1. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 1 Short presentation on the CCC protocol for mesh MAC Mathilde Benveniste mbenven@avaya.com

2. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 2 Introduction This presentation covers the CCC (Common Control Channel) MMAC protocol CCC provides a way for mesh points to access their assigned/selected channels in order to forward received frames to next hop Topology Discovery/Link Establishment/Routing are determined independently References: Doc. IEEE 802.11-05/0610r1 Doc. IEEE 802.11-05/0666r1

3. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 3 Attributes of CCC –Easy to administer/ implement –Extends battery life –Self-healing –QoS and congestion aware –Solves ‘hidden’ and ‘exposed’ node problems –Accommodates mobility –Good throughput –Applies to meshes with mix of single and multi-radio MPs –Allows mesh and BSS traffic to share radio/channel –Can co-exist with independent WiFi use

4. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 4 Logical channels Control Channel –For MP message exchange to reserve time on MT channel(s) –One control channel in mesh * –Does not change dynamically (i.e., fast) –The same control channel can serve diverse PHY MT channels (11b, g, a, …) Mesh Traffic (MT) Channel –For mesh traffic (i.e. forwarding traffic) transmitted between mesh neighbors –Transmission by reservation on control channel only –The MT channel may be either fixed, or selected on demand –It can be a channel used by the BSS of a mesh AP ___________ * MPs with a single radio will use the same physical channel for control and MT functions

5. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 5 Definitions/Frames Definition MTXOP (mesh transmit opportunity): a sequence of frames transmitted between a pair of MPs following a single contention/reservation for the channel (similar to 11e TXOP) FRAMES MRTS (mesh RTS): an expanded RTS used to reserve a MT channel Contents: Forwarding MP address; Receiving MP address; Transmit channel; MTXOP Duration; Access Priority, Number of Frames; Transmit rate; other MCTS (mesh CTS): an expanded CTS used by a MP to respond to a MRTS Contents: Forwarding MP address; Transmit channel; Duration; Number of Frames; Transmit rate/Busy time ; Radio Counter; other MACK (mesh Ack): a group Ack transmitted on the control channel (optional) Contents: Receiving MP address; Forwarding MP address; Transmit channel; MTXOP frame receipt status

6. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 6 MCTS Common Control Channel MT Channel 3 MT Channel 1 MT Channel 2 time MPs reserve time on MT channels by exchanging MRTS/MCTS on the control channel The reserved MT channel must be free when exchange completes Reserve MT channel 1 MRTS Reserve MT channel 3 MRTS Reserve MT channel 1 Reserve MT channel 2 MTXOP MCTS Reservations on control channel

7. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 7 CCC Overview MT channel NAVs –MPs listen to the control channel to set NAVs of MT channels MRTS/MCTS to reserve MT channel –MRTS transmitted on control channel by contention MT channel selection –The MRTS indicates reserved MT channel, selected from permissible set –The channel with shortest NAV Reservation response –The receiving MP accepts, extends, or declines the channel reservation request by sending a MCTS within SIFS –If MCTS not received, MRTS re-transmitted –A reservation request is declined if the receiving MP deems the requested channel busy, or if it has no radios available to receive the transmission MCTS Duration field set to 0

8. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 8 CCC Overview - 2 Three-way handshake –A second MRTS is sent by the forwarding MP if a reservation request is Extended – neighbors are notified of the longer reserved time Declined – neighbors are notified to reclaim the unused reserved time Declined reservation retry –If Radio Counter = 0, Busy Time indicates when a radio will become available; reservation retried then –If the Number of Radios > 0, Busy Time indicates when MT channel will become idle – update NAV at forwarding MP MRTS can be sent for another MT channel Mesh transmission on MT channel follows 802.11e EDCA –CSMA/CA needed because of independent WiFi use Acknowledgement: two options –on the MT channel (as done in 802.11), or –on the control channel, as a group acknowledgement – MACK

9. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 9 Control Channel Access Prioritization of mesh traffic transmissions occurs when MRTS transmitted on control channel The access priority of a MRTS depends on user priority of the MTXOP frames their residual lifetime, and congestion (e.g. queue length adjusted for the number of transmit radios ) Two prioritization schemes: 1.EDCA (802.11e distributed MAC protocol) –The channel access parameters (AIFSN, CWMin, CWMax) and MTXOP_limit depend on the assigned AC 2.NAV time filtering (described on next page) –Filters contention access and backoff countdown to increase priority differentiation

10. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 10 MCTS NAV time filtering A MT channel can be reserved only if NAV about to expire –For better prioritization and MT channel utilization AIFR (advance interval for reservation) –AIFR: time before NAV expiration when a reservation may start –Cannot be longer than duration of MRTS/MCTS exchange MT channel NAV Control Channel time AIFR MRTS New Reservation Duration The NAV is a reservation timer maintained by a MP for a channel

11. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 11 NAV time filtering - 2 Prioritization by AIFR A longer AIFR is used for a higher priority reservation CSMA/CA with ‘NAV filtering’ Unless the NAV of a MT channel is expired or due to expire within AIFR time –MRTS cannot be transmitted by forwarding MP –Backoff timer cannot be decremented Unless the NAV of a MT channel is expired or due to expire within AIFR – RTS_TxTime –MRTS is declined by receiving MP AIFS Busy Medium Backoff Delay MCTS for MT1 MRTS for MT1 AIFR[i] AIFR[j] MT1 NAV expires

12. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 12 MRTS residual lifetime Priority given to delayed transmissions Differentiate between traffic type w.r.t. delay tolerance –Assign max lifetime according to user priority and discard frames with longer age Priority is raised by MP if residual lifetime drops below a threshold level –Residual lifetime (RLT) is the time remaining until expiration of a frame –When a frame enters the mesh the expiration time (ET) is added to the frame –Residual lifetime is derived at each hop from expiration time and local clock time Benefits –bounds the delay to traverse the mesh –increases throughput (reduces dropped-frame rate) –prevents looping (equivalent to TTL) ET = T 0 +RLT T0T0 T2T2 T1T1 RLT = ET-T 1 RLT = ET-T 2 MAPMP2MP1 Time

13. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 13 MP priority Priority given to MPs with heavy traffic load –Mesh bottlenecks are avoided Queue size differentiation –The mesh point priority depends on the queued traffic adjusted for the number of transmit radios at the MP

14. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 14 Power consumption CCC preserves battery life –A single radio, the control radio, must be always on –With other MMAC protocols, a multi-radio MP must have all its radios powered all the time Tx power control –MRTS/MCTS, transmitted at known power levels, provide the signal attenuation between end points used for Tx power control CCC increases MT channel re-use with Tx power control –When control and MT channels are distinct, the MRTS/MCTS (longer Tx range) do not collide with MT channel traffic

15. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 15 The ‘hidden node’ problem A ‘hidden node’ is one that, if it transmits, will cause interference to the receiver of a concurrent transmission by a node that the hidden node cannot hear –The problem is addressed with RTS/CTS in BSS Because of the spatial separation of mesh points, the hidden node problem is harder to address in mesh networks –Hidden nodes are within the interference range of destination node but outside its Tx range RTS/CTS must be sent at more robust PHY mode than data frames Interference Range Tx Range 1 4 5 2 6 3 Single RTS/CTS reservation for entire MTXOP If RTS denied, forwarding MP must cancel reservation, to avoid channel waste Resetting NAVs must be avoided –All other outstanding NAV-setting requests would be also cancelled if NAVs were reset

16. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 16 2-Step NAV update for MRTS 1.Upon receipt of initial MRTS, a MP updates the NAV for the specified MT channel for the time interval RTSHSHK needed to transmit a MCTS and another MRTS 2.If a cancellation MRTS is not received, a node sets the NAV for the remaining Duration from the initial or subsequent MRTS MRTS NAV Cancel reservation MCTSMRTS NAV Extend reservation MCTS SIFS MRTS cancelled MRTS not cancelled First Step Second Step Initial MRTS Duration RTSHSHK

17. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 17 ‘Exposed node’ problem Node B becomes ‘exposed’ when C transmits. It cannot receive but it could transmit Node B becomes ‘exposed’ when D transmits. It cannot transmit but it could receive The existing NAV rules prevent an ‘exposed’ node from both transmitting and receiving The NAV must distinguish whether a node can transmit or receive Sensing Range A BCD RTS CTS RTS CTS Sensing Range A BCD RTS CTS RTS CTS

18. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 18 NAV rules for ‘exposed’ node problem 1.The NAV for each MT channel has two components: –MNAV_RTS is updated when receiving a MRTS with Duration>0 If MNAV_RTS >0 for a MT channel, a MP must decline a MRTS request for that channel –MNAV_CTS is updated when receiving a MCTS with Duration>0 If MNAV_CTS >0 for a MT channel, a MP must not transmit on that channel Both MNAV_RTS and MNAV_CTS are updated for all other NAV- setting requests 2.Ack on the control channel (MACK) –If the Ack on the MT channel, transmission by the exposed node would cause collision

19. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 19 Channel sharing by mesh and BSS traffic A MT channel can be shared by mesh and BSS traffic of MAPs without collisions MCTS to self A MAP reserves a MT channel for use in its BSS, by sending a MCTS addressed to itself –Especially important when a BSS uses HCCA, which assumes a contention-free channel QoS CF-Poll to self To silence BSS stations, the MAP sends a 802.11e QoS CF-Poll addressed to itself –The Duration field contains the time interval for which the BSS stations must set their NAV MPs ignore such a frame

20. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 20 Dual use of a radio for mesh and BSS traffic A MT radio can be shared by mesh and BSS traffic of a MAP QoS CF-Poll to self To switch a radio between serving the BSS and the mesh, a MAP silences its BSS stations by sending a 802.11e QoS CF-Poll addressed to itself –While the NAV of the BSS stations is non-zero, the MAP radio can be set to any MT channel, reserved by an MRTS sent to a mesh neighbor, to transmit mesh traffic

21. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 21 Challenges met with the CCC protocol Easy to administer/ implement –Requires no channel assignment for MT channels –Beacon monitoring fast and simple Extends battery life –Only one radio must be powered constantly in a multi-radio MP –Tx power control increases channel re-use Self-healing –Fast recovery from MP outage; channel assignment not prerequisite for control traffic exchange QoS and congestion aware – Robust prioritization with NAV filtering –Generalized adaptive prioritization bounds delay & reduces bottlenecks Solves ‘hidden’ node problem –2-step NAV setting for MRTS Solves ‘exposed’ node problem –Double NAV + MACK

22. doc.: IEEE 802.11-05/0707r0 Submission July 2005 Mathilde Benveniste, Avaya LabsSlide 22 Challenges met with the CCC protocol Accommodates mobility –Requires no channel re-assignment before control signaling can be exchanged Good throughput –Dynamic MT channel assignment enhances throughput Applies to meshes with mix of single and multi-radio MPs Allows mesh and BSS traffic to share radio/channel –Prevents collisions between mesh and BSS traffic Robust to heavy independent WiFi use –Lightly-loaded control channel can co-exist well in dense independent WiFi areas –Dynamic MT channel assignment makes it easier to find free channel time to transmit mesh traffic