Mobile Communications Chapter 7: Wireless LANs

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1 Characteristics IEEE PHY MAC Roaming IEEE a, b, g, e HIPERLAN Bluetooth Comparisons Prof. Dr.-Ing. Jochen Schiller, MC SS Comparison: infrastructure vs. ad-hoc networks infrastructure network AP AP wired network AP: Access Point AP Infrastructure networks: provide access to other networks typically communications between the wireless node and the access point, but not directly between the wireless nodes. the access points controls medium access, but also act as a bridge to other wireless or wired networks in the figure three WLANs with different coverage areas are connected most of the network functionality lies within the access points, whereas the wireless clients can remain quite simple. the MAC can be centralised to the access point or be distributed oven the wireless clients. these wireless networks do rely on the access points. Prof. Dr.-Ing. Jochen Schiller, MC SS

2 Comparison: infrastructure vs. ad-hoc networks ad-hoc network Ad-hoc networks: no need for any infrastructure to work. each node can communicate directly with other nodes no access point controlling media access is necessary nodes within an ad-hoc network can communicate directly or via other nodes the complexity of each node is higher because each node has - implement MAC-mechanisms - handle hidden or exposed terminal problem - perhaps priority mechanisms to provide a certain QoS Prof. Dr.-Ing. Jochen Schiller, MC SS IEEE standard Protocol architecture The basic IEEE standard!! mobile terminal fixed terminal application TCP IP LLC MAC PHY access point LLC MAC MAC PHY PHY infrastructure network application TCP IP LLC MAC PHY Prof. Dr.-Ing. Jochen Schiller, MC SS

3 PHY DLC University of Karlsruhe Synchronization DSSS PHY packet format (More important!!!) synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) indicates the start of a frame Signal data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: compliant HEC (Header Error Check) protection of signal, service and length, x 16 +x 12 +x 5 +1 length of the payload variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header Prof. Dr.-Ing. Jochen Schiller, MC SS Traffic services MAC layer I DFWMAC Distributed Foundation Wireless Medium Access Control Asynchronous Data Service (mandatory) exchange of data packets based on best-effort support of broadcast and multicast Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods DFWMAC-DCF CSMA/CA (mandatory) (DCF: distributed coordination function)* collision avoidance via randomized back-off mechanism ACK packet for acknowledgements (not for broadcasts) DFWMAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem DFWMAC- PCF (optional) (PCF: point coordination function) access point polls terminals according to a list LLC MAC PLCP PMD Prof. Dr.-Ing. Jochen Schiller, MC SS

4 MAC layer II General! Priorities defined through different inter frame spaces (IFS) no guaranteed hard priorities (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF, Point Coordination Function IFS) medium priority, for time-bounded service using PCF (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service medium PIFS contention next frame direct access if medium is free t Prof. Dr.-Ing. Jochen Schiller, MC SS competing stations - simple version bo r bo r station 1 station 2 station 3 bo r station 4 bo r bo r station 5 t medium not idle (frame, ack etc.) elapsed backoff time packet arrival at MAC bo r residual backoff time Prof. Dr.-Ing. Jochen Schiller, MC SS

5 Motivation - hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is hidden for C Exposed terminals A B C B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is exposed to B Prof. Dr.-Ing. Jochen Schiller, MC SS Fragmentation If a node senses the wireless channel to be very disturbed, the node may choose to fragment its sending frame into small sub frames (frags). sender receiver RTS CTS frag 1 ACK 1 frag 2 ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) contention data t In the frags, the control field gives information to other stations of how to set their NAV. Prof. Dr.-Ing. Jochen Schiller, MC SS

6 Frame format MAC frames Types control frames, management frames, data frames Sequence numbers important against duplicated frames due to lost ACKs es receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous bytes sending time, checksum, frame control, data Frame Control Duration/ ID Sequence Control 4 Data CRC bits Protocol version Type Subtype To DS From DS More Frag Power Retry Mgmt More Data WEP Order Prof. Dr.-Ing. Jochen Schiller, MC SS Special Frames: ACK, RTS, CTS Acknowledgement ACK bytes Frame Control Duration Receiver CRC Request To Send Clear To Send RTS CTS bytes bytes Frame Control Frame Control Duration Receiver Duration Receiver Transmitter CRC CRC Prof. Dr.-Ing. Jochen Schiller, MC SS

7 Synchronization using a Beacon (infrastructure) Each node of an network maintains an internal clock! To synchronize the clocks of all nodes, IEEE specifies a Timer synchronization function (TSF) Within a BSS (basic server set), timing is conveyed by the (quasi)periodic transmission of a beacon. A beacon contains a timestamp and other management information used for power management and roaming (e.g. identification of the BSS). Within infrastructure-base networks, the access point performs synch by transmitting beacons beacon interval access point medium B B B B value of the timestamp B beacon frame t Prof. Dr.-Ing. Jochen Schiller, MC SS Power saving with wake-up patterns (infrastructure) Example: one access point and one station The AP transmits a beacon frame each beacon interval. This interval is equal to the TIM interval. The AP maintains a delivery traffic indication map (DTIM) interval. TIM interval DTIM interval access point medium D B T T d D B station T TIM D DTIM p awake d t B broadcast/multicast p PS poll d data transmission to/from the station Prof. Dr.-Ing. Jochen Schiller, MC SS

8 Channel plan for IEEE b Channel Frequency (MHz) US/Canada Europé Japan x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Altogether 14 channels have been defined as the table shows. For each channel a center frequency is given. Depending on national restrictions 11 (US/Canada), 13 (Europe) or 14 channels (Japan) can be used. Prof. Dr.-Ing. Jochen Schiller, MC SS Channel selection (non-overlapping) Non-overlapping usage of channels for an IEEE b installation with minimum interference. The spacing between center frequencies should be at least 25 MHz. The occupied bandwidth of the main lobe of the signal is 22 MHz. Europe (ETSI) Users can install overlapping cells for WLANs using three non-overlapping chanells. channel 1 channel 7 channel [MHz] US (FCC)/Canada (IC) 22 MHz channel 1 channel 6 channel [MHz] 22 MHz Prof. Dr.-Ing. Jochen Schiller, MC SS

9 WLAN: IEEE some examples : A new release of the standard that includes amendments a, b, d, e, g, h, i & j. (July 2007) e: MAC Enhancements QoS Enhance the current MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol g: Data Rates at 2.4 GHz; 54 Mbit/s, OFDM, (backwards compatible with b) (2003) s: Mesh Networking, Extended Service Set (ESS) (June 2011) ad: Very High Throughput 60 GHz (~Dec 2012) Prof. Dr.-Ing. Jochen Schiller, MC SS Ad Hoc Networks Power is an issue All stations are mobile Prof. Dr.-Ing. Jochen Schiller, MC SS

10 Broadband Internet access via mesh networks Power is NOT an issue Stations are NOT mobile FL, UniK 4290: Introduction, Mesh networking with Ad hoc networks FL, UniK 4290: Introduction,

11 Sensor networks Power is an issue Stations are NOT mobile Prof. Dr.-Ing. Jochen Schiller, MC SS End Prof. Dr.-Ing. Jochen Schiller, MC SS

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