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    Media Access Control (MAC layer)

    Definition: Media access control (MAC) and logical link control (LLC) are the sublayers of the data link layer (Layer 2) in OSI Reference Model. 'MAC' is also refer to as MAC layer. It use MAC protocols to provides unique addressing identification and channel access control mechanism for network nodes to communicate with other nodes across a shared channel.

    Media Access Control (MAC layer) – Definition

    By Dinesh Thakur

    Definition: Media access control (MAC) and logical link control (LLC) are the sublayers of the data link layer (Layer 2) in OSI Reference Model. ‘MAC’ is also refer to as MAC layer. It use MAC protocols to provides unique addressing identification and channel access control mechanism for network nodes to communicate with other nodes across a shared channel.

    MAC describes the process that is employed to control the basis on which devices can access the shared network. Some level of control is required to ensure the ability of all devices to access the network within a reasonable period of time, thereby resulting in acceptable access and response times.

    It is also important that some method exists to either detect or avoid data collisions, which are caused by multiple transmissions being placed on the shared medium simultaneously. Media Access Control can be accomplished on either a centralized or decentralized basis, and can be characterized as either deterministic or non-deterministic in nature.

    We’ll be covering the following topics in this tutorial:

    Centralized Control

    A centralized controller polls devises to determine when access and transmission by each station is allowed to occur. Stations transmit when requested to do so, or when a station transmission request is acknowledged and granted. This process of polling requires the passing of control packets, adding overhead and reducing the amount of throughput relative to the raw bandwidth available. Additionally, the failure of the central controller will disrupt the entire network; in such an event, the controller is taken off-line and a backup controller assumes responsibility. Centrally, controlled networks generally employ deterministic access control; Token Ring and FDDl networks are centrally controlled.

    Deterministic Access

    Deterministic access is a media access control convention that allows both the centralized master station and each slaved station to determine the maximum length of time which will pass before access is provided to the network. In other words, each station can be guaranteed the right to communicate within a certain time frame. Additionally, the system administrator can assign access priorities. Deterministic access is also known as noncontentious, because the devices do not contend for access, rather access is controlled on a centralized basis.

    Deterministic access employs token passing. The token, which consists of a specific bit pattern, indicates the status of the network whether it is available or unavailable. The token is generated by a centralized master control station and transmitted across the network. The station in possession of the token is in control of access to the network. It may transmit or may require other stations to respond. After transmitting, the station will pass the token to a successor station in a predetermined sequence while the process is complex and overhead intensive, it yields careful control over the network.

    Deterministic access is especially effective in high-traffic environments where a lack of control would cause chaos in the form of frequent data collisions.

    General characteristics of token-based networks include a high level of access control, which is centralized. Access delay is measured and assured, with priority access being supported. Throughput is very close to raw bandwidth, as data collisions are avoided; throughput also improves under load, although absolute overhead is higher than is the case with non-deterministic access techniques. Deterministic access standards include Token-Passing Ring, IBM Token Ring, and Token-Passing Bus.

    Token-based LAN technologies are somewhat overhead intensive, due to the token passing and management processes. However, they can more than compensate for that fact by virtue of the avoidance of data collisions. Token Ring, for instance comes in 4, 16and 20 Mbps. In each case bandwidth utilization is virtually 100%.

    Non-deterministic Access

    Non-deterministic media access control, places access control responsibilities on the individual stations. This is popularly known as Carrier Sense Multiple Access (CSMA), and is most effective in low-traffic environments. There are two variations, CSMA/CD and CSMA/CA.

    CSMA is a decentralized, contentious media access control method used in Ethernet and other bus oriented LANs. Each of multiple stations, or nodes, must sense the carrier to determine network availability before access to the medium to transmit data: further, each station must monitor the network to determine if a collision has occurred. Collisions render the transmission invalid and require retransmission. In the event of a busy condition, the station will back off the network for a calculated random time interval before attempting subsequent access.

    CSMA is implemented in two standard means, CSMA/CD and CSMA/CA. In either case, latency and throughput degrade under heavy loads of traffic. For example, an Ethernet network running at a theoretical speed of 10Mbps typically provides about 4 to 6 Mbps throughput. While it is less costly than Token Ring networking, it also delivers less efficient use of bandwidth.

    Carrier Sense Multiple Access with Collision Detection (CSMA/CD).This is the most common media access control method used in bus networks. At that point, all devices back off the network, calculating. a random time interval before attempting a retransmission .

    स्रोत : ecomputernotes.com

    Media Access Control: What is it and Overview

    The essence of the media access control protocol is to ensure non-collision and eases the transfer of data packets between two computer terminals

    Access control

    Media Access Control: What is it and General Overview

    The essence of the MAC protocol is to ensure non-collision and eases the transfer of data packets.

    Updated on December 01, 2022

    Written by Bernhard Mehl

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    What is Media Access Control?

    A media access control is a network data transfer policy that determines how data is transmitted between two computer terminals through a network cable. The media access control policy involves sub-layers of the data link layer 2 in the OSI reference model.

    The essence of the MAC protocol is to ensure non-collision and eases the transfer of data packets between two computer terminals. A collision takes place when two or more terminals transmit data/information simultaneously. This leads to a breakdown of communication, which can prove costly for organizations that lean heavily on data transmission.

    Media Access Control Methods

    This network channel through which data is transmitted between terminal nodes to avoid collision has three various ways of accomplishing this purpose. They include:

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

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

    Demand priority Token passing

    Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

    Carrier sense multiple access with collision avoidance (CSMA/CA) is a media access control policy that regulates how data packets are transmitted between two computer nodes. This method avoids collision by configuring each computer terminal to make a signal before transmission. The signal is carried out by the transmitting computer to avoid a collision.

    Multiple access implies that many computers are attempting to transmit data. Collision avoidance means that when a computer node transmitting data states its intention, the other waits at a specific length of time before resending the data.

    CSMA/CA is data traffic regulation is slow and adds cost in having each computer node signal its intention before transmitting data. It used only on Apple networks.

    Want to learn more about the technicalities?

    Check out our Academy for lessons on access control.

    Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

    Carrier sense multiple access with collision detection (CSMA/CD) is the opposite of CSMA/CA. Instead of detecting data to transmit signal intention to prevent a collision, it observes the cable to detect the signal before transmitting.

    Collision detection means that when a collision is detected by the media access control policy, transmitting by the network stations stops at a random length of time before transmitting starts again.

    It is faster than CSMA/CA as it functions in a network station that involves fewer data frames being transmitted. CSMA/CD is not as efficient as CSMA/CA in preventing network collisions. This is because it only detects huge data traffic in the network cable. Huge data traffic increases the possibility of a collision taking place. It is used on the Ethernet network.

    Demand Priority

    The demand priority is an improved version of the Carrier sense multiple access with collision detection (CSMA/CD). This data control policy uses an ‘active hub’ in regulating how a network is accessed. Demand priority requires that the network terminals obtain authorization from the active hub before data can be transmitted.

    Another distinct feature of this MAC control policy is that data can be transmitted between the two network terminals at the same time without collision. In the Ethernet media, demand priority directs that data is transmitted directly to the receiving network terminal.

    Token Passing

    This media access control method uses free token passing to prevent a collision. Only a computer that possesses a free token, which is a small data frame, is authorized to transmit. Transmission occurs from a network terminal that has a higher priority than one with a low priority.

    Token passing flourishes in an environment where a large number of short data frames are transmitted. This media access control policy is highly efficient in avoiding a collision. Possession of the free token is the only key to transmitting data by a network node. Each terminal holds this free token for a specific amount of time if the network with the high priority does not have data to transmit, the token is passed to the adjoining station in the network.

    स्रोत : www.getkisi.com

    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.

    स्रोत : www.geeksforgeeks.org

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