Data and Computer Communications. Chapter 13 Wireless LANs

Transcription

1 Data and Computer Communications Chapter 13 Wireless LANs

2 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)

3 Example: Infrastructure LAN Adjacent cells use different frequencies Single cell Multiple cell

4 IEEE 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

5 IEEE 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 conformant MAC and physical layer

6 Key IEEE Standards Standard IEEE a IEEE b IEEE c IEEE d IEEE e IEEE g IEEE i IEEE n IEEE T IEEE ac IEEE ad 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 MAC layer Physical layer: Extend operation of WLANs to new regulatory domains (countries) MAC: Enhance to improve quality of service and enhance security mechanisms Physical layer: Extend b to data rates >20 Mbps MAC: Enhance security and authentication mechanisms Physical/MAC: Enhancements to enable higher throughput Recommended practice for the evaluation of wireless performance Physical/MAC: Enhancements to support Gbps in 5-GHz band Physical/MAC: Enhancements to support 1 Gbps in the 60- GHz band

7 IEEE Physical Layer Mostly operate in 2.4 or 5 Ghz ISM (industrial, scientific, and medical) bands No licensing is required Standard a b g n ac ad Year introduced Maximum data transfer speed Frequency band Channel bandwidth Highest order modulation Spectrum usage Antenna configuration 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

8 Throughput (Mbps) IEEE a/b/g/n b Extension of With data rates of 5.5 and 11 Mbps a Uses a relatively uncluttered frequency spectrum (5 GHz) Supports higher data rates, is less cluttered Orthogonal frequency division multiplexing (OFDM) g 25 Higher-speed extension to b 20 Operates in 2.4Ghz band n 10 Multiple-input-multiple-output (MIMO) antenna architecture Most significant change is to aggregate multiple MAC frames into a single block for transmission n g Simultaneous users/ap 25 Figure Average Throughput per User

9 IEEE ac/ad ac 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 ad A version of 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

10 Medium Access Control Access control Reliable data delivery Security MAC layer covers three functional areas:

11 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 physical and MAC layers unreliable Noise, interference, and other propagation effects result in loss of frames

12 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

13 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

14 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

15 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

16 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

17 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

18 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

19 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 Medium Access Control Logic

20 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

21 Example A DIFS E D RTS RTS A Beacon B ACK F B PIFS Beacon C SIFS ACK C ACK

22 Example - Fragmentation DIFS Src RTS Fragment1 Fragment2 SIFS SIFS SIFS SIFS SIFS Dest CTS ACK1 ACK2

23 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

24 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

25 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

26 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

27 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

28 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

29 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

30 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

31 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

32 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 supports several authentication schemes Does not mandate any particular scheme From relatively insecure handshaking to public-key encryption requires mutually acceptable, successful authentication before association Privacy Used to prevent messages being read by others allows optional use of encryption Original WEP security features were weak Subsequently i and WPA alternatives evolved giving better security

33 Summary IEEE architecture IEEE medium access control Reliable data delivery Medium access control IEEE security considerations