This document provides an overview of medium access control sublayer topics, including channel allocation, multiple access protocols, Ethernet, wireless LANs, and data link layer switching. It discusses static and dynamic channel allocation approaches. For multiple access protocols, it describes ALOHA, carrier sense multiple access, and collision-free protocols. It provides details on pure ALOHA, slotted ALOHA, 1-persistent CSMA, non-persistent CSMA, CSMA/CD, bit-map protocols, token passing, and binary countdown.
SLOTTED ALOHA and pure aloha are the category of alohaAkshathaM29
The data link layer is used in a computer network to transmit the data between two devices or nodes. It divides the layer into parts such as data link control and the multiple access resolution/protocol. The upper layer has the responsibility to flow control and the error control in the data link layer, and hence it is termed as logical of data link control. Whereas the lower sub-layer is used to handle and reduce the collision or multiple access on a channel. Hence it is termed as media access control or the multiple access resolutions. ALOHA, an acronym for Advocates of Linux Open-source Hawaii Association, is a multiple access protocol that allows data to be transmitted over a public network channel. It operates within the Medium Access Control (MAC) sublayer of the Open Systems Interconnection (OSI) model.When a sender and receiver have a dedicated link to transmit data packets, the data link control is enough to handle the channel. Suppose there is no dedicated path to communicate or transfer the data between two devices. In that case, multiple stations access the channel and simultaneously transmits the data over the channel. It may create collision and cross talk. Hence, the multiple access protocol is required to reduce the collision and avoid crosstalk between the channels.
ALOHA is a multiple access protocol for transmission of data via a shared network channel. It operates in the medium access control sublayer (MAC sublayer) of the open systems interconnection (OSI) model. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel.
this protocol, all the station has the equal priority to send the data over a channel. In random access protocol, one or more stations cannot depend on another station nor any station control another station. Depending on the channel's state (idle or busy), each station transmits the data frame. However, if more than one station sends the data over a channel, there may be a collision or data conflict. Due to the collision, the data frame packets may be lost or changed. And hence, it does not receive by the receiver end.
In ALOHA, each node or station transmits a frame without trying to detect whether the transmission channel is idle or busy. If the channel is idle, then the frames will be successfully transmitted. If two frames attempt to occupy the channel simultaneously, collision of frames will occur and the frames will be discarded. These stations may choose to retransmit the corrupted frames repeatedly until successful transmission occurs
In pure ALOHA, the time of transmission is continuous. Whenever a station has an available frame, it sends the frame. If there is collision and the frame is destroyed, the sender waits for a random amount of time before retransmitting it.
Slotted ALOHA reduces the number of collisions and doubles the capacity of pure ALOHA. The shared channel is divided into a number of discrete time intervals
This is the bottom sublayer of the Data Link Layer. This Chapter is especially relevant for LANs.
4.1 The Channel Allocation Problem
How to allocate a single channel among multiple users.
4.2 Multiple Access Protocols
How to handle contention for the use of a channel.
4.3 IEEE Standards for LANs
How do the protocols of the last sections apply to real systems. Here we talk about the actual standards in use.
4.4 Bridges
Ways of connecting networks together.
4.5 High Speed LANs
Directions in high speed networks.
satellite communication jntuh
Earth Station Technology: Introduction, Transmitters, Receivers, Antennas, Tracking Systems,
Terrestrial Interface, Primary Power Test Methods.
Introduction to Multiple Access Protocol.pptxupamatechverse
Data link layer is divided into two sub layers:
Logical link control (LLC) layer: The upper sub layer is responsible for data link control i.e. for flow and error control.
Media access control (MAC) layer: The lower sub layer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sub layer.
SLOTTED ALOHA and pure aloha are the category of alohaAkshathaM29
The data link layer is used in a computer network to transmit the data between two devices or nodes. It divides the layer into parts such as data link control and the multiple access resolution/protocol. The upper layer has the responsibility to flow control and the error control in the data link layer, and hence it is termed as logical of data link control. Whereas the lower sub-layer is used to handle and reduce the collision or multiple access on a channel. Hence it is termed as media access control or the multiple access resolutions. ALOHA, an acronym for Advocates of Linux Open-source Hawaii Association, is a multiple access protocol that allows data to be transmitted over a public network channel. It operates within the Medium Access Control (MAC) sublayer of the Open Systems Interconnection (OSI) model.When a sender and receiver have a dedicated link to transmit data packets, the data link control is enough to handle the channel. Suppose there is no dedicated path to communicate or transfer the data between two devices. In that case, multiple stations access the channel and simultaneously transmits the data over the channel. It may create collision and cross talk. Hence, the multiple access protocol is required to reduce the collision and avoid crosstalk between the channels.
ALOHA is a multiple access protocol for transmission of data via a shared network channel. It operates in the medium access control sublayer (MAC sublayer) of the open systems interconnection (OSI) model. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel.
this protocol, all the station has the equal priority to send the data over a channel. In random access protocol, one or more stations cannot depend on another station nor any station control another station. Depending on the channel's state (idle or busy), each station transmits the data frame. However, if more than one station sends the data over a channel, there may be a collision or data conflict. Due to the collision, the data frame packets may be lost or changed. And hence, it does not receive by the receiver end.
In ALOHA, each node or station transmits a frame without trying to detect whether the transmission channel is idle or busy. If the channel is idle, then the frames will be successfully transmitted. If two frames attempt to occupy the channel simultaneously, collision of frames will occur and the frames will be discarded. These stations may choose to retransmit the corrupted frames repeatedly until successful transmission occurs
In pure ALOHA, the time of transmission is continuous. Whenever a station has an available frame, it sends the frame. If there is collision and the frame is destroyed, the sender waits for a random amount of time before retransmitting it.
Slotted ALOHA reduces the number of collisions and doubles the capacity of pure ALOHA. The shared channel is divided into a number of discrete time intervals
This is the bottom sublayer of the Data Link Layer. This Chapter is especially relevant for LANs.
4.1 The Channel Allocation Problem
How to allocate a single channel among multiple users.
4.2 Multiple Access Protocols
How to handle contention for the use of a channel.
4.3 IEEE Standards for LANs
How do the protocols of the last sections apply to real systems. Here we talk about the actual standards in use.
4.4 Bridges
Ways of connecting networks together.
4.5 High Speed LANs
Directions in high speed networks.
satellite communication jntuh
Earth Station Technology: Introduction, Transmitters, Receivers, Antennas, Tracking Systems,
Terrestrial Interface, Primary Power Test Methods.
Introduction to Multiple Access Protocol.pptxupamatechverse
Data link layer is divided into two sub layers:
Logical link control (LLC) layer: The upper sub layer is responsible for data link control i.e. for flow and error control.
Media access control (MAC) layer: The lower sub layer is responsible for resolving access to the shared media. If the channel is dedicated, we do not need the lower sub layer.
If you are looking for a pi coin investor. Then look no further because I have the right one he is a pi vendor (he buy and resell to whales in China). I met him on a crypto conference and ever since I and my friends have sold more than 10k pi coins to him And he bought all and still want more. I will drop his telegram handle below just send him a message.
@Pi_vendor_247
how to swap pi coins to foreign currency withdrawable.DOT TECH
As of my last update, Pi is still in the testing phase and is not tradable on any exchanges.
However, Pi Network has announced plans to launch its Testnet and Mainnet in the future, which may include listing Pi on exchanges.
The current method for selling pi coins involves exchanging them with a pi vendor who purchases pi coins for investment reasons.
If you want to sell your pi coins, reach out to a pi vendor and sell them to anyone looking to sell pi coins from any country around the globe.
Below is the contact information for my personal pi vendor.
Telegram: @Pi_vendor_247
how to sell pi coins at high rate quickly.DOT TECH
Where can I sell my pi coins at a high rate.
Pi is not launched yet on any exchange. But one can easily sell his or her pi coins to investors who want to hold pi till mainnet launch.
This means crypto whales want to hold pi. And you can get a good rate for selling pi to them. I will leave the telegram contact of my personal pi vendor below.
A vendor is someone who buys from a miner and resell it to a holder or crypto whale.
Here is the telegram contact of my vendor:
@Pi_vendor_247
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how can I sell my pi coins for cash in a pi APPDOT TECH
You can't sell your pi coins in the pi network app. because it is not listed yet on any exchange.
The only way you can sell is by trading your pi coins with an investor (a person looking forward to hold massive amounts of pi coins before mainnet launch) .
You don't need to meet the investor directly all the trades are done with a pi vendor/merchant (a person that buys the pi coins from miners and resell it to investors)
I Will leave The telegram contact of my personal pi vendor, if you are finding a legitimate one.
@Pi_vendor_247
#pi network
#pi coins
#money
when will pi network coin be available on crypto exchange.DOT TECH
There is no set date for when Pi coins will enter the market.
However, the developers are working hard to get them released as soon as possible.
Once they are available, users will be able to exchange other cryptocurrencies for Pi coins on designated exchanges.
But for now the only way to sell your pi coins is through verified pi vendor.
Here is the telegram contact of my personal pi vendor
@Pi_vendor_247
Introduction to Indian Financial System ()Avanish Goel
The financial system of a country is an important tool for economic development of the country, as it helps in creation of wealth by linking savings with investments.
It facilitates the flow of funds form the households (savers) to business firms (investors) to aid in wealth creation and development of both the parties
what is the future of Pi Network currency.DOT TECH
The future of the Pi cryptocurrency is uncertain, and its success will depend on several factors. Pi is a relatively new cryptocurrency that aims to be user-friendly and accessible to a wide audience. Here are a few key considerations for its future:
Message: @Pi_vendor_247 on telegram if u want to sell PI COINS.
1. Mainnet Launch: As of my last knowledge update in January 2022, Pi was still in the testnet phase. Its success will depend on a successful transition to a mainnet, where actual transactions can take place.
2. User Adoption: Pi's success will be closely tied to user adoption. The more users who join the network and actively participate, the stronger the ecosystem can become.
3. Utility and Use Cases: For a cryptocurrency to thrive, it must offer utility and practical use cases. The Pi team has talked about various applications, including peer-to-peer transactions, smart contracts, and more. The development and implementation of these features will be essential.
4. Regulatory Environment: The regulatory environment for cryptocurrencies is evolving globally. How Pi navigates and complies with regulations in various jurisdictions will significantly impact its future.
5. Technology Development: The Pi network must continue to develop and improve its technology, security, and scalability to compete with established cryptocurrencies.
6. Community Engagement: The Pi community plays a critical role in its future. Engaged users can help build trust and grow the network.
7. Monetization and Sustainability: The Pi team's monetization strategy, such as fees, partnerships, or other revenue sources, will affect its long-term sustainability.
It's essential to approach Pi or any new cryptocurrency with caution and conduct due diligence. Cryptocurrency investments involve risks, and potential rewards can be uncertain. The success and future of Pi will depend on the collective efforts of its team, community, and the broader cryptocurrency market dynamics. It's advisable to stay updated on Pi's development and follow any updates from the official Pi Network website or announcements from the team.
Currently pi network is not tradable on binance or any other exchange because we are still in the enclosed mainnet.
Right now the only way to sell pi coins is by trading with a verified merchant.
What is a pi merchant?
A pi merchant is someone verified by pi network team and allowed to barter pi coins for goods and services.
Since pi network is not doing any pre-sale The only way exchanges like binance/huobi or crypto whales can get pi is by buying from miners. And a merchant stands in between the exchanges and the miners.
I will leave the telegram contact of my personal pi merchant. I and my friends has traded more than 6000pi coins successfully
Tele-gram
@Pi_vendor_247
Turin Startup Ecosystem 2024 - Ricerca sulle Startup e il Sistema dell'Innov...Quotidiano Piemontese
Turin Startup Ecosystem 2024
Una ricerca de il Club degli Investitori, in collaborazione con ToTeM Torino Tech Map e con il supporto della ESCP Business School e di Growth Capital
Resume
• Real GDP growth slowed down due to problems with access to electricity caused by the destruction of manoeuvrable electricity generation by Russian drones and missiles.
• Exports and imports continued growing due to better logistics through the Ukrainian sea corridor and road. Polish farmers and drivers stopped blocking borders at the end of April.
• In April, both the Tax and Customs Services over-executed the revenue plan. Moreover, the NBU transferred twice the planned profit to the budget.
• The European side approved the Ukraine Plan, which the government adopted to determine indicators for the Ukraine Facility. That approval will allow Ukraine to receive a EUR 1.9 bn loan from the EU in May. At the same time, the EU provided Ukraine with a EUR 1.5 bn loan in April, as the government fulfilled five indicators under the Ukraine Plan.
• The USA has finally approved an aid package for Ukraine, which includes USD 7.8 bn of budget support; however, the conditions and timing of the assistance are still unknown.
• As in March, annual consumer inflation amounted to 3.2% yoy in April.
• At the April monetary policy meeting, the NBU again reduced the key policy rate from 14.5% to 13.5% per annum.
• Over the past four weeks, the hryvnia exchange rate has stabilized in the UAH 39-40 per USD range.
how can i use my minded pi coins I need some funds.DOT TECH
If you are interested in selling your pi coins, i have a verified pi merchant, who buys pi coins and resell them to exchanges looking forward to hold till mainnet launch.
Because the core team has announced that pi network will not be doing any pre-sale. The only way exchanges like huobi, bitmart and hotbit can get pi is by buying from miners.
Now a merchant stands in between these exchanges and the miners. As a link to make transactions smooth. Because right now in the enclosed mainnet you can't sell pi coins your self. You need the help of a merchant,
i will leave the telegram contact of my personal pi merchant below. 👇 I and my friends has traded more than 3000pi coins with him successfully.
@Pi_vendor_247
US Economic Outlook - Being Decided - M Capital Group August 2021.pdfpchutichetpong
The U.S. economy is continuing its impressive recovery from the COVID-19 pandemic and not slowing down despite re-occurring bumps. The U.S. savings rate reached its highest ever recorded level at 34% in April 2020 and Americans seem ready to spend. The sectors that had been hurt the most by the pandemic specifically reduced consumer spending, like retail, leisure, hospitality, and travel, are now experiencing massive growth in revenue and job openings.
Could this growth lead to a “Roaring Twenties”? As quickly as the U.S. economy contracted, experiencing a 9.1% drop in economic output relative to the business cycle in Q2 2020, the largest in recorded history, it has rebounded beyond expectations. This surprising growth seems to be fueled by the U.S. government’s aggressive fiscal and monetary policies, and an increase in consumer spending as mobility restrictions are lifted. Unemployment rates between June 2020 and June 2021 decreased by 5.2%, while the demand for labor is increasing, coupled with increasing wages to incentivize Americans to rejoin the labor force. Schools and businesses are expected to fully reopen soon. In parallel, vaccination rates across the country and the world continue to rise, with full vaccination rates of 50% and 14.8% respectively.
However, it is not completely smooth sailing from here. According to M Capital Group, the main risks that threaten the continued growth of the U.S. economy are inflation, unsettled trade relations, and another wave of Covid-19 mutations that could shut down the world again. Have we learned from the past year of COVID-19 and adapted our economy accordingly?
“In order for the U.S. economy to continue growing, whether there is another wave or not, the U.S. needs to focus on diversifying supply chains, supporting business investment, and maintaining consumer spending,” says Grace Feeley, a research analyst at M Capital Group.
While the economic indicators are positive, the risks are coming closer to manifesting and threatening such growth. The new variants spreading throughout the world, Delta, Lambda, and Gamma, are vaccine-resistant and muddy the predictions made about the economy and health of the country. These variants bring back the feeling of uncertainty that has wreaked havoc not only on the stock market but the mindset of people around the world. MCG provides unique insight on how to mitigate these risks to possibly ensure a bright economic future.
5. The Channel Allocation Problem
5/24/2023 | Sushama Deshmukh 5
Channel allocation is a process in which a single channel is divided and allotted
to multiple users in order to carry user specific tasks.
user’s quantity may vary every time the process takes place.
The channel might be :
– a portion of the wireless spectrum in a geographic region,
– or a single wire or optical fiber to which multiple nodes are connected.
Channel allocation problem is how to allocate a single broadcast channel among
competing users.
Channel allocation problem can be solved by two schemes:
Static Channel Allocation in LANs and MANs, and Dynamic Channel Allocation.
6. The Channel Allocation Problem
5/24/2023 | Sushama Deshmukh 6
Static Channel allocation:
The traditional approach of allocating a single channel among multiple competing users
using Frequency Division Multiplexing (FDM).
FDM - if there are N users, the frequency channel is divided into N equal sized portions
(bandwidth), each user being assigned one portion. since each user has a private
frequency band, there is no interference between users.
It is not efficient to divide into fixed number of chunks.
Where,
T = mean time delay,
C = capacity of channel,
L = arrival rate of frames,
1/U = bits/frame,
N = number of sub channels,
T(FDM) = Frequency Division Multiplexing Time
7. The Channel Allocation Problem
5/24/2023 | Sushama Deshmukh 7
When the number of senders is large and varying or the traffic is bursty,
FDM presents some problems:
•If (number of spectrum portions < number of stations)
»some of them will be denied permission for lack of bandwidth.
•If (number of spectrum portions > number of stations)
»a large piece of valuable spectrum will be wasted.
•when some stations are quiescent,
»their bandwidth is simply wasted. They are not using it, and no one
else is allowed to use it either.
8. The Channel Allocation Problem
5/24/2023 | Sushama Deshmukh 8
5 Key Assumptions for Dynamic Channel Allocation
Station Model: The model consists of N independent stations (e.g., computers,
telephones, or personal communicators), each with a program or user that generates
frames for transmission. Stations are sometimes called terminals. The probability of a
frame being generated in an interval of length Dt is lDt, where l is a constant (the arrival
rate of new frames). Once a frame has been generated, the station is blocked and does
nothing until the frame has been successfully transmitted.
Single Channel Assumption: A single channel is available for all communication. All
stations can transmit on it and all can receive from it. As far as the hardware is concerned,
all stations are equivalent, although protocol software may assign priorities to them.
Collision Assumption: If two frames are transmitted simultaneously, they overlap in time
and the resulting signal is garbled. This event is called a collision. All stations can detect
collisions. A collided frame must be transmitted again later. There are no errors other than
those generated by collisions.
9. The Channel Allocation Problem
5/24/2023 | Sushama Deshmukh 9
Key Assumptions for Dynamic Channel Allocation continued……
4a. Continuous Time. Frame transmission can begin at any instant. There is no master
clock dividing time into discrete intervals.
4b. Slotted Time. Time is divided into discrete intervals (slots). Frame transmissions
always begin at the start of a slot. A slot may contain 0, 1, or more frames,
corresponding to an idle slot, a successful transmission, or a collision, respectively.
5a. Carrier Sense. Stations can tell if the channel is in use before trying to use it. If the
channel is sensed as busy, no station will attempt to use it until it goes idle.
5b. No Carrier Sense. Stations cannot sense the channel before trying to use it. They
just go ahead and transmit. Only later can they determine whether the transmission
was successful.
10. Multiple Access Protocols
5/24/2023 | Sushama Deshmukh 10
Multiple Access: enabling many devices to gain access to the channel at the same
time.
Multiple Access Protocols:
- Contention Protocols
Aloha Protocols
Carrier Sense Multiple Access(CSMA)protocols
- Collision-Free Protocols
- Limited-Contention Protocols
11. Multiple Access Protocols
ALOHA:
5/24/2023 | Sushama Deshmukh 11
ALOHA:
Abramson’s work, called the ALOHA system, used ground based radio broadcasting,
the basic idea is applicable to any system in which uncoordinated users are competing
for the use of a single shared channel.
Two versions of ALOHA here: Pure and Slotted.
They differ with respect to whether time is continuous, as in the pure version,
or divided into discrete slots into which all frames must fit.
13. Multiple Access Protocols
ALOHA:
5/24/2023 | Sushama Deshmukh 13
In pure ALOHA, the stations transmit frames whenever they have data to send.
When two or more stations transmit simultaneously, there is collision and the
frames are destroyed.
Whenever any station transmits a frame, it expects the acknowledgement from the
receiver.
If acknowledgement is not received within specified time, the station assumes that
the frame (or acknowledgement) has been destroyed.
If the frame is destroyed because of collision the station waits for a random amount
of time and sends it again. This waiting time must be random otherwise same frames
will collide again and again.
Therefore pure ALOHA dictates that when time-out period passes, each station must
wait for a random amount of time before resending its frame.
This randomness will help avoid more collisions.
14. Multiple Access Protocols
ALOHA:
5/24/2023 | Sushama Deshmukh 14
Vulnerable time
Let us find the length of time, the
vulnerable time, in which there is
a possibility of collision.
We assume that the stations send
fixed length frames with each
frame taking Tfr S to send.
15. Multiple Access Protocols
ALOHA:
5/24/2023 | Sushama Deshmukh 15
Station A sends a frame at time t. Now imagine
station B has already sent a frame between t -
Tfr and t. This leads to a collision between the
frames from station A and station B.
The end of B's frame collides with the
beginning of A's frame. On the other hand,
suppose that station C sends a frame between t
and t + Tfr .
Here, there is a collision between frames from
station A and station C.
The beginning of C's frame collides with the
end of A's frame. Looking at Figure, we see that
the vulnerable time, during which a collision
may occur in pure ALOHA, is 2 times the frame
transmission time.
Pure ALOHA vulnerable time = 2 x Tfr
Figure shows the vulnerable time for station A.
16. Multiple Access Protocols
ALOHA:
5/24/2023 | Sushama Deshmukh 16
Slotted ALOHA: Pure ALOHA has a vulnerable time of 2 x Tfr .
This is so because there is no rule that defines when the station can send. A station may send
soon after another station has started or soon before another station has finished.
Slotted ALOHA was invented to improve the efficiency of pure ALOHA.
In slotted ALOHA we divide the time into slots of Tfr s and force the station to send only at the
beginning of the time slot.
Because a station is allowed to send only at the beginning of the synchronized time slot, if a
station misses this moment, it must wait until the beginning of the next time slot. This means
that the station which started at the beginning of this slot has already finished sending its
frame. Of course, there is still the possibility of collision if two stations try to send at the
beginning of the same time slot.
However, the vulnerable time is now reduced to one-half, equal to Tfr. Figure 4 shows the
situation
18. Multiple Access Protocols
ALOHA:
5/24/2023 | Sushama Deshmukh 18
Slotted ALOHA:
Figure 4. shows the situation that the vulnerable time for slotted ALOHA is one-half that of pure
ALOHA. Slotted ALOHA vulnerable time = Tfr
The throughput for slotted ALOHA is,
S = G × e−G .
The maximum throughput Smax = 0.368
when G = 1.
19. Multiple Access Protocols
5/24/2023 | Sushama Deshmukh 19
Carrier Sense Multiple Access (CSMA):
To minimize the chance of collision and, therefore, increase the performance, the CSMA
method was developed. The chance of collision can be reduced if a station senses the medium
before trying to use it.
CSMA requires that each station first listen to the medium (or check the state of the medium)
before sending. In other words, CSMA is based on the principle "sense before transmit" or
"listen before talk."
CSMA can reduce the possibility of collision, but it cannot eliminate it. The reason for this is,
Stations are connected to a shared channel (usually a dedicated medium).
The possibility of collision still exists because of propagation delay; station may sense the
medium and find it idle, only because the first bit sent by another station has not yet been
received.
Versions of CSMA:–
- 1-persistent CSMA
–nonpersistent CSMA
–CSMA with Collision Detection
20. Multiple Access Protocols
5/24/2023 | Sushama Deshmukh 20
Carrier Sense Multiple Access (CSMA):
Space/time model of the collision in CSMA
At time tI' station B senses the medium
and finds it idle, so it sends a frame.
At time t2 (t2> tI)‘ station C senses the
medium and finds it idle because, at this
time, the first bits from station B have not
reached station C.
Station C also sends a frame.
The two signals collide and both frames
are destroyed.
21. Multiple Access Protocols
5/24/2023 | Sushama Deshmukh 21
Carrier Sense Multiple Access (CSMA):
Vulnerable Time
The vulnerable time for CSMA is the
propagation time Tp.
This is the time needed for a signal to
propagate from one end of the medium
to the other.
When a station sends a frame, and any
other station tries to send a frame
during this time, a collision will result.
But if the first bit of the frame reaches
the end of the medium, every station will
already have heard the bit and will
refrain from sending.
Vulnerable Time in CSMA
22. Multiple Access Protocols
5/24/2023 | Sushama Deshmukh 22
Carrier Sense Multiple Access (CSMA):
1 Persistence Methods:
•What should a station do if the channel is busy?
•What should a station do if the channel is idle?
• When a station has a frame to send, it first listens to the channel (idle or busy)
•If the channel is idle, the station sends its frame.
•if the channel is busy, the station continuously listen to tothe channel until it becomes
idle.Then the station transmits a frame.
•If a collision occurs, the station waits a random amount of time and starts all over again.
23. Multiple Access Protocols
5/24/2023 | Sushama Deshmukh 23
Carrier Sense Multiple Access (CSMA):
Non-persistent:
When a station has a frame to send, it first listens to the channel (idle or busy)
•If the channel is idle, the station sends its data.
•if the channel is busy, the station waits a random period of time and then repeats the
algorithm
•this algorithm leads to better channel utilization but longer delays than 1-persistent CSMA.
24. Multiple Access Protocols
5/24/2023 | Sushama Deshmukh 24
Carrier Sense Multiple Access (CSMA):
p-Persistent: This is used if the channel has time slots with a slot duration equal to or greater
than the maximum propagation time. The p-persistent approach combines the advantages of
the other two strategies. It reduces the chance of collision and improves efficiency.
In this method, after the station finds the line idle it follows these steps:
1.With probability p, the station sends its frame.
2.With probability q = 1 - p, the station waits for the beginning of the next time slot and checks
the line again.
a. If the line is idle, it goes to step 1.
b. If the line is busy, it acts as though a collision has occurred and uses the backoff procedure.
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Carrier Sense Multiple Access (CSMA):
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
The CSMA method does not specify the procedure following a collision.
Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm
to handle the collision.
In this method, a station monitors the medium after it sends a frame to see if the
transmission was successful. If so, the station is finished. If, however, there is a collision, the
frame is sent again.
To better understand CSMA/CD, let us
look at the first bits transmitted by the
two stations involved in the collision.
Although each station continues to send
bits in the frame until it detects the
collision, we show what happens as the
first bits collide. In the Figure, stations A
and C are involved in the collision.
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Collision Free Protocol:
Collision-Free Protocols
•there will never be any collisions (0% collision)
•Versions of Collision-Free Protocols
–A Bit-Map Protocol
–Token Passing
–Binary Countdown.
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Collision Free Protocol:
A Bit-Map Protocol
–(before any transmission happens) there is contention period consists of number of
slots, each slot is assigned to a station.
–Each station (has a frame to be sent) sends a 1 bit during the its slot
–At the end of the contention period the order of the transmissions (that are going to
happen) are known.
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Collision Free Protocol:
Token Passing protocol
•Token Passing is to pass a small message called a token from one station to the next in
the same predefined order. (The token represents permission to send).
•If a station has a frame queued for transmission when it receives the token, it can send
that frame before it passes the token to the next station.
•If it has no queued frame, it simply passes the token.
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Collision Free Protocol:
Binary Countdown
• A station wanting to use the channel
broadcasts its address as a binary bit string,
starting with the high order bit.
• The bits in each address position from
different stations are BOOLEAN ORed
together by the channel when they are sent
at the same time.
• The higher-numbered stations have a
higher priority than lower-numbered
stations.
Network links can be divided into two categories: those using point-to-point connections and those using broadcast channels. We studied point-to-point links in Chap. 2; this chapter deals with broadcast links and their protocols.
In any broadcast network, the key issue is how to determine who gets to use the channel when there is competition for it.
To make this point, consider a conference call in which six people, on six different telephones, are all connected so that each one can hear and talk to all the others. It is very likely that when one of them stops speaking, two or more will start talking at once, leading to chaos.
In a face-to-face meeting, chaos is avoided by external means. For example, at a meeting, people raise their hands to request permission to speak. When only a single channel is available, it is much harder to determine who should go next.
Many protocols for solving the problem are known.
In the literature, broadcast channels are sometimes referred to as Multiaccess channels or random access channels.
The protocols used to determine who goes next on a multiaccess channel belong to a sublayer of the data link layer called the MAC (Medium Access Control) sublayer.
The MAC sublayer is especially important in LANs, particularly wireless ones because wireless is naturally a broadcast channel. WANs, in contrast, use point-to-point links, except for satellite networks. Because multiaccess channels and LANs are so closely related, in this chapter we will discuss LANs in general, including a few issues that are not strictly part of the MAC sublayer, but the main subject here will be control of the channel.
Channel allocation problem is how to allocate a single broadcast channel among competing users.
The channel might be a portion of the wireless spectrum in a geographic region, or a single wire or optical fiber to which multiple nodes
are connected. It does not matter. In both cases, the channel connects each user to all other users and any user who makes full use of the channel interferes with other users who also wish to use the channel.
If there are N number of users and channel is divided into N equal-sized sub channels, Each user is assigned one portion.
If the number of users are small and don’t vary at times, then Frequency Division Multiplexing can be used as it is a simple and efficient channel bandwidth allocating technique.
This scheme is also referred as fixed channel allocation or fixed channel assignment.
In this allocation scheme, there is no interference between the users since each user is assigned a fixed channel. However, it is not suitable in case of a large number of users with variable bandwidth requirements.
In dynamic channel allocation scheme, frequency bands are not permanently assigned to the users. Instead channels are allotted to users dynamically as needed, from a central pool. The allocation is done considering a number of parameters so that transmission interference is minimized.
This allocation scheme optimises bandwidth usage and results is faster transmissions.
Dynamic channel allocation is further divided into centralised and distributed allocation.
The original ALOHA protocol is called pure ALOHA. This is a simple, but elegant protocol.
The idea is that each station sends a frame whenever it has a frame to send. However, since there is only one channel to share, there is the
possibility of collision between frames from different stations.
Below Figure shows an example of frame collisions in pure ALOHA.
The original ALOHA protocol is called pure ALOHA. This is a simple, but elegant protocol.
The idea is that each station sends a frame whenever it has a frame to send. However, since there is only one channel to share, there is the
possibility of collision between frames from different stations.
Below Figure shows an example of frame collisions in pure ALOHA.
In this protocol, the station listen to the channel while it is transmitting, and when it detects the collision, it stops transmitting to save time and bandwidth.–This protocol isthe basis of the Ethernet LAN.
Collision of the first bit in CSMA/CD
At time t 1, station A has executed its persistence procedure and starts sending the bits of its frame. At time t2, station C has not yet sensed the first
bit sent by A. Station C executes its persistence procedure and starts sending the bits in its frame, which propagate both to the left and to the right. The
collision occurs sometime after time t2.Station C detects a collision at time t3 when it receives the first bit of A's frame. Station C immediately (or after a
short time, but we assume immediately) aborts transmission.
Station A detects collision at time t4 when it receives the first bit of C's frame; it also immediately aborts transmission. Looking at the figure, we see that A
transmits for the duration t4 - tl; C transmits for the duration t3 - t2.