1. An Introduction to Computer Networks
Lecture 4: Point-to-Point link
University of Tehran
Dept. of EE and Computer Engineering
By:
Dr. Nasser Yazdani
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Introduction to Computer Network

2. Introduction to Computer Network
Outline
Concepts:
Data
signal
Links
Link functions
Encoding
Framing
Error Detection
Error correction or reliable transmission
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3. Introduction to Computer Network
Data
Discrete data: an instance is binary. Computer works with discrete data. Discrete is encoded in 0s and 1s.
Continuous data: change with time or space.
It is converted to discrete data by sampling
Data is delivered by signals in the links
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4. Introduction to Computer Network
Link Functions
Signal
Adaptor
Adaptor
Adaptor: convert bits into physical signal and physical signal back into bits
Functions
Construct Frame with Error Detection Code
Encode bit sequence into analog signal
Transmit bit sequence on a physical medium (Modulation)
Receive analog signal
Convert Analog Signal to Bit Sequence
Recover errors through error correction and/or ARQ
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5. Introduction to Computer Network
Link Components
NRZI
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6. Introduction to Computer Network
Link Properties
Function
Duplex/Half Duplex
One stream, multiple streams
Characteristics
Bit Error Rate
Data Rate (this sometimes mistakenly called bandwidth!)
Degradation with distance
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7. Example: Optical Links
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8. Introduction to Computer Network
Signals
Electromagnetic waves propagating in the light speed.
Frequency
Wavelength
A (periodic) signal can be viewed as a sum of sine waves of different frequencies and strengths.
Every signal has an equivalent representation in the frequency domain.
» What frequencies are present and what is their strength (energy)
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9. Introduction to Computer Network
Signals (cont)
Example- The following signal is the sum of sine waves.
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10. Introduction to Computer Network
Modulation
Sender changes the nature of the signal in a way that the receiver can recognize.
» Similar to radio: AM or FM
Digital transmission: encodes the values 0 or 1 in the signal.
» It is also possible to encode multi-valued symbols
Amplitude modulation: change the strength of the signal, typically between on and off.
» Sender and receiver agree on a “rate”
» On means 1, Off means 0
Similar: frequency or phase modulation.
Can also combine method modulation types.
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11. Introduction to Computer Network
Modulation
The function of transmitting the encoded signal over a link, often by combining it with another (carrier signal)
E.g. Frequency Modulation (FM)
Combine the signal with a carrier signal in such a way that the instantaneous frequency of the received signal contains the information of the carrier
E.g. Frequency Hopping (OFDM)
Signal transmitted over multiple frequencies
Sequence of frequencies is pseudo random
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12. Introduction to Computer Network
Link rate and Distance
Links become slower with distance because of attenuation of the signal Amplifiers and repeaters can help
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13. Introduction to Computer Network
Limits in sending signals
Noise: “random” energy is added to the signal.
Attenuation: some of the energy in the signal leaks away. We need repeaters.
Dispersion: attenuation and propagation speed are frequency dependent.
» Changes the shape of the signal
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14. Introduction to Computer Network
Noise
A signal s(t) sent over a link is generally
Distorted by the physical nature of the medium
This distortion may be known and reversible at the receiver
Affected by random physical effects
Shot noise
Fading
Multipath Effects
Also interference from other links
Wireless
Crosstalk
Dealing with noise is what communications engineers do
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15. Noise limits the link rate
Suppose there were no noise
E.g. Send s(t) always receive s(t+Δ)
Take a message of N bits say b1b2….bN, and send a pulse of amplitude of size 0.b1b2….bN
Can send at an arbitrarily high rate
This is true even if the link distorts the signal but in a known way
In practice the signal always gets distorted in an unpredictable (random) way
Receiver tries to estimate the effects but this lowers the effective rate
One way to mitigate noise is to jack up the power of the signal
Signal to Noise ratio (SNR) measures the extent of the distortion effects
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16. Introduction to Computer Network
Channel capacity
Every transmission medium supports transmission in a certain frequency range. This is called channel capacity.
» The channel bandwidth is determined by the transmission medium and the nature of the transmitter and receivers
A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H.
» E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second Assumes binary amplitude encoding
Shannon extended this result by accounting for the effects of noise.
More aggressive encoding can increase the channel bandwidth.
» Example: modems
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17. Bandwidth affects the data rate
There is usually a fixed range of frequencies at which the analog wave can traverse a link
The physical characteristics of the link might govern this
Example:
Voice Grade Telephone line 300Hz – 3300Hz
The bandwidth is 3000Hz
For the same SNR, a higher bandwidth gives a higher rate
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18. Signaling bits on a link Multi-level Signaling
2-bits per symbol
3-bits per symbol 8 4 7 6 3 5
Levels
Levels 4 2 3 2 1 1 00 11 10 01 00
000
010
101
001
110
Ultimately, what limits the number of bits I can send per symbol?
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19. Maximum Capacity/Data Rate
Shannon Capacity:
Bandwidth of link
Signal-to-Noise ratio
For example:
Bandwidth of telephone link from telephone to a typical home is approx 3300Hz – 300Hz = 3kHz
Signal-to-noise ratio is approx 30dB = 10log10(S/N)
Therefore, C = 3000*log2(1001) ~= 30kb/s
Optical fiber has a higher capacity because the bandwidth, B, of a fiber is much greater than for wire; and it is less susceptible to noise, N.
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20. Sampling Result (Nyquist)
Suppose a signal s(t) has a bandwidth B.
Sampling Result: Suppose we sample it (accurately) every T seconds.
If T≤ 1/2B then it is possible to reconstruct the s(t) correctly
Only one signal with bandwidth B has these sample points
There are multiple signals with these sample points for signals with bandwidth greater than B
Increasing the bandwidth results in a richer signal space
No noise allowed in the sampling result
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21. Introduction to Computer Network
Copper Wire
Unshielded twisted pair
» Two copper wires twisted - avoid antenna effect
» Grouped into cables: multiple pairs with common sheath
» Category 3 (voice grade) versus category 5
» 100 Mbps up to 100 m, 1 Mbps up to a few km
» Cost: ~ 10cents/foot
Coax cables.
» One connector is placed inside the other connector
» Holds the signal in place and keeps out noise
» Gigabit up to a km
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22. Introduction to Computer Network
Copper Wire
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23. Introduction to Computer Network
Microwaves
High frequency electromagnetic waves (>1GHz)
q Line of sight terrestrial transmissions and for communications via satellites.
q Some atmospheric interference occurs but reliable transmission can be obtained over distances up to 50 Km.
q Microwaves is absorbed by rain and does not penetrate obstacles.
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24. Introduction to Computer Network
Fiber Optic
q Thin thread of glass or plastic
q Lightweight.
q Fibers act as wave-guides for light which is usually
produced by lasers.
q Visible light has frequency around 5*10 15 Hz,
which ensures an extremely high bandwidth.
q The raw materials are cheap.
q Immune to electrical interference.
q Difficult to join and tap.
q Security advantages
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25. Introduction to Computer Network
Leased Lines
Dedicated link from Telephone Companies
DS1 24 digital voice of 64Kbps
DS3 28 DS1
STS stands for Synch. Transfer signal
Line
Bandwidth
DS1 (T1)
1.544 Mbps
DS3 (T3)
Mbps
STS-1 (OC1)
Mbps
STS-3 (OC3)
Mbps
STS-12 (OC12)
Mbps
STS-48 (OC48)
2.48 Gbps
STS-192 (OC192)
9.95 MGbps
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26. Introduction to Computer Network
Last-Mile Links
Connect from home to network service providers
xDSl (Digital Subscriber line), runs on local loop on telephone line. ADSL, VDSL
CATV- Cabel TV, BW of 6 MHZ, is asymmetric.
Line
Bandwidth
POTS (modem)
Kbps
ISDN
Kbps
XDSL
16Kbps-55.2 Mbps
CATV
20-40 Mpbs
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27. Introduction to Computer Network
Encoding
Goal: send bits from one node to another node on the same physical media
Encode binary data onto signals
This service is provided by the physical layer
Bits flow is between network adaptors.
Problem: specify a robust and efficient encoding scheme to achieve this goal
How to send clock information?
Extract from changes in signals.
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28. Introduction to Computer Network
Encoding (crieria)
Criteria for encoding:
Bit rate (in limited BW)
Recovering time information (in LAN)
Error detecting
Immunity to noise and interference
Complexity and cost of implementation.
Bit rates V.s Baud Rates
Baud rates is the number of pulses sent over the link or changing in signals.
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29. Introduction to Computer Network
Assumptions
We use two discrete signals, high and low, to encode 0 and 1
The transmission is synchronous, i.e., there is a clock used to sample the signal
In general, the duration of one bit is equal to one or two clock ticks
If the amplitude and duration of the signals is large enough, the receiver can do a reasonable job of looking at the distorted signal and estimating what was sent.
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30. Non-Return to Zero (NRZ)
1  high signal; 0  low signal 1 1 1 1
NRZ
(non-return to zero)
Clock
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31. Introduction to Computer Network
Problem with NRZ
Consecutive 0’s or 1’s may create problems.
Synchronization problem because of difference in the sender or receiver clocks.
The average of signals which is used to distinguish between low or high may move and make the decoding difficult. This is called baseline wander
Unable to recover clock
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32. Non-Return to Zero Inverted (NRZI)
1  make transition; 0  stay at the same level
Solve previous problems for long sequences of 1’s, but not for 0’s 1 1 1 1
NRZI
(non-return to zero
intverted)
Clock
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33. Introduction to Computer Network
Manchester
1  high-to-low transition; 0  low-to-high transition
Addresses clock recovery and baseline wander problems
Disadvantage: needs a clock that is twice as fast as the transmission rate
Efficiency of 50% 1 1 1 1
Manchester
Clock
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34. Introduction to Computer Network
Encodings (cont)
Bits
NRZ
Clock
Manchester
NRZI 1
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35. 4-bit/5-bit (100Mb/s Ethernet)
Goal: address inefficiency of Manchester encoding, while avoiding long periods of low/high signals
Solution:
Use 5 bits to encode every sequence of four bits such that no 5 bit code has more than one leading 0 and two trailing 0’s
Use NRZI to encode the 5 bit codes
Efficiency is 80%
4-bit 5-bit
4-bit 5-bit
Unused code are used for control. I.e is for line idle or for the line is dead.
FDDI is using this scheme.
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36. Introduction to Computer Network
Framing
How to distinguish between data and garbage.
Break sequence of bits into a frame
Typically implemented by network adaptor
Bits
Node A
Adaptor
Adaptor
Node B
Frames
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37. Byte-oriented Approaches
Sentinel-based
delineate frame with special pattern: or STX, ETX, etc characters.
e.g., HDLC, SDLC, PPP
Problem: special pattern or characters appear in the payload.
Character escaping or stuffing: in BISYNC (from IBM) with DLE character. 8 16 16 8
Beginning
Ending
Header
Body
CRC
sequence
sequence
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38. Byte-oriented Approaches (cont)
Counter-based
include payload length in the header
e.g., DDCMP protocol from DEC.
problem: count field corrupted
solution: catch when CRC fails
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39. Bit-oriented Approaches
Frame is a collection of bits, SDLC (IMB) (synch. Data Link Control), later changed to HDLC (high) by OSI
Beginning and end with
problem: How if the sequence appear in the body.
solution: Bit stuffing: add after 5 consecutive 1 a zero. 16 16 8 8
Beginning Sequence
Header
Body …….
CRC
Ending Sequence
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40. Introduction to Computer Network
Clock-Based Framing
e.g., SONET: Synchronous Optical Network
By Bellcore
Use NRZ , but scramble it for enough transition
Multiplex multiple low-speed links into a high sp
STS-n (STS-1 = Mbps), each frame 125 us
Overhead
Payload
90 columns
9 rows
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41. Introduction to Computer Network
Error Detection
To send extra information to find error in the frame.
The simplest form is sending two copies, inefficient.
Sending the sum of values (?) in the frame, checksum. In Internet, consider 16 bits sequences and then use one-complement to find the result.
Sending parity, Odd or even parity.
Two dimensional parity.
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42. Error Detecting (goals)
Reduce overhead, i.e., reduce the number of redundancy bits
Increase the number and the type of bit error patterns that can be detected
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43. Internet Checksum Algorithm
View message as a sequence of 16-bit integers; sum using 16-bit ones-complement arithmetic; take ones-complement of the result.
u_short
cksum(u_short *buf, int count) {
register u_long sum = 0;
while (count--)
sum += *buf++;
if (sum & 0xFFFF0000)
/* carry occurred, so wrap around */
sum &= 0xFFFF;
sum++; }
return ~(sum & 0xFFFF);
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44. Introduction to Computer Network
Even Parity Check
EDC field has 1 bit
Sender: If number of 1’s in payload is odd, the check bit is 1, else it is 0
Receiver: Accept the packet if payload number of 1’s match the value of the check bit.
Can detect an odd number of bit errors
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45. Two-dimensional Parity
Add one extra bit to a 7-bit code such that the number of 1’s in the resulting 8 bits is even (for even parity, and odd for odd parity)
Add a parity byte for the packet
Example: five 7-bit character packet, even parity 1 1 1 1
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46. How Many Errors Can you Detect?
All 1-bit errors
Example: 1 1
odd number of 1’s
error bit 1
Can detect AND
correct 1-bit errors 1
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47. Cyclic Redundancy Code (CRC)
Add r bits of redundant data to an n-bit message
Where r << n
e.g., r= 32 and n = 12,000 (1500 bytes) T R M r m
MSB
i.e. T = M.2r + R
Modulo-2 addition (XOR)
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48. Introduction to Computer Network
CRC(cont)
Represent n-bit message as n-1 degree polynomial
e.g., MSG= as M(x) = x7 + x4 + x3 + x1
Let k be the degree of some divisor polynomial
e.g., C(x) = x3 + x2 + 1
Transmit polynomial T (x) that is evenly divisible by C(x)
shift left k bits, i.e., M(x)xk
subtract remainder of M(x)xk / C(x) from M(x)xk
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CRC (cont)
Receiver polynomial T(x) + E(x)
E(x) = 0 implies no errors
Divide (T(x) + E(x)) by C(x); remainder zero if:
E(x) was zero (no error), or
E(x) is exactly divisible by C(x)
All operation is done in modulo 2 in which there is no carry. Then, the operation can be done by XOR only.
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CRC(cont)
Example: M(x)= , C(x) = 1001
0 1 1
The final transmitted message is:
T(x) = R
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Selecting C(x)
All single-bit errors, as long as the xk and x0 terms have non-zero coefficients.
All double-bit errors, as long as C(x) contains a factor with at least three terms
Any odd number of errors, as long as C(x) contains the factor (x + 1)
Any ‘burst’ error (i.e., sequence of consecutive error bits) for which the length of the burst is less than k bits.
Most burst errors of larger than k bits can also be detected
See Table 2.6 on page 102 for common C(x)
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52. Introduction to Computer Network
Hamming Distance
Given codewords A and B, the Hamming distance between them is the number of bits in A that need to be flipped to turn it into B
Example H(011101,000000)=4
If all codewords are at least d Hamming distance apart, then up to d-1 bit errors can be detected
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53. Reliable Transmission
Error detection code
Very costly
Acknowledgment and timeout
Send Acks in opposite direction.
Retransmit if sender did not receive Ack.
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54. Acknowledgements & Timeouts
Sender
Receiver
Sender
Receiver
Frame
Frame
Timeout
Timeout
ACK
ACK T T
Time
Frame
Timeout T
ACK
(a)
(c)
Receiver
Sender
Receiver
Sender
Frame
Frame
imeout
imeout
ACK T T
Frame
Frame
imeout
imeout T
ACK T
ACK
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(d)

55. Introduction to Computer Network
Stop-and-Wait
Sender
Receiver
Problem 1: keeping the pipe full
Example
1.5Mbps link x 45ms RTT = 67.5Kb (8KB)
1KB frames imples 1/8th link utilization
Problem 2: How about repeated frames?
Add a sequence number to frames.
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56. Introduction to Computer Network
Sliding Window
Allow multiple outstanding (un-ACKed) frames
Upper bound on un-ACKed frames, called window
Sender
Receiver T
ime …
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57. Introduction to Computer Network
SW: Sender
Assign sequence number to each frame (SeqNum)
Maintain three state variables:
send window size (SWS)
last acknowledgment received (LAR)
last frame sent (LFS)
Maintain invariant: LFS - LAR <= SWS
Advance LAR when ACK arrives
Buffer up to SWS frames
SWS
LAR
LFS …
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58. Introduction to Computer Network
SW: Receiver
Maintain three state variables
receive window size (RWS)
largest frame acceptable (LFA)
last frame received (LFR)
Maintain invariant: LFA - LFR <= RWS
Frame SeqNum arrives:
if LFR < SeqNum < = LFA accept
if SeqNum < = LFR or SeqNum > LFA discarded
Send cumulative ACKs
RWS
LFR
LFA …
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59. Introduction to Computer Network
Sequence Number Space
SeqNum field is finite; sequence numbers wrap around
Sequence number space must be larger then number of outstanding frames
SWS <= MaxSeqNum-1 is not sufficient
suppose 3-bit SeqNum field (0..7)
SWS=RWS=7
sender transmit frames 0..6
arrive successfully, but ACKs lost
sender retransmits 0..6
receiver expecting 7, 0..5, but receives second incarnation of 0..5
SWS < (MaxSeqNum+1)/2 is correct rule
Intuitively, SeqNum “slides” between two halves of sequence number space
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60. Concurrent Logical Channels
Multiplex 8 logical channels over a single link
Run stop-and-wait on each logical channel
Maintain three state bits per channel
channel busy
current sequence number out
next sequence number in
Header: 3-bit channel num, 1-bit sequence num
4-bits total
same as sliding window protocol
Separates reliability from order
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