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Synchronous Collision Resolution Follow-up Questions & Answers

Synchronous Collision Resolution Follow-up Questions & Answers. Date: 2008-04-03. Authors:. Patent Statement. Methods described in this presentation are covered in claims in patents and patents pending. The holder of these patents is John A. Stine

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Synchronous Collision Resolution Follow-up Questions & Answers

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  1. Synchronous Collision ResolutionFollow-up Questions & Answers Date: 2008-04-03 Authors: John A. Stine, The MITRE Corporation

  2. Patent Statement • Methods described in this presentation are covered in claims in patents and patents pending. • The holder of these patents is John A. Stine • A letter of assurance has not been submitted but would be if the methods are included in future standards John A. Stine, The MITRE Corporation

  3. Abstract This submission consolidates questions previously raised concerning the suitability of SCR for an 802.11 VTS protocol specifically addressing its compatibility with the physical layer, and its coexistence with legacy 802.11 implementations. Where possible definitive answers are provided, otherwise approaches are provided to arrive at answers with the help of those that have better knowledge of technical details of 802.11 modems. John A. Stine, The MITRE Corporation

  4. Problem Statement • Synchronous Collision Resolution (SCR) • Provides a distributed means to • Prioritize access • Reserve bandwidth for streaming traffic • The precedence and reservation mechanisms are highly effective • SCR may not be suitable • The 802.11 PHY layers may be too inefficient performing the collision resolution signaling • It may not work with the legacy MAC John A. Stine, The MITRE Corporation

  5. Physical Layer Questions • Does SCR require changes in the PHY for implementation? • If changes are required, how extensive would they be? • If only MAC changes are made, would SCR still provide a performance gain? John A. Stine, The MITRE Corporation

  6. Where should we be concerned about PHY changes? • Concerns are with the ability to do signaling efficiently. The portion where packets are exchanged is identical to the existing protocols John A. Stine, The MITRE Corporation

  7. Criticial Assumptions About Signaling • The presence of signals is detected and there is no requirement to recover symbols or bits (PHY) • A signal is detected as present when receiving many signals (PHY) • The signaling slot in which a signal was sent is unambiguous (PHY or MAC) John A. Stine, The MITRE Corporation

  8. Desired (not necessary) characteristics of signals • Short • Contributes to efficiency • Easily distinguished from noise and other transmissions • Allows operation in noisy environments where physical layer capabilities can reject interference • Have unique characteristics that are associated with the signaling slot in which they are sent • Reduces overhead requirements to prevent slot of transmission ambiguity • Provides security preventing some cases of malicious DoS John A. Stine, The MITRE Corporation

  9. Timing Parameters Table 1. Design Choices Table 2. Modem Capabilities and Physics John A. Stine, The MITRE Corporation

  10. Signaling Slot Assumptions • Assume the signal slot size is selected as • Assume signal transmission or signal reception starts at the beginning of a signaling slot • Assume required sensing time, tsf, can be specified • ts is selected to account for PHY limitations in sending and sensing signals, propagation times, and synchronization differences • tg is selected to account for PHY transitions between receiving and transmitting states, propagation times, and synchronization differences ts tg ts tg tsignaling slot tsignaling slot John A. Stine, The MITRE Corporation

  11. Late Signal Transmission • We select the minimum time to sense a signal, tsf, such that tsf > tsn and tsftss • Observatons John A. Stine, The MITRE Corporation

  12. Early Signal Transmission • Observations • Recall late signal transmission observations Largest tsn Smallest tss John A. Stine, The MITRE Corporation

  13. Design Equations – Specified Sensing Time Ensures tsf > tsn Ensures tss≥tsf Ensures tsf > tsn John A. Stine, The MITRE Corporation

  14. Design Equations – Variable Sensing Time • Assume tsm1 is the minimum time to sense and tsm2 is the maximum time it takes to sense then the design seeks tsm1 > tsn and tsm2tss Ensures tsf > tsn Ensures tss≥tsm2 Ensures tsm1 > tsn John A. Stine, The MITRE Corporation

  15. Using tp • If signals can be differentiated between slots, tp rather than tp can be used in the previous designs and the equations become Ensures tss≥tsm2 John A. Stine, The MITRE Corporation

  16. Questions that need to be answered about each PHY layer • What is the minimum duration signal, tsd, it can send? • By what increments can the signal be increased in duration? • Can you cause the signals to be different between signaling slots? • What is the minimum time, tsm1, for a receiver to detect a signal? • What is the maximum time, tsm2, for a receiver to detect a signal reliably? • Can the sensing time be specified? • How long does it take to transition, trt and ttr? John A. Stine, The MITRE Corporation

  17. Questions that need to be answered about the operating conditions • What type of synchronization tolerance can we achieve? • What is the maximum propagation distance that we should expect? John A. Stine, The MITRE Corporation

  18. What would be the total cost of signaling? 7 phases 8 phases 9 phases • Assume a static echoing design • Choices • Acceptable collision resolution reliability, i.e. number of CRS phases, n (Does not affect reservations) • Number of precedence levels, r • Reservations require two steps • Total time signaling 6 phases 5 phases POne Survivor 4 phases Number of Contenders John A. Stine, The MITRE Corporation

  19. What is the efficiency of the protocol? … RTS CTS PDU ACK • Efficiency tIFS_4 tIFS_3 tIFS_2 tIFS_5 tCTS tIFS_1 tRTS t tACK tPDU tsignaling_slot tCRS tslot … PDU ACK t tIFS_4 tIFS_5 tIFS_1 tACK tPDU tsignaling_slot tCRS tslot John A. Stine, The MITRE Corporation

  20. Legacy Compatibility Questions • How do we coexist with legacy 802.11 systems that are unaware of SCR? John A. Stine, The MITRE Corporation

  21. Assumptions • Compatibility means • Legacy nodes can contend for access and exchange packets with SCR nodes • Legacy nodes can receive packets from SCR nodes (can receive streams from SCR nodes) • SCR nodes are allowed to have precedence over legacy nodes John A. Stine, The MITRE Corporation

  22. Observations • Compatibility does not just occur but must be designed into the protocol • For compatibility we must ensure • Legacy does not interfere with SCR contentions • That legacy has an opportunity to contend • Features of SCR we will exploit • All nodes with packets to send contend in the next transmission slot • Features of legacy we will exploit • The duration field in packet headers can cause virtual sensing beyond a packet exchange John A. Stine, The MITRE Corporation

  23. Basic Idea Space in the back of the period available for legacy contention SCR traffic is front loaded • Establish a CBR period • Push SCR traffic to the front • Cause legacy to defer to SCR contentions • Allow legacy traffic at the end of the period if space available • Protect the beginning of the next CBR period CBR Period t John A. Stine, The MITRE Corporation

  24. How do SCR Nodes Cooperate • SCR contention ceases in a frame once there is a transmission slot without a contention • Indicates that no SCR node has a packet to send • If a packet arrives at the modem after this slot the SCR modem either • Waits to the next CBR period to contend to send it using SCR • Uses the legacy DCF • SCR reservations push themselves to the front of the frame • All nodes with CBR reservations contend in all preceding transmission slots in a period in an attempt to move the reservation forward in the period John A. Stine, The MITRE Corporation

  25. Protecting SCR Contentions • Problem – Silences during signaling may be long enough for legacy to backoff and to begin transmitting in the middle of an SCR contention • Goal – Cause legacy nodes to defer during SCR contentions • Proposal – Use the NAV field in the PDU header and ACK of SCR packet exchanges to cause virtual sensing by legacy through the next SCR transmission slot John A. Stine, The MITRE Corporation

  26. Protecting CBR Periods • Problem – Legacy exchanges may extend across the CBR period boundary • Goal – Prevent legacy packet exchanges that would extend across the CBR period boundary • Proposal – Give an SCR node an advantage in sending a packet, e.g. self CTS, close to the boundary in a way that allows it to establish a NAV that protects the boundary John A. Stine, The MITRE Corporation

  27. Conclusion • Determining the PHY ramifications on signaling requires more data about each PHY • Modifying PHYs to enable more efficient signaling may be warranted • The requirements for signals are not that stringent • An SCR implementation can be designed to be compatible with legacy John A. Stine, The MITRE Corporation

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