Media Access Control
- 1. S.VIJAYALAKSHMI M.SC(CS) NADAR SARASWATHI COLLEGE OF ARTS AND SCIENCE THENI
- 2. Media Access Control Multiple access links There is ‘collision if more than one node sends at the same ime only one node can send successfully at a time.
- 3. When a "collision" occurs, the signals will get distorted and the frame will be lose the link bandwidth is wasted during collision Question: How to coordinate the access of multiple sending and receiving nodes to the shared link ? Solution: We need a protocol to determine how nodes share channelMedium Access control (MAC) protocol The main task of a MAC protocol is to minimize collisions in order to utilize the bandwidth by: Determining when a node can use the link (medium) What a node should do when the link is busy What the node should do when it is involved in collision
- 4. Channel partitioning MAC protocols: Share channel efficiently and fairly at high load Inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! “Taking turns” protocols Eliminates empty slots without causing collisions Vulnerable to failures (e.g., failed node or lost token) Random access MAC protocols Efficient at low load: single node can fully utilize channel High load: collision overhead
- 6. Developed in the 1970s for a packet radio network Improvement: Start transmission only at fixed times (slots) CSMA = Carrier Sense Multiple Access Improvement: Start transmission only if no transmission is ongoing CD = Collision Detection Improvement: Stop ongoing transmission if a collision is detected (e.g. Ethernet) Evolution of Contention Protocols Slotted Aloha CSMA Aloha CSMA/CD
- 7. ALOHA Pure ALOHA Developed by Abramson in the 1970s for a packet radio network by Hawaii University. Whenever a station has a data, it transmits immediately. Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. Sender retransmits after some random time if there is a collision. Slotted ALOHA Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. Thus, it can reduce the collision duration.
- 8. ALOHA Mountainous islands – land network difficult to install Fully decentralized protocol The node waits for an ACK for time-out equals to the maximum round-trip propagation delay = 2* tprop
- 9. Frame Transmission in pure ALOHA If the frame is collided (no ACK was received) the stations wait for a random time and retransmit the frame again.
- 10. ThroughputAnalysis A frame (red frame) will be in a collision if and only if another transmission begins in the vulnerable period of the frame Vulnerable period has the length of 2 frame times Frame which collides with start of red frame Frame t0 t0+F Vulnerable Period of red frame Time Frame which collides with end of red frame
- 11. Vulnerable time- example A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the requirement to make this frame collision-free? Solution Average frame transmission time Tfr is 200 bits/200 kbps or 1 ms. The vulnerable time is 2 × 1 ms = 2 ms. This means no station should send later than 1 ms before this station starts transmission and no station should start sending during the one 1-ms period that this station is sending.
- 12. For small G: S ≈ G, there is nearly no collision, S is small because the load is small For large G: G >> S, there are many backlogged users, S is small because there are many collisions ThroughputAnalysis
- 13. SlottedALOHA time divided into discrete intervals (1 interval = 1 frame) the sending station waits until the beginning of the next discrete interval
- 15. Pure and Slotted ALOHA Throughput Simple improvement but big impact 8 0.4 0.3 0.2 0.1 00 SlottedAloha Aloha 0.368 0.184 G S
- 16. CSMA (Carrier Sense MultipleAccess Collisions hurt the efficiency of ALOHA protocol At best, channel is useful 37% of the time CSMA gives improved throughput compared to Aloha protocols. CSMA: listen before transmit If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission Human analogy: don’t interrupt others!
- 17. Collisions can still occur: propagation delay means two nodes may not hear each other’s transmission Collision: entire packet transmission time wasted
- 18. Kinds of CSMA Non-persistent CSMA CSMA 1-persistent CSMA Persistent CSMA p-persistent CSMA
- 19. reduces chance of collisions reduces the efficiency increases the chance for collisions 1-persistan p-persistent increase the chance for collisions Improves efficiency Nonpersistent vs. persistent