1 / 41

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.

jui
Download Presentation

Chapter 6

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 Node 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 (e.g., a bit-map protocol and binary countdown) 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= gT to 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 Easy to improve, just listen to the channel before transmitting a packet and thus avoid avoidable collisions  carrier sense CSMA gives improved throughput compared to Aloha protocols when MS can sense the transmissions of all other MSs. We’ll assume that the propagation delay is small compared to the transmission times (a = t/T  small). More than two MSs 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 Five mobile stations 1  5 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 slotted & unslotted Slotted  contention period

  17. Nonpersistent CSMA Protocols • Nonpersistent CSMA Protocol: Step 1: If the medium is idle, transmit immediately (same as 1-persistent or 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 • Wastes 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 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 or more nodes are waiting to transmit since they will all start at the same time which is more likely to happen with heavy traffic For unslotted 1-persistent CSMA, the throughput is given by: For slotted 1-persistent CSMA, the throughput is given by:

  19. Step 1: If the medium is idle, transmit in first available slot with probability p or delay for one slot with probability (1-p) Worst case slot size is the propagation delay of one packet. Step 2: If the medium is busy, continue to listen until medium becomes idle, then go to Step 1 Step 3: If a collision occurs, wait for a random amount of time and continue with Step 1.\ This is a good tradeoff between nonpersistent and1-persistent CSMA for a shared channel with a heavy traffic load. p-persistent CSMA Protocols (slotted)

  20. How to Select Probability p? • Assume that N nodes have a packet to send and the medium is busy • Then, Np is the expected number of nodes that will attempt to transmit once the medium becomes idle • If Np > 1 then a collision is expected to occur Therefore, network must make sure that to avoid collision, where N is the maximum number of nodes that can be active at a time.

  21. p-persistent CSMA Protocol If N terminals have packets to send, Np terminals will attemptto transmit once the medium becomes idle. If Np > 1, thencollision is expected. Therefore The throughput is: Where G is offered traffic rate a=t/T= propagation delay/packet transmission time Where are given by the following equations (pg 138):

  22. p-persistent CSMA Protocol Where:

  23. Persistent vsNonpersistent CSMA • Nonpersistent may be inefficient by unnecessarily waiting a random time to re-sense the medium when traffic is light but this will reduce collisions when traffic is heavy. • 1-persistent will have good channel utilization when traffic is light but will probably result in collisions when traffic is heavy. • p-persistent works better when traffic is heavy and provides a good balance between 1-persistent and nonpersistent CSMA

  24. 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 1-persistent CSMA S: Throughput Slotted Aloha Aloha 0 1 2 3 4 5 6 7 8 9 Traffic Load G

  25. In CSMA, if two terminals begin sending packet at the same time, each will transmit its complete packet (although a collision is taking place while the transmission continues) 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 (increased voltage on wires), cease transmitting.Detection not possible by wireless devices Step 4: Wait a random amount of time and then repeat the same algorithm CSMA/CD (CSMA with Collision Detection)

  26. 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

  27. 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

  28. All terminals listen to the same medium as CSMA/CD A Terminal ready to transmit senses the medium If medium is busy it waits until the end of current transmission It then waits for an additional predetermined time period DIFS (Distributed Inter Frame Space) It then picks a random number of slots (the initial value of a 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 backoff counter It resumes the 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

  29. 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 Time MS A’s frame MSs C starts transmitting

  30. 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 MS attempting to transmit its data finds the medium busy so it defers access until the medium is free for longer than the DIFS, then picks a random backoff period within its contention window before transmitting the Next Frame.

  31. wait data data wait MSB = 10 Random Delay helps CSMA/CA MSA = 25 MSA = 5 Another request MSB = 15 MSB = 20 MSA and MSB are backoff intervals at nodes A and B CW = 31 • MSA and MSB are the back-off intervals of MSs A and B • We assume for this example that CW = 31 • MS A and MS B have chosen a back-off interval of 25 and 20, respectively • MS B will reach zero before five units of time earlier than MS A • When this happens, MS A will notice that the medium became busy and freezes its back-off interval currently at 5 • As soon as the medium becomes idle again, MS A resumes its back-off countdown and transmits its data once the back-off interval reaches zero

  32. Contention Window Considerations • Chose window size over a large range of values where the range should be more than the number of nodes present on the network. Otherwise collisions will be more certain. • When the traffic load is heavy, the contention window should be large. Smaller if the load is small and/or the number of nodes is also small. • At times, change the window size to give priority to different nodes. Higher priority nodes should be able to choose lower contention windows. • After a collision, double the contention window range in order to decrease the probability of two or more nodes choosing the same slot • Typical contention window size: 32, 64, 128, 256, ……

  33. CSMA - CD versus CA • Collision Detection means if a collision is detected during transmission, the sender stops immediately and then waits a random time before trying again. • CSMA/CD is mainly used in Ethernet (wired). • Collision Avoidance tries to avoid collisions in advance since the wireless medium can’t detect collisions during transmission. CA is implemented by an additional waiting time of DIFS and a random backoff time. • CSMA/CA is used in wireless LANs.

  34. CSMA/CA and IEEE 802.11 • CSMA/CA used in wireless LANs (Access Point) and wireless ad hoc networks (peer-to-peer). • 802.11a  5 GHz band, can achieve bandwidths up to 54 Mbps, non overlapping channels (as compared to .11b) • 802.11b  2.4 GHZ ISM band, achieves 11 Mbps, frequencies susceptible to interference, crowded/busy • 802.11g  2.4 GHz, 54 Mbps using OFDM • 802.11n  either band, MIMO (up to 4 data streams)

  35. 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

  36. 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

  37. Hidden Terminal Problem Radio transmission/reception range R R R A B C A and C are hidden with respect to each othersince A can’t sense C and C can’t sense A

  38. 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 ad hoc networks

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

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

  41. Exposed Terminal Problem Each terminal can only communicate with the two closest terminals Radio Transmission range R R R R A B C D Transmission at A forces C (Exposed) to stop transmission to D B sends RTS to A but C will hear it and will not transmit to D This results in lower network throughput/utilization.

More Related