1Physical Layer II: Framing, SONET, SDH, etc. CS 4251: Computer Networking II Nick Feamster Spring 2008
2From Signals to Packets Analog Signal“Digital” SignalBit StreamPacketsHeader/BodyReceiverSenderPacketTransmission
3Analog versus Digital Encoding Digital transmissions.Interpret the signal as a series of 1’s and 0’sE.g. data transmission over the InternetAnalog transmissionDo not interpret the contentsE.g broadcast radioWhy digital transmission?
4Why Do We Need Encoding? Meet certain electrical constraints. Receiver needs enough “transitions” to keep track of the transmit clockAvoid receiver saturationCreate control symbols, besides regular data symbols.E.g. start or end of frame, escape, ...Error detection or error corrections.Some codes are illegal so receiver can detect certain classes of errorsMinor errors can be corrected by having multiple adjacent signals mapped to the same data symbolEncoding can be very complex, e.g. wireless.
5Encoding Use two discrete signals, high and low, to encode 0 and 1. Transmission is synchronous, i.e., a clock is used to sample the signal.In general, the duration of one bit is equal to one or two clock ticksReceiver’s clock must be synchronized with the sender’s clockEncoding can be done one bit at a time or in blocks of, e.g., 4 or 8 bits.
6Nonreturn to Zero (NRZ) Level: A positive constant voltage represents one binary value, and a negative contant voltage represents the otherDisadvantages:In the presence of noise, may be difficult to distinguish binary valuesSynchronization may be an issue
7Non-Return to Zero (NRZ) 1111.85V-.851 -> high signal; 0 -> low signalLong sequences of 1’s or 0’s can cause problems:Sensitive to clock skew, i.e. hard to recover clockDifficult to interpret 0’s and 1’s
8Improvement: Differential Encoding Example: Nonreturn to Zero InvertedZero: No transition at the beginning of an intervalOne: Transition at the beginning of an intervalAdvantageSince bits are represented by transitions, may be more resistant to noiseDisadvantageClocking still requires time synchronization
9Non-Return to Zero Inverted (NRZI) 1111.85V-.851 -> make transition; 0 -> signal stays the sameSolves the problem for long sequences of 1’s, but not for 0’s.
10Biphase Encoding Transition in the middle of the bit period Transition serves two purposesClocking mechanismDataExample: Manchester encodingOne represented as low to high transitionZero represented as high to low transition
11Aspects of Biphase Encoding AdvantagesSynchronization: Receiver can synchronize on the predictable transition in each bit-timeNo DC componentEasier error detectionDisadvantageAs many as two transitions per bit-timeModulation rate is twice that of other schemesRequires additional bandwidth
12Ethernet Manchester Encoding 11.85V-.85.1sPositive transition for 0, negative for 1Transition every cycle communicates clock (but need 2 transition times per bit)DC balance has good electrical properties
13Digital Data, Analog Signals Example: Transmitting digital data over the public telephone networkAmplitude Shift KeyingFrequency Shift KeyingPhase Shift Keying
14Amplitude-Shift Keying One binary digit represented by presence of carrier, at constant amplitudeOther binary digit represented by absence of carrier where the carrier signal is Acos(2πfc
16Amplitude-Shift Keying Used to transmit digital data over optical fiberSusceptible to sudden gain changesInefficient modulation technique for data
17Binary Frequency-Shift Keying (BFSK) Two binary digits represented by two different frequencies near the carrier frequencyf1 and f2 are offset from carrier frequency fc by equal but opposite amountsLess susceptible to error than ASKOn voice-grade lines, used up to 1200bpsUsed for high-frequency (3 to 30 MHz) radio transmissionCan be used at higher frequencies on LANs w/coaxial cable
18Multiple Frequency-Shift Keying More than two frequencies are usedMore bandwidth efficient but more susceptible to errorf i = f c + (2i – 1 – M)f df c = the carrier frequencyf d = the difference frequencyM = number of different signal elements = 2 LL = number of bits per signal element
19Phase-Shift Keying (PSK) Two-level PSK (BPSK)Uses two phases to represent binary digits
20Modulation: Supporting Multiple Channels Multiple channels can coexist if they transmit at a different frequency, or at a different time, or in a different part of the space.Space can be limited using wires or using transmit power of wireless transmitters.Frequency multiplexing means that different users use a different part of the spectrum.Controlling time is a datalink protocol issue.Media Access Control (MAC): who gets to send when?
21Time Division Multiplexing Users use the wire at different points in time.Aggregate bandwidth also requires more spectrum.FrequencyFrequency
22Plesiosynchronous Digital Hierarchy (PDH) Infrastructure based on phone networkSpoken word not intelligible above 3400 HzNyquist: 8000 samples per second256 quantization levels (8 bits)Hence, each voice call is 64Kbps data stream“Almost synchronous”: Individual streams are clocked at slightly different ratesStuff bits at the beginning of each frame allow for clock alignment (more complicated schemes called “B8ZS”, “HDB3”)
23Points in the Hierarchy: TDM LevelData RateDS064DS11,544DS344,736
24TDM: Moving up the Hierarchy Additional bits are stuffed into frames to allow for clock alignment at the start of every frameIn North America, a DS0 data link is provisioned at 56 Kbps. Elsewhere, it is 64 Kbps.Circuits can be provided in composite bundles
25Synchronous Digital Hierarchy (SDH) Tightly synchronized clocks remove the need for any complicated demultiplexingTypically allows for higher data ratesOC3: MbpsOC12: Mbps…
26Baseband versus Carrier Modulation Baseband modulation: send the “bare” signal.Carrier modulation: use the signal to modulate a higher frequency signal (carrier).Can be viewed as the product of the two signalsCorresponds to a shift in the frequency domainSame idea applies to frequency and phase modulation.E.g. change frequency of the carrier instead of its amplitude
29Frequency vs. Time-division Multiplexing With frequency-division multiplexing different users use different parts of the frequency spectrum.I.e. each user can send all the time at reduced rateExample: roommatesWith time-division multiplexing different users send at different times.I.e. each user can sent at full speed some of the timeExample: a time-share condoThe two solutions can be combinedFrequencyFrequencyBandsSlotFrameTime
30Wavelength-Division Multiplexing Send multiple wavelengths through the same fiber.Multiplex and demultiplex the optical signal on the fiberEach wavelength represents an optical carrier that can carry a separate signal.E.g., 16 colors of 2.4 Gbit/secondLike radio, but optical and much fasterOpticalSplitterFrequency