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MAC ProtocolsMAC Protocolsfor Ad Hoc Wireless for Ad Hoc Wireless NetworksNetworks

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Wireless InterferenceWireless Interference

A radio interface either transmits or receives (half-duplex).

A receiver must get a minimum SINR for successful receptionThis means no other transmitter (interferer) in

vicinity. If untrue -> collision (SINR insufficient).

Quintessential MAC problemSchedule transmissions on links conflict-free.

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Time Division Multiple Access Time Division Multiple Access (TDMA)(TDMA) Use slotted time. Schedule conflicting transmissions at

different time slots. Problem equivalent to graph coloring

Optimal solution is computationally hard. Significant research since the days of

packet radio. Often not deemed practical

Hard to compute good schedules in a distributed fashion.

Schedule needs to be traffic dependent. Need synchronized clocks in hardware to implement

slots

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Carrier Sense Multiple Access Carrier Sense Multiple Access (CSMA)(CSMA) Transmit when ready

Use a combination of carrier-sense and randomization to avoid conflict.

Not foolproof. Carrier sense not foolproof

Propagation delay (also a problem in wireline).Can sense only at transmitter; but collision

happens at receiver (a wireless problem).

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Virtual Carrier SensingVirtual Carrier Sensing

Any node hearing RTS or CTS sets up their NAV (network allocation vector) until end of ACK.

NAV set -> node silent (act as if carrier busy).

A B C D

RTSRTS

CTS

DATA

ACK

E

CTS

DATAACK

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802.11 Timeline802.11 Timeline

If carrier busy (physical or virtual), schedule transmission after a random backoff when carrier is free.

Average backoff interval is doubled for each failed attempt.

t

SIFS

DIFSACK

Defer access

Nodes that hear transmitter

Receiver

TransmitterDATA

Randombackoff

RTS

CTS

SIFS SIFS

NAV (RTS)NAV (CTS)

DIFS

Nodes that hear receiver Another

transfer

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Hidden and Exposed Terminal Problems Hidden and Exposed Terminal Problems (Revisited)(Revisited) In Ad Hoc networks, HTP and ETP would

happen frequently. Conventional CSMA severely suffer from both HTP and ETP !

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Design Goals of MAC Protocol for AHWNDesign Goals of MAC Protocol for AHWN

Distributed operation QoS support for real-time traffic Low access delay Bandwidth efficiency Fair allocation of BW to nodes Low control overhead Minimize the effects of hidden and exposed terminal

problems Scalable Efficient power control mechanism Adaptive data rate control, taking into consideration

of network load and neighbor status Try to use of directional antennas for reducing

interference, increasing spectrum reuse, and reducing power consumption

Time synchronization for BW reservation

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Classification of MAC Protocols for Classification of MAC Protocols for AHWNAHWN

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Contention-basedContention-basedSender-initiatedSender-initiatedSingle-channel ProtocolsSingle-channel Protocols

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MACA: Multiple Access Collision MACA: Multiple Access Collision AvoidanceAvoidance Proposed by Phil Karn (1990) as an alternative to

the CSMA Inspired by the CSMA/CA method

Extend and Enhance the CA part of the CSMA/CA – Every one overhearing CTS knows just how long to wait to avoid collision.

Get rid of the CS in CSMA/CA and become MACA. Lack of carrier doesn’t always mean it’s OK to transmit Presence of carrier doesn’t always mean it’s bad to transmit It’s too hard to build a good DCD (Data Carrier Detect) circuit

MACA uses signaling packets for CA RTS/CTS Contain: sender address, receiver address, packet size

If a packet transmitted is lost, use BEB algorithm Variants of this method are used in IEEE 802.11

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Data

MACA exampleMACA example MACA avoids HTP MACA avoids ETP

S1 S2R

RTS

CTSCTS

Data

R1 S2S1

CTS

RTSRTS

Data

R2

Vulnerable period is known to C by

CTS

Overhearing RTS

No CTS

RTSRTS

CTS

Data DataMACA could greatly relieve both problems,

but not completely solve them.

RTS

Data

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MACAW: MACA for Wireless LANsMACAW: MACA for Wireless LANs

Problems in MACA

Enhancement of the MACA by V. Bharghavan (1994) RTS-CTS-DS-DATA-ACK

R1 S2S1

CTS

RTSRTS

Data

Exposed Terminal Situation

R2

Overhearing RTS

No CTS

RTSRTS

CTSData

Back-off

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MACAW Packet Exchange: MACAW Packet Exchange: RTS-CTS-DS-DATA-ACKRTS-CTS-DS-DATA-ACK ACK for the fast error

recovery DS (Data Sending) packet

to ensure successful RTC-CTS dialog to solve exposed terminal

Include RRTS (request for RTS) to inform neighbor sender of tx timing

CTSRTS

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BEB in MACA may starves flows

Back-off counter carried in packet header is copied by receiving node

Reset to min value after every successful transmission

MILD back-off ( x1.5, -1 ) implements per flow fairness

Run back-off algorithm for each queue (per flow)

high volumeof traffic

collisionBEB

MACAW Back-off ModificationMACAW Back-off Modification

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FAMA: Floor Acquisition Multiple FAMA: Floor Acquisition Multiple Access Access C. Fullmer, J. Garcia-Luna-

Aceves (1995) In MACA,

data packets are prone to collisions with RTS packets (because of no CS)

Tx of bursts of packets is not possible

Floor acquisition Floor (channel) is acquired by

means of exchanging control packets (RTS-CTS) before transmission

Refinement of the MACA Duration of RTS >= 2τ (max

channel propagation delay)• To ensures that data packets

are always transmitted without collision

The length of the CTS is made longer than the RTS to deal with HTP of MACA

• The dominating CTS plays the role of Busy Tone

S1 S2R

Data

Hidden Terminal Situation

Data

MACA

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FAMA (Cont’d)FAMA (Cont’d)

2 FAMA protocols RTS-CTS exchange with no CS: ALOHA + RTS/CTS RTS-CTS exchange with non-persistent CS: non-

persistent CSMA• FAMA-NTR (non-persistent transmit request)

FAMA-NTR If channel is busy, sender backs off for a random period

and retry later If channel is idle,

• sender listens to the channel after RTS tx• If no CTS received within 2τ or corrupted, then take

random back-off and retry later• If CTS received, transmit a burst of data packets

Packet burst transmission• Receiver: wait RTS for τ seconds after each data packet

received• Sender: wait CTS for 2 τ seconds after tx RTS

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Contention-basedContention-basedSender-initiatedSender-initiatedMulti-channel ProtocolsMulti-channel Protocols

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BTMA: Busy Tone Multiple AccessBTMA: Busy Tone Multiple Access 2 channels: data/control ch.

Control ch for Tx busy tone signal

Carrier sense on busy tone before transmission.

If idle, turn on busy tone and start Tx

Any other nodes which sense carrier on the incoming data channel also Tx busy tone signal

No other nodes in the 2-hop neighborhood of the Tx node is permitted to simultaneously transmit

Perfect solution. But need a busy tone channel and extra interface. Channel gains on data and busy tone channels may be different.

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DBTMA: Dual BTMADBTMA: Dual BTMA

Control channel for RTS-CTS Busy tones

• BTt : to indicate Tx on data ch• BTr : to indicate Rx on data ch

RTS/CTS-based MAC (MACA and MACAW) block both the forward and backward Tx

But, DBTMA blocks reverse Tx

If no BTr,Tx RTS

If no BTt,Tx CTS

Block other nodes’ Rx

Block other nodes’ Tx

S1 R1S2R2 S3

BTt BTr

Rx cleared

Tx cleared

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Contention-basedContention-basedReceiver-initiated Receiver-initiated ProtocolsProtocols

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Receiver Initiated ProtocolsReceiver Initiated Protocols

Features Receiver polls its neighbor asking for data Reduce the number of control packets More efficient than sender-initiated collision avoidance

Design Issues – How to Poll the neighbors ? Polling Rate

• Whether the polling rate is independent of the data rate at polling nodes

• Independent Polling / Data Driven Polling Intended Audience

• Whether the poll is sent to a particular neighbor or to all neighbors

Intent of a polling packet• Whether the polling packet asks for permission to transmit

as well

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RI-BTMA: Receiver-Initiated BTMARI-BTMA: Receiver-Initiated BTMA

Data packet: preamble (P) + DATA Preamble carries ID of

intended DEST node Data and control

channels are slotted Each slot equal to

preamle Busy tone means

ACK the sender about successful reception of preamble

Block hidden node’s Tx

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MACA-BI (MACA - By Invitation)MACA-BI (MACA - By Invitation) F. Taluci, M. Gerla (1997) CTS RTR (Ready to Receive)

RTR packet carries time interval during DATA Tx Traffic prediction by receiver: Time interval is estimated by

• DATA packet modified to carry control information regarding backlog such as # of packets queued and packet lengths

• Or RTS from sender to declare it backlog, if RTR is not received within a given time period

But, RTR may collide DATA may collide with RTR

RTR

(1) (2)

DATA Hidden terminal:

Blocked fromTransmission

RTR3DATA DATA

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MARCH: Media Access MARCH: Media Access with Reduced Handshakewith Reduced Handshake Does not require any

traffic prediction Neighbor

overhearing CTS transmit CTS to receive DATA RTS is used only for the

first packet of the stream

Route identification CTS contains: MAC-SA,

MAC-DA, RTid

Lower # of control packets improves throughput and reduce E-E delay

MACA MARCH

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RIMA: Receiver Initiated Multiple RIMA: Receiver Initiated Multiple AccessAccess A. Tzamaloukas, J. Garcia-Luna-

Aceves (1999) RIMA-SP (Simple Polling), RIMA-DP (Dual-use Polling), RIMA-BP (Broadcast Polling)

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Contention-based Contention-based Synchronous Protocols Synchronous Protocols with Reservationwith Reservation

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Quality of Service Quality of Service

Difficulties in Quality of Service Support in Ad-hoc Networks No centralized coordinator : ex) IEEE 802.11 PCF (Point

Coordination Function) To guarantee a QoS request, a Distributed/Dynamic

Reservation Scheme is needed. IEEE 802.11.e EDCF (Enhanced DCF)

Provides Differentiated Access; Up to 8 Access Categories (AC)

• A Station should have separate Queues for each AC• Each AC may have different Values for Contention Window and

AIFS

aSlotTime

Backof Window Next FrameBusy Medium

Defer Access

PIFS

SIFS

DIFS/AIFS

DIFS/AIFS

Contention Window

Select slot and DecrementBackoff as long as medium is idle

Immediate accesswhen medium is free>= DIFS/AIFS[i] AIFS[i]

AIFS[j]

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D-PRMA: Distributed Packet RSV D-PRMA: Distributed Packet RSV MAMA Extends PRMA protocol

for voice support in AHWN

Contention only during reservation. Once reserved, CF

Slot-reservation A certain period at the

beginning of each minislot is reserved for CS

First minislot is used to contend the slot; if no node wins, the remaining minislots are used for contention until a contending node wins (RTS/CTS exchange)

Within reserved slot, communication between source and receiver nodes takes place by means of TDD or FDD

Prioritization Contention with

probability p• For first minislot, p=1 for

voice, p < 1 for data• For remaining minislots, p

< 1 for voice and data Only if a voice node wins,

reserve the same slot in each subsequent frame

• Receiver transmit BI through RTS/BI part of minislot 1 (eliminate HTP)

• Sender transmit BI through CTS/BI part of minislot 1

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CATA: CA Time AllocationCATA: CA Time Allocation Supports unicast, broadcast, and multicast

Works well with simple single-channel half-duplex radios Minislots

CMS1: receiver tx SR (slot rsv) packet to sender CMS2: sender tx RTS (for uni/broad/multicast session) Unicast session

• CMS3: Receiver tx CTS (rsv the same slot in subsequent frames)• CMS4: if sender sense idle, rsv was successful. Tx packets during

DMS Multicast session

• CMS3: Receiver remains idle. And listen• CMS4:

– Receiver: If listen anything during CMS3, tx NTS (not-to-send) packet to sender

– Sender: If receive NTS or noise, reservation had failed. Otherwise, reservation was successful. Tx packets during DMS

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HRMA: Hop RSV MAHRMA: Hop RSV MA Mulitichannel MAC

protocol based on simple half-duplex, very slow FHSS radios Reserve a FH Guarantee collision-free data

tx Time slot reservation where

each time slot is assigned a separate frequency channel

Frequency: slot fo : synchronizing frequency

for synchnorizing slot (fi, fi*), i=1,2, …, M slots

• fi: used for HR, RTS, CTS, data packet tx

• fi*: used for sending and receivng ACK packets

Each time slot divided into Synchronizing period: all idle

nodes exchange synchronization information with freq fo

HR (hop rsv) period• If hear HR packet, random

back-off RTS/CTS period

• If free, RTS/CTS exchange• If source hear CTS, successful

reservation of the current hop

Merging of subnets

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Contention-based Contention-based Asynchronous Protocols Asynchronous Protocols with Reservationwith Reservation

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MACA/PR (Piggyback Reservation)MACA/PR (Piggyback Reservation) C. Lin, M. Gerla (1999) BW reservation for real-time

packet Each node maintains a

reservation table (RT) that records all the reserved tx and rx slots/windows of all nodes within its transmission range

Periodically exchanges RT (overcome HTP)

For a non real-time packet, MACAW-based MAC is used

For a real-time traffic, slots are periodically guaranteed at each links on the path (per superframe/CYCLE)

The first data packet is transmitted just as best-effort packet, but reservation information is piggy-backed

Receiver node updates it RT, and pigg-backs the rsv confirmation information on ACK packet

QoS routing protocol used in MACA/PR: DSDV routing protocol

Adv: Does not require global synchronization among nodes

Drawback: A free slot can be reserved only if it can fit RTS-CTS-DATA-ACK exchange

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RTMAC: Real-Time MACRTMAC: Real-Time MAC Real-time extension of IEEE

802.11 DCF RTS, CTS, ACK for best-effort

packets ResvRTS, ResvCTS, ResvACK for

real-time packets• IFS for real-time packet = ½ DIFS

BW reservation Reserves a variable length

connection-slot (a set of resv-slot) on successive superframes

Each node maintains a RT containing information such as sender id, receiver id, starting/ending times of reservation

No time synchronization is assumed

• Superframe may not strictly align with the other nodes

• Protocol uses relative time for reservation

– Relative time + local clock absolute time

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RTMAC (Cont’d)RTMAC (Cont’d) 3-way handshake for reservation process

ResvRTS-ResvCTS-ResvACK if reservation OK If receiver rx ResvRTS on a slot reserved by neighbor,

• does not responde (because ACK/NACK packet may cause collision)

If receiver rx ResvRTS on a free slot, but requested connection-slot is not free on reveiver,

• tx negative CTS (resvNCTS) back to sender Reservation release

Sender broadcasts the release RTS (ResvRelRTS)• Nodes hearing this packet update their RT in order to free

the connection Receiver node respondes by broadcasting ResvRelCTS

packet• Receiver’s neighbor nodes update their RT in order to free

the connection QoS routing protocol

An extension of DSDV routing protocol