UNIT 4
MAC Protocols
1.1Introduction
ALOHA is a system for which coordinating and arbitrating access to give a shared communication Networks channel. It was developed in the 1970s by Norman Abramson and his colleagues at the University of Hawaii. The original system is used for ground based radio broadcasting, but the system has been implemented for the satellite communication systems.
A shared communication system like ALOHA requires a method for handling collisions that can occur when two or morethen two systems attempt to transmit on the channel at the same time. In the ALOHA system, a node gets transmits whenever data is available for sending. If another node transmits the data at the same time, a collision occurs, and the frames that were transmitted are lost. However, a node can listen to broadcasts on the medium, even on its own, and determine whether the frames were transmitted or not.
Aloha means "Hello". Aloha is a multiple access protocol at the datalink layer and proposes that how multiple terminals access to the medium without interference or collision. In 1972 Roberts developed a protocol that would be increasing the capacity of aloha two fold. The Slotted Aloha protocol involves dividing the time of the interval into discrete slots and each slot interval corresponds to the same time period of one frame. This method requires synchronization between the sending nodes to prevent collisions of the data.
There are two different versions of ALOHA
• In pure ALOHA, the stations transmit frames whenever the data is ready to send.
• When two or more stations transmit data at the same time, there is collision and the frames get destroyed.
• In pure ALOHA, whenever any station transmits a frame, it willexpects the acknowledgement from the receiver end.
• If acknowledgement is not received within a particular time, the station assumes that the frame (or acknowledgement) has been destroyed by the sender.
• If the frame is destroyed because of collision then the station waits for a random amount of time and then sends it again. This waiting time must be random because same frames will collide again and again.
• Therefore pure ALOHA dictates that when time-out period passes away, each station must wait for a random amount of time before sending its frame. This randomness will help to avoid more collisions.
• Figure shows an example of frame collisions in pure ALOHA.
• In fig there are four stations that are contended with one another for access to share the channel. All these stations are transmitting frames one by one. Some of these frames collide because of multiple frames are in contention for the shared channel. Only two frames, frame 1.1 and frame 2.2 survive. All other frames get destroyed.
• Whenever two frames try to occupy the channel at the same time, collision occurs and both will be damaged. If first bit of a new frame overlaps the other with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted again.
• Slotted ALOHA was invented to improve the efficiency of pure ALOHA as there are more chances of collision in pure ALOHA.
• In slotted ALOHA, the time of the shared channel is divided into discrete intervals known as slots.
• The stations can send a frame only at the beginning of the slot and only one frame is sent in each slot.
• In slotted ALOHA, if any of the station is not able to place the frame onto the channel at the beginning of the slot that isit misses the time slot then the station has to wait until the beginning of the next time slot.
• In slotted ALOHA, there is still a possibility of collision if two of the stations try to send at the beginning of the same time slot as shown in fig.
• Slotted ALOHA still has an edge to improve over pure ALOHA as chances of collision are reduced.
References:
- Data Communication & Networking by Forouzan, Tata McGraw Hill.
- Computer Network, 4e, by Andrew S. Tenenbaum, Pearson Education/ PHI.
- Data Communication and Computer Networks, by Prakash C.Gupta, PHI.
- Networking Ali-in-one Desk Reference by Doug Lowe, Wiley Dreamtech
- Computer Networking: A Top-Down Approach featuring the Internet, 3e by James F.Kurose.
- Computer Network by Godbole, Tata McGraw Hill.
- Computer Networking, by Stanford H. Rowe, Marsha L. Schuh
UNIT 4
MAC Protocols
1.1Introduction
ALOHA is a system for which coordinating and arbitrating access to give a shared communication Networks channel. It was developed in the 1970s by Norman Abramson and his colleagues at the University of Hawaii. The original system is used for ground based radio broadcasting, but the system has been implemented for the satellite communication systems.
A shared communication system like ALOHA requires a method for handling collisions that can occur when two or morethen two systems attempt to transmit on the channel at the same time. In the ALOHA system, a node gets transmits whenever data is available for sending. If another node transmits the data at the same time, a collision occurs, and the frames that were transmitted are lost. However, a node can listen to broadcasts on the medium, even on its own, and determine whether the frames were transmitted or not.
Aloha means "Hello". Aloha is a multiple access protocol at the datalink layer and proposes that how multiple terminals access to the medium without interference or collision. In 1972 Roberts developed a protocol that would be increasing the capacity of aloha two fold. The Slotted Aloha protocol involves dividing the time of the interval into discrete slots and each slot interval corresponds to the same time period of one frame. This method requires synchronization between the sending nodes to prevent collisions of the data.
There are two different versions of ALOHA
• In pure ALOHA, the stations transmit frames whenever the data is ready to send.
• When two or more stations transmit data at the same time, there is collision and the frames get destroyed.
• In pure ALOHA, whenever any station transmits a frame, it willexpects the acknowledgement from the receiver end.
• If acknowledgement is not received within a particular time, the station assumes that the frame (or acknowledgement) has been destroyed by the sender.
• If the frame is destroyed because of collision then the station waits for a random amount of time and then sends it again. This waiting time must be random because same frames will collide again and again.
• Therefore pure ALOHA dictates that when time-out period passes away, each station must wait for a random amount of time before sending its frame. This randomness will help to avoid more collisions.
• Figure shows an example of frame collisions in pure ALOHA.
• In fig there are four stations that are contended with one another for access to share the channel. All these stations are transmitting frames one by one. Some of these frames collide because of multiple frames are in contention for the shared channel. Only two frames, frame 1.1 and frame 2.2 survive. All other frames get destroyed.
• Whenever two frames try to occupy the channel at the same time, collision occurs and both will be damaged. If first bit of a new frame overlaps the other with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted again.
• Slotted ALOHA was invented to improve the efficiency of pure ALOHA as there are more chances of collision in pure ALOHA.
• In slotted ALOHA, the time of the shared channel is divided into discrete intervals known as slots.
• The stations can send a frame only at the beginning of the slot and only one frame is sent in each slot.
• In slotted ALOHA, if any of the station is not able to place the frame onto the channel at the beginning of the slot that isit misses the time slot then the station has to wait until the beginning of the next time slot.
• In slotted ALOHA, there is still a possibility of collision if two of the stations try to send at the beginning of the same time slot as shown in fig.
• Slotted ALOHA still has an edge to improve over pure ALOHA as chances of collision are reduced.
References:
- Data Communication & Networking by Forouzan, Tata McGraw Hill.
- Computer Network, 4e, by Andrew S. Tenenbaum, Pearson Education/ PHI.
- Data Communication and Computer Networks, by Prakash C.Gupta, PHI.
- Networking Ali-in-one Desk Reference by Doug Lowe, Wiley Dreamtech
- Computer Networking: A Top-Down Approach featuring the Internet, 3e by James F.Kurose.
- Computer Network by Godbole, Tata McGraw Hill.
- Computer Networking, by Stanford H. Rowe, Marsha L. Schuh
