Title: Chapter 2 Physical
1Chapter 2Physical Data-Link Layers
- Professor Rick Han
- University of Colorado at Boulder
- rhan_at_cs.colorado.edu
2Announcements
- Read all of Chapter 1,
- Read Chapter 2, sections 2.1-2.8, skip Section
2.9 Network Adaptors - Lecture slides for this week will be online soon
- Follow the Lectures link on class Web site
- Homework 1 up soon on Web, due date either next
Thursday or Tues 1/28 - Today, Chapters 1 and 2
3Homework 1 Ping
- Ping program allows you to send a packet to a
host and have it echoed back - Used to probe the network for roundtrip times
packet losses - Is server is alive?
- What might influence roundtrip times?
- Network delays (bandwidth, propagation,
congestion), server delays, time of day,
location,
4The Layered Network Stack
Internet Stack
Application Layer
Transport Layer (TCP/UDP)
Network Layer (IP)
Data Link Layer
Physical Layer
5Layer 1 The Physical Layer
- Host A encodes the bit into an analog signal.
Host B decodes the analog signal into a received
bit.
6Encoding Mapping Digital Bits to Analog Waveforms
- 1 -gt 5 volts
- 0 -gt -5 volts on copper wire
- Could also correspond to light intensity on
optical fiber - In general, called Pulse Amplitude Modulation
(PAM)
7Encoding Mapping Digital Bits to Analog
Waveforms (2)
- The waveform below is a special form of PAM
called NRZ (Non-Return to Zero)
- NRZ Problems
- DC bias/drift, aka baseline wander
- Losing clock synchronization
8Encoding Mapping Digital Bits to Analog
Waveforms (3)
- To fix baseline wander, encode bits as
transitions, rather than levels - this is called Manchester encoding
- 1 -gt upward transition, 0 -gt downward
transition
- D.C. balanced to zero and easy to recover clock
- 10 Mbps Ethernet uses Manchester encoding
9Encoding Mapping Digital Bits to Analog
Waveforms (4)
- Some other possible D/A mappings
- Pulse Width Modulation Morse code
- Pulse Position Modulation
- FSK (Frequency Shift Key) encodes bits as diff.
frequencies. 1200 bps modems used FSK. - 100 Mbps Fast Ethernet has 3 possible physical
layers - 100-BaseTX 2 twisted pair copper wires
- 100-BaseFX Optical fiber using 4B/5B NRZI
signalling (see text) - 100-BaseT4 4 twisted pair copper wires
- Gigabit Ethernet supports fiber optics and copper
wires
10Sending More Bits Per Second
- How would you increase the bit rate?
- Pack the pulses tighter together
- There are other more clever ways
- What problems might you encounter when packing
pulses more tightly? - Noise/interference
- Smearing of the pulse
11Sending More Bits Per Second (2)
Wire
- Fourier transform theory shows that a square
pulse is sum of many different sinusoidal
frequencies - Example a stereo equalizer breaks down audio
into its sinusoidal frequencies - High frequencies get attenuated in wire, thereby
smearing the square pulse - Closely packing time slots causes more smear
from previous slot to interfere with current slot - This is called InterSymbol Interference (ISI)
12Sending More Bits Per Second (3)
- Can send more bits per time slot using
multi-level D/A converter - 8 discrete levels
- 111 -gt 7 V, , 000 -gt -7 V
- 100110 -gt 1, 5,
Symbols
13Sending More Bits Per Second (4)
- Transmit a sequence of symbols S11, S25, S3,
- Baud rate Symbols/sec
- If baud rate is 1 symbol/sec, then bit rate 3
bits/sec - Tripled the bit rate!
- Limitations noise
- Datam(t)Sm(t)SqWv(t-mT)
- Where Sm(t) is the level of mth symbol, T
duration of each symbol, ttime, SqWv is square
wave
14Sending More Bits Per Second (5)
- Can send more bits over different frequencies
- AM/FM radio Frequency Division Multiplexing
(FDM)
Frequency (Hz)
GHz
KHz
MHz
- Example send data at 3 kHz and 6 kHz
- Modulate one bit stream Sm(t) up to 3000 Hz,
Sm(t)cos(2p3000t) , similarly Sm(t) at 6000 Hz - Datam(t) Sm(t)cos(2p3000t) Sm(t)cos
(2p6000t) simplified sq wave not
shown - demodulate down by multiplying by cosines again
and low-pass filtering
15Sending More Bits Per Second (6)
- Can send more bits over the same frequency!
- Datam(t) Sm(t)cos(2p3000t) Sm(t)sin
(2p3000t) - This is called Quadrature Amplitude Modulation
(QAM). - If Sm(t), and Sm(t) both have 4 levels, then
this is called 16-QAM. 9600 bps modems use this.
Baud?
16Code Division Multiple Access (CDMA)
- Another physical layer signalling technique
- In addition to multiplying the data signal by a
cosine, you also multiply by another signal a
chipping sequence - spreads the signal
- Different chipping sequences are orthogonal, so
multiple users can share the same bandwidth - CDMA is a form of spread spectrum
- As an aside, frequency hopping is another form of
spread spectrum, diff. from CDMA - Wireless Ethernet 802.11b uses this physical
layer technique
17Physical Layer Effects
- Goal maximize the Signal-to-Noise ratio (SNR) to
minimize the probability of bit error, then pass
the bit up to Data-Link Layer - Unreliability
- due toSmearing, Interference, (Wireless
shadowing, multi-path, doppler, ) - Apply advanced adaptive filtering and digital
signal processing (DSP) to improve SNR - Propagation Delay
- Speed of light c 3x108 m/s
- Over copper wires, propagation speed is 2/3 of c
- Satellite links have long prop. delays (120 ms
one-way) - Interactivity requires lt400 ms roundtrip
18Layer 2 The Data Link Layer
- Next Problem How do I send a message from Host A
to Host B?
- Data Link Layer, also called Layer 2, ensures
that host B can decode a digital message from a
stream of bits sent by host A - Examples PPP (Point-to-Point Protocol), HDLC,
LAPB, LAPD, Frame Relay,
19The Data Link Layer (cont.)
- A Data Link Layer Protocol implements
- Delimiting/framing of a message
- Fragmenting of a long message
- Retransmission of a lost message
1011000
Host A
Host B
20Defining a Protocol
- A protocol is an agreement between two parties or
endpoints as to how information is to be
transmitted - A protocol implements this agreement via
- A Header
- How each endpoint responds to control info in the
header ( external input)
Host A
Host B
21Framing
Receivers A/D
data
0110011011011000100110100001110001001
- How would you identify the start of the data?
- How would you identify the end of the data?
- What if this flag appears in the data?
- Character stuffing for byte-oriented protocols
- Bit stuffing for bit-oriented protocols
- What if a bit error causes a flag to appear in
data?
22Character/Byte Stuffing
- BiSync and PPP (common over modems)
- Data is divided into 8-bit bytes
- Byte boundaries synchronized btwn sender and
receiver - Define a special start-of-packet N-byte flag,
e.g. let flag be X (one byte flag) - Stuff X in data replace X with escape character
E (DLE in textbook) and X, i.e X -gt (E, X) - Stuff E in data replace E with (E, E)
- At rcvr, first X is start of packet (variants
exist)
23Character/Byte Stuffing (2)
- Sentinel X -gt X, at both start and end of packet
- Data X -gt (E,X)
- Data E -gt (E,E)
- Example Data (E,X) -gt (E,E,E,X)
- Example Trailing E -gt (E,E) followed by last X
24Bit Stuffing
01001000001111110000001110001001
- HDLC uses this
- Similar to byte stuffing, except bit stuffing is
not confined to byte boundaries - HDLC denotes beginning and end of a packet/frame
with 01111110 flag - Since 01111110 may occur anywhere (across byte
boundaries) in data, then stuff it - At sender, after 5 consecutive ones, insert a 0
- At receiver, 0111110 gt stuffing, so destuff,
01111110 gt end of frame, 01111111 gt error