Ch 5. MAC in WMNs

Ch 5. MAC in WMNs

Myungchul Kim

[email protected]

– 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


– 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


– 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)


• IEEE 802.11 DCF protocol


• 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


• 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.


• IEEE 802.11e MAC protocol


• IEEE 802.11e MAC protocol


• 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


• Protocols for mesh nodes equipped with directional antennas– Circular directional RTS

• Inform the neighbors about the intended transmissions

– Exploit the benefits of beam-forming


• Protocols for mesh nodes equipped with directional antennas


• 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


• 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



• 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.



• Multichannel MAC (MMAC)– Stringent syncronized requirements– No broadcasting– the channel negotiation in on a per-packet basis -> high control

overhead


• 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


• 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



• RTS and CTS with data channel index



• Combining routing with intelligent channel assignment -> a factor of 6 to 7 throuput improvement compared to a conventional single-channel scheme



• 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


• Contention-free MAC protocols for synchronized mesh networks


– 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


• Common channel framework– Simultaneous transmissions on multiple channels

– Request-to-switch (RTX) and clear-to-switch (CTX)


• Mesh deterministic access– Better QoS

– Contention-free time period: MDA opportunity (MDAOP)

– Set up procedure for an MADOP set


Documents

Ch 5. MAC in WMNs