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