Notes on NMA

7/31/2019 Notes on NMA

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Network Management

and ApplicationNotes By : Bijay Mishra


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Static Channel Allocation

ProblemThe history of broadcast networks includes satelliteand packet radio networks.

Let us view a satellite as a repeater amplifying andrebroadcasting everything that comes in.

To generalize this problem, consider networkswhere every frame sent is automatically receivedby every site (node).


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Satellite Channel

=fin

=fout


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Static Channel Allocation Problem

We model this situation as n independentusers (one per node), each wanting tocommunicate with another user and they

have no other form of communication.Channel Allocation Problem

To manage a single broadcast channel which must be shared

efficientlyandfairlyamong n uncoordinated users.


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Possible Model Assumptions for

Channel Allocation Problem

0. Listen property:: (applies to satellites)

The sender is able to listen to sent frame oneround-trip after sending it.

no need for explicit ACKs

1. Model consists ofn independent stations.2. A single channel is available for communications.


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Channel Allocation Problem

3. Collision Assumption :: If two frames are transmitted

simultaneously, they overlap in time and the

resulting signal is garbled. This event is a collision.

4a. Continuous Time Assumption :: frame transmissionscan begin at any time instant.

4b. Slotted Time Assumption :: time is divided into

discrete intervals (slots). Frame transmissionsalways begin at the start of a time slot.


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Channel Allocation Problem5a. Carrier Sense Assumption ::

Stations can tell if the channel is busy (in use) before tryingto use it. If the channel is busy, no station will attempt to

use the channel until it is idle.

5b. No Carrier Sense Assumption ::

Stations are unable to sense channel beforeattempting to send a frame. They just go ahead andtransmit a frame.


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ALOHA

Abramson solved the channel allocation problem forground radio at University of Hawaii in 1970s.

Aloha Transmission Strategy

Stations transmit whenever they have data to send.

Collisions will occur and colliding frames are

destroyed.

Aloha Retransmission Strategy

Station waits a random amount of time before sending again.


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Pure ALOHA

Figure: Vulnerable period for the shaded frame.


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t

t0t0-X t0+X t0+X+2tprop t0+X+2tprop

Vulnerable

periodTime-out Backoff

periodRetransmission

if necessary

First transmission Retransmission

random backoff period B

ALOHA


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ALOHA

S = G e-2 (1+a) G

Vulnerable period :: t0 X to t0 + X two frame transmission times

Assume: Poisson Arrivals with average number of arrivals of 2G arrivals/ 2 X


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Slotted ALOHA (Roberts 1972)

uses discrete time intervals as slots (i.e., slot = one

packet transmission time) and synchronize send

time (e.g., use pip from a satellite).

Slotted Aloha Strategy

Station transmits ONLY at the beginning of a time slot.

Collisions will occur and colliding frames are

destroyed.Slotted Aloha Retransmission Strategy

Station waits a random amount of time before sending again.


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t

(k+1)XkX t0+X+2tpropt0+X+2tprop

Vulnerable

period

Time-out Backoff

periodRetransmission

if necessary

Slotted ALOHA

random backoff period B slots


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Slotted ALOHA

S = G e- (1+a) G

Vulnerable period :: t0 X to t0 one frame transmission timeAssume: Poisson Arrivals with average number of arrivals of

G arrivals/ X

P0 = P[k=0, t=1] = eG

S = G P0

S = G eG

and an adjustment for a yields


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0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.

01563

0.

03125

0

.0625

0.

125

0.

25

0.

5 1 2 4 8

Ge-G

Ge-2G

G

S0.184

0.368

ALOHA and Slotted ALOHA

Throughput versus Load

Aloha

Slotted Aloha


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CSMA

(Carrier Sense with Multiple Access)

nonpersistent CSMA {less greedy}

1. Sense the channel.

2. IF the channel is idle, THEN transmit.

3. IF the channel is busy, THEN wait a

random amount of time and start over.


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1 - Persistent CSMA

1 - persistent CSMA {selfish}


2. IF the channel is idle, THEN transmit.

3. IF the channel is busy, THEN continue to

listen until channel is idle and transmitimmediately.


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P - Persistent CSMA

p - persistent CSMA {a slotted approximation}


2. IF the channel is idle, THEN with probability ptransmit and with probability (1-p) delay onetime slotand start over.

3. IF the channel is busy, THEN delay one time slotand start over.


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P Persistent CSMA details

the time slot is usually set to the maximumpropagation delay.

as p decreases, stations wait longer to

transmit but the number of collisionsdecreases.

Considerations for the choice ofp :

(n x p) must be < 1 for stability, where n ismaximum number of stations, i.e.,

p < 1/n


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CSMA Collisions

In all three cases a collision is possible.

CSMA determines collisions by the lack ofan ACK which results in a TIMEOUT. {This isextremely expensive with respect toperformance.}

If a collision occurs, THEN wait a randomamount of time and start over.


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CSMA/CDCSMA with Collision Detection

If a collision is detected during transmission,THEN immediately cease transmitting the frame.

The first station to detect a collision sends a jamsignal to all stations to indicate that there hasbeen a collision.

After receiving a jam signal, a station that wasattempting to transmit waits a random amount oftime before attempting to retransmit.

The maximum time needed to detect a collision =2 x propagation delay.


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CSMA vs CSMA/CD CSMA is essentially a historical technology until we

include Wireless LANs.

If propagation time is short compared totransmission time, station can be listening before

sending with CSMA.

Collision detection (CD) accomplished by detectingvoltage levels outside acceptable range. Thusattenuation limits distance without a repeater.

If the collision time is short compared to packet time(i.e., small a), performance will increase due to CD.


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Probability of 1 successful transmission:

frame contention frame

Ps u c c e s s np (1 p )n 1

Psuccess is maximized atp=1/n:

Ps u c c e s smax

n(1 1

n)n 1

1

e

0

0.1

0.2

0.3

0.4

0.5

0.6

2 4 6 8 10 12 14 16

n

Pmax


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0

0.1

0.2

0.3

0.4

0.5

0.6

0

.02

0

.03

0

.06

0

.13

0

.25

0.5 1 2 4 8

16

32

64

1-Persistent

CSMA

0.53

0.45

0.16

S

G

Throughput vs Load

with varying a

a = 1

a = 0.01

a = 0.1


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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0

.02

0

.03

0

.06

0

.13

0

.25

0.5 1 2 4 8

16

32

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Non-Persistent

CSMA

0.81

0.51

0.14

S

G

Throughput vs Load

With varying a

a = 0.01

a = 0.1

a = 1


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0

0.2

0.4

0.6

0.8

1

0.01 0.1 1

Aloha

Slotted Aloha

1-P CSMANon-P CSMA

CSMA/CD

a

max

Maximum Achievable Throughputs


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CSMA-CD

0

5

10

15

20

25

30

0

0.0

6

0.1

2

0.1

8

0.2

4

0.

3

0.3

6

0.4

2

0.4

8

0.5

4

0.

6

0.6

6

0.7

2

0.7

8

0.8

4

0.

9

0.9

6

Load

Avg.

TransferDelay

a = 0.01a = 0.1a = 0.2

Frame Delay varying a


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Dynamic Channel Allocation Parameters

Station Model. N independent stations, each acting as a Poisson Process for the purpose

protocol analysis

Single Channel Assumption. A single channel is available for all communication.

Collision Assumption. If transmitted frames overlap in time, the resulting signal is garbled.

Transmission Discipline: Continuous time

Frames can be transmitted at any time

Slotted time Frames can be transmitted at particular time points

Sensing capability: Station cannot sense the channel before trying to use it.

Stations can tell if the channel is in use before trying to use it


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Pure ALOHA Protocol

While there is a new frame A to send DO

1. Send frame A and wait for ACK

2. If after some time ACK is not received

(timer times out), wait a random amount

of time and go to 1.

End


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Pure ALOHA

In pure ALOHA, frames are transmitted at

completely arbitrary times.


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Analysis of Pure ALOHA

Notation: Tf= frame time (processing, transmission, propagation)

S: Average number of successful transmissions per Tf; that is,the throughputor efficiency.

G: Average number of total frames transmitted per Tf D: Average delay between the time a packet is ready for

transmission and the completion of successful transmission.

We will make the following assumptions

All frames are of constant length The channel is noise-free; the errors are only due to

collisions.

Frames do not queue at individual stations

The channel acts as a Poisson process.


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Since S represents the number of good

transmissions perframe time, and G represents

the total number of attempted transmissions

perframe time, then we have:

S = G (Probability of good transmission)

The vulnerable time for a successful

transmission is2Tf

So, the probability of good transmission is not to

have an arrival during the vulnerable time .


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t0 t0 + t t0 + 2t t0 + 3t

Collides with

the start of

the shaded

frame

Collides with

the end of

the shaded

frame

Vulnerable Time

Vulnerable period for the shaded frame

t


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Using:

!

)()(

k

ettP

tk

k

And setting t= 2Tfand k= 0, we get

20

2

0

2

( 2 )(2 )

0!becasue . Thus,

fT

f G

f

G

f

T eP T e

GS G e

T


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If we differentiate S = Ge-2G with respect to G andset the result to 0 and solve for G, we find thatthe maximum occurs when

G = 0.5,

and for that S = 1/2e = 0.18. So, the maximumthroughput is only 18% of capacity.

ALOHANET uses a data rate of 9600bps. This

means the maximum total throughput (sum ofdata arriving from all user nodes) is only 0.18 9600 = 1728bps.


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Pure ALOHA

Throughput versus offered traffic for ALOHA

systems.


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Analysis of Pure ALOHA; another approach

There are N stations Each station transmits with probabilityp

For a typical node i to have a successful transmissionmeans that there was no prior overlapping

transmissions before or after, each with probability (1-p)N-1

Thus the probability of node i having a successfultransmission isp (1-p)2(N-1)

Therefore, the probability of a successful transmissionis Np (1 p)2(N-1)

The maximum value for the above term when N is

large is 1/2e


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Slotted ALOHA

Channel is organized into uniform slots whose size equals theframe transmission time. Transmission is permitted only to beginat a slot boundary.

Here is the procedure:

While there is a new frame A to send do

1. Send frame A at a slot boundary and wait for ACK

2. If after some time ACK is not received, wait a randomamount of time and go to 1.

End


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Slotted ALOHA

Throughput versus offered traffic for ALOHA systems.


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Non-persistent CSMA

While there is a new frame A to send DO

1. Check the medium

2. If the medium is busy, wait some time, and go to

1.3. (medium idle) Send frame A and wait for ACK

4. If after some time ACK is not received (timertimes out), wait a random amount of time and

go to 1.End


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1-persistent CSMA

While there is a new frame A to send do1. Check the medium

2. If the medium is busy, go to 1.

3. (medium idle) Send frame A and wait forACK

4. If after some time ACK is not received(timer times out), wait a random amount

of time and go to 1.End.


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p-persistent CSMA

While there is a new frame A to send do

1. Check the medium

2. If the medium is busy, go to 1.

3. (medium idle) With probability p send frame Aand the go to 4, and probability (1- p) delay onetime slot and go to 1.

4. If after some time ACK is not received (timer

times out), wait a random amount of time andgo to 1.

End.


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CSMA Summary

Non-persistent:

Transmit if idle

Otherwise, delay, try againConstant or variable

Delay

Channel busy

Ready

1-persistent:

Transmit as soon as channel goes idle. If

collision, back off and try again

Time

p-persistent:Transmit as soon as channel goes idle with

probability p. Otherwise, delay one slot,

repeat process

CSMA persistence and backoff

Nonpersistent

1-persistent

p-persistent


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Persistent and Non-persistent CSMA

Comparison of throughput versus load for

various random access protocols.


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CSMA with Collision Detection

Stations can sense the medium while

transmitting

A station aborts its transmission if it senses

another transmission is also happening (that is,

it detects collision)

Question: When can a station be sure that it

has seizedthe channel?

Minimum time to detect collision is the time it takes

for a signal to traverse between two farthest apart

stations.


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CSMA with Collision Detection

CSMA/CD can be in one of three states:

contention, transmission, or idle.


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CSMA/CD

A station is said to seize the channel if all

the other stations become aware of its

transmission.

There has to be a lower bound on the

length of each frame for the collision

detection feature to work out. Ethernet uses CSMA/CD


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CSMA/CA

Identical to CSMA/CD but used when listeningis not possible while transmitting

Idle channel reservation is done by sending a

short request message asking other nodes todefer transmission

If collison is detected then, then random waitis used

Wireless IEEE 802.11 uses CSMA/CA with anRTS/CTS mechanism

OSI R f M d l


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OSI Reference Model

Data Transmission


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Data Transmission

The Open Systems Interconnect (OSI) reference model


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The Open Systems Interconnect (OSI) reference modeloutlines 7 layers for an ideal network architecture.

Physical LayerThe nuts and bolts layer, where the cable, connector

and signaling specifications are defined

Describes the electrical, mechanical, and functionalinterface to the carrier

It includes:

Voltages and pulse coding of bits Media and media interface

Line discipline (full or half duplex)

Pin Assignments

i k


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Data Link LayerGets data packets on and off the wire

Does error detection and correction andretransmission

The primary purpose of the Data Link Layer is to

provide error-free transmission of informationbetween two end stations

The MAC (Medium Access Control) on the lowerhalf, deals with getting the data on and off the

wire

The LLC (Logical Link Control) on the upper half,does the error checking


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Network LayerThe Network Layer controls the operation of the

network orsub-network

Routing and flow control are performed here

This is the lowest layer of the OSI model that can

remainignorant of the physical network

The general functions are: Addressing messages

Routing messages Controlling congestion

Translating addresses

Counting packets


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Transport Layer

Ensures the performance of the lower 3

layers

It provides a transparent, logical data

stream between the end user and thenetwork service

This is the lower layer that provides local

user servicesIt provides the session layer with reliable

message transfer facilities


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Session Layer

Control the communications betweenapplications across a network

Testing for out-of-sequence packets

and handling two-way communication

are handled here

Presentation Layer


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Presentation LayerThe Presentation Layer formats the data to be presented

to the Application Layer

Differences in data representation are dealt with at this

level

For example, UNIX-style line endings (CR only) might be

converted to MS-DOS style (CRLF), or EBCIDIC to ASCII

character sets

It can be viewed as the translator for the network

It also does:

Encryption

Encoding

Compression of data


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Applications Layer

Where the user applications softwarelies

Handles issues such as:

File access and transfer

Virtual terminal emulation

Inter process communication

Electronic Mail

Network Management


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TCP/IP Reference Model

TCP/IP Transmission Control


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TCP/IP = Transmission ControlProtocol/Internet ProtocolIs the basic communication language or

protocol of the Internet

It can also be used as a communications

protocol in the private networks ( intranets andin extranets)

TCP/IP is a two-layered program

Transmission Control Protocol - Manages theassembling of a message or file into smaller packets

Internet Protocol - Handles the address part of eachpacket so that it gets to the right destination

Application Layer


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Application Layer It contains all the higher level protocols such as Telnet, File

Transfer (FTP), Simple Mail Transfer(SMTP), Domain Name

Service(DNS), Hypertext Transfer (HTTP)

Transport Layer

Designed to allow peer entities on the source anddestination hosts carry on a conversation

TCP and UDP(end-to-end Protocols)defined here

TCP (Transmission Control) - Manages the assembling of a

message or file into smaller packets that are transmitted over theInternet

UDP (User Datagram) - Connectionless protocol for applicationsthat do not want TCPs sequencing or flow control( Speech orVideo)

Internet Layer


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Internet Layer

Defines an official packet format and protocol

calledIP(Internet Protocol) Internet Layer delivers IP packets

to where they are supposed to go(packet routing)

Host-to-Network Layer

Host connects to the network using relevant

protocols so it can send IP packets over it


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Repeater: PHY device that restores data and


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Repeater: PHY device that restores data and

collision signals: a digital amplifier

Hub: Multi-port repeater + fault detectionBridge: Data link layer device connecting two

or more collision domains. MAC multicasts are

propagated throughout extendedLANRouter: Network layer device. IP, IPX,

AppleTalk. Does not propagate MAC

multicasts

Switch: Multi-port bridge with parallel paths

IEEE 802 3


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IEEE 802.3The IEEE 802 LAN/MAN Standards Committee

develops Local Area Network standards andMetropolitan Area Network standards

The most widely used standards are for the

Ethernet family, Token Ring, Wireless LAN,Bridging and Virtual Bridged LANs

The IEEE 802.3 Working Group develops

standards for CSMA/CD (Ethernet) based LANs


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Gigabit Ethernet

Is an open forum with the purpose of

promoting industry cooperation in the

development of Gigabit Ethernet

http://www.gigabit-ethernet.orgFunded in 1996 by:

3COM, Bay Networks, Cisco, Compaq, Granit

System, Intel,LSI Logic, Packet Engine, SUNMicrosystem, UB Networks, VLSI


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