Random Access For Mac Layer
2021年2月9日Download here: http://gg.gg/o94zn
In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. Within the data link layer, the LLC provides flow control and multiplexing for the logical link (i.e. EtherType, 802.1Q VLAN tag etc), while the MAC provides flow control and multiplexing for the transmission medium.
*Random Mac Address
*Random Access For Mac Layers
This module discusses the need for medium access control (MAC), and introduces representative random access and scheduling MAC protocols - including the carrier-sense multiple access with collision detection CSMA-CS protocol which forms the basis for the Ethernet LAN standard. If the S-MAC determines to continue the S-RA, S-MAC may reselect the random access resource and instruct again to S-PHY to transmit the preamble after predetermined backoff time. If the S-MAC determines to stop the S-RA, S-MAC may discard the random access resource and flush the HARQ buffer for random access procedure.
These two sublayers together correspond to layer 2 of the OSI model. For compatibility reasons, LLC is optional for implementations of IEEE 802.3 (the frames are then ’raw’), but compulsory for implementations of other IEEE 802 physical layer standards. Within the hierarchy of the OSI model and IEEE 802 standards, the MAC sublayer provides a control abstraction of the physical layer such that the complexities of physical link control are invisible to the LLC and upper layers of the network stack. Thus any LLC sublayer (and higher layers) may be used with any MAC. In turn, the medium access control block is formally connected to the PHY via a media-independent interface. Although the MAC block is today typically integrated with the PHY within the same device package, historically any MAC could be used with any PHY, independent of the transmission medium.
When sending data to another device on the network, the MAC sublayer encapsulates higher-level frames into frames appropriate for the transmission medium (i.e. the MAC adds a syncword preamble and also padding if necessary), adds a frame check sequence to identify transmission errors, and then forwards the data to the physical layer as soon as the appropriate channel access method permits it. For topologies with a collision domain (bus, ring, mesh, point-to-multipoint topologies), controlling when data is sent and when to wait is necessary to avoid collisions. Additionally, the MAC is also responsible for compensating for collisions by initiating retransmission if a jam signal is detected. When receiving data from the physical layer, the MAC block ensures data integrity by verifying the sender’s frame check sequences, and strips off the sender’s preamble and padding before passing the data up to the higher layers.Functions performed in the MAC sublayer[edit]
According to IEEE Std 802-2001 section 6.2.3 ’MAC sublayer’, the primary functions performed by the MAC layer are:[1]
*Frame delimiting and recognition
*Addressing of destination stations (both as individual stations and as groups of stations)
*Conveyance of source-station addressing information
*Transparent data transfer of LLC PDUs, or of equivalent information in the Ethernet sublayer
*Protection against errors, generally by means of generating and checking frame check sequences
*Control of access to the physical transmission medium
In the case of Ethernet, the functions required of a MAC are:[2]
*receive/transmit normal frames
*half-duplex retransmission and backoff functions
*append/check FCS (frame check sequence)
*interframe gap enforcement
*discard malformed frames
*prepend(tx)/remove(rx) preamble, SFD (start frame delimiter), and padding
*half-duplex compatibility: append(tx)/remove(rx) MAC addressAddressing mechanism[edit]
The local network addresses used in IEEE 802 networks and FDDI networks are called media access control addresses; they are based on the addressing scheme that was used in early Ethernet implementations. A MAC address is intended as a unique serial number. MAC addresses are typically assigned to network interface hardware at the time of manufacture. The most significant part of the address identifies the manufacturer, who assigns the remainder of the address, thus provide a potentially unique address. This makes it possible for frames to be delivered on a network link that interconnects hosts by some combination of repeaters, hubs, bridges and switches, but not by network layerrouters. Thus, for example, when an IP packet reaches its destination (sub)network, the destination IP address (a layer 3 or network layer concept) is resolved with the Address Resolution Protocol for IPv4, or by Neighbor Discovery Protocol (IPv6) into the MAC address (a layer 2 concept) of the destination host.
Examples of physical networks are Ethernet networks and Wi-Fi networks, both of which are IEEE 802 networks and use IEEE 802 48-bit MAC addresses.
A MAC layer is not required in full-duplexpoint-to-point communication, but address fields are included in some point-to-point protocols for compatibility reasons.Channel access control mechanism[edit]
The channel access control mechanisms provided by the MAC layer are also known as a multiple access method. This makes it possible for several stations connected to the same physical medium to share it. Examples of shared physical media are bus networks, ring networks, hub networks, wireless networks and half-duplex point-to-point links. The multiple access method may detect or avoid data packet collisions if a packet mode contention based channel access method is used, or reserve resources to establish a logical channel if a circuit-switched or channelization-based channel access method is used. The channel access control mechanism relies on a physical layer multiplex scheme.
The most widespread multiple access method is the contention-based CSMA/CD used in Ethernet networks. This mechanism is only utilized within a network collision domain, for example an Ethernet bus network or a hub-based star topology network. An Ethernet network may be divided into several collision domains, interconnected by bridges and switches.
A multiple access method is not required in a switched full-duplex network, such as today’s switched Ethernet networks, but is often available in the equipment for compatibility reasons.Channel access control mechanism for concurrent transmission[edit]
Use of directional antennas and millimeter-wave communication in a wireless personal area network increases the probability of concurrent scheduling of non‐interfering transmissions in a localized area, which results in an immense increase in network throughput. However, the optimum scheduling of concurrent transmission is an NP-hard problem.[3]Cellular networks[edit]
Cellular networks, such as GSM, UMTS or LTE networks, also use a MAC layer. The MAC protocol in cellular networks is designed to maximize the utilization of the expensive licensed spectrum.[4] The air interface of a cellular network is at layers 1 and 2 of the OSI model; at layer 2, it is divided into multiple protocol layers. In UMTS and LTE, those protocols are the Packet Data Convergence Protocol (PDCP), the Radio Link Control (RLC) protocol, and the MAC protocol. The base station has absolute control over the air interface and schedules the downlink access as well as the uplink access of all devices. The MAC protocol is specified by 3GPP in TS 25.321[5] for UMTS, TS 36.321[6] for LTE and TS 38.321[7] for 5G New Radio (NR).See also[edit]
*MACsec (IEEE 802.1AE)References[edit]
*^’IEEE 802-2001 (R2007) IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture’(PDF). IEEE.
*^’4.1.4’, IEEE 802.3-2002, IEEE
*^Bilal, Muhammad; et al. (2014). ’Time‐Slotted Scheduling Schemes for Multi‐hop Concurrent Transmission in WPANs with Directional Antenna’. ETRI Journal. 36 (3): 374–384. arXiv:1801.06018. doi:10.4218/etrij.14.0113.0703.
*^Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN978-1107143210.
*^3GPP TS 25.321 Medium Access Control (MAC) protocol specification
*^3GPP TS 36.321 Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification
*^3GPP TS 38.321 NR; Medium Access Control (MAC) protocol specificationRetrieved from ’https://en.wikipedia.org/w/index.php?title=Medium_access_control&oldid=1001405903’
Random Access Protocols is a Multiple access protocol that is divided into four categories which are ALOHA, CSMA, CSMA/CD, and CSMA/CA. In this article, we will cover all of these Random Access Protocols in detail.
Have you ever been to a railway station? And noticed the ticket counter over there?
Above are the scenarios for approaching a ticket counter. Which one do you think is more productive? The ordered one, right? And we all know the reason why. Just to get things working and avoid problems we have some rules or protocols, like ’please stand in the queue’, ’do not push each other’, ’wait for your turn’, etc. in the same way computer network channels also have protocols like multiple access protocols, random access protocols, etc.
Let’s say you are talking to your friend using a mobile phone. This means there is a link established between you and him. But the point to be remembered is that the communication channel between you and him (the sender & the receiver or vice-versa) is not always a dedicated link, which means the channels are not only providing service to you at that time but to others as well. This means multiple users might be communicating through the same channel.
How is that possible? The reason behind this is the multiple access protocols. If you refer to the OSI model you will come across the data link layer. Now divide the layers into 2 parts, the upper part of the layer will take care of the data link control, and the lower half will be taking care in resolving the access to the shared media, as shown in the above diagram.
The following diagram classifies the multiple-access protocol. In this article, we are going to cover Random Access Protocol.Random Access Protocols
Once again, let’s use the example of mobile phone communication. Whenever you call someone, a connection between you and the desired person is established, also anyone can call anyone. So here we have all the users (stations) at an equal priority, where any station can send data depending on medium’s state whether it is idle or busy, meaning that if you friend is talking to someone else through the mobile phone, then its status is busy and you cannot establish a connection and since all the users are assigned equal priority you can not disconnect your friend’s ongoing call and connect yours.
The random access protocols consist of the following characteristics:
*
There is no time restriction for sending the data (you can talk to your friend without a time restriction).
*
There is a fixed sequence of stations which are transmitting the data.
As in the above diagram you might have observed that the random-access protocol is further divided into four categories, which are:
*
ALOHA
*
CSMA
*
CSMA/CD
*
CSMA/CA
Let’s cover each one of them, one by one.ALOHA Random Access Protocol
The ALOHA protocol or also known as the ALOHA method is a simple communication scheme in which every transmitting station or source in a network will send the data whenever a frame is available for transmission. If we succeed and the frame reaches its destination, then the next frame is lined-up for transmission. But remember, if the data frame is not received by the receiver (maybe due to collision) then the frame is sent again until it successfully reaches the receiver’s end.
Whenever we talk about a wireless broadcast system or a half-duplex two-way link, the ALOHA method works efficiently. But as the network becomes more and more complex e.g. the ethernet. Now here in the ethernet, the system involves multiple sources and destinations which share a common data path or channel, then the conflict occurs because cellpadding=’10’ cellspacing=’0’>
PURE ALOHA
SLOTTED ALOHA
Data transmission
Stations can transmit the data randomly i.e. any number of stations can transmit data at any time.
here, any random station can transmit the data at the beginning of any random time slot
Time status
Here, the time is continuous and is not globally synchronized with any other station.
Here, the time is discrete unlike pure ALOHA and is also globally synchronized
Vulnerable time
2*Frame transmission time
Frame transmission time
Probability of successful transmission of a data packet
G*e-2G
where, G = no. of stations willing to transmit data
G*e-G
Maximum efficiency
18.4%
36.8%
Collision status
It does not reduce the total number of collisions to halfRandom Mac Address
Here, it reduces the total number of collisions to half and doubles the efficiency of pure ALOHACSMA Random Access Protocol
CSMA stands for Carrier Sense Multiple Access. Till now we have understood that when 2 or more stations start sending data, then a collision occurs, so this CSMA method was developed to decrease the chances of collisions when 2 or more stations start sending their signals over the data link layer. But how do they do it? The CSMA makes each station to first check the medium (whether it is busy or not) before sending any data packet.
Here, Vulnerable time = Propagation Time
But, what to do if the channels are busy? Now, here the persistence methods can be applied to help the station act when the channel is busy or idle.
The CSMA has 4 access modes:
*
1-persistent mode: In this, first the node checks the channel, if the channel is idle then the node or station transmits data, otherwise it keeps on waiting and whenever the channel is idle, the stations transmit the data-frame.
*
Non-persistent mode: In this, the station checks the channel similarly as 1-persistent mode, but the only difference is that when the channel is busy it checks it again after a random amount of time, unlike the 1-persistent where the stations keep on checking continuously.
*
P-persistent mode: In this, the station checks the channel and if found idle then it transmits the data frame with the probability of P and if the data is not transmitted (1-P) then the station waits for a random amount of time and again transmits the data with the probability P and this cycle goes on continuously until the data-frame is successfully sent.
*
O-persistent: In this, the transmission occurs based on the superiority of stations which is decided beforehand and transmission occurs in that order. If the channel is idle, then the station waits for its turn to send the data-frame.Throughput & Efficiency of CSMA:
It is comparatively much greater than the throughput of pure and slotted ALOHA. Here, for the 1-persistent mode, the throughput is 50% when G=1 and for Non-persistent mode, the throughput can reach up to 90%.CSMA/CD Random Access Protocol
CSMA/CD means CSMA with Collision Detection.
In this, whenever station transmits data-frame it then monitors the channel or the medium to acknowledge the state of the transmission i.e. successfully transmitted or failed. If the transmission succeeds, then it prepares for the next frame otherwise it resends the previously failed data-frame. The point to remember here is, that the frame transmission time should be at least twice the maximum propagation time, which can be deduced when the distance between the two stations involved in a collision is maximum.CSMA/CA Random Access Protocol
CSMA/CA means CSMA with collision avoidance.
To detect the possible collisions, the sender receives the acknowledgement and if there is only one acknowledgment present (it’s own) then this means that the data-frame has been sent successfully. But, if there are 2 or more acknowledgment signals then this indicates that the collision has occurred.
This method avoids collisions by:Random Access For Mac Layers
*
Interframe space: in this case, assume that your station waits for the channel to become idle and found that the channel is idle, then it will not send the data-frame immediately (in order to avoid collision due to propagation delay) it rather waits for some time called interframe space or IFS, and after this time the station again checks the medium for being idle. But it should be kept in mind that the IFS duration depends on the priority of the station.
*
Contention Window: here, the time is divided into slots. Say, if the sender is ready for transmission of the data, it then chooses a random number of slots as waiting time which doubles every time whenever the channel is busy. But, if the channel is not idle at that moment, then it does not restart the entire process but restarts the timer when the channel is found idle again.
*
Acknowledgment: as we discussed above that the sender station will re-transmits the data if acknowledgment is not received before the timer expires.
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https://diarynote.indered.space
In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublayer and the logical link control (LLC) sublayer together make up the data link layer. Within the data link layer, the LLC provides flow control and multiplexing for the logical link (i.e. EtherType, 802.1Q VLAN tag etc), while the MAC provides flow control and multiplexing for the transmission medium.
*Random Mac Address
*Random Access For Mac Layers
This module discusses the need for medium access control (MAC), and introduces representative random access and scheduling MAC protocols - including the carrier-sense multiple access with collision detection CSMA-CS protocol which forms the basis for the Ethernet LAN standard. If the S-MAC determines to continue the S-RA, S-MAC may reselect the random access resource and instruct again to S-PHY to transmit the preamble after predetermined backoff time. If the S-MAC determines to stop the S-RA, S-MAC may discard the random access resource and flush the HARQ buffer for random access procedure.
These two sublayers together correspond to layer 2 of the OSI model. For compatibility reasons, LLC is optional for implementations of IEEE 802.3 (the frames are then ’raw’), but compulsory for implementations of other IEEE 802 physical layer standards. Within the hierarchy of the OSI model and IEEE 802 standards, the MAC sublayer provides a control abstraction of the physical layer such that the complexities of physical link control are invisible to the LLC and upper layers of the network stack. Thus any LLC sublayer (and higher layers) may be used with any MAC. In turn, the medium access control block is formally connected to the PHY via a media-independent interface. Although the MAC block is today typically integrated with the PHY within the same device package, historically any MAC could be used with any PHY, independent of the transmission medium.
When sending data to another device on the network, the MAC sublayer encapsulates higher-level frames into frames appropriate for the transmission medium (i.e. the MAC adds a syncword preamble and also padding if necessary), adds a frame check sequence to identify transmission errors, and then forwards the data to the physical layer as soon as the appropriate channel access method permits it. For topologies with a collision domain (bus, ring, mesh, point-to-multipoint topologies), controlling when data is sent and when to wait is necessary to avoid collisions. Additionally, the MAC is also responsible for compensating for collisions by initiating retransmission if a jam signal is detected. When receiving data from the physical layer, the MAC block ensures data integrity by verifying the sender’s frame check sequences, and strips off the sender’s preamble and padding before passing the data up to the higher layers.Functions performed in the MAC sublayer[edit]
According to IEEE Std 802-2001 section 6.2.3 ’MAC sublayer’, the primary functions performed by the MAC layer are:[1]
*Frame delimiting and recognition
*Addressing of destination stations (both as individual stations and as groups of stations)
*Conveyance of source-station addressing information
*Transparent data transfer of LLC PDUs, or of equivalent information in the Ethernet sublayer
*Protection against errors, generally by means of generating and checking frame check sequences
*Control of access to the physical transmission medium
In the case of Ethernet, the functions required of a MAC are:[2]
*receive/transmit normal frames
*half-duplex retransmission and backoff functions
*append/check FCS (frame check sequence)
*interframe gap enforcement
*discard malformed frames
*prepend(tx)/remove(rx) preamble, SFD (start frame delimiter), and padding
*half-duplex compatibility: append(tx)/remove(rx) MAC addressAddressing mechanism[edit]
The local network addresses used in IEEE 802 networks and FDDI networks are called media access control addresses; they are based on the addressing scheme that was used in early Ethernet implementations. A MAC address is intended as a unique serial number. MAC addresses are typically assigned to network interface hardware at the time of manufacture. The most significant part of the address identifies the manufacturer, who assigns the remainder of the address, thus provide a potentially unique address. This makes it possible for frames to be delivered on a network link that interconnects hosts by some combination of repeaters, hubs, bridges and switches, but not by network layerrouters. Thus, for example, when an IP packet reaches its destination (sub)network, the destination IP address (a layer 3 or network layer concept) is resolved with the Address Resolution Protocol for IPv4, or by Neighbor Discovery Protocol (IPv6) into the MAC address (a layer 2 concept) of the destination host.
Examples of physical networks are Ethernet networks and Wi-Fi networks, both of which are IEEE 802 networks and use IEEE 802 48-bit MAC addresses.
A MAC layer is not required in full-duplexpoint-to-point communication, but address fields are included in some point-to-point protocols for compatibility reasons.Channel access control mechanism[edit]
The channel access control mechanisms provided by the MAC layer are also known as a multiple access method. This makes it possible for several stations connected to the same physical medium to share it. Examples of shared physical media are bus networks, ring networks, hub networks, wireless networks and half-duplex point-to-point links. The multiple access method may detect or avoid data packet collisions if a packet mode contention based channel access method is used, or reserve resources to establish a logical channel if a circuit-switched or channelization-based channel access method is used. The channel access control mechanism relies on a physical layer multiplex scheme.
The most widespread multiple access method is the contention-based CSMA/CD used in Ethernet networks. This mechanism is only utilized within a network collision domain, for example an Ethernet bus network or a hub-based star topology network. An Ethernet network may be divided into several collision domains, interconnected by bridges and switches.
A multiple access method is not required in a switched full-duplex network, such as today’s switched Ethernet networks, but is often available in the equipment for compatibility reasons.Channel access control mechanism for concurrent transmission[edit]
Use of directional antennas and millimeter-wave communication in a wireless personal area network increases the probability of concurrent scheduling of non‐interfering transmissions in a localized area, which results in an immense increase in network throughput. However, the optimum scheduling of concurrent transmission is an NP-hard problem.[3]Cellular networks[edit]
Cellular networks, such as GSM, UMTS or LTE networks, also use a MAC layer. The MAC protocol in cellular networks is designed to maximize the utilization of the expensive licensed spectrum.[4] The air interface of a cellular network is at layers 1 and 2 of the OSI model; at layer 2, it is divided into multiple protocol layers. In UMTS and LTE, those protocols are the Packet Data Convergence Protocol (PDCP), the Radio Link Control (RLC) protocol, and the MAC protocol. The base station has absolute control over the air interface and schedules the downlink access as well as the uplink access of all devices. The MAC protocol is specified by 3GPP in TS 25.321[5] for UMTS, TS 36.321[6] for LTE and TS 38.321[7] for 5G New Radio (NR).See also[edit]
*MACsec (IEEE 802.1AE)References[edit]
*^’IEEE 802-2001 (R2007) IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture’(PDF). IEEE.
*^’4.1.4’, IEEE 802.3-2002, IEEE
*^Bilal, Muhammad; et al. (2014). ’Time‐Slotted Scheduling Schemes for Multi‐hop Concurrent Transmission in WPANs with Directional Antenna’. ETRI Journal. 36 (3): 374–384. arXiv:1801.06018. doi:10.4218/etrij.14.0113.0703.
*^Guowang Miao; Jens Zander; Ki Won Sung; Ben Slimane (2016). Fundamentals of Mobile Data Networks. Cambridge University Press. ISBN978-1107143210.
*^3GPP TS 25.321 Medium Access Control (MAC) protocol specification
*^3GPP TS 36.321 Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification
*^3GPP TS 38.321 NR; Medium Access Control (MAC) protocol specificationRetrieved from ’https://en.wikipedia.org/w/index.php?title=Medium_access_control&oldid=1001405903’
Random Access Protocols is a Multiple access protocol that is divided into four categories which are ALOHA, CSMA, CSMA/CD, and CSMA/CA. In this article, we will cover all of these Random Access Protocols in detail.
Have you ever been to a railway station? And noticed the ticket counter over there?
Above are the scenarios for approaching a ticket counter. Which one do you think is more productive? The ordered one, right? And we all know the reason why. Just to get things working and avoid problems we have some rules or protocols, like ’please stand in the queue’, ’do not push each other’, ’wait for your turn’, etc. in the same way computer network channels also have protocols like multiple access protocols, random access protocols, etc.
Let’s say you are talking to your friend using a mobile phone. This means there is a link established between you and him. But the point to be remembered is that the communication channel between you and him (the sender & the receiver or vice-versa) is not always a dedicated link, which means the channels are not only providing service to you at that time but to others as well. This means multiple users might be communicating through the same channel.
How is that possible? The reason behind this is the multiple access protocols. If you refer to the OSI model you will come across the data link layer. Now divide the layers into 2 parts, the upper part of the layer will take care of the data link control, and the lower half will be taking care in resolving the access to the shared media, as shown in the above diagram.
The following diagram classifies the multiple-access protocol. In this article, we are going to cover Random Access Protocol.Random Access Protocols
Once again, let’s use the example of mobile phone communication. Whenever you call someone, a connection between you and the desired person is established, also anyone can call anyone. So here we have all the users (stations) at an equal priority, where any station can send data depending on medium’s state whether it is idle or busy, meaning that if you friend is talking to someone else through the mobile phone, then its status is busy and you cannot establish a connection and since all the users are assigned equal priority you can not disconnect your friend’s ongoing call and connect yours.
The random access protocols consist of the following characteristics:
*
There is no time restriction for sending the data (you can talk to your friend without a time restriction).
*
There is a fixed sequence of stations which are transmitting the data.
As in the above diagram you might have observed that the random-access protocol is further divided into four categories, which are:
*
ALOHA
*
CSMA
*
CSMA/CD
*
CSMA/CA
Let’s cover each one of them, one by one.ALOHA Random Access Protocol
The ALOHA protocol or also known as the ALOHA method is a simple communication scheme in which every transmitting station or source in a network will send the data whenever a frame is available for transmission. If we succeed and the frame reaches its destination, then the next frame is lined-up for transmission. But remember, if the data frame is not received by the receiver (maybe due to collision) then the frame is sent again until it successfully reaches the receiver’s end.
Whenever we talk about a wireless broadcast system or a half-duplex two-way link, the ALOHA method works efficiently. But as the network becomes more and more complex e.g. the ethernet. Now here in the ethernet, the system involves multiple sources and destinations which share a common data path or channel, then the conflict occurs because cellpadding=’10’ cellspacing=’0’>
PURE ALOHA
SLOTTED ALOHA
Data transmission
Stations can transmit the data randomly i.e. any number of stations can transmit data at any time.
here, any random station can transmit the data at the beginning of any random time slot
Time status
Here, the time is continuous and is not globally synchronized with any other station.
Here, the time is discrete unlike pure ALOHA and is also globally synchronized
Vulnerable time
2*Frame transmission time
Frame transmission time
Probability of successful transmission of a data packet
G*e-2G
where, G = no. of stations willing to transmit data
G*e-G
Maximum efficiency
18.4%
36.8%
Collision status
It does not reduce the total number of collisions to halfRandom Mac Address
Here, it reduces the total number of collisions to half and doubles the efficiency of pure ALOHACSMA Random Access Protocol
CSMA stands for Carrier Sense Multiple Access. Till now we have understood that when 2 or more stations start sending data, then a collision occurs, so this CSMA method was developed to decrease the chances of collisions when 2 or more stations start sending their signals over the data link layer. But how do they do it? The CSMA makes each station to first check the medium (whether it is busy or not) before sending any data packet.
Here, Vulnerable time = Propagation Time
But, what to do if the channels are busy? Now, here the persistence methods can be applied to help the station act when the channel is busy or idle.
The CSMA has 4 access modes:
*
1-persistent mode: In this, first the node checks the channel, if the channel is idle then the node or station transmits data, otherwise it keeps on waiting and whenever the channel is idle, the stations transmit the data-frame.
*
Non-persistent mode: In this, the station checks the channel similarly as 1-persistent mode, but the only difference is that when the channel is busy it checks it again after a random amount of time, unlike the 1-persistent where the stations keep on checking continuously.
*
P-persistent mode: In this, the station checks the channel and if found idle then it transmits the data frame with the probability of P and if the data is not transmitted (1-P) then the station waits for a random amount of time and again transmits the data with the probability P and this cycle goes on continuously until the data-frame is successfully sent.
*
O-persistent: In this, the transmission occurs based on the superiority of stations which is decided beforehand and transmission occurs in that order. If the channel is idle, then the station waits for its turn to send the data-frame.Throughput & Efficiency of CSMA:
It is comparatively much greater than the throughput of pure and slotted ALOHA. Here, for the 1-persistent mode, the throughput is 50% when G=1 and for Non-persistent mode, the throughput can reach up to 90%.CSMA/CD Random Access Protocol
CSMA/CD means CSMA with Collision Detection.
In this, whenever station transmits data-frame it then monitors the channel or the medium to acknowledge the state of the transmission i.e. successfully transmitted or failed. If the transmission succeeds, then it prepares for the next frame otherwise it resends the previously failed data-frame. The point to remember here is, that the frame transmission time should be at least twice the maximum propagation time, which can be deduced when the distance between the two stations involved in a collision is maximum.CSMA/CA Random Access Protocol
CSMA/CA means CSMA with collision avoidance.
To detect the possible collisions, the sender receives the acknowledgement and if there is only one acknowledgment present (it’s own) then this means that the data-frame has been sent successfully. But, if there are 2 or more acknowledgment signals then this indicates that the collision has occurred.
This method avoids collisions by:Random Access For Mac Layers
*
Interframe space: in this case, assume that your station waits for the channel to become idle and found that the channel is idle, then it will not send the data-frame immediately (in order to avoid collision due to propagation delay) it rather waits for some time called interframe space or IFS, and after this time the station again checks the medium for being idle. But it should be kept in mind that the IFS duration depends on the priority of the station.
*
Contention Window: here, the time is divided into slots. Say, if the sender is ready for transmission of the data, it then chooses a random number of slots as waiting time which doubles every time whenever the channel is busy. But, if the channel is not idle at that moment, then it does not restart the entire process but restarts the timer when the channel is found idle again.
*
Acknowledgment: as we discussed above that the sender station will re-transmits the data if acknowledgment is not received before the timer expires.
Download here: http://gg.gg/o94zn
https://diarynote.indered.space
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