Ad-hoc networks

1. Ad-hoc & Sensor Based Networks
ECE752
P. Uday Ashish
M. Tech (E&C)
A2315320006
Contention based MAC Protocols

3.  These protocols follow a contention based channel access policy. Resource
reservation is not a priority
 Whenever a nodes receives a packet to be transmitted it contends with it
neighbor nodes for access to the shared channel
 These protocols cannot provide QoS guarantees since the nodes are not
guaranteed regular access to the channel
 Two Types: -
• Sender Initiated
• Receiver Initiated

4. MACA (Multiple Access with Collision Avoidance)
RTS: Request to Send
CTS : Clear to Send
• After receiving an RTS
from the sender all
terminal will defer until a
CTS packet is received by
the sender
• All terminals hearing CTS
defer for the length of data
transmission

5. • Binary Exponential Back off (BEB)
Algorithm is used in MACA
• It maintains a Back off Counter (BO)
• After every collision BO is doubled
• After every successful RTS-CTS exchange
BO is reduced to minimal.
Limitations: -
• No ACK Packet
• Unfair sharing of bandwidth

6. MACAW (MACA for Wireless)
RTS: Request to Send
CTS : Clear to Send
ACK: Acknowledgment
DS: Data Sending
RRTS: Ready for RTS
• After receiving an RTS from the sender all
terminal will defer until a CTS packet is
received by the sender
• All terminals hearing CTS defer for the length
of data transmission
• Neighbor terminals defer until an ACK packet
is received after data transmission

7. • Multiplicative Linear Increase & Decrease
(MILD) Algorithm is used in MACAW
• After every collision BO is multiplied by
1.5 units of time
• After every successful RTS-CTS exchange
BO is decreased by 1 unit of time.
Advantages over MACA
• Has an ACK Packet
• Hidden & exposed terminal problem
are handled
Limitations: -
• Does not completely solve hidden &
exposed node problem

8. MACA-BI (MACA By Invitation)
RTR: Ready to receive
• Receiver initiated Protocol
• Data transmission occurs
through a two way
handshake mechanism
• Requires information
about the traffic at
neighboring nodes

9. Limitations: -
• Cannot work properly with irregular traffic

10. BTMA (Busy Tone Multiple Access)
• Proposed for overcoming hidden
terminal problem
• Uses a busy tone
• Transmission channel is split into 2 –
 Data Channel
 Control Channel
• Low probability of collision

11. DBTMA (Dual Busy Tone Multiple Access)
• Transmission channel is split into 2 –
 Data Channel – Data Packet
Transmission
 Control Channel – RTS, CTS, Busy Tones
• Two busy tones BTt & BTr are used
• Low probability of collision
• Exhibits better network utilization

12. RI-BTMA (Receiver Initiated Busy Tone Multiple Access)
• Data packet is divided into two
portions- Preamble
Actual data Packet
• Preamble carries identification of the
intended destination node
• Data and control channels are slotted,
with each slot equal to the size of
preamble

13. Comparison
MAC Protocols Hidden & exposed issues Throughput Fairness Signals Used Channel Type
MACA Not Solved Poor RTS, CTS Single Channel Sender Initiated
MACAW Not Solved Higher than MACA RTS, CTS, ACK Single Channel Sender Initiated
MACA-BI Partially Solved Fair RTR Single Channel Receiver Initiated
BTMA Solved Average Busy Tone Multiple Channel Sender Initiated
DBTMA Solved Average RTS, CTS, Busy Tone Multiple Channel Sender Initiated
RI-DBTMA Solved Average RTS, CTS, Busy Tone Multiple Channel Receiver Initiated