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802.11a/b/g Networks

802.11a/b/g Networks. Herbert Rubens herb@cs.jhu.edu. Some slides taken from UIUC Wireless Networking Group. 802.11a/b/g. Operates in 2 different modes: Infrastructure mode Associates with an access point All communication goes through the access point

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802.11a/b/g Networks

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  1. 802.11a/b/g Networks Herbert Rubens herb@cs.jhu.edu Some slides taken from UIUC Wireless Networking Group

  2. 802.11a/b/g • Operates in 2 different modes: • Infrastructure mode • Associates with an access point • All communication goes through the access point • Used for wireless access at a company or campus • Peer-to-Peer Ad Hoc Mode • If two nodes are within range of each other they can communicate directly with no access point • A few users in a room could quickly exchange files with no access point required

  3. Infrastructure Access • Access Points: • Provide infrastructure access to mobile users • Cover a fixed area • Wired into LAN

  4. Peer to Peer Ad Hoc Mode

  5. Infrastructure Access

  6. 802.11a/b/g are multi-rate devices 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps

  7. MAC Layer Fairness Models • Per Packet Fairness: If two adjacent senders continuously are attempting to send packets, they should each send the same number of packets. • Temporal Fairness: If two adjacent senders are continuously attempting to send packets, they should each be able to send for the same amount of medium time. • In single rate networks these are the SAME!

  8. Temporal Fairness Example Per Packet Fairness 11 Mbps 1 Mbps Temporal Fairness 11 Mbps 1 Mbps

  9. 802.11b Channels • 11 available channels (in US) • Only 3 are non-overlapping!

  10. Channel 1 Channel 6 Channel 11

  11. Problems • Access Point placement depends on wired network availability • Obstructions make it difficult to provide total coverage of an area • Site surveys are performed to determine coverage areas • Security Concerns: rogue access points in companies etc.. • Each Access Point has limited range

  12. Peer to Peer Ad Hoc Mode

  13. Peer to Peer Ad Hoc Mode X X X

  14. Problems • Communication is only possible between nodes which are directly in range of each other

  15. Problems for both Infrastructure and Ad hoc Mode • If nodes move out of range of the access point (Infrastructure Mode) • OR nodes are not in direct range of each other (Ad Hoc Mode) • Then communication is not possible!!

  16. What if ?? Multi-hop Infrastructure Access Multi-hop Ad Hoc Network OR

  17. Multi-hop Infrastructure Access • Nodes might be out of range of the access point, BUT in range of other nodes. • The nodes in range of the access point could relay packets to allow out of range nodes to communicate. • NOT part of 802.11

  18. Multi-hop Ad Hoc Network • If communication is required between two nodes which are out of range of each other, intermediary nodes can forward the packets. • NOT part of 802.11 Destination Source

  19. How can this be done? • ROUTING!! • Wired Networks: • Hierarchical Routing • Network is divided into subnets • Nodes look at netmask and determine if the address is directly reachable. If not, just forward to the default gateway. • Different protocols for different levels of the hierarchy • RIP, OSPF, BGP

  20. Wireless Routing • Flat routing • You can’t assume that since a node is in your subnet that it is directly accessible • Node must maintain or discover routes to the destination • All nodes are routers

  21. Mobile Ad Hoc Networks • Formed by wireless hosts which may be mobile • Without (necessarily) using a pre-existing infrastructure • Routes between nodes may potentially contain multiple hops

  22. Mobile Ad Hoc Networks • May need to traverse multiple links to reach a destination

  23. Mobile Ad Hoc Networks (MANET) • Mobility causes route changes

  24. Why Ad Hoc Networks ? • Ease of deployment • Speed of deployment • Decreased dependence on infrastructure

  25. Many Applications • Personal area networking • cell phone, laptop, ear phone, wrist watch • Military environments • soldiers, tanks, planes • Civilian environments • taxi cab network • meeting rooms • sports stadiums • boats, small aircraft • Emergency operations • search-and-rescue • policing and fire fighting

  26. Challenges • Limited wireless transmission range • Broadcast nature of the wireless medium • Packet losses due to transmission errors • Mobility-induced route changes • Mobility-induced packet losses • Battery constraints • Potentially frequent network partitions • Ease of snooping on wireless transmissions (security hazard)

  27. Unicast RoutinginMobile Ad Hoc Networks

  28. Why is Routing in MANET different ? • Host mobility • link failure/repair due to mobility may have different characteristics than those due to other causes • Rate of link failure/repair may be high when nodes move fast • New performance criteria may be used • route stability despite mobility • energy consumption

  29. Unicast Routing Protocols • Many protocols have been proposed • Some have been invented specifically for MANET • Others are adapted from previously proposed protocols for wired networks • No single protocol works well in all environments • some attempts made to develop adaptive protocols

  30. Routing Protocols • Proactive protocols • Determine routes independent of traffic pattern • Traditional link-state and distance-vector routing protocols are proactive • Reactive protocols • Maintain routes only if needed • Hybrid protocols

  31. Trade-Off • Latency of route discovery • Proactive protocols may have lower latency since routes are maintained at all times • Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y • Overhead of route discovery/maintenance • Reactive protocols may have lower overhead since routes are determined only if needed • Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating • Which approach achieves a better trade-off depends on the traffic and mobility patterns

  32. Overview of Unicast Routing Protocols

  33. Flooding for Data Delivery • Sender S broadcasts data packet P to all its neighbors • Each node receiving P forwards P to its neighbors • Sequence numbers used to avoid the possibility of forwarding the same packet more than once • Packet P reaches destination D provided that D is reachable from sender S • Node D does not forward the packet

  34. Flooding for Data Delivery Y Z S E F B C M L J A G H D K I N Represents a node that has received packet P Represents that connected nodes are within each other’s transmission range

  35. Flooding for Data Delivery Y Broadcast transmission Z S E F B C M L J A G H D K I N Represents a node that receives packet P for the first time Represents transmission of packet P

  36. Flooding for Data Delivery Y Z S E F B C M L J A G H D K I N • Node H receives packet P from two neighbors: • potential for collision

  37. Flooding for Data Delivery Y Z S E F B C M L J A G H D K I N • Node C receives packet P from G and H, but does not forward • it again, because node C has already forwarded packet P once

  38. Flooding for Data Delivery Y Z S E F B C M L J A G H D K I N • Nodes J and K both broadcast packet P to node D • Since nodes J and K are hidden from each other, their • transmissions may collide • =>Packet P may not be delivered to node D at all, • despite the use of flooding

  39. Flooding for Data Delivery Y Z S E F B C M L J A G H D K I N • Node D does not forward packet P, because node D • is the intended destination of packet P

  40. Flooding for Data Delivery Y Z S E F B C M L J A G H D K I N • Flooding completed • Nodes unreachable from S do not receive packet P (e.g., node Z) • Nodes for which all paths from S go through the destination D • also do not receive packet P (example: node N)

  41. Flooding for Data Delivery Y Z S E F B C M L J A G H D K I N • Flooding may deliver packets to too many nodes • (in the worst case, all nodes reachable from sender • may receive the packet)

  42. Flooding for Data Delivery: Advantages • Simplicity • May be more efficient than other protocols when rate of information transmission is low enough that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher • this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions • Potentially higher reliability of data delivery • Because packets may be delivered to the destination on multiple paths

  43. Flooding for Data Delivery: Disadvantages • Potentially, very high overhead • Data packets may be delivered to too many nodes who do not need to receive them • Potentially lower reliability of data delivery • Flooding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead • Broadcasting in IEEE 802.11 MAC is unreliable • In our example, nodes J and K may transmit to node D simultaneously, resulting in loss of the packet • in this case, destination would not receive the packet at all

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