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 0 0 1 0 1 1 1 0 0 0 1 Packets 0100010101011100101010101011101110000001111010101110101010101101011010111001 Header/Body Receiver Sender Packet Transmission

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) 1 1 1 1 .85 V -.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) 1 1 1 1 .85 V -.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 1 1 .85 V -.85 .1s 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

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 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 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

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 Data Rate DS0 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 Carrier Modulation Signal Carrier Frequency Modulated Carrier

Frequency Division Multiplexing: Multiple Channels Determines Bandwidth of Link Amplitude Determines Bandwidth of Channel Different Carrier Frequencies

Frequency 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 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 Frequency Frequency Bands Slot Frame Time

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