Important Concepts at the Physical Layer

Transmission Media

Various Characteristics Guided media v.s. unguided media E.g., twisted pair v.s. air Point-to-point v.s. multipoint link E.g., A cross-over Ethernet cable connecting two PCs v.s. an Ethernet coaxial cable connecting multiple PCs Full-duplex v.s. half-duplex v.s. simplex E.g., Ethernet v.s. Ethernet v.s. ?

Transmission Signal

A Signal Is Made of Many Frequencies

A Signal Can Be Expressed in the Frequency Domain 4/π[sin(2πft) + (1/3)sin(2π(3f)t)]

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

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.

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

Transmission Impairments Limit a Link’s Bandwidth Attenuation and attenuation distortion Different frequency components are attenuated at different factors.

Delay distortion Different frequency components are delayed at different factors.

Noise Thermal, impulse, crosstalk, etc.

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)

Channel Capacity Data rate, bandwidth, noise, and error rate are closely related. Nyquist bandwidth C = B log2(M), M is the number of discrete signals or voltage levels. Shannon capacity C = B log2( 1 + SNR), SNR is signal to noise ratio (SNR)db = 10 log10 (signal power/noise power)

Some Facts The 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).

Data Encoding

The Four Different Applications Are All Possible Digital data -> digital signal E.g. Ethernet (our focus) Digital data -> analog signal E.g., Modem Analog data -> digital signal Voice/Video over IP Analog data -> analog signal E.g. AM/FM

Metrics for Data Encoding Schemes Required bandwidth A lack of high-frequency components means that less bandwidth is required for transmission. Clocking The transmitter and receiver’s clocks need to be precisely synchronized. Error detection Is it easy to detect an error? Noise immunity Is the code robust to errors? Cost and complexity Is the code easy to implement?

802.3 Ethernet 802.5 token ring

No way to know when a string of bits has started or ended if a string of 0s is transmitted.

The Advantages and Disadvantages of Manchester Encoding Synchronization is embedded in the signal No DC component No 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 immunity To invert a bit is harder. You need to precisely invert the first and second half of a bit signal. Disadvantage: Require higher bandwidth In every bit time, there is a transition. Therefore, 100 Mbps Fast Ethernet and Gigabit Ethernet do not use this scheme.

Digital to Analog Modulation Schemes

Analog to Analog Modulation Schemes Why do we use a modulation scheme to transmit an analog signal?

Propagation Delay

Propagation Delay Cannot be Improved Propagation 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)

Propagation Delay Has Nothing to Do With Bandwidth No 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!

Data Transmission Time + Signal Propagation Delay For 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 link Why? 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.