notes on nma
TRANSCRIPT
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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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Possible Model Assumptions for
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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Possible Model Assumptions for
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}
1. Sense the channel.
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}
1. Sense the channel.
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
64
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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Analysis of Pure ALOHA
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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Analysis of Pure ALOHA
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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Analysis of Pure ALOHA
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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Analysis of Pure ALOHA
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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