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Chapter 6

Multiple Radio Access. Chapter 6. Introduction Multiple Radio Access Protocols Contention-based Protocols Pure ALOHA Slotted ALOHA CSMA (Carrier Sense Multiple Access) CSMA/CD (CSMA with Collision Detection) CSMA/CA (CSMA with Collision Avoidance) Summary. Outline.

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Chapter 6

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  1. Multiple Radio Access Chapter 6

  2. Introduction Multiple Radio Access Protocols Contention-based Protocols Pure ALOHA Slotted ALOHA CSMA (Carrier Sense Multiple Access) CSMA/CD (CSMA with Collision Detection) CSMA/CA (CSMA with Collision Avoidance) Summary Outline

  3. Multiple access control channels Each Mobile Station (MS) is attached to a transmitter / receiver which communicates via a channel shared by other nodes Transmission from any MS is received by other MSs Introduction MS 3 MS 4 MS 2 Shared Multiple Access Medium … MS 1 MS N

  4. Multiple access issues If more than one MS transmit at a time on the control channel to BS, a collision occurs How to determine which MS can transmit to BS? Multiple access protocols Solving multiple access issues Different types: Contention protocols resolve a collision after it occurs. These protocols execute a collision resolution protocol after each collision Collision-free protocols ensure that a collision can never occur Introduction (Cont’d)

  5. Static Channelization Scheduling Dynamic Medium Access Control Random Access Channel Sharing Techniques Medium - Sharing Techniques

  6. Contention-based Conflict-free Collision resolution Random access TREE, WINDOW, etc. ALOHA, CSMA, BTMA, ISMA, etc. FDMA, TDMA, CDMA, Token Bus, DQDB, etc. DQDB: Distributed Queue Dual Bus Classification of Multiple Access Protocols Multiple-access protocols BTMA: Busy Tone Multiple Access ISMA: Internet Streaming Media Alliance

  7. ALOHA Developed in the 1970s for a packet radio network by Hawaii University Whenever a terminal (MS) has data, it transmits. Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. If there is a collision, sender retransmits after some random time 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 Contention-based Protocols

  8. Node 2 Packet Retransmission 2 2 3 Collision Node 3 Packet Pure ALOHA Node 1 Packet Waiting a random time Retransmission 1 3 Time Collision mechanism in ALOHA

  9. The throughput Sth of pure Aloha as: • Differentiating Sth with respect to G and equating to zero gives • The Maximum throughput of ALOHA is Throughput of Pure ALOHA • The probability of successful transmission Ps is the probability no other packet is scheduled in an interval of length 2T where g is the packet rate of the traffic • Defining G= gTto normalize offered load, we have

  10. Retransmission Retransmission 2 3 Slotted ALOHA Node 1 Packet Nodes 2 & 3 Packets No transmission 1 2 3 Time Collision Slot Collision mechanism in slotted ALOHA

  11. The throughput Sth of pure Aloha as: • Differentiating Sth with respect to G and equating to zero gives • The Maximum throughput of ALOHA is Throughput of Slotted ALOHA • The probability of successful transmission Ps is the probability no other packet is scheduled in an interval of length T n where g is the packet rate of the traffic • Defining G= gT to normalize offered load, we have

  12. 0.5 0.4 0.368 0.3 Slotted Aloha S: Throughput 0.184 0.2 Aloha 0.1 0 0 2 4 6 8 G = gT Throughput

  13. CSMA (Carrier Sense Multiple Access) Improvement: Start transmission only if no transmission is ongoing CSMA/CD (CSMA with Collision Detection) Improvement: Stop ongoing transmission if a collision is detected CSMA/CA (CSMA with Collision Avoidance) Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit. CSMA/CA with ACK CSMA/CA with RTS/CTS Contention Protocols (Cont’d)

  14. Max throughput achievable by slotted ALOHA is 0.368 CSMA gives improved throughput compared to Aloha protocols Listens to the channel before transmitting a packet (avoid avoidable collisions) CSMA (Carrier Sense Multiple Access)

  15. MS 5 sense MS 2 Packet Delay for MS 5 MS 3 Packet 2 3 5 Delay for MS 4 Collision MS 4 senses Collision Mechanism in CSMA MS 1 Packet 1 4 Time

  16. Unslotted Nonpersistent CSMA Nonpersistent CSMA (no wait) Slotted Nonpersistent CSMA Unslotted persistent CSMA Persistent CSMA (wait) Slotted persistent CSMA 1-persistent CSMA p-persistent CSMA Kinds of CSMA CSMA

  17. Nonpersistent CSMA Protocols • Nonpersistent CSMA Protocol: • Step 1: If the medium is idle, transmit immediately (same as p = 1) • Step 2: If the medium is busy, wait a random amount of time and repeat Step 1 • Random backoff reduces probability of collisions • Waste idle time if the backoff time is too long • For unslotted nonpersistent CSMA, the throughput is given by: • For slotted nonpersistent CSMA, the throughput is given by:

  18. 1-persistent CSMA Protocols • 1-persistent CSMA Protocol: • Step 1: If the medium is idle, transmit immediately • Step 2: If the medium is busy, continue to listen until medium becomes idle, and then transmit immediately • There will always be a collision if two nodes want to retransmit (usually you stop transmission attempts after few tries) • For unslotted 1-persistent CSMA, the throughput is given by: • For slotted 1-persistent CSMA, the throughput is given by:

  19. p-persistent CSMA Protocol (time is slotted): Step 1: If the medium is idle, transmit with probability p, or delay the transmission with probability (1 - p) until the next slot Step 2: If the medium is busy, waits until the next slot and continue to listen until medium becomes idle, then go to Step 1 A good tradeoff between nonpersistent and 1-persistent CSMA p-persistent CSMA Protocols

  20. Throughput 0.01-persistent CSMA 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Nonpersistent CSMA 0.1-persistent CSMA 0.5-persistent CSMA S: Throughput 1-persistent CSMA Slotted Aloha Aloha 0 1 2 3 4 5 6 7 8 9 Traffic Load G

  21. In CSMA, if 2 terminals begin sending packet at the same time, each will transmit its complete packet (although collision is taking place) Wasting medium for an entire packet time CSMA/CD Step 1: If the medium is idle, transmit Step 2: If the medium is busy, continue to listen until the channel is idle then transmit Step 3: If a collision is detected during transmission, cease transmitting (detection not possible by wireless devices) Step 4: Wait a random amount of time and repeats the same algorithm CSMA/CD (CSMA with Collision Detection)

  22. T0+t- B begins transmission A B T0+t+tcd B detects collision A B T0+2t+tcd+tcr- A detects collision just before end of transmission A B CSMA/CD in Ethernet (Cont’d) T0 A begins transmission A B (tis the propagation time)  Time

  23. 1.2 α = t /T= 0 1 0.8 α = 0.01 S: Throughput 0.6 0.4 α = 0.1 α = 1 0.2 0 0.001 0.01 0.1 1 10 100 1000 Traffic load G Throughput of slotted non-persistent CSMA/CD

  24. All terminals listen to the same medium as CSMA/CD Terminal ready to transmit senses the medium If medium is busy it waits until the end of current transmission It again waits for an additional predetermined time period DIFS (Distributed inter frame Space) Then picks up a random number of slots (the initial value of backoff counter) within a contention window to wait before transmitting its frame If there are transmissions by other MSs during this time period (backoff time), the MS freezes its counter It resumes count down after other MSs finish transmission + DIFS. The MS can start its transmission when the counter reaches to zero CSMA/CA (CSMA with collision Avoidance) for wireless devices

  25. MS B’s frame MS C’s frame Delay: B Delay: C MSs B & C sense the medium MSs B resenses the medium and transmits its frame MSs C resenses the medium but defers to MS B CSMA/CA (Cont’d) for Wireless Devices MS A’s frame Time MSs C starts transmitting

  26. Contention window DIFS DIFS Medium Busy Defer access Slot Backoff after defer CSMA/CA Explained Contention window Next Frame Time DIFS – Distributed Inter Frame Spacing

  27. Example of Backoff Intervals Backoff=2 (2) (3) DIFS DIFS Backoff=9 DIFS Backoff=4 DIFS (5) busy Station 1 Backoff=5 Packet arrival at MAC busy Station 2 (1) busy Station 3 Backoff=7 Backoff=2 (4) busy Station 4 • After packet arrival at MAC, station 3 senses medium free for DIFS, so it starts transmission immediately (without backoff interval). • For station 1, 2, and 4, their DIFS intervals are interrupted by station 3. Thus, backoff intervals for station 1, 2, and 4, are generated randomly (i.e. 9, 5, and 7, respectively). • After transmission of station 2, the remaining backoff interval of station 1 is (9-5)=4. • After transmission of station 2, the remaining backoff interval of station 4 is (7-5)=2. • After transmission of station 4, the remaining backoff interval of station 1 is (4-2)=2.

  28. Backoff time = CW* Random() * Slot time CW = starts at CWmin, and doubles after each failure until reaching CWmax (e.g., CWmin = 7, CWmax = 255) Random() = (0,1) Slot Time = Transmitter turn on delay + medium propagation delay + medium busy detect response time CWmax 255 255 127 8 63 31 15 CWmin 7 第三次重送 初始值 第二次重送 第一次重送 Random Backoff Time

  29. Priorities of frames are distinguished by the IFS (inter-frame spacing) between two consecutive frames. 4 IFS's: SIFS: the highest priority ACK, CTS, the second or subsequent MPDU of a fragmented burst, and to respond to a poll from the PCF. PIFS (PCF-IFS): 2nd highest by PCF to send any of the Contention Free Period frames. DIFS (DCF-IFS): 3nd highest by the DCF to transmit MPDUs or MMPDUs EIFS (Extended-IFS): lowest by the DCF to transmit a frame when previous frame was not received correctly Priority Scheme in MAC

  30. Immediate Acknowledgements from receiver upon reception of data frame without any need for sensing the medium ACK frame transmitted after time interval SIFS (Short Inter-Frame Space) (SIFS <DIFS) Receiver transmits ACK without sensing the medium If ACK is lost, retransmission done CSMA/CA with ACK for Ad Hoc Networks

  31. DIFS SIFS DIFS Contention window Backoff after defer CSMA/CA/ACK Time Data Source MS ACK BS Next Frame Other MSs Defer access SIFS – Short Inter Frame Spacing

  32. Hidden Terminal Problem Radio transmission range R R R A B C A and C are hidden with respect to each other

  33. Transmitter sends an RTS (request to send) after medium has been idle for time interval more than DIFS Receiver responds with CTS (clear to send) after medium has been idle for SIFS Then Data is exchanged RTS/CTS is used for reserving channel for data transmission so that the collision can only occur in control message CSMA/CA with RTS/CTS for Hidden Terminal Problem

  34. DIFS SIFS SIFS SIFS CTS DIFS Contention window Next Frame Backoff Defer access CSMA/CA with RTS/CTS Time Data Source MS RTS ACK Destination MS Other MSs

  35. RTS Propagation delay CTS Data ACK RTS/CTS MS A MS B This helps avoid hidden terminal problem in Ad hoc networks

  36. Exposed Terminal Problem Radio Transmission range R R R R A A B C D Transmission at B forces C (Exposed) to stop transmission to D

  37. Homework • Problems: 6.7, 6.12, 6.17 (Due Nov. 1) • Practice at home: 6.8, 6.10, 6.16, 6.21

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