seminar 2 multi channel multi radio wireless mesh networks
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Seminar 2: Multi-Channel, Multi-Radio Wireless


  1. �������������������������������� ��������������������������������� Seminar 2: Multi-Channel, Multi-Radio Wireless Mesh Networks r e l s s a K Andreas J. Kassler . J s kassler@ieee.org a e r d n A Topics in Computer Networks 2009 • Seminar 3 1

  2. ��������������������������������� Overview � Principles of Mesh Networks � Multi Radio Multi Channel � Conclusions ��������������������������������� r e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 2

  3. ��������������������������������� Overview � Principles of Mesh Networks � Multi Radio Multi Channel � Conclusions ��������������������������������� r e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 3

  4. ����������������� �������� Current Wireless Networks � Infrastructure based – needs “wired” connectivity to access points. – Deployment slow and expensive R Internet Wired Backbone ��������������������������������� r e l s s Node X a K . out of J s a Reach e r d n A Topics in Computer Networks 2009 • Seminar 3 4

  5. ����������������� �������� Multi Hop Wireless Networks � Get rid of the wires! – mesh routing backbone created by grid of wireless APs – Clients can associate with any access point. – Complete transparency: nodes forward voice, R video and data traffic to and from nearby nodes Internet wirelessly and ultimately to the internet ��������������������������������� Every node is now Every node is now Access Point AND Router Access Point AND Router r e l s Node Reachable! Node Reachable! s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 5

  6. ����������������� �������� Multi Hop Wireless Networks � Wireless mesh networks – Small number of wireless hops to gateway R Internet ��������������������������������� r e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 6

  7. ����������������� �������� Multi Hop Wireless Networks � Why interesting and study? – No Wires! – Properties: • Robust & Fault tolerant • Self organising • Self configuring • Self healing ��������������������������������� • No centralized management – Empowers the individual and community r e l s s A WMN is dynamically self-organized and self-configured, with the a A WMN is dynamically self-organized and self-configured, with the K . J nodes in the network automatically establishing and maintaining nodes in the network automatically establishing and maintaining s a e r mesh connectivity among themselves mesh connectivity among themselves d n A Topics in Computer Networks 2009 • Seminar 3 7

  8. !����"���������������������������������� Broadband Internet Access for rural/urban areas � Metropolitan scale mesh networks � chaska.net – City of Chaska (8000 homes, 23.000 residents) � 28% uptake after 2 years – Nomadic broadband service for $17.99 per month – Based on Tropos mesh products • $600,000 infrastructure plus 2 month deployment �������������� • 365 mesh routers � 95% coverage • 60 backhaul links ��������������������������������� �������������� r e l s s a K . J s a e r �������������� d n A Topics in Computer Networks 2009 • Seminar 3 8

  9. !����"���������������������������������� WiFi Mesh for Rural Networks Extend Internet access into areas � which do not have wired networking infrastructure. � Reduced Infrastructure cost � Typically semi infrastructured backbone network (Mesh) Long distance links can be common � Cheap, Off the shelf hardware � Mission to support both social & � ��������������������������������� economic development � Useful for developing areas r e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 9

  10. �������#��������� �������� An Early Multi Hop Wireless Network ��������������������������������� r What Challenges can we identify? What Challenges can we identify? e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 10

  11. �������#��������� �������� Channel Access � Wired networking protocols such as Ethernet perform poorly when used in wireless communication Why? Because of media dependent differences � You Should know: ��������������������������������� – Hidden terminal problem – Exposed terminal problem – Collision detection problem r – Interference problem e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 11

  12. � ����$�� Medium Access Coordination no parallel transmission no parallel reception ��������������������������������� Goal for MAC layer design: r e • avoid parallel interfering transmissions l s s a K • do not hinder parallel non-interfering transmissions . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 12

  13. � ����$�� Random Access without reservation IEEE 802.11 Distributed Coordination Function – DCF (CSMA CA) � – Stations have to equally compete for access to the medium – Acknowledgment scheme is used for error indication CW = 4 CW = 11 Node 1 DIFS DIFS Busy Data Frame (transmit) Time SIFS ACK Node 2 (receive) Time ��������������������������������� CW = 9 CW=9-4=5 Node 3 DIFS DIFS Busy Busy NAV (transmit) Time � simple, well accepted, most frequently implemented and used r e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 13

  14. � ����$�� Hidden Node Problem � Hidden Node Problem – A mesh node is hidden for an ongoing transmission if it is not able to sense the ongoing transmission but its transmission would disturb the reception. – A node not in the sensing range of the transmitter but within the interference range of the receiver hidden nodes � HN induced problems – Throughput degradation ��������������������������������� – Unfairness r e l s s a K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 14

  15. � ����$�� Simple Reservation – Two way Handshake � IEEE 802.11 RTS/CTS – RTS: Request to send – CTS: Clear to send hidden nodes � Nodes receiving RTS or CTS might not get involved in new transmissions CTS ��������������������������������� RTS � RTS/CTS – ��������� solves hidden node problem – induces increased overhead and delay r e l s s a – also virtual carrier sensing K . J s a e r d n A Topics in Computer Networks 2009 • Seminar 3 15

  16. � ����$�� RTS/CTS CW = 4 SIFS DIFS DIFS DATA Busy RTS sender Time SIFS SIFS DIFS CTS ACK receiver Time CW = 9 other 1 DIFS DIFS Busy NAV(RTS) Time ��������������������������������� CW = 13 DIFS DIFS other 2 Busy NAV(CTS) Time r e l s s a K . J receiver other 2 other 1 sender s a e r d n A Topics in Computer Networks 2009 • Seminar 3 16

  17. � ����$�� RTS/CTS Problems CTS sender receiver hidden node ��������������������������������� interfering range receiving range A node unable to decode the CTS might nevertheless disturb the transmission. � � Hidden node problem still exists. r e l s s Critical if adaptive modulation and coding is used. � a K . J s Threshold on packet size for RTS/CTS usage typically maximum packet size. � a e r d n A Topics in Computer Networks 2009 • Seminar 3 17

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