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    Multiple Access Protocols in Computer Networks

    Multiple Access Protocols in Computer Networks - Multiple access protocols are a set of protocols operating in the Medium Access Control sublayer (MAC sublayer) ...

    Multiple Access Protocols in Computer Networks

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    Multiple access protocols are a set of protocols operating in the Medium Access Control sublayer (MAC sublayer) of the Open Systems Interconnection (OSI) model. These protocols allow a number of nodes or users to access a shared network channel. Several data streams originating from several nodes are transferred through the multi-point transmission channel.

    The objectives of multiple access protocols are optimization of transmission time, minimization of collisions and avoidance of crosstalks.

    Categories of Multiple Access Protocols

    Multiple access protocols can be broadly classified into three categories - random access protocols, controlled access protocols and channelization protocols.

    Random Access Protocols

    Random access protocols assign uniform priority to all connected nodes. Any node can send data if the transmission channel is idle. No fixed time or fixed sequence is given for data transmission.

    The four random access protocols are−

    ALOHA

    Carrier sense multiple access (CMSA)

    Carrier sense multiple access with collision detection (CMSA/CD)

    Carrier sense multiple access with collision avoidance (CMSA/CA)

    Controlled Access Protocols

    Controlled access protocols allow only one node to send data at a given time.Before initiating transmission, a node seeks information from other nodes to determine which station has the right to send. This avoids collision of messages on the shared channel.

    The station can be assigned the right to send by the following three methods−

    Reservation Polling Token Passing

    Channelization

    Channelization are a set of methods by which the available bandwidth is divided among the different nodes for simultaneous data transfer.

    The three channelization methods are−

    Frequency division multiple access (FDMA)

    Time division multiple access (TDMA)

    Code division multiple access (CDMA)

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    Multiple Access Protocols in Computer Network

    A Computer Science portal for geeks. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions.

    Multiple Access Protocols in Computer Network

    Difficulty Level : Medium

    Last Updated : 02 Jul, 2021

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    The Data Link Layer is responsible for transmission of data between two nodes. Its main functions are-

    Data Link Control

    Multiple Access Control

    Data Link control –

    The data link control is responsible for reliable transmission of message over transmission channel by using techniques like framing, error control and flow control. For Data link control refer to – Stop and Wait ARQ

    Multiple Access Control –

    If there is a dedicated link between the sender and the receiver then data link control layer is sufficient, however if there is no dedicated link present then multiple stations can access the channel simultaneously. Hence multiple access protocols are required to decrease collision and avoid crosstalk. For example, in a classroom full of students, when a teacher asks a question and all the students (or stations) start answering simultaneously (send data at same time) then a lot of chaos is created( data overlap or data lost) then it is the job of the teacher (multiple access protocols) to manage the students and make them answer one at a time.

    Thus, protocols are required for sharing data on non dedicated channels. Multiple access protocols can be subdivided further as –

    1. Random Access Protocol: In this, all stations have same superiority that is no station has more priority than another station. Any station can send data depending on medium’s state( idle or busy). It has two features:

    There is no fixed time for sending data

    There is no fixed sequence of stations sending data

    The Random access protocols are further subdivided as:

    (a) ALOHA – It was designed for wireless LAN but is also applicable for shared medium. In this, multiple stations can transmit data at the same time and can hence lead to collision and data being garbled.Pure Aloha:

    When a station sends data it waits for an acknowledgement. If the acknowledgement doesn’t come within the allotted time then the station waits for a random amount of time called back-off time (Tb) and re-sends the data. Since different stations wait for different amount of time, the probability of further collision decreases.

    Vulnerable Time = 2* Frame transmission time

    Throughput = G exp{-2*G}

    Maximum throughput = 0.184 for G=0.5

    Slotted Aloha:

    It is similar to pure aloha, except that we divide time into slots and sending of data is allowed only at the beginning of these slots. If a station misses out the allowed time, it must wait for the next slot. This reduces the probability of collision.

    Vulnerable Time = Frame transmission time

    Throughput = G exp{-*G}

    Maximum throughput = 0.368 for G=1

    For more information on ALOHA refer – LAN Technologies

    (b) CSMA – Carrier Sense Multiple Access ensures fewer collisions as the station is required to first sense the medium (for idle or busy) before transmitting data. If it is idle then it sends data, otherwise it waits till the channel becomes idle. However there is still chance of collision in CSMA due to propagation delay. For example, if station A wants to send data, it will first sense the medium.If it finds the channel idle, it will start sending data. However, by the time the first bit of data is transmitted (delayed due to propagation delay) from station A, if station B requests to send data and senses the medium it will also find it idle and will also send data. This will result in collision of data from station A and B.

    CSMA access modes-

    1-persistent: The node senses the channel, if idle it sends the data, otherwise it continuously keeps on checking the medium for being idle and transmits unconditionally(with 1 probability) as soon as the channel gets idle.Non-Persistent: The node senses the channel, if idle it sends the data, otherwise it checks the medium after a random amount of time (not continuously) and transmits when found idle.P-persistent: The node senses the medium, if idle it sends the data with p probability. If the data is not transmitted ((1-p) probability) then it waits for some time and checks the medium again, now if it is found idle then it send with p probability. This repeat continues until the frame is sent. It is used in Wifi and packet radio systems.O-persistent: Superiority of nodes is decided beforehand and transmission occurs in that order. If the medium is idle, node waits for its time slot to send data.(c) CSMA/CD – Carrier sense multiple access with collision detection. Stations can terminate transmission of data if collision is detected. For more details refer – Efficiency of CSMA/CD(d) CSMA/CA – Carrier sense multiple access with collision avoidance. The process of collisions detection involves sender receiving acknowledgement signals. If there is just one signal(its own) then the data is successfully sent but if there are two signals(its own and the one with which it has collided) then it means a collision has occurred. To distinguish between these two cases, collision must have a lot of impact on received signal. However it is not so in wired networks, so CSMA/CA is used in this case.

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    Multiple Access Protocols

    Multiple Access Protocols: A group of protocols known as multiple access protocols work within the Open Systems Interconnection (OSI) model's Medium Access Control (MAC) sublayer. Visit to learn more about Multiple Access Protocols.

    BYJU'S GATEGATEGATE Study MaterialGATE Notes For CSEMultiple Access Protocols Notes

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    Multiple Access Protocols

    A group of protocols known as multiple access protocols work within the Open Systems Interconnection (OSI) model’s Medium Access Control (MAC) sublayer. Multiple nodes or users can access a shared network channel due to these technologies.

    In this article, we will look more into the Multiple Access Protocols according to the GATE Syllabus for (Computer Science Engineering) CSE. We will read ahead to find out more about it.

    Table of Contents

    What are Multiple Access Protocols?

    Data Link Control

    Multiple Access Control

    Classification of Multiple Access Protocol

    Random Access Protocol

    ALOHA CSMA CSMA/CD CSMA/CA

    Controlled Access Protocol

    Channelization

    Frequency Division Multiple Access

    Time Division Multiple Access

    Code Division Multiple Access

    What are Multiple Access Protocols?

    Data transmission between two nodes is handled by the Data Link Layer. Its primary duties include data link control and multiple access control.

    Data Link Control

    By utilising methods like framing, error control, and flow control, the data link control is in charge of ensuring that messages are reliably transmitted via transmission channels. To regulate a data link, use the Stop and Wait ARQ command.

    Multiple Access Control

    The data connection control layer is sufficient if there is a dedicated link present between the sender and the receiver, but numerous stations can access the channel at once if there isn’t. Therefore, it is necessary to use several access protocols to reduce collision and prevent crosstalk.

    When a teacher asks a question in a classroom full of students and all the students start responding simultaneously (send data at the same time), a lot of chaos is created (data overlap or data lost), so the teacher’s job is to manage the students and force them to respond one at a time using multiple access protocols. Protocols are therefore necessary for data sharing over non-dedicated channels.

    Classification of Multiple Access Protocol

    Multiple access protocols may also be classified into: Random Access Protocol, Controlled Access and Channelization.

    1. Random Access Protocol

    All stations in the random access protocol have equal superiority, which means that no station has higher priority than any other station. Depending on the status of the medium, any station may send data ( idle or busy). It has two attributes:

    There is no set timing for data transmission.

    The order of the stations delivering data is not fixed.

    The following are divisions of the random access protocols:

    (a) ALOHA

    Although ALOHA was created for wireless LAN, it can also be used for shared mediums. This allows for simultaneous data transmission from numerous stations, which might cause collisions and jumbled data.

    (b) CSMA

    Fewer collisions are guaranteed by carrier sensing multiple access (CSMA) since the station must first determine whether the medium is busy or idle before delivering data. If it isn’t idle, it waits for the channel to become idle before sending data. Due to propagation latency, there is still a potential for collision in CSMA.

    For instance, station A will sense the medium before sending any data. It will begin sending data if it discovers that the channel is empty. However, if station B wishes to send data and senses the medium, it will also find it idle and send data at the same time the first bits of information are transferred from station A (delayed owing to propagation delay). As a result, data from stations A and B will collide.

    Here are the CSMA access modes:

    Non-persistent: The node senses the channel; if it is free, it sends the data; if not, it checks the medium once or twice (not continuously) and sends the data when it is.1-persistent: The node senses the channel, sends the data if it is idle, or constantly checks the medium for idleness before sending data unconditionally (with a probability of 1) when the channel becomes idle.P-persistent: The node senses the media and sends data with p probability if it is idle. If the data is not transferred ((1-p) probability), the system waits a while before checking the media once more. If the medium is still empty, the system sends the data with a p probability. This process will repeat until the frame is sent. It is used in packet radio and Wifi systems.O-persistent: Transmission takes place in the sequence determined by the superiority of nodes. The node waits because of its time slot in order to send data if the medium is not in use.

    (c) CSMA/CD

    CSMA/CD is an abbreviation of Carrier Sense Multiple Access/Collision Detection. It refers to the multiple access carrier with collision detection. In CSMA/CD, all the stations have the ability to stop data transmission if a collision is found anywhere.

    (d) CSMA/CA

    CSMA/CA is an abbreviation of Carrier Sense Multiple Access/Collision Avoidance. Multiple access with carrier awareness and collision avoidance, sender receipt of acknowledgement signals is a necessary step in the collision detection process. The data is successfully delivered if there is just one signal (its own), but a collision has occurred if there are two signals (its own and that with which it collided). The collision must significantly affect the received signal in order to discriminate between these two scenarios. However, this is not the case in wired networks, which is why CSMA/CA is employed here.

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