22-07-2014, 02:10 PM
Mobile Computing
[attachment=66117]
ISSUES in Wireless MAC
Bandwidth Efficiency
BW available is very limited
MAC should be designed such that the scarce bandwidth is utilized in an efficient manner
Hidden and Exposed Node Problem
Collision-prone shared channel
Multiple nodes may contend for the medium leading to collision
MAC should make sure that collision is minimized
Design Goals
Available bandwidth should be utilized efficiently
Fair allocation of bandwidth
Control overhead should be kept low
Should minimize the effect of hidden and exposed node
Should be scalable to large network
Should have power control mechanisms to manage energy consumption of the nodes
MACA
Proposed as an alternative to the traditional CSMA
CSMA senses the state of the channel only at the transmitter
Leads to hidden node problem
Does not use carrier sensing
Nodes start transmitting after a random backoff
MACA uses RTS and CTS to overcome hidden node problem and exposed node problem
Node which only hears CTS (but no RTS), stops from transmitting (hidden node)
Node which only hears RTS (but no CTS), is free to transmit (exposed node)
RTS and CTS carry the expected duration of data transmission
When there is a collision, it uses binary exponential backoff (BEB) before retrying
CATA (Cont’d)
If a sender wants to reserve DMS slot, it first checks that CMS1 slot is idle
Sends RTS in CMS2
Rcvr should send CTS in CMS3
the reservation is then successful
Sender sends data in DMS
Once reservation is successful in a slot, from next slot sender and rcvr do not send anything in CMS3, but sender sends not-to-send (NTS) during CMS4
CATA (Cont’d)
NTS is a negative ACK
any potential broadcast or multicast source which detects NTS (or noise) during CMS4, understand that the reservation is made by someone else and hence no broadcast or multicast is done in the slot
If CMS4 was idle, then broadcast/multicast source starts transmitting
[attachment=66117]
ISSUES in Wireless MAC
Bandwidth Efficiency
BW available is very limited
MAC should be designed such that the scarce bandwidth is utilized in an efficient manner
Hidden and Exposed Node Problem
Collision-prone shared channel
Multiple nodes may contend for the medium leading to collision
MAC should make sure that collision is minimized
Design Goals
Available bandwidth should be utilized efficiently
Fair allocation of bandwidth
Control overhead should be kept low
Should minimize the effect of hidden and exposed node
Should be scalable to large network
Should have power control mechanisms to manage energy consumption of the nodes
MACA
Proposed as an alternative to the traditional CSMA
CSMA senses the state of the channel only at the transmitter
Leads to hidden node problem
Does not use carrier sensing
Nodes start transmitting after a random backoff
MACA uses RTS and CTS to overcome hidden node problem and exposed node problem
Node which only hears CTS (but no RTS), stops from transmitting (hidden node)
Node which only hears RTS (but no CTS), is free to transmit (exposed node)
RTS and CTS carry the expected duration of data transmission
When there is a collision, it uses binary exponential backoff (BEB) before retrying
CATA (Cont’d)
If a sender wants to reserve DMS slot, it first checks that CMS1 slot is idle
Sends RTS in CMS2
Rcvr should send CTS in CMS3
the reservation is then successful
Sender sends data in DMS
Once reservation is successful in a slot, from next slot sender and rcvr do not send anything in CMS3, but sender sends not-to-send (NTS) during CMS4
CATA (Cont’d)
NTS is a negative ACK
any potential broadcast or multicast source which detects NTS (or noise) during CMS4, understand that the reservation is made by someone else and hence no broadcast or multicast is done in the slot
If CMS4 was idle, then broadcast/multicast source starts transmitting