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

– Throughput improvement• 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.

– Hidden node problem

Design objectives and challenges


- Expose node problem

1. Distributed MAC, distributed channel selection and/or channel assignment

2. Multihop transmission: hidden node problem and exposed node problem

3. Directional antennas or a multichannel MAC protocol may suffer from a deafness problem

4. Dynamic nature: variations in link quality, changing congestion levels, and user mobility

5. 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 function (PCF) on CSMA/CA– Distributed inter frame space (DIFS)– Short inter frame space (SIFS)– Virtual carrier sensing: 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– Preferable 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 syncronization among nodes


• Multichannel MAC protocols– Cross-layer channel assignment

• Combing channel assignment with the routing protocol -> simple MAC

• Separation 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 throughput 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 (optional)– Simultaneous transmissions on multiple channels

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


• Mesh deterministic access (optional)– Better QoS

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

– Set up procedure for an MADOP set


Documents

Ch 5. MAC in WMNs