ĐIỆN tử VIỄN THÔNG l1 802 11 ACN2016 khotailieu

Wireless LANs ACN2016 Wireless LANs: Characteristics • Advantages – Flexible deployment; Minimal wiring problems – More robust against disasters – Historic buildings, conferences, … • Disadvantages – Low bandwidth compared to wired networks – Need to follow wireless spectrum regulations ACN2016 Infrastructure and Adhoc Networks infrastructure network AP AP wired network AP: Access Point AP ad-hoc network ACN2016 Source: Schiller Wireless LANs are different… • Destination address does not equal destination location • The media impact the design – wireless LANs intended to cover reasonable geographic distances must be built from basic coverage blocks • Impact of handling mobile (portable) stations – Propagation effects – Mobility management – power management ACN2016 Difference Between Wired and Wireless Ethernet LAN Wireless LAN B A B C C A • If both A and C sense the channel to be idle at the same time, they send at the same time • Collision can be detected at sender in Ethernet ACN2016 Wireless PHY – Medium has neither absolute nor readily observable boundaries outside which stations are unable to receive frames – Are unprotected from outside signals and are significantly less reliable than wired PHYs – Have time varying and asymmetric propagation properties – Lack full connectivity • the assumption that every station (STA) can hear every other STA in invalid ACN2016 Wireless MAC: Motivation • Can we apply media access methods from fixed networks? • Example CSMA/CD – Carrier Sense Multiple Access with Collision Detection – send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) ACN2016 Wireless MAC – signal strength decreases inversely proportional to the square of the distance – sender would apply CS and CD, but the collisions happen at the receiver – sender may not “hear” the collision, i.e., CD does not work – CS might not work, e.g if a terminal is “hidden” ACN2016 Hidden Terminal Problem A B C – A and C cannot hear each other – A sends to B, C cannot receive A – C wants to send to B, C senses a “free” medium (CS fails) – Collision occurs at B – A cannot receive the collision (CD fails) – A is “hidden” for C ACN2016 Exposed Terminal Problem D A B C – A starts sending to B – C senses carrier, finds medium in use and has to wait for A->B to end – D is outside the range of A, therefore waiting is not necessary ACN2016 10 Power saving with wake-up patterns (adhoc) ATIM window station1 beacon interval B1 station2 A B2 B2 B1 D a d t B beacon frame awake random delay A transmit ATIM D transmit data a acknowledge ATIM d acknowledge data ACN2016 68 802.11 - Roaming • Scanning – scan the environment, i.e., – passive scanning • listen into the medium for beacon signals (to detect other network) – active scanning • send probes into the medium on each channel and wait for an answer • Station then selects the best AP (e.g based on signal strength) – sends association Request to the AP • association Response – success: AP has answered, station is now associated with the new AP – failure: continue scanning ACN2016 69 Roaming (contd.) • AP accepts Association Request – signal the new station to the distribution system – the distribution system updates its data base (i.e., location information) – typically, the distribution system now informs the old AP so it can release resources ACN2016 70 Hardware • Original WaveLAN card (NCR) – – – – 914 MHz Radio Frequency Transmit power 281.8 mW Transmission Range ~250 m (outdoors) at 2Mbps SNRT 10 dB • WaveLAN II (Lucent) – 2.4 GHz radio frequency range – Transmit Power 30mW – Transmission range 376 m (outdoors) at Mbps (60m indoors) – Receive Threshold = –81dBm – Carrier Sense Threshold ACN2016 = -111dBm 71 802.11 status LLC 802.11i security WEP 802.11f MAC Mgmt MAC Inter Access Point Protocol 802.11e MIB PHY QoS enhancements DSSS 802.11b 5,11 Mbps 802.11g 20+ Mbps ACN2016 FH IR OFDM 802.11a 6,9,12,18,24 36,48,54 Mbps 72 IEEE 802.11 Summary • Infrastructure (PCF) and adhoc (DCF) modes • Signaling packets for collision avoidance – Medium is reserved for the duration of the transmission – Beacons in PCF – RTS-CTS in DCF • Acknowledgements for reliability • Binary exponential backoff for congestion control • Power save mode for energy conservation ACN2016 73 HIPERLAN • Wireless LAN ratified by ETSI • HIPERLAN1 – – – – First of the series of spec Supports five different priorities Data rate of 23.5 Mbps Forwards packets using several relays • Extends communication beyond the radio range – Power conservation by specific sleep and wakeup pattern – MSDU lifetime can be set to have time bound services – MAC layer uses residual lifetime and user priority to choose the next MSDU to be transmitted ACN2016 74 HIPERLAN2 • • • • Operates at GHz Data rates up to 54 Mbps OFDM in the physical layer and a dynamic TDMA/TDD based MAC QoS support – Each connection has its QoS parameters (delay, jitter, bit error) • Connection oriented – Negotiation of QoS parameter during connection establishment • Dynamic frequency selection – Best frequency chosen based on interference level and usage of radio channels • Power save – Mobile devices can negotiate certain sleep and wakeup pattern for power save • Access Points can have multiple transceivers ACN2016 • APs can have sectorized antenna 75 HIPERLAN2 • Two modes of operation • Centralized Mode (CM) – Like the infrastructure mode in 802.11, APs are connected to a core network and Mobile Stations (MS) are associated with APs • Direct Mode (DM) – This is the optional ad hoc mode of HiperLAN2 – Data is directly exchanged between MS • But the network is still controlled • Done via an AP that has the central controller (CC) functionality or via an MS that has CC functionality • This ensures QoS support in ad hoc mode also ACN2016 76 HIPERLAN2 AP AP/CC data control MS control MS control MS MS data Centralized mode Direct mode Different modes of operation of HiperLAN2 ACN2016 77 Bluetooth • Design goal was to set up short range ad hoc network (called piconets) • 79 channels in the 2.4 GHz band with MHz carrier spacing • Devices perform frequency hopping at 1600 hops/s • Maximum data rate of 1Mbps • Range of about 10m ACN2016 78 Bluetooth P S S M SB S P M – Master S - Slave SB – Standby P - Park SB Bluetooth Piconet ACN2016 79 Bluetooth • Piconet – A collection of bluetooth devices which are synchronized to the same hopping sequence – One of the devices is the master, all others are slaves – Master determines the hopping pattern in the piconet and the slaves have to synchronize to this pattern – Each piconet has a unique hopping pattern – Parked devices • Cannot participate in the piconet, but are known and can be activated within few msec • Devices in standby not participate in piconet ACN2016 80 Bluetooth • Piconet – Active members assigned a 3-bit active member address (AMA) • Upto devices can be active in a piconet – Parked devices use 8-bit parked member address (PMA) – Standby devices not need address ACN2016 81 Bluetooth • Scatternet – Only having piconet within 80 MHz in total is not very efficient – Many piconets with overlapping coverage can exist simulatenously • A device may participate in two different piconets – Bluetooth uses FH-CDMA for separation of piconet – A slave first syncs to one piconet and communicates, then leaves that piconet and enters the other piconet (of scatternet) by syncing to its FH sequence – A master cannot be shared between two piconets of a scatternet – Master can leave one piconet and enter the other as a slave • All traffic in the former piconet is suspended until the master returns ACN2016 82 ... application application TCP TCP IP IP LLC LLC LLC 802. 11 MAC 802. 11 MAC 802. 3 MAC 802. 3 MAC 802. 11 PHY 802. 11 PHY 802. 3 PHY 802. 3 PHY ACN2016 21 802. 11 - Layers and functions • MAC • PLCP Physical... frequency 802. 11 LAN ACN2016 16 Source: Schiller 802. 11 - infrastructure 802. 11 LAN STA1 802. x LAN BSS1 Portal Access Point Distribution System Access Point ESS BSS2 STA2 802. 11 LAN ACN2016 STA3... layer in the GHz band ACN2016 13 802. 11 architecture • The basic service set (BSS) is the basic building block of an IEEE 802. 11 LAN ad-hoc network BSS1 ACN2016 BSS2 14 802. 11 architecture (contd.)