CS698T Wireless Networks: Principles and Practice

Transcription

1 CS698T Wireless Networks: Principles and Practice IEEE (WLAN/WiFi) Bhaskaran Raman, Department of CSE, IIT Kanpur

2 IEEE (WiFi) Part of 802.x series is Ethernet a, b, g specify three different PHY layers MAC is the same a: 5.2 to 5.7GHz b/g: 2.4 to GHz

3 802.11: What does it specify? Source: IEEE specification

4 802.11: What does it Specify? PHY sub-layer a, b, g MAC sub-layer Independent of the PHY DCF (Distributed Coodrination Function) CSMA/CA PCF (Point Coordination Function) MAC management

5 PHY b data-rates (modulation schemes): 1Mbps (BPSK), 2Mbps (QPSK), 5.5Mbps (CCK), 11Mbps (CCK) a and g data-rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps OFDM + BPSK/QPSK/16QAM/64QAM g also supports b data-rates and modulations (backwards compatible)

6 802.11b Channels Non-overlapping channels Mhz Each channel is 22MHz wide Adjacent channels overlap Non-overlapping channels: 1, 6, Band recently delicensed in India for indoor, outdoor usage

7 802.11a Channels Each channel is 20MHz wide As of now, band is not free in India for outdoor use Source: Cisco presentation

8 Service Sets Basic Service Set: set of STAs in a region IBSS: Independent Basic Service Set Infrastructure Basic Service Set

9 Extended Service Set (ESS) Wired LAN (ethernet), or Wireless (WDS)

10 MAC Classification Based on what dimension is used for multiplexing: SDMA, TDMA, FDMA, CDMA Based on how control is achieved: Central (+) Easy to design/implement (-) Single point of failure, bottleneck Distributed (+) Natural when there is no central information specifies DCF (Distributed Coordination Function), PCF (Point Coordination Function) PCF uses central control

11 Ethernet CSMA/CD: Prelude to CSMA/CA CSMA/CD: Carrier-Sense Multiple Access with Collision Detection Listen before transmit (CS) Tx when (as soon as) medium is free (1-persistent) Collision Detection (CD) Backoff (exponential) on collision

12 CSMA/CA Collision detection impossible in wireless Conceptual name is CSMA/CA: Carrier-Sense Multiple Access with Collision Avoidance Tx power is relatively very high near the transmitter calls it DCF (Distributed Coordination Function) Collision Avoidance: Back-off before tx (even when no collision) Contention Window (CW) in terms of number slots

13 The Backoff Procedure A data DIFS B C Slot-time data Contention window: Num slots=random[0, CW) CW = 32, 64, 128, 256, 512, 1024 Double CW on collision Question: Why DIFS?

14 CSMA/CA + ACK ACK data A DIFS Slot-time SIFS DIFS data B data C ACK missing ==> Deduce collision SIFS should be < DIFS Retransmit (have to content anew) Else, ACK timeout may occur unnecessarily

15 DCF Timing Relations Source: IEEE Specifications

16 Propagation Time is Important Ignore delay between A & B r=1 A One-way delay = d C r=0 B (1)A finishes tx at time t (2)B senses the channel to be free at t, C senses the channel to be free at t+d (3)C starts sending at t+d+difs, this reaches B at t+d+difs+d (4)B should not have started tx by then ==> slottime should be < 2d

17 PCF Mode of Operation Source: IEEE Specification

18 IFS Relations Source: IEEE Specification

19 The Hidden Node Problem A C B Medium is free DOES NOT IMPLY ok-to-transmit

20 The Exposed Node Problem A D C B Medium is busy DOES NOT IMPLY not-ok-to-transmit

21 Hidden Node Solution: RTS/CTS CTS A C B RTS

22 RTS/CTS Exchange Example Source: IEEE Specification

23 Beacons and Probes Client Beacon has: AP capabilities, beacon period, SSID, Traffic Indication Map (TIM) A client may be in the coverage area of many APs APs send periodic beacons Client may passively scan these Or, probe-response for active scanning

24 Authentication and Association Client A client has to Authenticate itself to an AP Then Associate itself A client may authenticate itself to many APs to speed-up roaming

25 Frame Format Source: IEEE Specification Source: IEEE Specification

26 Throughput estimation in PLCP preamble + header: 24 bytes RTS: 20 bytes, CTS: 14 bytes MAC header: 28 or 34 bytes IP header: 20 bytes TCP header: 20 bytes UDP header: 8 bytes Bottomline: too much per-packet overhead!

27 Alphabet Soup Source: Cisco presentation

28 Enterprise Deployment

29 Community Networks Source: Roofnet SIGCOMM 04 paper

30 Long-Dist.: Digital Gangetic Plains for last-hop access within a village Sep 02 Point-toPoint link Dec Km 5.1 Km 12 Km 2.3 Km MS3 Jun 02 IITK 17.3 Km Nov 02 Bithoor 12 Km Jun 02 5 Km 39 Km Sarauhan Mandhana Apr 02 Rasoolabad Safipur 22.5 Km Lodhar Mar 03 Rajajipuram/Lucknow Mar 04 Jun Km 23 Km Banthar End to end distance ~80 Km 22 Km Dec 03 River Ganges Sawayajpur Land-line access point (close to high-population density area) Not to scale

31 The Ashwini Network

32 Wireless Technologies Source: Cisco presentation