver, the total time needed is:The transmission time of the data on the link + the signal propagation delay of the linkWhy? We need the data transmission time to put the last bit of the data onto the link.Then the last bit needs the link propagation delay to reach the receiver.Only at that time, the whole piece of data can be picked up by the receiver.2- *