MAC Protocols for Ad Hoc Wireless Networks

1

Wireless Interference



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



A receiver must get a minimum SINR for successful reception

 This means no other transmitter (interferer) in vicinity.

 If untrue -> collision (SINR insufficient).



Quintessential MAC problem

 Schedule transmissions on links conflict-free.

2

Time Division Multiple Access (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 3

Carrier Sense Multiple Access (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).

4

Virtual Carrier Sensing

A B RTS C DATA RTS CTS DATA ACK D CTS ACK E

 

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).

5

802.11 Timeline

Transmitter Receiver DIFS RTS SIFS SIFS CTS DATA ACK SIFS Nodes that hear DIFS transmitter NAV (RTS) NAV (CTS) Nodes that hear receiver Defer access Random Another backoff transfer 

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 6

Hidden and Exposed Terminal Problems (Revisited)



In Ad Hoc networks, HTP and ETP would happen frequently. Conventional CSMA severely suffer from both HTP and ETP !

7

Design 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

8

Classification of MAC Protocols for AHWN

9

Contention-based Sender-initiated Single-channel Protocols

10

MACA: Multiple Access Collision Avoidance

     

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

11

MACA example



MACA avoids HTP RTS

S1

RTS CTS

R

CTS

S2 

MACA avoids ETP

R1 S1 S2

RTS CTS RTS

Overhearing RTS R2 Data Data Vulnerable period is known to C by CTS

MACA could greatly relieve both problems, but not completely solve them.

12 Data Data

RTS

No CTS

RTS CTS

Data Data

MACAW: MACA for Wireless LANs



Problems in MACA Exposed Terminal Situation

R1 S1 S2 R2 

Enhancement of the MACA by V. Bharghavan (1994)

 RTS-CTS-DS-DATA-ACK

RTS CTS

Data

RTS

Overhearing RTS

RTS

Data No CTS

RTS CTS

Back-off 13

MACAW Packet Exchange: RTS-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

14 RTS CTS

MACAW Back-off Modification



BEB in MACA may starves flows

high volume of traffic collision BEB 

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) 15

FAMA: Floor Acquisition Multiple 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

MACA Hidden Terminal Situation

S1 R S2 Data Data 16

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 17

Contention-based Sender-initiated Multi-channel Protocols

18

BTMA: 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.

19

R2

DBTMA: Dual BTMA

  

Control channel for

 RTS-CTS  Busy tones • • BT t : to indicate Tx on data ch BT r : 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

Rx cleared Tx cleared BTt BTr S2 S1 R1 S3 If no BTr, Tx RTS Block other nodes’ Rx If no BTt, Tx CTS Block other nodes’ Tx 20

Contention-based Receiver-initiated Protocols

21

Receiver 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 22

RI-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 23

MACA-BI (MACA - By Invitation)

 

F. Taluci, M. Gerla (1997) CTS



RTR (Ready to Receive)

 

(1)

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

(2) DATA RTR

Hidden terminal: Blocked from Transmission 

But, RTR may collide



DATA may collide with RTR

DATA RTR 3 DATA 24

MARCH: Media Access with 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 MACA  

Route identification

 CTS contains: MAC-SA, MAC-DA, RT id

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

25 MARCH

RIMA: Receiver Initiated Multiple Access



A. Tzamaloukas, J. Garcia-Luna-Aceves (1999)



RIMA-SP (Simple Polling),



RIMA-DP (Dual-use Polling),



RIMA-BP (Broadcast Polling)

26

Contention-based Synchronous Protocols with Reservation

27

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 Immediate access when medium is free >= DIFS/AIFS[i] DIFS/ AIFS AIFS[j] AIFS[i] DIFS/ AIFS PIFS Busy Medium SIFS Contention Window Backof Window Next Frame aSlotTime Defer Access 28 Select slot and Decrement Backoff as long as medium is idle

D-PRMA: Distributed Packet RSV MA

  

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 29

CATA: 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 30

HRMA: 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 31

Contention-based Asynchronous Protocols with Reservation

32

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

33

RTMAC: 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 slot superframes connection (a set of resv-slot) on successive   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  time absolute 34

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 35