ECE 4450:427/527 - Computer Networks Spring 2017 Dr. Nghi Tran Department of Electrical & Computer Engineering Lecture 5.6: Wireless Networks - MAC Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 1
Wireless Mobile Networks Brief Introduction Wireless Channel Characteristics WiFi: CSMA/CA - Collision Avoidance Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 2
Elements of Wireless Networks network infrastructure wireless hosts laptop, PDA, IP phone run applications may be stationary (nonmobile) or mobile wireless does not always mean mobility Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 3
Elements of Wireless Mobile Networks network infrastructure 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 (AP) Handoff: Mobile changes station Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 4
Characteristics of selected wireless link standards 200 802.11n Data rate (Mbps) 54 5-11 4 1.384 802.15 802.11a,g 802.11b 802.11a,g point-to-point 802.16 (WiMAX) UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO UMTS/WCDMA, CDMA2000 3G data 3G cellular enhanced.056 IS-95, CDMA, GSM 2G Indoor 10-30m Outdoor 50-200m Mid-range outdoor 200m 4 Km Long-range outdoor 5Km 20 Km Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 5
Wireless Mobile Networks 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 Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 6
Wireless Network Taxonomy infrastructure (e.g., APs) no infrastructure single hop 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) 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 Dr. Nghi Tran (ECE-University of Akron) ECE 4450:427/527 Computer Networks 7
Wireless Channel Characteristics 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 ECE-University of Akron ECE 4450:427/527 Computer Networks 8
Multipath propagation Constructive and destructive interference: channel strengths change randomly with time Fading When channel is weak, i.e., bad quality Low reliability ECE-University of Akron ECE 4450:427/527 Computer Networks 9
Hidden Terminal Problem Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B Hidden terminal problem: Signals blocked B, A hear each other B, C hear each other A, C can not hear each other means A, C unaware of their interference at B ECE-University of Akron ECE 4450:427/527 Computer Networks 10
Hidden Terminal Problem A B C A s signal strength C s signal strength space Hidden terminal problem due to signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other interfering at B ECE-University of Akron ECE 4450:427/527 Computer Networks 11
Exposed Node Problem Suppose B is sending to A. Node C is aware of this communication because it hears B s transmission. It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B s transmission. Suppose C wants to transmit to node D. This is not a problem since C s transmission to D will not interfere with A s ability to receive from B. ECE-University of Akron ECE 4450:427/527 Computer Networks 12
IEEE 802.11 Wireless LAN 802.11b 2.4-5 GHz unlicensed spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps 802.11n: multiple antenna 2.4-5 GHz range up to 600 Mbps all use CSMA/CA for multiple access all have base-station and ad-hoc network versions ECE-University of Akron ECE 4450:427/527 Computer Networks 13
80211. Wireless LAN Architecture Internet wireless host communicates with base station base station = access point (AP) AP hub, switch or router Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts BSS 1 AP access point (AP): base station ad hoc mode: hosts only BSS 2 ECE-University of Akron ECE 4450:427/527 Computer Networks 14
IEEE 802.11 Multiple Access avoid collisions: 2 + nodes transmitting at same time 802.11: CSMA - sense before transmitting don t collide with ongoing transmission by other node 802.11: no collision detection as in Ethernet! difficult to receive (sense collisions) and transmitting at the same time due to weak received signals: swamps the receiving circuitry can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) ECE-University of Akron ECE 4450:427/527 Computer Networks 15
IEEE 802.11 MAC Protocol: CSMA/CA To deal with transmitting and sensing problems 802.11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) cannot sense when transmitting 2 if sense channel busy then - start random backoff time - timer counts down while channel idle. If busy, timer frozen - transmit when timer expires. Wait for ACK - ACK: New frame, go to step 1; if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS DIFS sender data ACK receiver SIFS ECE-University of Akron ECE 4450:427/527 Computer Networks 16
IEEE 802.11 MAC Protocol: CSMA/CA To deal with hidden terminal problem: Using RTS and CTS idea: allow sender to reserve channel rather than random access of data frames: avoid collisions of data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) BS broadcasts clear-to-send (CTS) in response to RTS CTS heard by all nodes sender transmits data frame other stations defer transmissions avoid data frame collisions completely using small reservation packets! ECE-University of Akron ECE 4450:427/527 Computer Networks 17
IEEE 802.11 MAC Protocol: CSMA/CA A AP B reservation collision DATA (A) defer time ECE-University of Akron ECE 4450:427/527 Computer Networks 18
IEEE 802.11 MAC Protocol: CSMA/CA Applets to check out: http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csmaca/withhidden.html http://media.pearsoncmg.com/aw/aw_kurose_network_2/applets/csmaca/withouthidden.html ECE-University of Akron ECE 4450:427/527 Computer Networks 19