1. MAC Protocols for Ad Hoc Wireless Networks

2. 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.

3. 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

4. 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).

5. Virtual Carrier SensingB C D E
RTS
RTS
CTS
CTS
DATA
DATA
ACK
ACK
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).

6. 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.
RTS
DATA
Transmitter
SIFS
SIFS
DIFS
CTS
ACK
Receiver
SIFS
Nodes that hear
transmitter
DIFS
NAV (RTS)
NAV (CTS) t
Nodes that hear
receiver
Defer access
Random
backoff
Another
transfer

7. 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 !

8. 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

9. Classification of MAC Protocols for AHWN

10. Contention-based Sender-initiated Single-channel Protocols

11. 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

12. MACA example MACA avoids HTP MACA avoids ETPR S2 R1 S1 S2 R2
RTS
RTS
RTS
RTS
CTS
CTS
CTS
Overhearing RTS
Data
Data
Data
Data
No CTS
RTS
RTS
CTS
Vulnerable period is known to C by CTS
Data
Data
MACA could greatly relieve both problems,
but not completely solve them.

13. MACAW: MACA for Wireless LANs
Problems in MACA
Enhancement of the MACA by V. Bharghavan (1994)
RTS-CTS-DS-DATA-ACK
Exposed Terminal Situation R1 S1 S2 R2
RTS
RTS
CTS
Overhearing RTS
No CTS
RTS
RTS
CTS
Data
Data
Back-off

14. 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
CTS
RTS

15. MACAW Back-off Modification
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 volume
of traffic
collision
BEB

16. FAMA: Floor Acquisition Multiple Access
MACA
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
Hidden Terminal Situation S1 R S2
Data
Data

17. FAMA (Cont’d) 2 FAMA protocols FAMA-NTR
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

18. Contention-based Sender-initiated Multi-channel Protocols

19. 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.

20. DBTMA: 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 R1 S2 R2 S3
BTt
BTr
Rx cleared
Tx cleared

21. Contention-based Receiver-initiated Protocols

22. 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

23. 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

24. 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
(1)
(2)
DATA
RTR
Hidden terminal:
Blocked from
Transmission
RTR3
DATA

25. 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
Route identification
CTS contains: MAC-SA, MAC-DA, RTid
Lower # of control packets improves throughput and reduce E-E delay
MACA
MARCH

26. 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)

27. Contention-based Synchronous Protocols with Reservation

28. Quality of Service
Difficulties in Quality of Service Support in Ad-hoc Networks
No centralized coordinator : ex) IEEE PCF (Point Coordination Function)
To guarantee a QoS request, a Distributed/Dynamic Reservation Scheme is needed.
IEEE 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

29. 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

30. 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

31. 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

32. Contention-based Asynchronous Protocols with Reservation

33. 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

34. 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 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

35. 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