Ch 5. MAC in WMNs
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Transcript of Ch 5. MAC in WMNs
– 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 802.11 MAC and IEEE 802.11e MAC: single channel
– Directional antennas
– Multiple channels at mesh nodes
– IEEE 802.11b: 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
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 802.11 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 802.11 DCF protocol
Conventional wireless MAC protocols
• IEEE 802.11e 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 802.11e 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 802.11e MAC protocol
Conventional wireless MAC protocols
• IEEE 802.11e 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
Advanced MAC protocols for WMNs
• Contention-free MAC protocols for synchronized mesh networks– IEEE 802.16, 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 802.11 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 802.11 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 802.11 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 802.11 TDs group