2a Wireless MAC, TCP

2a Wireless MAC, TCP - 2.a Wireless LAN (04/06) 1....

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1 2.a Wireless LAN (04/06) 1. Introduction 2. Wireless LAN Protocols a. MACA b. MACAW 3. Wireless communication protocol stack 4. MAC 802.11 Sublayer Protocol 5. CSMA/CA 6. The 802.11 Frame Structure 7. Services 8. TCP Congestion Control 9. 802.11b MAC Protocol (Review) 10. TCP/UDP over 802.11b Wireless Networks 11. Appendix 12. Conclusion (Recommended:T.208-211, 462-464, 553-555) (T. 68-71,134-137, 267-270,292-310)
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2 802.11 Wireless LAN Configurations BS The standardized in mid 1990 , to send an IP packet over the WLAN, make 802.11 compatible with Ethernet above the DLL. 1. Ad-Hoc Networking 2. Peer-to-peer Networking
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3 IEEE 802.11 Wireless LAN Applications: Internet access, portable connection, Ad Hoc networking (multi-hopping) IEEE 802.11 standards define MAC protocol; unlicensed frequency spectrum bands: 900Mhz, 2.4 GHz -5.7 GHz BS Like a bridged LAN (flat MAC address) Portal connection
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4 c c c f t f t t f SDMA Guard space SDMA is always used in combination with one or more other schemes. Cells
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5 FDMA (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel.
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6 Random Access protocols ( Review) Ethernet A node transmits at random (no priority among nodes). If collide ”, they retransmit at random times The random access MAC protocol specifies how to detect collisions and how to recover from them ( via delayed retransmissions, binary back off ) Random access MAC protocols: (a) ALOHA (b) SLOTTED ALOHA (c) CSMA and CSMA/CD
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7 Collision Sender A Sender B Sender C Sender A Sender B Sender C Aloha (18%) Slotted Aloha (37%) Aloha Multiple Access
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8 Differences with Ethernet Ethernet operates with CSMA/CD protocol. With wireless LANs that idea does not work well. 1. Problems with multipath fading of a radio signal 2. Mobility of wireless communication stations. 3. Base station-to-base station movement . 4. Exposed and hidden stations problems From the outside, the entire system should look like a single Ethernet. The connection between the 802.11 system and the outside world is called Portal.
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9 CSMA (Carrier Sense Multiple Access) CSMA : listen before transmit. If channel is sensed busy , listens again. (Leonard Kleinrock, 1975). Persistent CSMA : retry immediately when collide (this may cause instability) Non persistent CSMA : retry after random interval Note: collisions may still exist, since two stations may sense the channel idle at the same time In case of collision, the entire packet transmission time is wasted CDMA with Collision Avoidance (CA) access schemes used in wireless LANs. Here sensing the carrier combined with a back-off scheme. (MACA)
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10 IEEE standard 802.11 mobile terminal access point (BS) server fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC LLC Each LAN cell has only one channel, Unlike cellular systems, covering the entire available bandwidth and covering all the stations in this cell. Typically, its Bit Rate is 11 to 54 Mbps.
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11 Wireless LAN Protocols A B C D A B C D Radio range a b Hidden station problem Exposed station problem Two Major Problems Possible interference at B
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2a Wireless MAC, TCP - 2.a Wireless LAN (04/06) 1....

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