CMPE 257 Wireless and Mobile Networking Katia Obraczka Computer Engineering UCSC Baskin Engineering Lecture 2 CMPE 257 Winter 11 1 Announcements CMPE 257 Winter 11 2 Today Finish covering fundamentals and terminology Medium access control MAC CMPE 257 Winter 11 3 Elements of a Wireless Network network infrastructure wireless hosts laptop PDA IP phone run applications may be stationary non mobile or mobile wireless does not always mean mobility CMPE 257 Winter 11 4 Elements of a Wireless Network network infrastructure CMPE 257 Winter 11 base station typically connected to wired network relay responsible for sending packets between wired network and wireless host s in its area e g cell towers 802 11 access points 5 Elements of a Wireless Network network infrastructure CMPE 257 Winter 11 wireless link typically used to connect mobile s to base station also used as backbone link multiple access protocol coordinates link access 6 Wireless networking modes network infrastructure CMPE 257 Winter 11 infrastructure mode base station connects mobiles into network infrastructure handoff mobile changes base station as it moves 7 Wireless networking modes ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network route among themselves CMPE 257 Winter 11 8 Wireless Network Taxonomy single hop infrastructure e g APs no infrastructure host connects to base station WiFi WiMAX cellular which connects to larger Internet no base station no connection to larger Internet Bluetooth ad hoc nets Mobile Adhoc Networks CMPE 257 Winter 11 multiple hops host may have to relay through several wireless nodes to connect to larger Internet mesh net no base station no connection to larger Internet May have to relay to reach other a given wireless node MANET VANET Vehicular Adhoc Networks 9 Wireless versus Wired Networking What complicates wireless networking vs wired networking What are the challenges raised by wireless communication CMPE 257 Winter 11 10 Wireless Networking Challenges Wireless channel E g for satellite we get extended propagation delays High bit error rate BER higher than optical fiber and coax Asymmetry in bandwidth and delay Wireless channel impairements effects of wave propagation attenuation etc Mobility introduces topology dynamics Power constraints CMPE 257 Winter 11 11 Wireless Link Characteristics 1 Differences from wired link decreased signal strength radio signal attenuates as it propagates through matter path loss interference from other sources standardized wireless network frequencies e g 2 4 GHz shared by other devices e g phone devices motors interfere as well multipath propagation radio signal reflects off objects ground arriving ad destination at slightly different times make communication across even a point to point wireless link much more difficult CMPE 257 Winter 11 12 Wireless Link Characteristics 2 10 1 SNR signal to noise ratio SNR versus BER tradeoffs given physical layer increase power increase SNR decrease BER given SNR choose physical layer that meets BER requirement 10 3 BER larger SNR easier to extract signal from noise a good thing 10 2 10 4 10 5 10 6 10 7 10 20 30 40 SNR dB QAM256 8 Mbps QAM16 4 Mbps BPSK 1 Mbps Quadrature Amplitude Modulation QAM Binary Phase Shift Keying BPSK CMPE 257 Winter 11 13 The Wireless Link CMPE 257 Winter 11 14 Noise and Interference CMPE 257 Winter 11 15 Signal Propagation CMPE 257 Winter 11 16 Radio Channel Propagation Effects Diffraction Reflection Scattering CMPE 257 Winter 11 17 Path Loss Wireless Channel Impairements Spatial and temporal effects Radio propagation models Indoor versus outdoor Large scale versus small scale CMPE 257 Winter 11 18 Large and Small Scale Propagation Models Large scale models Large T R distances 100s or 1000s of meters and long time scales Captures average channel conditions Small scale models Short T R distances and short time scales seconds CMPE 257 Winter 11 19 Medium Access Control MAC protocols Aka multiple access protocols CMPE 257 Winter 11 20 When do we need MAC 2 types of links Point to point Shared shared wire e g cabled Ethernet shared RF e g 802 11 WiFi shared RF satellite CMPE 257 Winter 11 humans at a cocktail party shared air acoustical 21 Shared Links If more than 1 node transmits at the same time Collision at receiver MAC protocol Arbitrate access to medium Determine who can transmit when CMPE 257 Winter 11 22 Expanded Data Link Layer Sublayers of data link layer Logical Link Control LLC flow and error control Multiple Access Control MAC multiple access resolution CMPE 257 Winter 11 23 Types of MAC Who makes decision Centralized versus distributed MACs Channel access policy CMPE 257 Winter 11 24 Types of MAC Channel Access Policy Random access or contention based Controlled access No scheduled time for transmissions No order for transmissions Stations coordinate access to channel Station only transmits when it has right to send Channelization Bandwidth of channel is statically partitioned CMPE 257 Winter 11 25 Another way to look at it Control Distributed Centralized How they coordinate medium access Round robin Scheduled access Contention based A priori partitioning channel CMPE 257 Winter 11 26 Contention Based Protocols Aka Random Access MAC ALOHA Slotted Aloha CSMA CSMA CD Ethernet IEEE 802 3 CSMA CA WiFi IEEE 802 11 CMPE 257 Winter 11 27 CSMA Carrier sense Stations listen first whether another transmission is in progress If medium in use wait If not transmit Can collisions still occur CMPE 257 Winter 11 28 CSMA CD CSMA collision detection Performance improvement over CSMA How Listen while transmitting If collision transmit brief jamming signal and abort transmission How does this improve performance Wait random time and try again CMPE 257 Winter 11 29 Collision Avoidance CA CSMA CD assumes stations can detect collision Not valid in all contexts e g wireless Attenuation too great to detect collision at all stations Hard for transmitter to distinguish its own transmission from incoming weak signals and noise Radios are usually half duplex CSMA CA tries to avoid collisions CMPE 257 Winter 11 30 CSMA CA Tries to avoid collisions by avoiding the hidden terminal problem CMPE 257 Winter 11 31 Hidden Terminal Problem CMPE 257 Winter 11 32 Hidden and Exposed Terminals Hidden Terminals CMPE 257 Winter 11 Exposed Terminals 33 CSMA CA RTS CTS Solution Channel reservation With collision avoidance stations exchange small