Universität Karlsruhe Institut für Telematik ECE 591 Mobilkommunikation SS 1998 Lecture 5: IEEE 802.11 ECE 591 Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

IEEE 802.11 Requirements Wi-Fi often used by the public as a synonym for IEEE 802.11-wireless LAN (WLAN). Design for small coverage (e.g. office, home) Low/no mobility High data-rate applications Ability to integrate real time applications and non-real-time applications Use un-licensed spectrum

802.11: Infrastructure Mode Architecture similar to cellular networks station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point access point (AP) station integrated into the wireless LAN and the distribution system basic service set (BSS) group of stations using the same AP portal bridge to other (wired) networks distribution system interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 802.11 LAN 802.x LAN STA1 BSS1 Portal Access Point Distribution System Access Point ESS BSS2 STA2 STA3 802.11 LAN 9

802.11 - Architecture of an ad-hoc network Freie Universität Berlin Institut of Computer Science Mobile Communications 2002 802.11 - Architecture of an ad-hoc network 802.11 LAN Direct communication within a limited range Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency STA1 IBSS1 STA3 STA2 IBSS2 STA5 STA4 802.11 LAN Prof. Dr.-Ing. Jochen Schiller

Freie Universität Berlin Institut of Computer Science Mobile Communications 2002 IEEE standard 802.11 fixed terminal mobile terminal infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY Prof. Dr.-Ing. Jochen Schiller

IEEE 802.11 Physical Layer Family of IEEE 802.11 standards: unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 300 MHz 5.15-5.35 GHz 5.725-5.825 GHz and 802.11b/g 802.11a

The IEEE 802.11 Family Protocol Release Data Freq. Rate (typical) Rate (max) Range (indoor) Legacy 1997 2.4 GHz 1 Mbps 2Mbps ? 802.11a 1999 5 GHz 25 Mbps 54 Mbps ~30 m 802.11b 6.5 Mbps 11 Mbps 802.11g 2003 802.11n 2008 2.4/5 GHz 200 Mbps 540 Mbps ~50 m

802.11a Modulation Use OFDM to divide each physical channel (20 MHz) into 52 subcarriers (312.5 KHz each) 48 data, 4 pilot Adaptive modulation BPSK: 6, 9 Mbps QPSK: 12, 18 Mbps 16-QAM: 24, 36 Mbps 64-QAM: 48, 54 Mbps

802.11 MAC Layer: Access Methods DFWMAC-DCF CSMA/CA (mandatory) collision avoidance via randomized “back-off“ ACK packet for acknowledgements DFWMAC-DCF w/ RTS/CTS (optional) additional virtual “carrier sensing: to avoid hidden terminal problem DFWMAC- PCF (optional) access point polls terminals according to a list

Hidden and Exposed Terminals Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Hidden and Exposed Terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” for C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B A B C Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Near and Far Terminals Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A’s signal C cannot receive A If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! A B C Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

MACA - collision avoidance Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 MACA - collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address receiver address packet size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC) Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 MACA examples MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS from A A C RTS CTS CTS B A C RTS RTS CTS B Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

MACA variant: DFWMAC in IEEE802.11 Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 MACA variant: DFWMAC in IEEE802.11 sender receiver idle idle packet ready to send; RTS data; ACK RxBusy time-out; RTS wait for the right to send RTS; CTS time-out  data; NAK ACK time-out  NAK; RTS CTS; data wait for data wait for ACK RTS; RxBusy ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller

802.11 CSMA/CA CSMA: Listen before transmit Collision avoidance when transmitting a packet, choose a backoff interval in the range [0, CW] CW is contention window Count down the backoff interval when medium is idle count-down is suspended if medium becomes busy Transmit when backoff interval reaches 0

Congestion Avoidance: Example busy B1 = 25 B2 = 20 B1 = 5 data wait data wait B2 = 10 B2 = 15 busy B1 and B2 are backoff intervals at nodes 1 and 2

802.11 – RTS/CTS + ACK Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) Receiver acknowledges via CTS (if ready to receive) CTS reserves channel for sender, notifying possibly hidden stations Sender can now send data at once, acknowledgement via ACK Other stations store NAV distributed via RTS and CTS DIFS RTS data sender SIFS SIFS CTS SIFS ACK receiver NAV (RTS) DIFS data other stations NAV (CTS) t defer access new contention

Universität Karlsruhe Institut für Telematik Mobilkommunikation SS 1998 Not END Prof. Dr. Dr. h.c. G. Krüger E. Dorner / Dr. J. Schiller