Random Access Techniques for Data Oriented Networks

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Random Access Techniques for Data Oriented Networks

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1. Random Access Techniques for Data Oriented Networks (Ch. 4.3 ? 4.5)

2. 2 Random access protocols Needed for 2 purposes: 1. To demand for channel assignment (e.g. in FDMA, TDMA or CDMA) 2. To transmit short packets without channel assignment

3. 3 ALOHA multiple access protocol Ha 363Ha 363

4. 4 Pure ALOHA

5. 5 Vulnerable period in (pure) ALOHA Ha 363Ha 363

6. 6 Slotted ALOHA

7. 7 Slotted ALOHA Skalar 501Skalar 501

8. 8 Vulnerable period in slotted ALOHA Ha 368Ha 368

9. 9 Throughput equations

10. 10 Throughput equations (ctd)

11. 11 Throughput in ALOHA channels Sklar 500Sklar 500

12. 12 Tree search for contention resolution Sklar 506Sklar 506

13. 13 Reservation ALOHA Sklar 503 Two types of slots: Minislots which are open to contention, used to request reservations Data (traffic) slots As the load decreases, more channels convert into minislots to reduce collisions.Sklar 503 Two types of slots: Minislots which are open to contention, used to request reservations Data (traffic) slots As the load decreases, more channels convert into minislots to reduce collisions.

14. 14 Over the axis txns of BSm below txns of MSs BS is sending a long message to MS3. At the same time MS1 is sending a short status info, while MS2 is requesting a reservation (Fig. Incorrectly shows it as MS3) The beginning of free (i.e. open to contention) slots, and their length is announced at the beginning of a frame. Then MS1 and MS2 tx their packets. Ack from MS3 comes in the contention free (reserved part). Over the axis txns of BSm below txns of MSs BS is sending a long message to MS3. At the same time MS1 is sending a short status info, while MS2 is requesting a reservation (Fig. Incorrectly shows it as MS3) The beginning of free (i.e. open to contention) slots, and their length is announced at the beginning of a frame. Then MS1 and MS2 tx their packets. Ack from MS3 comes in the contention free (reserved part).

15. 15 Example 1 A group of stations share a 56 kbps pure ALOHA channel. Each station generates a packet on the average of once every 10 seconds, even if the previous one has not yet been sent (i.e., the stations buffer the packets). Each packet is 3000 bits. What is the maximum number of stations that can share this channel, assuming the arrival process is Poisson.

16. 16 Maximum throughput is 18.4%. In terms of 56 kbps this is 56000 x 0.184 = 10300 bps Average traffic of each station is 300 bps. Thus 10300/300 = 34 stations

17. 17 Example 2 Measurements of a slotted-ALOHA channel show that 20% of the slots are idle. (a) What is the normalized total traffic on the channel? (b) What is the normalized throughput? (c) Is the channel underloaded or overloaded?

18. 18

19. 19 Listen before Talk or Carrier Sense Multiple Access (CSMA) If the channel is sensed as idle, transmit a packet If sensed busy, apply random delay At the new point in time, sense the channel and repeat the algorithm In ALOHA users are not paying attention what the others are doing (because of initial application being satcom, they could not sense, but they could easily detect collisions). User 1 sends 2 packets While the second packet of U1 is being sent, U2 has a packet, it senses the channel and realizes it is busy, after a random delay, sends. This time U1 & U3 have something to send. They sense and find it busy. After a random delay they find it idle, and both transmit, but because of the prop. Time delay they don?t sense each other. A collision occurs, they reschedule it After a random delay, packets are retxd.In ALOHA users are not paying attention what the others are doing (because of initial application being satcom, they could not sense, but they could easily detect collisions). User 1 sends 2 packets While the second packet of U1 is being sent, U2 has a packet, it senses the channel and realizes it is busy, after a random delay, sends. This time U1 & U3 have something to send. They sense and find it busy. After a random delay they find it idle, and both transmit, but because of the prop. Time delay they don?t sense each other. A collision occurs, they reschedule it After a random delay, packets are retxd.

20. 20

21. 21 Example: Normalized propagation delay Define ?a? for 10 Mbps 802.3 (Ethernet) and 2 Mbps 802.11 wireless LAN. In 802.3 max distance 200m, propagation speed 200 km/s; thus ? = 1?s. 1K bits/packet, T=100 ?s. Then a=0.01. In 802.11 max distance 100m, propagation speed 300 km/s; thus ? = 0.33?s. 1K bits/packet, T=500 ?s. Then a=0.00066.

22. 22 Persistent and nonpersistent Nonpersistent if after sensing busy, another sensing only after a random waiting period. Persistent if after sensing busy, continue sensing until channel becomes free. Transmit immediately after free, 1-persistent. Run a random generator, transmit with probability p, then p-persistent.

23. 23 Sensing and Retransmission Alternatives for CSMA

24. 24 Hidden Terminal Problem

25. 25 Hidden Terminal Problem Two terminals cannot sense the transmissions of each other but a third terminal senses them both. Above described CSMA does not prevent collisions between hidden terminals. In ad hoc networks: busy-tone multiple access (BTMA). Use two channels: message and busy-tone channels. Terminals hearing the busy tone also transmit busy-tones. In cellular, base is heard by all, sends busy-idle bit. This system is called data sense mlt. access (DSMA)

26. 26 CSMA/CD CD: Collision detection IEEE 802.3 (Ethernet) Stop transmission as soon as a collision is detected. Avoid wasting of channel occupancy. Critical when long packets are transmitted. Initiate a back-off procedure for retransmissions. In WLAN it is difficult to recognize a collision. Instead an approach called collision avoidance (CA) is employed.

27. 27 CSMA/CA adopted by IEEE 802.11 MS1 senses the medium is idle, but can not start txing right away. Waits for IFS duration. Since Medium is still idle IFS later, MS1 starts txing. MS2 when first senses, the medium is idle. When it senses before transmitting after IFS, medium is busy. Then waits. MS1 senses the medium is idle, but can not start txing right away. Waits for IFS duration. Since Medium is still idle IFS later, MS1 starts txing. MS2 when first senses, the medium is idle. When it senses before transmitting after IFS, medium is busy. Then waits.

28. 28 CSMA/CA While A is txing, B, C and D sense the channel to transmit B,C,D run random counters. C gets the shortest backoff time. C starts transmitting during CW. At this point D has as much time left in the counter as shown by black. Freezes it. After IFS counter starts. When counters Reaches 0, D starts. Although E got its packet after B, it has less time in the counter. It txs next. The3rd dashed IFS lines are incorrect. There should be only one. While A is txing, B, C and D sense the channel to transmit B,C,D run random counters. C gets the shortest backoff time. C starts transmitting during CW. At this point D has as much time left in the counter as shown by black. Freezes it. After IFS counter starts. When counters Reaches 0, D starts. Although E got its packet after B, it has less time in the counter. It txs next. The3rd dashed IFS lines are incorrect. There should be only one.

29. 29 Combing for Collision Avoidance Another approach to avoid collisions. A?s code 11101 B 11010 C 10001 In the first slot they all tx because the first bit of their code is 1. In the second C has a 0, while listening detects the other carriers. Therefore will not transmit when its time comes. B detects A?s presence in the 3rd slot, and backs off. A completes the cycle and transmits.Another approach to avoid collisions. A?s code 11101 B 11010 C 10001 In the first slot they all tx because the first bit of their code is 1. In the second C has a 0, while listening detects the other carriers. Therefore will not transmit when its time comes. B detects A?s presence in the 3rd slot, and backs off. A completes the cycle and transmits.

30. 30 Request to send/clear to send in IEEE 802.11

31. 31

32. 32

33. 33

34. 34 Effect of Capture

35. 35 Performance with Capture

36. 36 Integration of Voice and Data Voice packets can tolerate errors and packet loss (rather high!). Limited tolerance to delay. Data packets cannot tolerate high error rates, but can tolerate delays. Simplest approach separate channels for voice (isochronous) and data (asynchronous) packets. However integration is desirable (e.g. lip-synch).

37. 37 Data Integration in Voice-Networks FDMA: In AMPS 19.2 kb/s CDPD uses a voice channel. Voice inactivity not exploited. TDMA: in GPRS uses time slots for transmission up to 200 kb/s. Again voice activity not considered. CDMA: integration is natural.

38. 38 Moveable boundary TDMA for Voice and Data Integration

39. 39 Voice Integration in Data Networks QoS issue. Voice over Internet (VOIP) Not suitable for mobile applications. Suitable for WLAN Using IP via User datagram Protocol (UDP): no error control, sequencing, flow control Transmission control protocol (TCP): error and flow control but increased delay and reduced through Discuss difficulty of integrating voice in low data rate services where ALOHA or CSMA type MA makes them attractive for bursty, not high volume traffic. In cordless we could impose 64 kbps PCM (or 32 kbps ADPCM) standard for voice. In cellular we accepted a lower quality because there was no alternative.Discuss difficulty of integrating voice in low data rate services where ALOHA or CSMA type MA makes them attractive for bursty, not high volume traffic. In cordless we could impose 64 kbps PCM (or 32 kbps ADPCM) standard for voice. In cellular we accepted a lower quality because there was no alternative.

40. 40 Voice and Data over WLAN

41. 41 Prioritizing Voice Traffic

42. 42 Constant rate voice transmission

43. 43 Coping with voice packet jitters


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