# CS 716: Introduction to communication networks. - 9 th class; 19 th Aug Instructor: Sridhar Iyer IIT Bombay

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1 CS 716: Introduction to communication networks - 9 th class; 19 th Aug 2011 Instructor: Sridhar Iyer IIT Bombay

2 Contention-based MAC: ALOHA Users transmit whenever they have data to send Collisions occur, and if packet is lost, then source has to retransmit Collision are detected by listening while transmitting loss of acknowledgements If collision, then sender waits random time to avoid repeated collision IIT Bombay cs 716 2

3 Vulnerable interval For a given frame, the time when no other frame may be transmitted if a collision is to be avoided. Assume all packets have same length (L) and require Tp seconds for transmission Each packet vulnerable to collisions for time Vp =?? Packet B Packet C T p T p Packet A t IIT Bombay cs 716 3

4 ALOHA: Vulnerable interval Suppose packet A sent at time to If pkt B sent any time in [to - T p to t o ] end of packet B collides with beginning of packet A If pkt C sent any time in [to to t o + T p ] start of packet C will collide with end of packet A Total vulnerable interval for packet A is 2Tp Can we do something to improve the efficiency? IIT Bombay cs 716 4

5 Slotted ALOHA A simple way to double ALOHA s capacity Make sure transmissions start on a slot boundary Halves vulnerability interval Requires global synchronization Master station generates synchronization pulses for time-slots Used in cellular phone uplink IIT Bombay cs 716 5

6 Slotted ALOHA IIT Bombay cs 716 6

7 Carrier Sense Multiple Access Listen before you speak Check whether the medium is active before sending a packet (i.e carrier sensing) If medium is idle, then transmit If collision happens, then detect and resolve IIT Bombay cs 716 7

8 Activity: Group Discussion Suppose your group now has to work out the details: What are needed to enable collision detection? How does a station recover from collision? Hint: Typical Ethernet (10BaseT; 100BaseT) has A minimum frame size (64 bytes). A maximum segment length (100 meters?). Think about why these are required and how these numbers are arrived at. IIT Bombay cs 716 8

9 Concept: MAC Detailed design Example: Ethernet (IEEE 802.3) IIT Bombay cs 716 9

10 1 - Persistent CSMA Sense the channel. IF the channel is idle, THEN transmit. IF the channel is busy, THEN continue to listen until channel is idle. Now transmit immediately. IIT Bombay cs

11 P - Persistent CSMA Sense the channel. IF the channel is idle, THEN with probability p transmit and with probability (1-p) delay for one time slot and start over. IF the channel is busy, THEN delay one time-slot and start over. Time slot is usually set to the maximum propagation delay. as p decreases, stations wait longer to transmit, but the number of collisions decreases IIT Bombay cs

12 Non-Persistent CSMA Sense the channel. IF the channel is idle, THEN transmit. If the channel is busy, THEN wait a random amount of time and start over. Random time needs to be chosen appropriately. IIT Bombay cs

13 Collision detection (CSMA/CD) All aforementioned scheme can suffer from collision. Device can detect collision by: Listen while transmitting. Transmit for 2 * propagation delay. Jamming signal. IIT Bombay cs

14 Contention Interval - 2D t = 0: A begins transmission A B t = D - ε : packet almost at B B begins transmission A B t = D: B detects collision, stops transmitting A B t = 2D - ε : A detects collision A B IIT Bombay cs

15 Minimum frame size t = 0: A begins transmission A B t = D - ε : packet almost at B B begins transmission A B t = D: B detects collision, stops transmitting A B t = 2D - ε : A detects collision A B IIT Bombay cs

16 Minimum frame size It takes A a complete RTT (2D) to detect collision When B detects collision (gets more power than it is putting out) it generates 48-bit noise burst ( Jam bits) to warn all other stations Min. frame size equal to number of bits transmitted during one RTT: slottime: number of bits transmitted by a source during the max. RTT (2D = 51.2 μsec) for any Ethernet network. Collisions must be detected by sources while still transmitting All frames must be at least 1 slot (on 10Mbps, this is 512 bits) IIT Bombay cs

17 Collision recovery On collision detection wait for random time before retrying. Binary Exponential Backoff Algorithm: Reduces the chances of two waiting stations picking the same random time. IIT Bombay cs

18 Binary Exponential Backoff 1.On detecting 1 st collision for packet x station A chooses a number r between 0 and 1. waits for r * slot time and transmit. k. On detecting k th collision for packet x choose r between 0,1,..,(2 k 1) When value of k becomes high (10), give up. Randomization increase with larger window, but delay increases. Slot time is 2 * propagation delay. IIT Bombay cs

19 Frame: Ethernet (IEEE 802.3) CSMA/CD with jamming Ethernet Address (48 bits) Example: 08:00:0D:01:74:71 Ethernet Frame Format Why bytes? IIT Bombay cs

20 Activity: Pair-Solo Consider two nodes communicating using the CSMA/CD protocol (as in Ethernet). Suppose the bandwidth is 100 Mbps, the frame size is 1500 bytes and propagation speed is 3x10^8 m/sec. Calculate the maximum possible distance between the nodes such that the sender can detect any collision. Pair - Discuss the solution approach with your neighbour. Solo - Work out the answer by yourself. IIT Bombay cs

21 At the end of this topic You should be able to do: Determine what MAC protocol would be suitable for a given scenario. Evaluate tradeoff between two MAC protocols for a given scenario. Describe CSMA and its variations. Describe the collision detection mechanism in Ethernet. Perform binary exponential back-off calculations. Perform throughput calculations for TDMA and CSMA. IIT Bombay cs

22 Reflection What did I learn in today's class? Each student to mention one point. Take-home questions: How can collision detection (CD) be changed into a collision avoidance (CA) mechanism? What are the pros and cons of CD v/s CA? IIT Bombay cs

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