LM CCN Section 5.1: Wireless MAC
Computer and Communication Networks
Wireless Medium Access Control (MAC) John Easton
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Learning Objectives
o Understand wireless networking
– How use of a broadcast medium impacts access control
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Introduction
o Wireless networks are becoming ubiquitous
o The edge of the Internet is becoming wireless
– Single-hop
o Wireless LAN
o Cellular
– Multi-hop (e.g. Bluetooth)
o RCM oPAN
o Military
Coverage map generated by OpenSignal https://opensignal.com Last accessed 18th March 2018
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LM CCN Section 5.1: Wireless MAC
Network Growth (2016 – 2021)
o Cisco Visual Networking Index (VNI) 2016 – 2021
– Smartphone traffic to exceed PC traffic by 2020
– Wifi & mobile > 63% of IP by 2021
– Sevenfold increase in mobile data traffic
Cisco (June, 2017) “The Zettabyte Era: Trends and Analysis” Available online at https://www.cisco.com/c/en/us/solutions/collateral/service-provider/visual-networking-index-vni/vni-hyperconnectivity- wp.html Last accessed 18th March 2018
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Elements of Wireless Network
o Edge devices
– Run applications
– Stationary or mobile
o Access points
– Relay packets
– Transmission rate linked to
SNIR
o Multiple modes
– Infrastructure mode
o ED -> AP -> Ethernet – Ad-hoc mode
Router
Access Point
Edge Devices
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Ad-hoc Networks
o No base stations
o Node to node transmission
– Route independently
o Often used in the field where
infrastructure is limited – e.g. VANET
o Wireless sensor networks for
RCM purposes often use this model
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LM CCN Section 5.1: Wireless MAC
Range of Link Characteristics
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Why Wireless is Different
o Unlike other networks
o Concept of link as graph edge joining
nodes does not apply
– Node broadcasts messages
– Broadcast domains have poorly- defined boundaries
– Domains (almost always) overlap o By design!
– Interference range larger than useful communication range
– Nodes move
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Why Wireless is Different (Cont.)
o Media access
– Wired and wireless both shared media
o Wired
– Simultaneous Tx & Rx
o Collision detection
o Transmitted signal ≠ sensed signal → collision – Channel condition identical at Tx & Rx
o Wireless
– Simultaneous Tx & Rx not possible
o No collision detection
– Channel condition varies node to node, strength at Tx & Rx not identical
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LM CCN Section 5.1: Wireless MAC
Example – Undetected Interference
o Node A transmits to node B
– Mountain blocks signal from
C
– Likewise if C transmits to B
o Node A cannot sense node C o Nodes A and C interfere at B
– Collision rate high, wastes bandwidth & energy
o Hidden terminal problem
o Absence of carrier ≠ safe to transmit
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Example – Failure to Exploit Channel
o Node W transmits to X
o Node Y wants to transmit to Z – Senses transmission of W
– Defers transmission
o Potential for simultaneous
transmission missed
– High idle rate, wastes
bandwidth
o Exposed terminal problem
o Presence of carrier ≠ unsafe to
transmit
Z
Y
W
X
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Wireless MAC
o Proved to be non-trivial
– Even with knowledge of wired
MAC approaches
o IEEE 802.11 standards form the predominant wireless MAC protocol
o Handshake protocol with terminal silencing
– Exploits short messages to protect longer transmissions
– Lower collision probability
o Two modes
– CSMA/CA – contention based – Priority-based access
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LM CCN Section 5.1: Wireless MAC
Wireless MAC (Cont.)
o Howdoesitwork?(distributedversion)
o Allbackloggednodeschoosearandomnumber
– R=rand(0,CW_min)
o EachnodecountsdownR
– Waitminimumperiodofquiet=interframespace – Sensecarrierwhilecountingdown
o If carrier becomes busy freeze countdown o TransmitRTSwhencountreacheszero
– Neighboursfreezecountdown,decodeRTS
– RTScontainsNetworkAllocationVector(NAV)
o Expected duration for (CTS + DATA + ACK) = T_comm – NeighbourssilencedforT_commtime,thenresumecount
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Wireless MAC (Cont.)
o ReceiverreplieswithCTS
– IncludesNAVfor(DATA+ACK) – NeighboursupdateNAV
o TxsendsDATA,RxconfirmsreceiptwithACK – AfterACKnodesfreetoresumecountdown – TxchoosesnewR,beginstocount
o IfRTSorDATAcollide(noCTS/ACKreceived)
– Indicatescollision
– Txchosesnewrandomnumber(exponentialbackoff)
o R = rand(0, 2^i * CW_min)
– Oncetransmissionsuccessfulreverttonormalbackofffunction
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Wireless MAC (Cont.)
o Basic flow control doesn’t require ACK
– Sender issues RTS with NAV after DIFS – Receiver acknowledges after SIFS
o Channel reserved, other stations in range silenced for NAV
– Sender transmits data
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LM CCN Section 5.1: Wireless MAC
Wireless MAC (Cont.)
o Full flow control includes ACK – Improves reliability
o RTS/CTS is optional in 802.11
– May not be turned on, short data frames unlikely to benefit – Some NICs use only above threshold frame length
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Wireless MAC (Cont.)
o What about Infrastructure networks?
– AP acts as Point coordinator
– AP always has first chance to speak
– Polled stations speak without contention
o You’re hold the stick, it’s your turn
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Wireless MAC (Cont.)
o CSMA-CA combines DCF and PCF
o IFS variation allows prioritisation of access
– SIFS
o Highest priority (ACK, CTS, polling response)
– PIFS
o Medium priority, gives AP priority over others
– DIFS
o Lowest priority, used for asynchronous data service
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LM CCN Section 5.1: Wireless MAC
Wireless MAC (Cont.)
o Does RTS / CTS solve the hidden terminal problem?
– Assume carrier sensing zone = communication zone
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Wireless MAC (Cont.)
o What about exposed terminal problem? o B should be able to transmit to A
– RTS prevents this
o Broader carrier sensing would block even more
transmissions
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Wireless MAC (Cont.)
o 802.11 does not solve HT/ET completely o RTS / CTS alleviates problem
– Better to have RTS collision (20 bytes) than data frame collision (2k bytes)
o Recommends larger carrier sensing zone
– Helps with HT problem, but makes ET problem worse – Tradeoff to be made when configuring
o Backoff mechanism is effective but wasteful o 802.11 still being optimised
– Wireless MAC research is still an active field
B+
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LM CCN Section 5.1: Wireless MAC
Summary
o Understand wireless networking
– How use of a broadcast medium impacts access control
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