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 MAC

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  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 t 0 collides with other frames sent in [t 0 -1 and t 0 +1]

Pure ALOHA (Collision Mechanism)  collision probability increases:  frame sent at t 0 collides with other frames sent in [t 0 -1 and t 0 +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 ProsCons  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  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, N p is the expected number of nodes that will attempt to transmit once the medium becomes idle  If N p >1, then a collision is expected to occur Therefore, network must make sure that N p ≤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 ]  If node A fails to receive an ACK, it will retransmit the packet

The Incompleteness of the RTS-CTS method

Using a directional antenna to resolve the exposed 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 ??