Ch 5. MAC in WMNs
Myungchul Kim
mckim@icu.ac.kr
Introduction
– 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.
Design objectives and challenges
– 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
Conventional wireless MAC protocols
– 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
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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
Advanced MAC protocols for WMNs
• 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
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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 features proposed by the
802.11 TDs group
– 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