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Multiple Access Protocols
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So far, we have assumed that the end-to-end network (consisting of source and destination) is nothing more than point-to-point d. H.PPP (private) link. However, imagine a situation where a broadcast network is required (eal with multiple participating hosts in a cellular network). In this case, we need to look at media access control (Aloha and CSMA methods) for efficient networking. Therefore, we assume that there is a dedicated connection (or channel) available between the sender and receiver. Now conser a case where we are using our phone to connect to another phone where the channel (allocated frequency band) is not dedicated (and distributed among many members). Someone standing a few feet away from us may be using the same channel to talk to their friends. So we do need proprietary protocols (at the data link layer) to implement this kind of scenario in real life.
What is Media Access Control ?
In broadcast networks, each communication channel must be dedicated to one broadcast user. Other users connected to this medium should wait. The data link layer is dived into two function-oriented sublayers, namely H. The upper layer is mainly responsible for controlling data connections, and the lower layer is responsible for parsing access to shared media. Using a dedicated link (PPP network), the lower layers are omitted. When nodes or stations are connected and share a common link called a broadcast link, we need a multiple access protocol to coordinate access to the link. Let’s look at a more realistic case. Imagine you are in a meeting. We don’t want other people to start talking at the same time, we don’t want others to interrupt or interrupt us while we’re talking, etc. The same problem occurs during transmission in broadcast connection. Therefore, in order to maintain the desired efficiency in the network, we need to understand the specific media access control (also known as media access control) protocol implemented in the DLL. When multiple stations (users) use the common link of a communication system, we need to use multiple access protocols to coordinate access to the common link. Three techniques for dealing with multiple access problems are as follows: Random access method. Controlled Access. gue.
Random Access in Media Access Control (MAC)
In direct access technology, there is no control station. Every station has the right to use public media without any control. As the number of sites increases, so does the probability of conflicts or access violations. Conflicts occur when multiple users attempt to access shared media at the same time. Packets may be modified or completely corrupted due to possible collisions. In any case, each station that has data to send uses a procedure defined by the protocol to dece whether to send or not. This decision also depends on the state of the medium (le or busy). Sites compete with each other to access media. With the random access method, each station has the right to the medium (channel) without the control of other stations. The following are random access methods: Aloha (pure Aloha). Slotted Aloha. CSMA. CSMA-CD. CSMA – Approx.
Pure Aloha in random access (meaning ‘hello’ in Hawaii)
Its main principle is that any sender can broadcast at any time. When two signals colle, each station simply waits a random amount of time and tries again. Collisions are easily detected, when the central station receives a frame, it sends an acknowledgment on a different frequency. When the subscriber station receives the acknowledgment, it assumes that the transmitted frame was successfully received, and when it receives the acknowledgment, it assumes a collision and is ready to send again. Obviously, we need to resend frames that were corrupted in transit. The pure ALOHA protocol mainly relies on the confirmation of the receiver. Pure ALOHA specifies that after the timeout period expires, each station waits a random amount of time before retransmitting its frame. Randomness helps avo more collisions. We call this time the backoff time, i. H. Tb. Timeout equals the maximum possible round-trip propagation delay, which is twice the time required to send a frame between the two farthest stations. Algorithm used: transmit a frame and wait for acknowledgment. If an acknowledgment is received (i.e. no collision), go to step 1 to retransmit. Now let’s understand the concept of fragile time in Aloha. Fragile time is nothing but a time when a collision is possible. We assume that the station sends fixed length frames and each frame needs to be sent with “Tf”. Vulnerability time, i.e. the duration of a possible collision in pure ALOHA, is twice the frame transmission time. Now let us understand the concept of Slotted Aloha.
Slotted Aloha in Random Access
Slotted Aloha was introduced mainly to improve the efficiency of pure Aloha. As we have already seen, pure Aloha has a fragile time of 2 x Tf. Because there is basically no rule about when a station is allowed to transmit. In Slotted ALOHA, we dive time into slots of “Tf” seconds and force stations to transmit only at the beginning of the slot. If we conser the frame transfer time as “T”. A collision occurs when any part of two transmissions overlap. Assume “T” is the time required for transmission, and both stations need to transmit. The total time required for both stations to successfully complete this operation is 2T. In the case of pure ALOHA, allowing a site to transmit at any time can waste up to 2T of time. Alternatively, the slotted ALOHA scheme dives time into intervals (slots) of “T” units each, and requires each station to start each transmission at the beginning of the slot. In other words, even if a station is ready to transmit in the mdle of a time slot, it must wait for the start of the next time slot. Using this method, a collision occurs when both stations are ready in the same time slot. The vulnerable period is reduced to half that of pure ALOHA. The throughput efficiency of slotted ALOHA is twice that of a pure ALOHA system. Increasing the frame length or reducing the runtime can improve utilization.
Carrier Sense Multiple Access (CSMA) in Medium Sharing Technique
The working principle of the CSMA protocol is carrier detection. in this log. A station first listens for the presence of a transmission (carrier) on the cable and deces to respond accordingly. If the site gets the media before trying to use it, it reduces the chance of a conflict. Carrier Sense Multiple Access (CSMA) requires each station to first listen to the medium (or check the state of the medium) before transmitting. This reduces the chance of collisions by capturing the media before sending any of the frames in it. There is still the possibility of collisions due to propagation delays. When a station sends a frame, it still takes time (albeit very briefly) for the first bit to arrive (and capture) at each station. Now let’s look at three types of CSMA: Non-Persistent CSMA: In this scheme, when a station wants to send a frame and finds that the channel is busy (another station is sending), it waits a fixed amount of time interval. After this time, it checks the status of the channel again and sends when the channel is le. Persistent CSMA: With this scheme, the station wishing to transmit continuously monitors the channel until the channel is clear, and then transmits immediately. The disadvantage of this strategy is that if both stations are waiting, they will transmit at the same time and a collision will occur. This then requires a retransmission. P – Persistent CSMA: This possibility of collisions and retransmissions is reduced in p·persistent CSMA. Using this scheme, once the channel is free, all waiting stations are not allowed to transmit simultaneously. Suppose a station transmits with probability “p”. For example, if p = 1/5 and 5 stations are waiting, on average only one station will transmit and the others will wait.
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Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
The problem with CSMA discussed earlier is that the sending station continues to send its frames even if a collision occurs. This wastes channel time. In CSMA/CD, a specific collision can be detected if a station receives other transmissions while it is transmitting. The CSMA program does not specify what to do after a collision. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) improves collision handling algorithms. In this method, the station first monitors the medium after sending a frame to see if the transmission was successful. If so, the station is complete. If a collision occurs, the frame is resent. Once a collision is detected, the transmitting station sends out a jamming signal. Traffic jam signals alert other stations. Then, the station should not transmit immediately after the collision. After a certain “return” delay time, the station repeats the transmission again. In the event of another collision, the backoff time will be further increased. In order for CSMA/CD to work, we need a certain frame size limit. Before sending the last bit of the frame, the sending station must detect a possible collision and then abort the transmission.
Carrier Sense Multiple Access/Collision Avoance (CSMA/CA)
We need to avo collisions on wireless networks because they cannot be detected. Carrier Sense Multiple Access with Collision Avoance (CSMA/CA) is introduced for this network. Collision is avoed by using three strategies of CSMA/CA, namely H. Interframe space, contention windows, and acknowledgements. In the event of a collision, the station receives two signals, ie. H. Their own signals and signals are sent by the second station. In order to distinguish the two cases, the received signal must be significantly different in the two cases. Finally, we come to the end of this long article. Hope you like it. Stay tuned to learn more. report this ad
How is media access controlled using Aloha?
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.
What is Aloha CSMA?
It is a carrier sense multiple access based on media access protocol to sense the traffic on a channel (idle or busy) before transmitting the data. It means that if the channel is idle, the station can send data to the channel. Otherwise, it must wait until the channel becomes idle.
What are the different protocols used for media access control?
- Carrier sense multiple access with collision avoidance (CSMA/CA)
- Carrier sense multiple access with collision detection (CSMA/CD)
- Demand priority.
- Token passing.
What are different types of CSMA protocols?
- Persistent CSMA. In this method, station that wants to transmit data continuously senses the channel to check whether the channel is idle or busy. …
- Non-Persistent CSMA. …
- P-Persistent CSMA. …
- CSMA/CD.
What is ALOHA and types of ALOHA?
ALOHA is a medium access control (MAC) protocol for transmission of data via a shared network channel. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel. There are two types of ALOHA protocols – Pure ALOHA and Slotted ALOHA.
What are the two types of MAC protocols?
The classification of MAC protocols or types of MAC protocol or types of MAC are: Contention based protocols without reservation. Contention based protocols with reservation.
What is difference between Aloha and CSMA?
Main difference between Aloha and CSMA is that Aloha protocol does not try to detect whether the channel is free before transmitting but the CSMA protocol verifies that the channel is free before transmitting data.
How many types of Aloha are there?
Aloha is the type of Random access protocol, It have two types one is Pure Aloha and another is Slotted Aloha. In Pure Aloha, Stations transmit whenever data is available at arbitrary times and Colliding frames are destroyed.
Which is better Aloha or CSMA?
For slotted Aloha, the key optimization parameter is the medium access probability, for non-slotted Aloha we tune the mean back-off time, whereas for CSMA it is the carrier sense threshold that is adjusted. Our study shows that CSMA always outperforms slotted Aloha, which in turn outperforms its non-slotted version.
What is meant by MAC protocol?
Media access control (MAC) protocols enforce a methodology to allow multiple devices access to a shared media network. Before LANs, communication between computing devices had been point-to-point. That is, two devices were connected by a dedicated channel.
Why CSMA is extensively used in many MAC protocols?
CSMA gives equal transmission opportunities. A node sending at 10Mbps gets 10 times less time than a node sending at 1 Mbps. The network is limited by the slowest node. sending at 10Mbps gets 10 times less time than a node sending at 1 Mbps.
Is MAC a protocol?
No, the MAC is not a protocol in that you won’t find any ‘MAC spec’ that you can implement. MACs are typically embedded in hardware devices and expose functionality to send and receive frames to the media that they’re controlling.
What are the advantages of CSMA over ALOHA?
In a moderate path-loss scenario (path-loss exponent equal to 4), without fading and the SINR level required for capture equal to 10, CSMA offers about 2.4 times larger rate of successful transmissions than slotted Aloha and about 3.2 times larger than non-slotted Aloha.
How CSMA can be used for access control?
It enables stations to detect when interference is taking place and to cease transmission immediately, first sending a congestion signal to notify the other stations sharing the medium of the collision so that, if they have anything to transmit, they wait before transmitting.
What are the CSMA access modes?
There are several CSMA access modes: 1-persistent, P-persistent, and O-persistent. 1-persistent is used in CSMA/CD systems, like Ethernet. This mode waits for the medium to be idle, then transmits data. P-persistent is used in CSMA/CA (collision avoidance) systems, like Wi-Fi.
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