Wireless EE 122 Intro to Communication Networks Fall 2007 WF 4 5 30 in Cory 277 Jorge Ortiz Vern Paxson TAs Lisa Fowler Daniel Killebrew Jorge Ortiz http inst eecs berkeley edu ee122 Materials with thanks to Jennifer Rexford Ion Stoica and colleagues at Princeton and UC Berkeley 1 Announcements Prof Paxson has leftover exams in office I have leftover homework 2 s in my office 410 Soda Hall Next week the same lecture will be given both at the usual Wednesday time and on Monday Same time and room Reminder no lecture Friday due to holiday 2 Wired Communication Pros Very reliable o For Ethernet medium must be capable of a Bit Error Rate BER of 10 12 one error every one trillion bits Insulated wires wires placed underground and in walls Error Correction Techniques Very high transfer rates o Up to 100 Gbit s or more Long distance o Up to 40km 25 miles in 10 Gbit s Ethernet Cons Expensive to set up infrastructure Infrastructure is fixed once set up No mobility 3 Wireless Communication Pros Up to 248Mbps maximum rate 802 11n October 2008 Allows mobility Much cheaper and easier to deploy Cons Exposed unshielded medium o Susceptible to physical phenomena interference o Variable BER Error correction may not suffice in all cases Slower data rates for wider distances OSI layered stack designed for wired medium o Assumptions do not hold in wireless o Difficult to hide underlying behavior 4 Goals for Today s Lecture Characteristics of Wireless Media 802 11 Architecture and Media Access Control Protocol Collision Detection vs Collision Avoidance Hidden Terminal and Exposed Terminal Problem Request To Send RTS Clear To Send CTS mechanism Multihop Wireless Networks Sensor Networks TCP over Multihop Networks Wireless Security 5 Wireless Communication Standards Alphabet Soup Cellular 2G GSM CDMA 3G CDMA2000 IEEE 802 11 A 5 0Ghz band 54Mbps 25 Mbps operating rate B 2 4Ghz band 11Mbps 4 5 Mbps operating rate G 2 4Ghz 54Mbps 19 Mbps operating rate Other versions to come IEEE 802 15 lower power wireless 802 15 1 2 4Ghz 2 1 Mbps Bluetooth 802 15 4 2 4Ghz 250 Kbps Sensor Networks 6 Wireless Link Characteristics Figure Courtesy of Kurose and Ross 7 Other Wireless Link Characteristics Path loss Signal attenuation as a function of distance Signal to noise ratio SNR Signal Power Noise Power decreases make signal unrecoverable Multipath Propagation Signal reflects off surfaces effectively causing selfinterference Interference from other sources Internal Interference o Hosts within range of each other collide with one another s transmission remember Aloha External Interference o Microwave is turned on and blocks your signal 8 Path Loss Signal power attenuates by about r2 factor for omni directional antennas in free space Where r is the distance between the sender and the receiver The exponent in the factor is different depending on placement of antennas Less than 2 for directional antennas Greater than 2 when antennas are placed on the ground o Signal bounces off the ground and reduces the power of the signal 9 Multipath Effects Ceiling S R Floor Signals bounce off surface and interfere with one another What signals are out of phase Orthogonal signals cancel each other and nothing is received 10 Probability Distribution of Packet Reception based on Antenna Placement plot courtesty of Sam Madden Harvard 11 A Wireless Link courtesy of Gilman Tolle and Jonathan Hui ArchRock A Wireless Link courtesy of Gilman Tolle and Jonathan Hui ArchRock The Amoeboed cell courtesy of Prof David Culler UCB Signal Noise Distance 14 Wireless Bit Errors The lower the SNR the higher the Bit Error Rate BER How can we deal with this Make the signal stronger Why is this not always a good idea Increased signal strength requires more power Increases the interference range of the sender so you interfere with more nodes around you Error Correction schemes can correct some problems 15 802 11 Architecture Kurose and Ross 802 11 frames exchanges Designed for limited geographical area 802 3 Ethernet frames exchanged AP s are set to specific channel and broadcast beacon messages with SSID and MAC Address periodically Hosts scan all the channels to discover the AP s Host associates with AP actively or passively 16 802 11 frame Slide Adapted from Dr Ju Wang VCU Usually ARP or IP datagram Transmission time data ack 2 2 6 6 6 6 2 Address 1 MAC address of wireless host or AP to receive this frame 4 0 2312 frame address address address seq address duration control 1 2 3 4 control payload CRC Address 4 used only in ad hoc multihop mode Address 3 MAC address of router interface to Address 2 MAC address which AP is attached of wireless host or AP transmitting this frame 802 11 frame Slide Adapted from Dr Ju Wang VCU frame seq for reliable ARQ 2 2 6 6 6 2 6 0 2312 frame addressaddressaddress seq address duration control 1 2 3 4 control 2 Protocol version 2 Type 4 1 Subtype To AP 1 1 1 payload 1 1 From More Power More Retry AP frag mgt data frame type RTS CTS ACK data 4 CRC 1 1 WEP Rsvd Defines meaning of Address fields Ad hoc or Infrastructure Ethernet vs 802 11 adapted from S Savage UCSD Wireless MAC design Why not just use Ethernet algorithms o Ethernet one shared collision domain It s technically difficult to detect collisions Collisions are at receiver not sender even if we could it wouldn t work Different transmitters have different coverage areas In addition wireless links are much more prone to loss than wired links Carrier Sense CSMA is OK detection CD is not 19 Hidden Terminals A B C transmit range A and C can both send to B but can t hear each other A is a hidden terminal for C and vice versa CSMA CD will be ineffective need to sense at receiver 20 Exposed Terminals A B C D Exposed node B sends a packet to A C hears this and decides not to send a packet to D despite the fact that this will not cause interference 21 CSMA CA CSMA w Collision Avoidance Since we can t detect collisions we try to avoid them When medium busy choose random interval contention window Wait for that many idle timeslots to pass before sending When a collision is inferred retransmit with binary exponential backoff like Ethernet Use ACK from receiver to infer no collision Use exponential backoff to adapt contention window 22 Multiple Access with Collision Avoidance MACA sender RTS receiver other node in sender s range CTS data ACK Before every data transmission Sender sends a Request to Send RTS frame containing the length of the transmission Receiver respond with a Clear
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