Network Operations & administration CS 4592 Lecture 16

1. Network Operations & administration CS 4592Lecture 16 Instructor: Ibrahim Tariq

2. Data Link Layer

3. Protocols (Contd.)

4. Stop-and-Wait ARQ

5. Note In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic.(01010101)

6. Flow diagram for Example 11.3

7. Stop-and-Wait ARQ -Delayed ACK- Importance of ACK numbering

8. Drawbacks of Stop-and-Wait ARQ • After each frame sent the host must wait for an ACK • inefficient use of bandwidth • To improve efficiency ACK should be sent after multiple frames • Alternatives: Sliding Window protocols • Go-back-N ARQ • Selective Repeat ARQ

9. Sliding Window Protocols • Sequence numbers • sent frames are numbered sequentially • number of frames stored in the header • if the number of bits in the header is m than sequence number goes from 0 to 2^m-1 • Sliding window • to hold the unacknowledged outstanding frames • the receiver window size always 1 sequence number frame acknowledged frames

10. Go-Back-N ARQ

11. Note The send window is an abstract concept defining an imaginary box of size 2m − 1 with three variables: Sf, Sn, and Ssize.

12. S- holds the sequence number of the recently sent frame SF – holds sequence number of the first frame in the window SL – holds the sequence number of the last frame R – sequence number of the frame expected to be received Go-back-N-Control variables-

13. Note The send window can slide oneor more slots when a valid acknowledgment arrives.

14. Flow diagram for Example 11.6

15. Flow diagram for Example 11.7

16. Figure 11.13 Receive window for Go-Back-N ARQ

17. Note The receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn. The window slides when a correct frame has arrived; sliding occurs one slot at a time.

18. Figure 11.15 Window size for Go-Back-N ARQ

19. Note Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the size of the send window is 1.

20. Go-back-N • Inefficient • all out of order received packets are discarded • This is a problem in a noisy link • many frames must be retransmitted -> bandwidth consuming • Solution • re-send only the damaged frames • Selective Repeat ARQ • avoid unnecessary retransmissions

21. Selective Repeat ARQ

22. Selective Repeat ARQ • Processing at the receiver more complex • The window size is reduced to one half of 2m • Both the transmitter and the receiver have the same window size • Receiver expects frames within the range of the sequence numbers

23. Selective Repeat ARQ-lost frame- Note: retransmission triggered with NACK and not with expired timer

24. Selective Repeat ARQ-sender window size-

25. Note In the Go-Back-N Protocol, the sequence numbers are modulo 2m, where m is the size of the sequence number field in bits.

26. Figure 11.22 Delivery of data in Selective Repeat ARQ

27. Figure 11.23 Flow diagram for Example 11.8

28. Data Link Layer

29. Data Link Layer Topics to Cover Error Detection and Correction Data Link Control and Protocols Multiple Access Local Area Networks Wireless LANs

30. Multiple access problem • Example: • Cocktail party – many people gather together in a large room • Broadcast medium – air • Another example: a classroom • Human protocols: • “Give everyone a chance to speak” • “Don’t speak until you are spoken to” • “Don’t monopolize the conversation” • “Raise your hand if you have a question” • “Don’t interrupt when someone is speaking” • “Don’t fall asleep when someone else is talking”

31. Multiple access protocols • In LANs, WiFi, satellite networks, cocktail party • If more than 2 users send @ the same time - collision • All collided packets are lost -> waste of bandwidth • Ideally, the MAC protocol for a broadcast channel with the bit-rate R bps should satisfy: • if only 1 node is sending than the throughput is R • when M nodes have data to send than the throughput is R/M • decentralized protocol – no master • simple & inexpensive to implement

32. Taxonomy of Multiple-Access Protocols

33. Random Access Protocols • In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send.

34. ACK ACK ACK ACK ALOHA Network • Developed by Norm Abramson at the Univ. of Hawaii • the guy had interest in surfing and packet switching • mountainous islands → land-based network difficult to install • fully decentralized protocol

35. Pure Aloha • The node immediately transmits its frame completely • If the frame is collided it retransmits the frame again (after completely transmitting its collided frame) with the probability p

36. Assumptions all frames same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Slotted Aloha Operation • when node obtains fresh frame, it transmits in next slot • no collision, node can send new frame in next slot • if collision, node retransmits frame in each subsequent slot with prob. p until success

37. Frames in a Slotted ALOHA Network

38. Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Slotted Aloha Cons • collisions, wasting slots • idle slots • nodes may be able to detect collision in less than time to transmit packet • clock synchronization

39. Multiple Access Protocols ALOHA

40. Carrier Sense Multiple Access • Invented to minimize collisions and increase the performance • A station now “follows” the activity of other stations • Simple rules for a polite human conversation • Listen before talking • If someone else begins talking at the same time as you, stop talking • CSMA: • A node should not send if another node is already sending • carrier sensing • CD (collision detection): • A node should stop transmission if there is interference • collision detection

41. channel propagation delay Carrier Sense Multiple Access • If everyone is sensing the medium how come that collisions still occur?

42. CSMA (cnt’d) • Reduces the chance of collisions • reduces the efficiency • increases the chance for collisions • 1-persistant • p-persistant • Decreases the chance for collisions • Improves efficiency

43. Multiple Access Protocols ALOHA

45. CSMA with Collision Detection (CSMA/CD) • CSMA/CD can be in one of the three states: contention, transmission, or idle. • Example of CSMA/CD: Ethernet • How long does it take before stations realize that there has been a collision?

46. Collision Detection • How the station detects a collision? • There are many collision detection methods! • Most of them are analog processes. • Examples: • detecting voltage level on the line • detecting power level • detecting simultaneous transmission & reception

47. CSMA with Collision Avoidance -CSMA/CA- • no collisions

48. Multiple Access Protocols ALOHA