Background of Ad hoc Wireless Networks

Student Presentations

Wireless Communication Technology and Research

Ad hoc Routing and Mobile IP and Mobility

Wireless Sensor and Mesh Networks

Mobile and Ad hoc Networks

Adhoc Wireless MACAdhoc Wireless MAChttp://web.uettaxila.edu.pk/CMS/SP2012/teAWNms/

Outline Wireless MAC Issues

Hidden terminal problem Exposed terminal problem Capture

MAC Performance Metrics Wireless MAC Classification Distributed Wireless MAC Protocols

Aloha Slotted Aloha CSMA CSMA/CA 802.11 MAC

DCF Backoff

Hiper LAN MAC

Contention-based Protocols ALOHA

Developed in the 1970s for a packet radio network by Hawaii University.

Whenever a terminal (MS) has data, it transmits. Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. Sender retransmits after some random time if there is a collision.

Slotted ALOHA Improvement: Time is slotted and a packet can only be

transmitted at the beginning of one slot. Thus, it can reduce the collision duration.

Contention-based Protocols (Contd.) CSMA (Carrier Sense Multiple Access)

Improvement: Start transmission only if no transmission is ongoing

CSMA/CD (CSMA with Collision Detection) Improvement: Stop ongoing transmission if a

collision is detected CSMA/CA (CSMA with Collision Avoidance)

Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit

CSMA/CA with ACK CSMA/CA with RTS/CTS

Contention-based Protocols (Contd.) CSMA (Carrier Sense Multiple Access)

Improvement: Start transmission only if no transmission is ongoing

CSMA/CD (CSMA with Collision Detection) Improvement: Stop ongoing transmission if a

collision is detected CSMA/CA (CSMA with Collision Avoidance)

Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit

CSMA/CA with ACK CSMA/CA with RTS/CTS

Pure ALOHA (Collision Mechanism)

when frame first arrives - transmit immediately collision probability increases:

frame sent at t0 collides with other frames sent in [t0-1 and t0+1]

Pure ALOHA (Collision Mechanism) collision probability increases:

frame sent at t0 collides with other frames sent in [t0-1 and t0+1]

Slotted Aloha

all frames same size time is divided into

equal size slots (time to

transmit 1 frame) nodes start to transmit

frames only at beginning

of slots nodes are synchronized if 2 or more nodes transmit in

slot, all nodes detect collision

Assumptions Operation

when node obtains fresh frame, it transmits in next slot

no collision, node can send

new frame in next slot if collision, node retransmits

frame in each subsequent slot with prob. p until success

Slotted Aloha

single active node can continuously transmit at full rate of channel

highly decentralized: only slots in nodes need

to be in sync simple

Pros Cons

collisions, wasting slots idle slots nodes may spend more time

to detect collisions, leaving lesser time to transmit packet

clock synchronization

Throughput Performance of Aloha

Carrier Sense Multiple Access

Max throughput achievable by slotted ALOHA is 0.368.

CSMA gives improved throughput compared to ALOHA protocols.

Listens to the channel before transmitting a packet (avoids avoidable collisions).

CSMA Collision

Collision can still occur: propagation delay means

two nodes may not hear

each other’s transmission Collision:

entire packet transmission

time wasted Note:

role of distance & propagation

delay in determining collision

probability

Kinds of CSMA

Non Persistent CSMA Protocol

Non persistent CSMA Protocol: Step 1: If the medium is idle, transmit immediately Step 2: If the medium is busy, wait a random amount

of time and repeat Step 1 Random back-off reduces probability of collisions Waste idle time if the back-off time is too long

1-Persistent CSMA Protocol

1-Persistent CSMA Protocol: Step 1: If the medium is idle, transmit immediately Step 2: If the medium is busy, continue to listen until

medium becomes idle, and then transmit immediately There will always be a collision if two nodes want

to retransmit (usually you stop transmission attempts after few tries)

p-Persistent CSMA Protocol

p-Persistent CSMA Protocol: Step 1: If the medium is idle, transmit with probability

p, and delay (for worst case propagation delay) for one packet with probability (1-p)

Step 2: If the medium is busy, continue to listen until medium becomes idle, then go to Step 1

Step 3: If transmission is delayed by one time slot, continue with Step 1

A good tradeoff between non-persistent and 1-persistent CSMA

How to select Probability p?

Assume that N nodes have a packet to send and the

medium is busy

Then, Np is the expected number of nodes that will

attempt to transmit once the medium becomes idle

If Np >1, then a collision is expected to occur

Therefore, network must make sure that Np ≤1 to

avoid collision, where N is the maximum number of

nodes that can be active at a time

Throughput Comparison

Distributed MAC Protocols

Most distributed MAC protocols are based on the principle of carrier sensing & collision avoidance (CSMA/CA)

Hidden terminals play very dominant role in CSMA/CA based protocols Collisions that occur at the destination may not be

heard by the sender Therefore receiver has to send some kind of

feedback to sender

Distributed MAC – Hidden Node Problem

In the diagram simultaneous transmission will collide at B Half duplex operation of wireless terminals, transmission ➔

and listening simultaneously is not possible. A and C have no knowledge of this collision, Two ways A & C can know that collision has occurred at B

1. A & C periodically stop transmission and listen for feedback from B

2. B uses out of band signaling to inform A & C

Collision Avoidance Techniques

First option is very difficult, second option is possible but (inefficient) because it requires additional channel

Two well know approaches of collision avoidance (CA) Out-of-band approach Hand shaking approach

CA with Out of Band Signaling Busy Tone Multiple Access (BTMA) (Tobagi & Kleinrock,

Haas) protocol uses out of band signaling to solve hidden terminal problem

Any node hearing ongoing transmission, transmits busy tone All nodes hearing busy tone keep silent All nodes in 2R radius of the transmitter keep silence (R –

range) Avoids interference from hidden terminals Requires a separate channel for busy tone Con: Eliminates hidden nodes, increases exposed nodes

CA with Out of Band Signaling Receiver Initiated Busy Tone Multiple Access

(RIBTMA) (S. Wu. & V.O.K. Li, 1988) Only receiver transmits the busy tone The receiver decodes the message and verifies the

address that it is indeed the receiver The nodes in the vicinity of the receiver (Radius R) are

inhibited Does not eliminate hidden terminal problem

completely, but reduces the exposed nodes

CA with Control Handshaking – (MACA) Alternative to carrier sensing i.e. does not use CSMA Multiple Access with Collision Avoidance (MACA) uses three

way handshake to avoid hidden terminal problem (Karn, 90) When node B wants to send a packet to node C, node B first

sends a Request-to-Send (RTS) to C All nodes within one hop of the sending node hear the RTS and

defer their transmissions. On receiving RTS, node C responds by sending Clear-to-Send

(CTS), provided node C is able to receive the packet When a node (such as D) overhears a CTS, it keeps quiet for

the duration of the transfer Transfer duration is included in RTS and CTS both

Hidden Terminal Avoidance

MACA Examples

MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving

CTS from B MACA avoids the problem of

exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS

from A

CA with Control Handshaking (MACAW)

Does not completely solve the hidden terminal problem but does prevent to a large extent

Enhancements to RTS-CTS control hand shaking and more complete single channel solutions [(MACAW (1997), Bhargavan] [Fullmer et al, (1995, 1997)] MACA has no ACK but MACAW does

In these techniques tradeoff is in the overhead of handshaking and the number of hidden nodes removed

Reliability

Wireless links are prone to errors. High packet loss rate detrimental to transport-layer performance.

MACA delegates packet loss recovery to transport layer ➔ Higher delays Better to perform at the MAC layer

Mechanisms needed to reduce packet loss rate experienced by upper layers

A Simple solution to Improve Reliability (MACAW)

When node B receives a data packet from node A, node B sends an Acknowledgement (ACK). This approach adopted in many protocols [Bharghavan94,IEEE 802.11]

If node A fails to receive an ACK, it will retransmit the packet

The Incompleteness of the RTS-CTS method

The Incompleteness of the RTS-CTS method

Using a directional antenna to resolve theexposed node problem

Assignment #4

Go through the research papers highlighted with Yellow in today’s lecture and write their abstract and contribution in your own words.

Q&A

?