Data and Computer Communications Chapter 13 Wireless LANs
Wireless LAN Topology Infrastructure LAN Connect to stations on wired LAN and in other cells May do automatic handoff Ad hoc LAN No hub Peer-to-peer (direct communication)
Example: Infrastructure LAN Adjacent cells use different frequencies Single cell Multiple cell
IEEE 802.11 Architecture Basic service set (BSS) BSS generally corresponds to cell May connect to backbone distribution system (DS) through access point (AP) DS can be switch, wired network, or wireless network Extended service set (ESS) Two or more BSS interconnected by DS Appears as single logical LAN to LLC
IEEE 802.11 Terminology Access point (AP) Basic service set (BSS) Coordination function Distribution system (DS) Extended service set (ESS) Frame MAC protocol data unit (MPDU) MAC service data unit (MSDU) Any entity that has station functionality and provides access to the distribution system via the wireless medium for associated stations A set of stations controlled by a single coordination function The logical function that determines when a station operating within a BSS is permitted to transmit and may be able to receive PDUs A system used to interconnect a set of BSSs and integrated LANs to create an ESS A set of one or more interconnected BSSs and integrated LANs that appear as a single BSS to the LLC layer at any station associated with one of these BSSs Synonym for MAC protocol data unit The unit of data exchanged between two peer MAC entities using the services of the physical layer Information that is delivered as a unit between MAC users Station (Table can be Kyungpook found on page National 424 in the University textbook) Any device that contains an IEEE 802.11 conformant MAC and physical layer
Key IEEE 802.11 Standards Standard IEEE 802.11a IEEE 802.11b IEEE 802.11c IEEE 802.11d IEEE 802.11e IEEE 802.11g IEEE 802.11i IEEE 802.11n IEEE 802.11T IEEE 802.11ac IEEE 802.11ad Scope Physical layer: 5-GHz OFDM at rates from 6 to 54 Mbps Physical layer: 2.4-GHz DSSS at 5.5 and 11 Mbps Bridge operation at 802.11 MAC layer Physical layer: Extend operation of 802.11 WLANs to new regulatory domains (countries) MAC: Enhance to improve quality of service and enhance security mechanisms Physical layer: Extend 802.11b to data rates >20 Mbps MAC: Enhance security and authentication mechanisms Physical/MAC: Enhancements to enable higher throughput Recommended practice for the evaluation of 802.11 wireless performance Physical/MAC: Enhancements to support 0.5 1 Gbps in 5-GHz band Physical/MAC: Enhancements to support 1 Gbps in the 60- GHz band
IEEE 802.11 Physical Layer Mostly operate in 2.4 or 5 Ghz ISM (industrial, scientific, and medical) bands No licensing is required Standard 802.11a 802.11b 802.11g 802.11n 802.11ac 802.11ad Year introduced Maximum data transfer speed Frequency band Channel bandwidth Highest order modulation Spectrum usage Antenna configuration 1999 1999 2003 2000 2012 2014 54 Mbps 11 Mbps 54 Mbps 5 GHz 2.4 GHz 2.4 GHz 20 MHz 20 MHz 20 MHz 65 to 600 Mbps 2.4 or 5 GHz 20, 40 MHz 78 Mbps to 3.2 Gbps 6.76 Gbps 5 GHz 60 GHz 40, 80, 160 MHz 2160 MHz 64 QAM 11 CCK 64 QAM 64 QAM 256 QAM 64 QAM DSSS OFDM DSSS, OFDM 1 1 SISO 1 1 SISO 1 1 SISO Up to 4 4 MIMO OFDM SC-OFDM SC, OFDM Up to 8 8 MIMO, MU- MIMO 1 1 SISO
Throughput (Mbps) IEEE 802.11 a/b/g/n 802.11b Extension of 802.11 With data rates of 5.5 and 11 Mbps 802.11a Uses a relatively uncluttered frequency spectrum (5 GHz) Supports higher data rates, is less cluttered Orthogonal frequency division multiplexing (OFDM) 802.11g 25 Higher-speed extension to 802.11b 20 Operates in 2.4Ghz band 15 802.11n 10 Multiple-input-multiple-output (MIMO) antenna architecture Most significant change is to aggregate multiple MAC frames into a single block for transmission 5 0 5 802.11n 802.11g 10 15 20 Simultaneous users/ap 25 Figure 13.12 Average Throughput per User
IEEE 802.11 ac/ad 802.11ac Includes the option of multiuser MIMO (MU-MIMO) On the downlink, the transmitter is able to use its antenna resources to transmit multiple frames to different stations, all at the same time and over the same frequency spectrum Each antenna of a MU-MIMO AP can simultaneously communicate with a different single-antenna device, such as a smartphone or tablet 802.11ad A version of 802.11 operating in the 60-GHz frequency band Few devices operate in the 60-GHz which means communications would experience less interference Offers the potential for much wider channel bandwidth than the 5-GHz band Undesirable propagation characteristics: Multipath losses can be quite high in this range
Medium Access Control Access control Reliable data delivery Security MAC layer covers three functional areas:
Reliable Data Delivery Can be dealt with at a higher layer, but more efficient to deal with errors at MAC level Use frame exchange protocol Request to send (RTS) Clear to send (CTS) ACK Exchange treated as atomic unit Alleviate hidden node problem 802.11 physical and MAC layers unreliable Noise, interference, and other propagation effects result in loss of frames
Hidden Node Problem A talks to B C senses the channel C does not hear A s transmission (out of range) C talks to B Signals from A and C collide at B More collisions and wastage of resources Collision A B C
Exposed Node Problem Assuming that B talks to A, and C wants to talk to D C senses channel and finds it to be busy C stays quiet (when it could have ideally transmitted) Underutilization of channel, lower effective throughput Not possible A B C D
Frame Exchange Protocol 4-way (RTS, CTS, Data, ACK) exchange for every data frame transmission 1. Source issues RTS frame to destination 2. Destination responds with CTS 3. After receiving CTS, source transmits data 4. Destination responds with ACK 5. If no ACK within short period of time, retransmit A E RTS B D A E CTS D B C A E Data B D A E ACK B D C F C F F C F B A CTS RTS Data ACK time
RTS/CTS RTS alerts all stations within range of source that exchange is under way Other stations don t transmit to avoid collision CTS alerts all stations within range of destination Other stations don t transmit to avoid collision Alleviates hidden node problem Collision when two or more nodes send RTS at the same time But, bandwidth waste can be minimized (RTS/CTS packet is very short compared to data packet) Frame exchange protocol is required function of MAC but may be disabled
Access Control CFP (Contention Free Period) and CP (Contention Period) are alternated CP : controlled by DCF (Distributed Coordination Function) CFP : controlled by PCF (Point Coordination Function) Base station sends beacon frame periodically Stations can distinguish between CP or CFP period using beacon All implementations must support DCF, but PCF is optional
DCF Uses CSMA/CA (CSMA with Collision Avoidance) No collision detection Not practical on wireless network Dynamic range of signals very large Transmitting station cannot distinguish incoming weak signals from noise and effects of own transmission Contention-Based Designed for a best-effort service Does not support real-time application Includes a set of delays that amounts as a priority scheme
Basic Operation of CSMA/CA 1) If idle, wait to see if remains idle for one IFS. If so, transmit immediately frame 1) If busy (either initially or becomes busy during IFS), back-off Packet arrival at MAC
CSMA/CA Algorithm Wait for frame to transmit Medium idle? No Yes Wait IFS Still idle? No Wait until current transmission ends Yes Transmit frame Wait IFS Still idle? No Yes Exponential backoff while medium idle Transmit frame Figure 13.6 IEEE 802.11 Medium Access Control Logic
Interframe Spacing DIFS (DCF IFS) Used as minimum delay for asynchronous frames contending for access SIFS (Short IFS) Gives highest priority Acknowledgment (ACK) Clear to Send (CTS) Poll response Fragments of fragment burst PIFS (PCF IFS) Used by base station for issuing beacon EIFS (Extended IFS) Lowest priority interval used to report bad or unknown frame SIFS < PIFS < DIFS < EIFS
Example A DIFS E D RTS RTS A Beacon B ACK F B PIFS Beacon C SIFS ACK C ACK
Example - Fragmentation DIFS Src RTS Fragment1 Fragment2 SIFS SIFS SIFS SIFS SIFS Dest CTS ACK1 ACK2
Binary Exponential Backoff Choose a random number in a contention window (between 0 and CW-1) CW = 2 n+2 for n-th retransmission Initial value = 8 If collision, CW is doubled Once current transmission over, delay one IFS Countdown a back-off timer for each empty slot. Freeze the timer while channel is busy. Transmit when back-off timer reaches 0 Backoff _ Time UNIFORM (0, CW 1) Slot _ Time BEB is unfair since successful transmitters reset CW to minimum value. Hence, it is more likely that successful transmitters continue to be successful
Example Packet arrival time at MAC Backoff timer countdown A B C Backoff timer frozen A B C Packet DIFS B1 = 25 B2 = 20 Frozen Packet DIFS B1 = 5 Packet Frozen DIFS Packet B3 = 13 B3 = 8
Example DIFS DIFS BO E BO R DIFS BO E BO R DIFS BO E Busy Station 1 BO E Busy Station 2 Station 3 Busy BO E Busy BO E BO R Station 4 BO E BO R BO E Busy BO E BO R Station 5 t Busy Medium not idle (frame, ACK etc.) BO E Elapsed backoff time Packet arrival at MAC BO R Residual backoff time
Example - Retransmission DIFS CW is doubled CW is reset to 8 Src RTS Data (corrupted) Data Dest SIFS CTS SIFS SIFS SIFS ACK DIFS All Compete for channel access
Physical Carrier Sensing Carrier sensed by signal strength at wireless interface If idle for one IFS, transmit frame Interframe space is used as minimum delay for contending for access Amounts to priority scheme IFS Busy Medium Access if idle during IFS Defer Access
Virtual Channel Sensing Managed by MAC Limits the need for physical carrier sensing at the air interface in order to save power Sending station records some value in duration field of frame header Duration field specifies the transmission time required for the frame, in which time the channel will be busy Unit: usec The listening stations read the duration field and set their NAV (Network Allocation Vector), which is an indicator on how long it must defer from accessing Stations do not access the channel until their NAV reach zero When NAV reaches zero, the virtual CS indication is that the channel is idle
Example: Virtual Channel Sensing DIFS SIFS Src RTS SIFS Data SIFS Dest CTS ACK DIFS NAV(RTS) NAV(CTS) Contention Window Other NAV(DATA) Defer Access Backoff
Point Coordination Function (PCF) Alternative access method implemented on top of DCF Contention free-based. No collisions occur Supports time-bounded multimedia applications Polling by centralized polling master (point coordinator) Uses PIFS when issuing polls Point coordinator polls in round-robin to stations configured for polling When poll issued, polled station may respond using SIFS If point coordinator receives response, it issues another poll using PIFS If no response during expected turnaround time, coordinator issues poll Coordinator could lock out asynchronous traffic by issuing polls Have a superframe interval defined
Example Contention Free Repetition Interval (Super Frame) PIFS Contention Free Period (CFP) SIFS SIFS SIFS SIFS SIFS SIFS Contention Period (CP) DCF Beacon D1+poll D2+Ack +poll Poll+ Ack CF-End U1+Ack U2+Ack Null
Access and Privacy Services Authentication Used to establish station identity Wired LANs assume physical connection gives authority to use LAN Not a valid assumption for wireless LANs 802.11 supports several authentication schemes Does not mandate any particular scheme From relatively insecure handshaking to public-key encryption 802.11 requires mutually acceptable, successful authentication before association Privacy Used to prevent messages being read by others 802.11 allows optional use of encryption Original WEP security features were weak Subsequently 802.11i and WPA alternatives evolved giving better security
Summary IEEE 802.11 architecture IEEE 802.11 medium access control Reliable data delivery Medium access control IEEE 802.11 security considerations