issues and research directions in mac layer design for ad

September 19, 2005 MANETs

Issues and research directions in MAC layer design for Ad hoc networks


It’s all about sharing!


The Problem* RF occupies 360 steradians

causing collisions. * Common Space (HN)

‐ Efficiency: Maximize throughput of the entire network by minimizing collisions in a distributed manner.

‐ Fairness: Every node in the network be able to transmit equally.

* Identifying “non” common spots (EN)

* Scalability* Power efficiency* Topology changes* Multihop communication.


Outline

* Classification of published MAC protocols [Jurdak etal2004]

* New research directions

* Conclusion


Outline

* Classification of published MAC protocols

* New research directions

* Conclusion


Channel Separation and Access

* Single Channel:* Transmitter Initiated: Generalized networks with predictable traffic patterns, more intuitive. ‐ CSMA, CSMA/CD, CSMA/CA, MACAW (Ack).‐ DPC‐AP (Distr. Power Control‐Adaptive Probing)‐ PS‐DCC (802.11[standard]): Probabilistic back‐off.

* Receiver Initiated: Sensor networks, and high density networks.‐ MACA‐BI (By Invitation), RIMA‐SP (Simple polling).


Channel Separation and Access contd…

* Multiple Channels:* Generalized Separation: Data and control planes are separate, using busy tone or RTS/CTS‐ N‐ data channels and 1 control channel (DCAPC)‐ Channel borrowing in cells.

* Time Division Multiplexing : Division in time* For real‐time traffic* Needs Synchronization (GPS or other)

‐ Slot reservation by nodes,‐ Priority of nodes‐ Classification based on traffic (Hard/Soft real time, NRT)



* FDMA: Division in frequency.* Propose to use CSMA. (HN is not solved)* Need hardware compatibility…

* CDMA: N codes for data, one for control.* IEEE 802.11 Ad hoc mode: Uses combines CSMA with MACAW. * Splits the medium into two using FHSS or DSSS, with a busy channel signal.

* Contention window with random back off ensures fairness.



* SDMA: Similar to CDMA, uses full spectrum all the time, using directional antennas. * Polling using omni directional, after handshake, data is transmitted.

* Keep neighbors transceiver directions, sense the medium in that direction.

* Hybrid Protocols: Combine two or more of previous approaches.* Division in (upstream and down stream) frequency and then in time.

* Division time and frequency simultaneously (Hop reservation).

* Bluetooth: Combines CDMA and TDMA. Master to ~ 7 slaves.


Topology Based MAC* In Ad Hoc networks, topology is dynamic and nodes are heterogeneous in capabilities.* Single hop flat‐ Each node has to contend only with its immediate neighbors. ‐ Scale poorly with node density, as delay increase exponentially. ‐ Mobility increases power consumption.

* Multi‐hop flat‐ More scalable, power efficient.‐ Frequency channel reuse/Spatial reuse‐ Directional antennas improve the efficiency.


Topology Based MAC cntd..

* Clustered Topology:* Cluster head takes away control overhead from the nodes within the cluster.‐ Choose the lowest IP ied node as the head randomly.‐ Weighted clustering based on nodes ability to be the head. ‐ Eg, Hyperlan and Bluetooth

* Centralized Topology: Central base station coordinates medium access. –Not suitable for Ad hoc.


Power

* MAC is designed to optimize the usage of the power: More for data and less for control.* Adaptive power increase: Keep increasing the power so it is just enough to reach the receiver.

* Sleep mode : [PAMAS] On a common control channel knowing the duration of transmission, neighbors in the range turn power off. [HyperLan]: p (power)‐savers and p‐supporters manage sleeping patterns. [Bluetooth] : 3‐level operation.

* Battery level awareness ‐ Lower battery, higher priority.

* Reducing control overhead (March – Minimize handshakes)

* TDMA/multichannel has more overhead.


Other classifications* Transmission Initiation: Typically sender initiated

* For sensor networks, receiver initiation makes more sense. (More nodes have data to send/or in a are)

* Should make sure that only one node transmits.* Traffic Load and scalability:

* High load: Receiver initiated MAC, directional antennas.* Hotspots borrow channels from neighboring cells. (Channel

borrowing).* High Density: Transmission power control to avoid collisions.

* Directional antennas.* Voice and real time traffic: Priority based. Cluster based to allocate

circuits/manage synchronization.* Range: Very Short range : Blue tooth.

* Short range: 802.11a, Hyperlan* Medium range: 802.11b., Channel borrowoing. (Grid‐B)


Some research directions – Directional Antenna [Vaidya2003]

A

B

C

A silenced node

D


Directional Antennas* Transmit gain = receive gain,

therefore during communication, antenna is directional.

* Communication is allowed through “unblocked (directions)”antennas.

* DMAC Scenario1: Directional RTS and Omni CTS with location information:

* C knows the direction in which B is transmitting, can transmit/receive in a different direction. ACK collision is possible.

* DMAC scenario 2: DRTS/ORTS: Lesser ACK collision probability.

A

B

C

D


Antenna Model

* Directional transmission* Ability to beam form in any direction* Single beam /multi‐beam operation* Ability to “beam detect” .* Idle node is Omni directional.


Directional Antennas

* Better performance in stationary environments compared to omni directional.

* Capacity improvement: throughput improvement factor is 2π/α , 2π/β and 4π/αβ for directional transmission/omni reception, omni transmission/directional reception, and directional transmission/directional reception, where α, βare the transmission and receive angles. [Su etal2003]

* Spatial filtering and adaptive beam forming.* Can find fewer hop routes and energy efficient routes

‐ Ability to localize and find longer range neighbors, control topology.

* Directional virtual carrier sensing for MAC.‐ Deafness: ‐ Capture: Receive unwanted communication.


Cross layer designs

* Cross layer designs: Use “network status” to enhance MAC performance.

‐ Optimal Back off to achieve higher utilization‐ Solve hidden and exposed terminal problems by utilising “topology” information from higher layers.‐MobileMan[Conti etal2004]


Technologies

* Acoustic: Underwater communication* Optical: Indoor/Clear weather* RF

* Omni‐directional* Directional: Single /Multi beam.

* UWB: Very wide band, hence higher throughput.* Short range, high spatial reuse.


Conclusion

* MAC design is basically an optimization problem, the objective function is very specific to application and constraints are imposed by the technology at hand.

* New technologies continually shape ad hoc network design, so while designing a new Ad hoc MAC, one should capitalize on available technologies.


References:* Raja Jurdak, Videira Lopes and Pierre Baldi, A survey, classification and comparative analysis of Medium access controlprotocols for Ad Hoc networks. IEEE Communications Surveys.

* Romit Roy Choudhury, Nitin Vaidya, Deafness: A MAC problem in Ad hoc Networks when using directional Antennas, ICNP 2004.

* Nitin Vaidya, Tutorial: MAC and routing with directional Antennas.

* Su Yi, Yong Pei, and Shivkumar Kalyanaraman, ʺOn the Capacity Improvement of Ad Hoc Wireless Networks Using Directional Antennas,ʺ (Mobihoc), Annapolis, MD, June 2003

* Marco Conti, Gaia Maselli, Giovanni Turi, Cross layering in Mobile Ad hoc Network Design. Computer, Feb 2004..

* http://www.intel.com/technology/ultrawideband/downloads/Ultra‐Wideband.pdf

http://www.intel.com/technology/ultrawideband/downloads/Ultra-Wideband.pdf

http://www.intel.com/technology/ultrawideband/downloads/Ultra-Wideband.pdf

issues and research directions in mac layer design for ad

Documents