Wireless Medium Access
Control Protocols
A Survery by Ajay Chandra V.
Gummalla and John O. Limb
Introduction
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Survey
Distributed vs. Centralized Networks
Wireless MAC Issues
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Low Power Sensor Nodes
Random Access
Guaranteed Access
Hybrid Access
Introduction Cont’d.
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Distributed MAC Protocols
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Distributed Foundation Wirelesss MAC
(DFWMAC)
Eliminate Yield – Non-Preemptive Priority
Multiple Access (EY-NPMA)
Introduction Cont’d.
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Centralized MAC Protocols
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Random Access
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Idle Sense Multiple Acces (ISMA)
Randomly Addressed Polling (RAP)
Resource Auction Multiple Access (RAMA)
Guaranteed Access
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Zhang’s and Acampora’s Proposals
Disposable Token MAC Protocol (DTMP)
Introduction Cont’d.
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Hybrid Access
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Random Reservation Protocols (RRA)
Packet Reservation Multiple Access (PRMA)
Random Reservation Access – Independent
Stations Algorithm (RRA-ISA)
Distributed Queuing Request Updated Multiple
Access (DQRUMA)
Moble Access Scheme based on Contention and
Reservation for ATM (MASCARA)
Introduction Cont’d.
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Dynamic Slot Assignment ++ (DSA++)
Distributed Wireless Network
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ad hoc network
No central administration
Multi-hop wireless networks
Wireless Sensor Nets
Centralized Wireless Network
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Last Hop Network
Very common
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Corporate, Academic, and Cellular uses.
Has a controlling Base Station, with
variable intelligence
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Wireless Access Point
Cellular Tower
Wireless MAC Issues
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Half-Duplex
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No Collision Detection
Uplink and Downlink must be multiplexed
Time Varying Channel
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Reflection, Diffraction, and Scattering
Different version of signal are superimposed on
each other
Multipath Propagation
Coherence Time = time signal strength changes
by 3dB
Wireless MAC Issues Cont’d.
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Burst Channel Errors
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Higher BER
Errors occur in long bursts
Link Layer retransmission based on
immediate ACKs
Wireless MAC Issues Cont’d.
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Location Dependent Carrier Sensing
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Hidden Nodes: Node A doesn’t know Node B is
also talking to BS
Exposed Nodes: Node A knows node B is talking,
but doesn’t know that it will not affect Node A’s
conversation with BS
Capture: Node A and B are both transmitting to
BS, but since Node A’s signal strength is stronger
Node A’s transmission is used no collision is
detected.
Random Access
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Random Access is based on a “Talk
whenever you want” way of thinking
Collisions are resolved by a contention
resolution algorithm
Distributed Networks
Guaranteed Access
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Access to medium is scheduled
Round Robin
Master/Slave (Polling)
Tokens
TDMA, FDMA
Hybrid Access
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Melds best qualities of Random and
Guaranteed Access
Request-Grant mechanisms
Requests are Random Access, and once
reserved transmission is guaranteed
Random Reservation Access
Demand Assignment
Distributed Foundation
Wireless MAC (DFWMAC)
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802.11 Standard
4-way exchange: RTSCTS-DATA-ACK
No ACK causes sender
to retransmit
No CTS causes
exponential backoff
RTS and CTS contain a
NAV which details how
much data is to be sent
Elimination Yield – Non-Preemptive
Priority Multiple Access (EY-NPMA)
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HIPERLAN
Sense channel for time to send (TTS) 1700
bits, if clear, then send
If busy, N slots; When done listen again
If still busy, abort; Else listen again, and if
not busy then transmit until finished
Idle Sense Multiple Access
(ISMA)
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Carrier Sensing and Collision detection are
performed by the BS
When medium is idle BS broadcasts idle
signal (IS)
Nodes with data send
If collision BS cannot decode signal, does not
send ACK and broadcasts IS again
Otherwise BS sends ACK/ISA (ISA)
Efficiency is improved by using small
Reservation packets
Randomly Address Polling
(RAP)
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Nodes with data broadcast orthogonal
“codes” simultaneously
BS receives all codes, using a CDMA receiver
BS then polls each code
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All nodes with that code transmit
If only one node the BS sends ACK
More than one node with code causes BS to send
NACK
Reservation RAP supports nodes with
streaming traffic
RAP
Resource Auction Multiple
Access (RAMA)
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Each node has and N-bit ID and transmits it,
in contention phase
BS then echos back ID it heard bit-by-bit
Once a node receives a bit it did not transmit,
it drops out
Since BS does an OR operation on received
IDs then node with highest ID always wins
Zhang’s Proposal
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BS polls each node for data, round
robin
Node responds with data request, or a
keep alive if queue is empty
BS then polls each node that responded
with a data request
Disposable Token MAC
Protocol (DTMP)
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Improves on Zhang’s proposal
When polling nodes BS indicates if it
has data to send to nodes
If no data, then remain silent
Otherwise send short message
Transmit any data to send
Channel is assumed to be reciprocal
Acampora’s Proposal
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Poll, request, data phases
BS polls each node, if the node has
data to sends it responds
The BS the broadcasts this nodes ID so
that all nodes know the order in which
to send
BS then polls nodes each node in turn
for its data
Various Proposals
Random Reservation Protocols
(RRA)
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Uplink is time slotted
Each slot large enough to carry one voice
packet
Downlink is broadcast channel
Nodes use random access to request
reservations for data to send
BS enforces a policy of reservations
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Stream Reservation
Complete BS scheduling
Packet Reservation Multiple
Access (PRMA)
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A node with a back-logged voice packet transmits with
probability p
If successful, reserves that slot for following packets
Data is similar, though no reservations are made
Different access probabilities are used for voice and data
Introduction of data packets into voice only network
decreases efficiency
Improvements include limited data reservations, separating
voice and data channels (FRMA), separating request and
data channels (PRMA++)
Centralized PRMA uses scheduling to achieve QOS
Random Reservation Access –
Independent Stations Algorithm
(RRA-ISA)
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BS polls a subset of all nodes
Subset is defined by the probability of a
single transmission in a slot is
maximized
BS uses channel history to compute
subsets
Distributed-Queuing Request Update
Multiple Access (DQRUMA)
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Uplink and Downlink are duplexed
Uplink has request channel and packet
channel
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Request channel is for contention requests
Packet channel is for data (and piggyback new
contention requests)
Downlink has 3 messages: ACK for current
slot, transmission permission for node to use
next uplink slot, and data to the nodes
Better than RAMA and PRMA
Mobile Access Scheme based on
Contention and Reservation for ATM
(MASCARA)
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Frame consists of three periods: broadcast,
reserved, and contention
Broadcast informs nodes of structure of
current frame and scheduled uplink
transmissions
Reserved period consists of downlink data,
and uplink data as defined in broadcast
period
Contention is random access and used to
send new requests to BS
Dynamic Slot Assignment++
(DSA++)
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MAC on uplink is TDMA
Both uplink and downlink are slotted
Each downlink slot contains some data and a
MAC message
MAC message contains ACK for transmission
on previous uplink slot and a reservation for
next uplink slot
BS collects all requests and schedules uplink
transmissions
Hybrids
Comparison Summary
Comparisons
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QoS guarantees are not suited to Random
Access protocols because delay cannot be
bounded
Demand Assignment protocols are best suited
to multimedia applications
Random Access lends itself to large networks
Polling protocols are efficient only for smaller
networks
TDD protocols perform poorly at high data
rates due to increase in switching