Physical Layer II: Framing, SONET, SDH, etc.

CS 4251: Computer Networking IINick FeamsterSpring 2008

From Signals to Packets

Analog Signal

“Digital” Signal

Bit Stream 0 0 1 0 1 1 1 0 0 0 1

Packets0100010101011100101010101011101110000001111010101110101010101101011010111001

Header/Body Header/Body Header/Body

ReceiverSenderPacket

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 -> 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

V 0

.85

-.85

0 0 0 11 0 1 0 1

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 -> make transition; 0 -> signal stays the same

• Solves the problem for long sequences of 1’s, but not for 0’s.

V 0

.85

-.85

0 0 0 11 0 1 0 1

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

• 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

V 0

.85

-.85

0 1 1 0

.1s

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

ts tfA c2cos0

1binary 0binary

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

ts tfA 12cos tfA 22cos

1binary 0binary

• 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

tfAts ii 2cos Mi 1

Phase-Shift Keying (PSK)• Two-level PSK (BPSK)

– Uses two phases to represent binary digits

ts tfA c2cos tfA c2cos

1binary 0binary

tfA c2cos

tfA c2cos1binary 0binary

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

DS0 64

DS1 1,544

DS3 44,736

Level Data Rate

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 ModulationA

mpl

itude

Signal CarrierFrequency

Am

plitu

de

ModulatedCarrier

Frequency Division Multiplexing:Multiple Channels

Am

plitu

de

Different CarrierFrequencies

DeterminesBandwidthof Channel

Determines Bandwidth of Link

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

Fre

quen

cy

Time

FrequencyBands

Slot 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

OpticalSplitter

Frequency