Transcript Link Layer
Physical Layer II: Framing, SONET, SDH, etc. CS 4251: Computer Networking II Nick Feamster Spring 2008 From Signals to Packets Analog Signal “Digital” Signal Bit Stream Packets 0 0 1 0 1 1 1 0 0 0 1 0100010101011100101010101011101110000001111010101110101010101101011010111001 Header/Body Packet Transmission Sender Header/Body Header/Body Receiver Analog versus Digital Encoding • Digital transmissions. – Interpret the signal as a series of 1’s and 0’s – E.g. data transmission over the Internet • Analog transmission – Do not interpret the contents – E.g broadcast radio • Why digital transmission? Why Do We Need Encoding? • Meet certain electrical constraints. – Receiver needs enough “transitions” to keep track of the transmit clock – Avoid receiver saturation • Create 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 errors – Minor errors can be corrected by having multiple adjacent signals mapped to the same data symbol • Encoding can be very complex, e.g. wireless. Encoding • 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 ticks – Receiver’s clock must be synchronized with the sender’s clock • Encoding can be done one bit at a time or in blocks of, e.g., 4 or 8 bits. Nonreturn to Zero (NRZ) • Level: A positive constant voltage represents one binary value, and a negative contant voltage represents the other • Disadvantages: – In the presence of noise, may be difficult to distinguish binary values – Synchronization may be an issue Non-Return to Zero (NRZ) 0 1 0 0 0 1 1 0 1 .85 V 0 -.85 • 1 -> high signal; 0 -> low signal • Long sequences of 1’s or 0’s can cause problems: – Sensitive to clock skew, i.e. hard to recover clock – Difficult to interpret 0’s and 1’s Improvement: Differential Encoding • Example: Nonreturn to Zero Inverted – Zero: No transition at the beginning of an interval – One: Transition at the beginning of an interval • Advantage – Since bits are represented by transitions, may be more resistant to noise • Disadvantage – Clocking still requires time synchronization Non-Return to Zero Inverted (NRZI) 0 1 0 0 0 1 1 0 1 .85 V 0 -.85 • 1 -> make transition; 0 -> signal stays the same • Solves the problem for long sequences of 1’s, but not for 0’s. Biphase Encoding • Transition in the middle of the bit period – Transition serves two purposes • Clocking mechanism • Data • Example: Manchester encoding – One represented as low to high transition – Zero represented as high to low transition Aspects of Biphase Encoding • Advantages – Synchronization: Receiver can synchronize on the predictable transition in each bit-time – No DC component – Easier error detection • Disadvantage – As many as two transitions per bit-time • Modulation rate is twice that of other schemes • Requires additional bandwidth Ethernet Manchester Encoding 0 1 1 0 .85 V 0 -.85 .1s • Positive transition for 0, negative for 1 • Transition every cycle communicates clock (but need 2 transition times per bit) • DC balance has good electrical properties Digital Data, Analog Signals • Example: Transmitting digital data over the public telephone network • Amplitude Shift Keying • Frequency Shift Keying • Phase Shift Keying Amplitude-Shift Keying • One binary digit represented by presence of carrier, at constant amplitude • Other binary digit represented by absence of carrier where the carrier signal is Acos(2πfc A cos2f ct s t 0 binary1 binary 0 Amplitude-Shift Keying • Used to transmit digital data over optical fiber • Susceptible to sudden gain changes • Inefficient modulation technique for data Binary Frequency-Shift Keying (BFSK) • Two binary digits represented by two different frequencies near the carrier frequency • f1 and f2 are offset from carrier frequency fc by equal but opposite amounts A cos2f1t s t A cos2f 2t • • • • binary1 binary 0 Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) radio transmission Can be used at higher frequencies on LANs w/coaxial cable Multiple Frequency-Shift Keying • More than two frequencies are used • More bandwidth efficient but more susceptible to error si t A cos2f i t • • • • • 1 i M f i = f c + (2i – 1 – M)f d f c = the carrier frequency f d = the difference frequency M = number of different signal elements = 2 L L = number of bits per signal element Phase-Shift Keying (PSK) • Two-level PSK (BPSK) – Uses two phases to represent binary digits binary1 A cos2f ct s t binary 0 A cos2f ct A cos2f ct A cos2f ct binary1 binary 0 Modulation: 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? Time Division Multiplexing • Users use the wire at different points in time. • Aggregate bandwidth also requires more spectrum. Frequency Frequency Plesiosynchronous Digital Hierarchy (PDH) • Infrastructure based on phone network – – – – Spoken word not intelligible above 3400 Hz Nyquist: 8000 samples per second 256 quantization levels (8 bits) Hence, each voice call is 64Kbps data stream • “Almost synchronous”: Individual streams are clocked at slightly different rates – Stuff bits at the beginning of each frame allow for clock alignment (more complicated schemes called “B8ZS”, “HDB3”) Points in the Hierarchy: TDM Level DS0 Data Rate 64 DS1 1,544 DS3 44,736 TDM: Moving up the Hierarchy • Additional bits are stuffed into frames to allow for clock alignment at the start of every frame • In North America, a DS0 data link is provisioned at 56 Kbps. Elsewhere, it is 64 Kbps. • Circuits can be provided in composite bundles Synchronous Digital Hierarchy (SDH) • Tightly synchronized clocks remove the need for any complicated demultiplexing • Typically allows for higher data rates – OC3: 155.52 Mbps – OC12: 622.08 Mbps – … Baseband 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 signals – Corresponds to a shift in the frequency domain • Same idea applies to frequency and phase modulation. – E.g. change frequency of the carrier instead of its amplitude Amplitude Amplitude Amplitude Carrier Modulation Signal Carrier Frequency Modulated Carrier Frequency Division Multiplexing: Multiple Channels Amplitude Determines Bandwidth of Link Determines Bandwidth of Channel Different Carrier Frequencies • With frequency-division multiplexing different users use different parts of the frequency spectrum. Frequency Frequency vs. Time-division Multiplexing Frequency Bands – I.e. each user can send all the time at reduced rate – Example: roommates • With time-division multiplexing different users send at different times. – I.e. each user can sent at full speed some of the time – Example: a time-share condo • The two solutions can be combined Slot Time Frame Wavelength-Division Multiplexing • Send multiple wavelengths through the same fiber. – Multiplex and demultiplex the optical signal on the fiber • Each wavelength represents an optical carrier that can carry a separate signal. – E.g., 16 colors of 2.4 Gbit/second • Like radio, but optical and much faster Optical Splitter Frequency