Ch 5. MAC in WMNs Myungchul Kim Wireless routers called wireless mesh points (MPs) MAC protocols with omni directional antennas for sharing a single channel: Aloha, Slotted Aloha, CSMA, CSMA/CA IEEE MAC and IEEE e MAC: single channel Directional antennas Multiple channels at mesh nodes IEEE b: three orthogonal channels could be used simultaneously in a neighborhood without interfering. Introduction Control and management of WMNs have to be distributed across all nodes. Distributed MAC, distributed channel selection and/or channel assignment Multihop transmission: hidden node problem and exposed node problem Design objectives and challenges Directional antennas or a multichannel MAC protocol may suffer from a deafness problem Dyanmic nature: variations in link quality, changing congestion levels, and user mobility Error-prone nature of wireless channels Design objectives and challenges Contention-based protocols vs collision-free channel partition protocols Aloha and slotted aloha Conventional wireless MAC protocols CSMA and CSMA/CA CSMA CSMA/CA Hidden node problem RTS/CTS Collision? IEEE DCF protocol Distributed coordination function (DCF) on CSMA/CA Point coordination funtion (PCF) on CSMA/CA Distributed inter frame space (DIFS) Short inter frame space (SIFS) Network allocation vector (NAV) Conventional wireless MAC protocols IEEE DCF protocol Conventional wireless MAC protocols IEEE e MAC protocol Define the channel access functions and the traffic specification (TSPEC) management Channel access function -> hybrid coordination function (HCF) A contention-based protocol called enhanced distributed channel access (EDCA) A polling mechanism called HCF controlled channel access (HCCA): central control Conventional wireless MAC protocols IEEE e MAC protocol EDCA: enhance the original DCF by providing prioritized medium access based on different traffic classes, access categories (ACs) TXOP: a bounded time interval in which a node is allowed to transmit a series of frames. Conventional wireless MAC protocols IEEE e MAC protocol Conventional wireless MAC protocols IEEE e MAC protocol Conventional wireless MAC protocols Protocols for mesh nodes equipped with directional antennas Spatial reuse New MAC A different kind of hidden node problem, a deafness problem, and a higher directional interference Advanced MAC protocols for WMNs Protocols for mesh nodes equipped with directional antennas Directional MAC (D-MAC) CTS is transmitted omnidirectionally to reduce the number of hidden nodes Or RTS packets are sent omnidirectionally Tone-based directional MAC (Tone MAC) Use omnidirectional out-of-band tones to indicate deafness to blocked transmitter After a DATA/ACK, send out tones to indicate that they were recently engaged in communication. Directional virtual carrier sensing (DCVS) Caching the angle of arrival, beam locking and unlocking, and use of directional NAV Advanced MAC protocols for WMNs Protocols for mesh nodes equipped with directional antennas Circular directional RTS Inform the neighbors about the intended transmissions Exploit the benefits of beam-forming Advanced MAC protocols for WMNs Protocols for mesh nodes equipped with directional antennas Advanced MAC protocols for WMNs Multichannel MAC protocols Channel selection techniques Handshake-based channel selection Channel hopping Cross-layer channel assignment Common control channel or not One transmitter and multiple receivers One transceiver Advanced MAC protocols for WMNs Multichannel MAC protocols Handshake-based channel selection Dynamic channel assignment (DCA) One control channel and n euqivalent data channels All nodes with a channel usage list (CUL) and a free channel list (FCL) RTS with FCL, CTS with the channel to use and RES through control channel Advanced MAC protocols for WMNs Multichannel MAC protocols Handshake-based channel selection Multichannel CSMA MAC Similar to DCA Multichannel MAC (MMAC) Becon intervals The first interval for channel negotiation and the second for data packets Ad hoc traffic indication message (ATIM) window Common control channel Prefearable channel list (PCL) All channel negotiation occur in ATIM windows over the common control channel. RTS, CTS, ACK and data are all transmitted on the negotiated data channel. Advanced MAC protocols for WMNs Multichannel MAC protocols Handshake-based channel selection Multichannel MAC (MMAC) Stringent syncronized requirements No broadcasting the channel negotiation in on a per-packet basis -> high control overhead Advanced MAC protocols for WMNs Multichannel MAC protocols Channel hopping Receiver-initiated channel-hop with dual polling (RICH-DP) All nodes in a network follow a common channel-hopping sequence and each hop lasts just long enough for the nodes to receive a collision-avoidance control packet from a neighbor. Slotted seeded channel hopping (SSCH) No dedicated control channel and clock sysncronization among nodes Advanced MAC protocols for WMNs Multichannel MAC protocols Cross-layer channel assignment Combing channel assignment with the routing protocol -> simple MAC Seperation of channel assignments and MAC One common control channel and multiple data channels Nodes are assigned data channels by the routing protocol All channel assignments are piggybacked onto routing protocol messages -> lower communication overhead Advanced MAC protocols for WMNs Multichannel MAC protocols Cross-layer channel assignment RTS and CTS with data channel index Advanced MAC protocols for WMNs Multichannel MAC protocols Cross-layer channel assignment Combining routing with intelligent channel assignment -> a factor of 6 to 7 throuput improvement compared to a conventional single-channel scheme Advanced MAC protocols for WMNs Contention-free MAC protocols for synchronized mesh networks IEEE , WiMax Point to multipoint and mesh network TDMA Connection oriented Subscriber station (SS), connection ID (CID), base station (BS) The MAC layer schedules the usage of the airlink resources and provide QoS differentiation Advanced MAC protocols for WMNs Contention-free MAC protocols for synchronized mesh networks Advanced MAC protocols for WMNs 802.11e, 11i and 11k for one-hop wireless networks Intramesh congestion control End-to-end flow control at the transport layer? Multimedia application over UDP TCP congestion control does not work well across a multihop wireless network Advanced MAC features proposed by the TDs group Intramesh congestion control Hop-by-hop congestion control mechanism that operates at the data link layer Broadcast neighborhood congestion announcement and/or unicast congestion control request Detecting congestion Monitor effective MAC transmission and receiving rate for the packets to be forwarded Monitor queue size Advanced MAC features proposed by the TDs group Common channel framework Simultaneous transmissions on multiple channels Request-to-switch (RTX) and clear-to-switch (CTX) Advanced MAC features proposed by the TDs group Mesh deterministic access Better QoS Contention-free time period: MDA opportunity (MDAOP) Set up procedure for an MADOP set Advanced MAC features proposed by the TDs group