Network Operations & administration CS 4592 Lecture 13

Network Operations & administration CS 4592

Lecture 13

Instructor: Ibrahim Tariq

Data Link Layer

Error Correction

1. By retransmission– flow and error control protocols

2. Forward Error Correction (FEC)– require more redundancy bits– should locate the invalid bit or bits– n-bit code word contains m data bits + r

redundancy bitsn=m+r

– m+r+1 bits discoverable by r bits– 2r>=m+r+1

Data and redundancy bits

Number ofdata bits

m

Number of redundancy bits

r

Total bits

m + r

1 2 3

2 3 5

3 3 6

4 3 7

5 4 9

6 4 10

7 4 11

Hamming Code

• Hamming codes provide for FEC using a “Block Parity”– i.e, instead of one parity bit send a block of parity

bits• Allows correction of single bit errors• This is accomplished by using more than one

parity bit• Each computed on different combination of

bits in the data

Positions of Redundancy Bits

Redundancy Bits Calculation

Example

Error Correction using Hamming Code

Revision

1.11

Categories of topology

1.12

A fully connected mesh topology (five devices)

1.13

A star topology connecting four stations

1.14

A bus topology connecting three stations

1.15

A ring topology connecting six stations

1.16

A hybrid topology: a star backbone with three bus networks

2.17

Figure 2.2 Seven layers of the OSI model

2.18

Figure 2.3 The interaction between layers in the OSI model

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Four Level of Addresses

20

Relationship of Layers & Addresses in TCP/IP

21

Communication NetworkCommunication networks

Broadcast networksEnd nodes share a common channel (TV,

radio…)

Switched networks End nodes send to one (or more) end nodes

Packet switchingData sent in discrete portions

(the Internet)

Circuit switchingDedicated circuit per call

(telephone, ISDN)(physical)

6.22

Figure 6.2 Categories of multiplexing

6.23

Figure 6.13 Synchronous time-division multiplexing

6.24

Figure 6.26 TDM slot comparison

25

Transmission Impairments

26

Bit Rate & Bit Interval (contd.)

27

Noiseless Channel: Nyquist Bit Rate

• Defines theoretical maximum bit rate for Noiseless Channel:

• Bit Rate=2 X Bandwidth X log2L

28

ExampleWe have a channel with a 1 MHz bandwidth. The SNR for this channel is 63; what is the appropriate bit rate and signal level?

Solution

C = B log2 (1 + SNR) = 106 log2 (1 + 63) = 106 log2 (64) = 6 Mbps

Then we use the Nyquist formula to find the number of signal levels.

6 Mbps = 2 1 MHz log2 L L = 8

First, we use the Shannon formula to find our upper limit.

29

Noisy Channel: Shannon Capacity

• Defines theoretical maximum bit rate for Noisy Channel:

• Capacity=Bandwidth X log2(1+SNR)

Data Communication & Networks, Summer 2009

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Line Coding Schemes

Data Communication & Networks, Summer 2009

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Unipolar Encoding

Data Communication & Networks, Summer 2009

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Polar: NRZ-L and NRZ-I Encoding

Data Communication & Networks, Summer 2009

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Polar: RZ Encoding

Data Communication & Networks, Summer 2009

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Polar: Manchester Encoding

Data Communication & Networks, Summer 2009

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Polar: Differential Manchester Encoding

4.36

Figure 4.6 Polar NRZ-L and NRZ-I schemes

4.37

Figure 4.9 Bipolar schemes: AMI and pseudoternary

Data Communication & Networks, Summer 2009

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4.39

ANALOG-TO-DIGITAL CONVERSION

Digital signal is superior to an analog signal. The tendency today is to change an analog signal to digital data. pulse code modulation

4.40

Components of PCM encoder

5.41

Types of digital-to-analog conversion

5.42

Concept of a constellation diagram

5.43

Figure 5.13 Three constellation diagrams

5.44

Types of analog-to-analog modulation

Circuit Switching

• There are three phases in circuit switching:– Establish– Transfer– Disconnect

• The telephone message is sent in one go, it is not broken up. The message arrives in the same order that it was originally sent.

Packet Switching

• In packet-based networks, the message gets broken into small data packets.

• These packets seek out the most efficient route to travel as circuits become available.

• Each packet may go a different route from the others.

9.47

Figure 9.11 Bandwidth division in ADSL

9.48

ADSL modem

9.49

DSLAM

9.50

Summary of DSL technologies