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An Introduction to Computer Networks Lecture 8: Wirless Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani 1 Outline Why wireless Networks What is special on wireless networks Challenges Bluetooth Zigbee 802.11 802.11 mac Univ. of Tehran Introduction to computer Network 2 Why wireless networks? Mobility: to support mobile applications Costs: reductions in infrastructure and operating costs: no cabling or cable replacement Special situations: No cabling is possible or it is very expensive. Reduce downtime: Moisture or hazards may cut connections. Why wireless networks? (cont) Rapidly growing market attests to public need for mobility and uninterrupted access Consumers are used to the flexibility and will demand instantaneous, uninterrupted, fast access regardless of the application. Consumers and businesses are willing to pay for it The Two Hottest Trends in Telecommunications Networks 700 600 Millions Mobile Telephone Users 500 400 Internet Users 300 200 100 0 1993 1994 1995 1996 1997 1998 1999 2000 2001 Univ. of Tehran Year Source: Ericsson Radio Systems, Inc. Introduction to computer Network Growth of Home wireless Why is it so popular? Flexible Low cost Easy to deploy Support mobility Applications ? Ubiquitous, Pervasive computing or nomadic access. Ad hoc networking: Where it is difficult or impossible to set infrastructure. LAN extensions: Robots or industrial equipment communicate each others. Sensor network where elements are two many and they can not be wired!. Sensor Networks: for monitoring, controlling, e Ad hoc networks Collection of wireless mobile nodes dynamically forming a temporary network without the use of any existing network infrastructure or centralized administration. Hop-by-hop routing due to limited range of each node Nodes may enter and leave the network Usage scenarios: Military Disaster relief Temporary groups of participants (conferences) Sensor networks Deployment of small, usually wireless sensor nodes. Collect data, stream to central site Maybe have actuators Hugely resource constrained Internet protocols have implicit assumptions about node capabilities Power cost to transmit each bit is very high relative to node battery lifetime Loss / etc., like other wireless Ad-hoc: Deployment is often somewhat random Summary Need to be connected from everywhere and anytime. Need to be connected on movement Need to good quality service on those situation. Interworking with the existing networks Classification of Wireless Networks Mobility: fixed wireless or mobile Analog or digital Ad hoc (decentralized) or centralized (fixed base stations) Services: voice (isochronous) or data (asynchronous) Ownership: public or private Classification of Wireless Networks Area: wide (WAN), metropolitan (MAN), local (LAN), or personal (PAN) area networks Switched (circuit- or packet-switched) or broadcast Low bit-rate (voice grade) or high bit-rate (video, multimedia) Terrestrial or satellite What is special on wireless? Mobility in the network elements Very diverse applications/devices. Connectivity and coverage (internetworking) is a problem. Maintaining quality of service over very unreliable links Security (privacy, authentication,...) is very serious here. Broadcast media. Cost efficiency Big issues! Integration with existing data networks sounds very difficult. It is not always possible to apply wired networks design methods/principles here. Layering is not work very well, mostly we need cross layer design Wireless Differences 1 Physical layer: signals travel in open space Subject to interference From other sources and self (multipath) Creates interference for other wireless devices Noisy lots of losses Channel conditions can be very dynamic Wireless Differences 2 Need to share airwaves rather than wire Don’t know what hosts are involved Hosts may not be using same link technology Interaction of multiple transmitters at receiver Collisions, capture, interference Use of spectrum: limited resource. Cannot “create” more capacity very easily More pressure to use spectrum efficiently Wireless Differences 3 Mobility Must update routing protocols to handle frequent changes Changes in the channel conditions. Requires hand off as mobile host moves in/out range Coarse time scale: distance/interference/obstacles change Other characteristics of wireless Slow Growing Application Diversity Collision Avoidance: Car Networks Mesh Networks Wired Internet Access Point Sensor Relay Node Ad-Hoc/Sensor Networks Wireless Home Multimedia Challenge: Diversity Wireless Edge Network INTERNET INTERNET Wireless Edge Network 2005 2010 New architectures must accommodate rapidly evolving technology Must accommodate different optimization goals Power, coverage, capacity, price Other Challenges Performance: Nothing is really work well Security: It is a broadcast media Cross layer interception TCP performance Ideal Wireless Area network? Wish List High speed (Efficiency) Low cost No use/minimal use of the mobile equipment battery Can work in the presence of other WLAN (Heterogeneity) Easy to install and use Etc Univ. of Tehran Computer Network 23 Wireless LAN Design Goals Wireless LAN Design Goals Portable product: Different countries have different regulations concerning RF band usage. Low power consumption License free operation Multiple networks should co-exist Univ. of Tehran Computer Network 24 Wireless LAN Design Alternatives Design Choices Physical Layer: diffused Infrared (IR) or Radio Frequency (RF)? Radio Technology: Direct-Sequence or FrequencyHopping? Which frequency range to use? Which MAC protocol to use. Peer-Peer architecture or Base-Station approach? Univ. of Tehran Computer Network 25 DSSS (Direct Sequence Spread Spectrum) XOR of the signal with pseudo-random number (chipping sequence) generate a signal with a wider range of frequency: spread spectrum tb user data 0 1 XOR tc chipping sequence 01101010110101 = resulting signal 01101011001010 tb: bit period tc: chip period Radio Technology Spread Spectrum Technologies Frequency Hopping: The sender keeps changing the carrier wave frequency at which its sending its data. Receiver must be in synch with transmitter, and know the ordering of frequencies. Direct-Sequence: The receiver listens to a set of frequencies at the same time. The subset of frequencies that actually contain data from the sender is determined by spreading code, which both the sender and receiver must know. This subset of frequencies changes during transmission. Non-Spread Spectrum requires licensing Univ. of Tehran Computer Network 27 Wireless Standards Univ. of Tehran Computer Network 28 Distance vs. Data Rate Univ. of Tehran Computer Network 29 Bluetooth Goals Original goal Ad-hoc wireless connectivity for everything! Low-cost replacement for annoying wire between cellphone and headset Result: Two modes of operation Point to point (serial wire replacement) Point to multipoint (ad-hoc networking) Univ. of Tehran Computer Network 30 Bluetooth devices Cellphones Headsets PDAs Laptops Two-way pagers Pads, tabs, etc… Univ. of Tehran Computer Network 31 Bluetooth design Specs Started with Ericsson's Bluetooth Project in 1994 ! Named after Danish king Herald Blatand (AD 940-981) who was fond of blueberries Radio-frequency communication between cell phones over short distances Intel, IBM, Nokia, Toshiba, and Ericsson formed Bluetooth SIG in May 1998 Version 1.0A of the specification came out in late 1999. IEEE 802.15.1 approved in early 2002 is based on Bluetooth Key Features: Lower Power: 10 μA in standby, 50 mA while transmitting Cheap: $5 per device Small: 9 mm2 single chips Univ. of Tehran Computer Network 32 Bluetooth design Specs Frequency Range: 2402 - 2480 MHz (total 79 MHz band) 23 MHz in some countries, e.g., Spain Data Rate:1 Mbps (Nominal) 720 kbps (User) Channel Bandwidth:1 MHz Range: Up to 10 m can be extended further RF hopping: 1600 times/s => 625 μs/hop Security: Challenge/Response Authentication. 128b Encryption TX Output Power: Class 1: 20 dBm Max. (0.1W) – 100m Class 2: 4 dBm (2.5 mW) Class 3: 0 dBm (1mW) – 10m Univ. of Tehran Computer Network 33 Piconet Piconet is formed by a master and many slaves Up to 7 active slaves. Slaves can only transmit when requested by master Up to 255 Parked slaves Active slaves are polled by master for transmission Each station gets a 8-bit parked address => 255 parked slaves/piconet The parked station can join in 2ms. Other stations can join in more time. A device can participate in multiple piconets => complex schedule Univ. of Tehran Computer Network 34 Bluetooth Operational States Univ. of Tehran Computer Network 35 Bluetooth Operational States (Cont) Standby: Initial state Inquiry: Master sends an inquiry packet. Slaves scan for inquiries and respond with their address and clock after a random delay (CSMA/CA) Page: Master in page state invites devices to join the piconet. Page message is sent in 3 consecutive slots (3 frequencies). Slave enters page response state and sends page response including its device access code. Master informs slave about its clock and address so that slave can participate in piconet. Slave computes the clock offset. Connected: A short 3-bit logical address is assigned Transmit: Univ. of Tehran Computer Network 36 Bluetooth Packet Format Packets can be up to five slots long. 2745 bits. Access codes: Channel access code identifies the piconet Device access code for paging requests and response Inquiry access code to discover units Header: member address (3b), type code (4b), flow control, ack/nack (1b), sequence number, and header error check (8b) 8b Header is encoded using 1/3 rate FEC resulting in 54b Synchronous traffic has periodic reserved slots. Other slots can be allocated for asynchronous traffic 54b 0-2754b 72b Access Code Univ. of Tehran Baseband/link Control Header Computer Network Data Payload 37 Bluetooth Energy Management Three inactive states: Hold: No ACL. SCO (Sync data) continues. Node can do something else: scan, page, inquire Sniff: Low-power mode. Slave listens only after fixed sniff intervals. Park: Very Low-power mode. Gives up its 3-bit active member address and gets an 8-bit parked member address. Packets for parked stations are broadcast to 3-bit zero address. Sniff Univ. of Tehran Computer Network 38 Bluetooth Protocol Stack RF = Frequency hopping GFSK modulation Baseband: Frequency hop selection, connection, MAC Univ. of Tehran Computer Network 39 Baseband Layer Each device has a 48-bit IEEE MAC address 3 parts: Lower address part (LAP) – 24 bits Upper address part (UAP) – 8 bits Non-significant address part (NAP) - 16 bits UAP+NAP = Organizationally Unique Identifier (OUI) from IEEE LAP is used in identifying the piconet and other operations Clock runs at 3200 cycles/sec or 312.5 μs (twice the hop rate) Univ. of Tehran Computer Network 40 Bluetooth Protocol Stack Logical Link Control and Adaptation Protocol (L2CAP) Host Controller Interface RFCOMM Layer: Protocol multiplexing Segmentation and reassembly Controls peak bandwidth, latency, and delay variation Presents a virtual serial port Sets up a connection to another RFCOMM Service Discovery Protocol (SDP): Each device has one SDP which acts as a server and client for service discovery messages IrDA Interoperability protocols: Allow existing IrDA applications to work w/o changes Univ. of Tehran Computer Network 41 Bluetooth Protocol Stack IrDA object Exchange (IrOBEX) and Infrared Mobile Communication (IrMC) for synchronization Audio is carried over 64 kbps over SCO links over baseband Telephony control specification binary (TCS-BIN) implements call control including group management (multiple extensions, call forwarding, and group calls) Application Profiles: Set of algorithms, options, and parameters. Standard profiles: Headset, Cordless telephony, Intercom, LAN, Fax, Serial line (RS232 and USB). Univ. of Tehran Computer Network 42 ZigBee Ultra-low power, low-data rate, industrial monitoring and control applications requiring small amounts of data, turned off most of the time (<1% duty cycle), e.g., wireless light switches, meter reading, patient monitoring IEEE 802.15.4 Less Complex. 32kB protocol stack vs 250kB for Bluetooth Range: 1 to 100 m, up to 65000 nodes. Tri-Band: 16 Channels at 250 kbps in 2.4GHz ISM 10 Channels at 40 kb/s in 915 MHz ISM band One Channel at 20 kb/s in European 868 MHz band ! Ref: ZigBee Alliance, http://www.ZigBee.org Univ. of Tehran Computer Network 43 ZigBee Two types of devices: Full Function Devices (FFD) for network routing and link coordination Reduced Function Devices (RFD): Simple send/receive devices Univ. of Tehran Computer Network 44 802.11 LAN Architectures Distributed wireless Networks: also called Ad-hoc networks Centralized wireless Networks: also called last hop networks. They are extension to wired networks. Univ. of Tehran Computer Network 45 Wireless LAN Architecture Ad Hoc Laptop Server Laptop DS Access Point Access Point Pager PDA Univ. of Tehran Laptop Computer Network Laptop 46 Access Point Functions Access point has three components Wireless LAN interface to communicate with nodes in its service area Wireline interface card to connect to the backbone network MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently Univ. of Tehran Computer Network 47 Medium Access Control Wireless channel is a shared medium Need access control mechanism to avoid interference MAC protocol design has been an active area of research for many years. See Survey. Univ. of Tehran Computer Network 48 MAC: A Simple Classification Wireless MAC Centralized Distributed On Demand MACs, Polling Guaranteed or controlled access Random access Our focus FDMA, TDMA, Polling Univ. of Tehran Computer Network 49 Wireless MAC issues Half duplex operations: difficult to receive data while sending Time varying channel: Multipath propagation, fading Burst Channel error: BER is as high as 10-3. We need a better strategy to overcome noises. Location dependant carrier sensing: signal decays with path length. Hidden nodes Exposed nodes Capture: when a receiver can cleanly receive data from two sources simultaneously, the farther one sounds a noise. Univ. of Tehran Computer Network 50 Performance Metrics Delay: ave time on the MAC queue Throughput: fraction used for data transmission. Fairness: Not preference any node Stability: handle instantaneous loads greater than its max. capacity. Robust against channel fading Power consumption: or power saving Support for multimedia Univ. of Tehran Computer Network 51 Wireless LAN Architecture, Cont… Logical Link Control Layer MAC Layer: Consist of two sub layer, physical Layer and physical convergence layer Physical convergence layer, shields LLC from the specifics of the physical medium. Together with LLC it constitutes equivalent of Link Layer of OSI Univ. of Tehran Computer Network 52 Power Management Battery life of mobile computers/PDAs are very short. Need to save The additional usage for wireless should be minimal Wireless stations have three states Sleep Awake Transmit Univ. of Tehran Computer Network 53 Power Management, Cont… AP knows the power management of each node AP buffers packets to the sleeping nodes AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when? The node is awake only when it is sending data, receiving data or listening to TDIM. Univ. of Tehran Computer Network 54 802.11 Features Power management: NICs to switch to lower-power standby modes periodically when not transmitting, reducing the drain on the battery. Put to sleep, etc. Bandwidth: To compress data Security: Addressing: destination address does not always correspond to location. Univ. of Tehran Computer Network 55 IEEE 802.11 Topology Independent basic service set (IBSS) networks (Ad-hoc) Basic service set (BSS), associated node with an AP Extended service set (ESS) BSS networks Distribution system (DS) as an element that interconnects BSSs within the ESS via APs. Univ. of Tehran Computer Network 56 ESS topology connectivity between multiple BSSs, They use a common DS Univ. of Tehran Computer Network 57 802.11 Logical Architecture •PLCP: Physical Layer Convergence Procedure •PMD: Physical Medium Dependent •MAC provides asynchronous, connectionless service •Single MAC and one of multiple PHYs like DSSS, OFDM, IR and FHSS. Univ. of Tehran Computer Network 58 802.11 MAC Frame Format Bytes 32 Preamble 34~2346 6 MPDU PLCP header MAC Header Frame Duration Addr 1 Addr 2 Addr 3 Sequence Address 4 User Control Control Data Bytes 2 2 6 6 2 6 6 CRC 4 Encrypted to WEP Bits 2 2 Protocol Version 4 1 1 1 Type Sub type To From DS DS Univ. of Tehran Last Retry Power Fragment Mgt Computer Network EP RSVD 59 802.11 MAC Frame Format Address Fields contains Source address Destination address AP address Transmitting station address DS = Distribution System User Data, up to 2304 bytes long Univ. of Tehran Computer Network 60 Special Frames: ACK, RTS, CTS bytes Acknowledgement 2 2 6 Frame Receiver Duration Control Address ACK 4 CRC bytes Request To SendRTS 2 2 6 6 Frame Receiver Transmitter Duration Control Address Address bytes Clear To Send CTS 2 2 6 Frame Receiver Duration Control Address 4 CRC 4 CRC IEEE 802.11 LLC Layer Provides three type of service for exchanging data between (mobile) devices connected to the same LAN Acknowledged connectionless Un-acknowledged connectionless, useful for broadcasting or multicasting. Connection oriented Higher layers expect error free transmission Univ. of Tehran Computer Network 62 IEEE 802.11 LLC Layer, Cont.. Destination Source SAP SAP Control Data Each SAP (Service Access Point) address is 7 bits. One bit is added to it to indicate whether it is order or response. Control has three values Information, carry user data Supervisory, for error control and flow control Unnumbered, other type of control packet Univ. of Tehran Computer Network 63 IEEE 802.11 LLC <-> MAC Primitives Four types of primitives are exchanged between LLC and MAC Layer Request: order to perform a function Confirm: response to Request Indication: inform an event Response: inform completion of process began by Indication Univ. of Tehran Computer Network 64 Reception of packets AP Buffer traffic to sleeping nodes Sleeping nodes wake up to listen to TIM (Traffic Indication Map) in the Beacon AP send a DTIM (Delivery TIM) followed by the data for that station. Beacon contains, time stamp, beacon interval, DTIM period, DTIM count, sync info, TIM broadcast indicator Univ. of Tehran Computer Network 65 Frame type and subtypes Three type of frames Management Control Asynchronous data Each type has subtypes Control RTS CTS ACK Univ. of Tehran Computer Network 66 Frame type and subtypes, Cont.. Management Association request/ response Re-association request/ response: transfer from AP to another. Probe request/ response privacy request/ response: encrypting content Authentication: to establish identity Beacon (Time stamp, beacon interval, channels sync info, etc.) Univ. of Tehran Computer Network 67 Frame type and subtypes, Cont.. Management… TIM (Traffic Indication Map) indicates traffic to a dozing node dissociation Univ. of Tehran Computer Network 68 802.11 Management Operations Scanning Association/Reassociation Time synchronization Power management Univ. of Tehran Computer Network 69 Scanning in 802.11 Goal: find networks in the area Passive scanning Not require transmission Move to each channel, and listen for Beacon frames Active scanning Require transmission Move to each channel, and send Probe Request frames to solicit Probe Responses from a network Univ. of Tehran Computer Network 70 Association in 802.11 1: Association request 2: Association response 3: Data traffic AP Client Univ. of Tehran Computer Network 71 Reassociation in 802.11 1: Reassociation request 3: Reassociation response 5: Send buffered frames Client 6: Data traffic New AP 2: verify previous association Old AP Univ. of Tehran Computer Network 4: send buffered 72 frames Time Synchronization in 802.11 Timing synchronization function (TSF) AP controls timing in infrastructure networks All stations maintain a local timer TSF keeps timer from all stations in sync Periodic Beacons convey timing Beacons are sent at well known intervals Timestamp from Beacons used to calibrate local clocks Local TSF timer mitigates loss of Beacons Univ. of Tehran Computer Network 73 Power Management in 802.11 A station is in one of the three states Transmitter on Receiver on Both transmitter and receiver off (dozing) AP buffers packets for dozing stations AP announces which stations have frames buffered in its Beacon frames Dozing stations wake up to listen to the beacons If there is data buffered for it, it sends a poll frame to get the buffered data Univ. of Tehran Computer Network 74 Authentication Three levels of authentication Open: AP does not challenge the identity of the node. Password: upon association, the AP demands a password from the node. Public Key: Each node has a public key. Upon association, the AP sends an encrypted message using the nodes public key. The node needs to respond correctly using it private key. Univ. of Tehran Computer Network 75 02.11 Activities IEEE 802.11c: Bridge Operation (Completed. Added to IEEE 802.1D) 802.11d: Global Harmonization (PHYs for other countries. Published as IEEE Std 802.11d-2001) 802.11e: Quality of Service. IEEE Std 802.11e-2005 802.11f: Inter-Access Point Protocol (Published as IEEE Std Std 802.11F-2003) 802.11h: Dynamic Frequency Selection and transmit power control to satisfy 5GHz band operation in Europe. Published as IEEE Std 802.11h-2003 802.11i: MAC Enhancements for Enhanced Security. Published as IEEE Std 802.11i-2004 802.11j: 4.9-5 GHz operation in Japan. IEEE Std 802.11j-2004 802.11k: Radio Resource Measurement interface to higher layers. Active. Univ. of Tehran Computer Network 76 02.11 Activities IEEE 802.11m: Maintenance. Correct editorial and technical issues in 802.11a/b/d/g/h. Active. 802.11n: Enhancements for higher throughput (100+ Mbps). Active. 802.11p: Inter-vehicle and vehicle-road side communication at 5.8GHz. Active. 802.11r: Fast Roaming. Started July 2003. Active. 802.11s: ESS Mesh Networks. Active. 802.11T: Wireless Performance Metrics. Active. 802.11u: Inter-working with External Networks. Active. 802.11v: Wireless Network Management enhancements for interface to upper layers. Extension to 80211.k. Active. Study Group ADS: Management frame security. Active Standing Committee Wireless Next Generation WNG: Globalization jointly w ETSI-BRAN and MMAC. Active. Univ. of Tehran Computer Network 77 IEEE 802.11 Wireless MAC Distributed and centralized MAC components Distributed Coordination Function (DCF) Point Coordination Function (PCF) DCF suitable for multi-hop and ad hoc networking DCF is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) protocol Univ. of Tehran Computer Network 78 IEEE 802.11 DCF Uses RTS-CTS exchange to avoid hidden terminal problem Any node overhearing a CTS cannot transmit for the duration of the transfer Uses ACK to achieve reliability Any node receiving the RTS cannot transmit for the duration of the transfer To prevent collision with ACK when it arrives at the sender When B is sending data to C, node A will keep quite A Univ. of Tehran B Computer Network C 79 Hidden Terminal Problem Node B can communicate with A and C both A and C cannot hear each other When A transmits to B, C cannot detect the transmission using the carrier sense mechanism If C transmits, collision will occur at node B A Univ. of Tehran B Computer Network C 80 MACA Solution for Hidden Terminal Problem When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to A On receiving RTS, node A responds by sending Clear-to-Send (CTS), provided node A is able to receive the packet A B C When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer Transfer duration is included in RTS and CTS both Univ. of Tehran Computer Network 81 IEEE 802.11 RTS = Request-to-Send RTS A B Univ. of Tehran C D E Computer Network F 82 IEEE 802.11 RTS = Request-to-Send RTS A B C D E F NAV = 10 NAV = remaining duration to keep quiet Univ. of Tehran Computer Network 83 IEEE 802.11 CTS = Clear-to-Send CTS A B Univ. of Tehran C D E Computer Network F 84 IEEE 802.11 •DATA packet follows CTS. Successful data reception acknowledged using ACK. CTS = Clear-to-Send CTS A B C D E F NAV = 8 Univ. of Tehran Computer Network 85 IEEE 802.11 DATA A B Univ. of Tehran C D E Computer Network F 86 IEEE 802.11 Reserved area ACK A Univ. of Tehran B C D Computer Network E F 87 IEEE 802.11 Carrier sense range Interference range DATA A B C D E F Transmit range Univ. of Tehran Computer Network 88 IEEE 802.11 ACK A B Univ. of Tehran C D E Computer Network F 89 CSMA/CA Carrier sense in 802.11 Physical carrier sense Virtual carrier sense using Network Allocation Vector (NAV) NAV is updated based on overheard RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission Collision avoidance Nodes stay silent when carrier sensed (physical/virtual) Backoff intervals used to reduce collision probability Univ. of Tehran Computer Network 90 Backoff Interval When transmitting a packet, choose a backoff interval in the range [0,cw] Count down the backoff interval when medium is idle cw is contention window Count-down is suspended if medium becomes busy When backoff interval reaches 0, transmit RTS Univ. of Tehran Computer Network 91 DCF Example B1 = 25 B1 = 5 wait data data B2 = 20 cw = 31 Univ. of Tehran wait B2 = 15 B2 = 10 B1 and B2 are backoff intervals at nodes 1 and 2 Computer Network 92 Backoff Interval The time spent counting down backoff intervals is a part of MAC overhead Choosing a large cw leads to large backoff intervals and can result in larger overhead Choosing a small cw leads to a larger number of collisions (when two nodes count down to 0 simultaneously) Univ. of Tehran Computer Network 93 Binary Exponential Backoff in DCF When a node fails to receive CTS in response to its RTS, it increases the contention window When a node successfully completes a data transfer, it restores cw to Cwmin cw is doubled (up to an upper bound) cw follows a sawtooth curve 802.11 has large room for improvement Random backoff Univ. of Tehran RTS/CTS Data Transmission/ACK Computer Network 94 Inter Frame Spacing SIFS = Short inter frame space = dependent on PHY PIFS = point coordination function (PCF) inter frame space = SIFS + slot time DIFS = distributed coordination function (DCF) inter frame space = PIFS + slot time The back-off timer is expressed in terms of number of time slots. Univ. of Tehran Computer Network 95 802.11 Frame Priorities Busy DIFS PIFS SIFS content window Frame transmission Time Short interframe space (SIFS) PCF interframe space (PIFS) For highest priority frames (e.g., RTS/CTS, ACK) Used by PCF during contention free operation DCF interframe space (DIFS) Minimum medium idle time for contention-based services Univ. of Tehran Computer Network 96 SIFS/DIFS SIFS makes RTS/CTS/Data/ACK atomic Example: Slot Time = 1, CW = 5, DIFS=3, PIFS=2, SIFS=1, Univ. of Tehran Computer Network 97 Priorities in 802.11 CTS and ACK have priority over RTS After channel becomes idle If a node wants to send CTS/ACK, it transmits SIFS duration after channel goes idle If a node wants to send RTS, it waits for DIFS > SIFS Univ. of Tehran Computer Network 98 SIFS and DIFS DATA1 ACK1 SIFS DIFS Univ. of Tehran backoff RTS SIFS Computer Network 99 Energy Conservation Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest Two approaches to reduce energy consumption Power save: Turn off wireless interface when desirable Power control: Reduce transmit power Univ. of Tehran Computer Network 100 Power Control with 802.11 Transmit RTS/CTS/DATA/ACK at least power level needed to communicate with the receiver A B C D A/B do not receive RTS/CTS from C/D. Also do not sense D’s data transmission B’s transmission to A at high power interferes with reception of ACK at C Univ. of Tehran Computer Network 101 Related Standards Activities IEEE 802.11 Hiperlan/2 http://www.etsi.org/technicalactiv/hiperlan2.htm BlueTooth http://grouper.ieee.org/groups/802/11/ http://www.bluetooth.com IETF manet (Mobile Ad-hoc Networks) working group http://www.ietf.org/html.charters/manet-charter.html Univ. of Tehran Computer Network 102