IEEE (WLAN) Standards. Osama Aboul-Magd

Transcription

1 1 IEEE (WLAN) Standards Osama Aboul-Magd

2 Abstract This presentation provides an overview of the IEEE Standards. Both MAC and PHY specifications are considered. 2

3 Outline Introduction IEEE 802 Overview IEEE Overview IEEE PHY Evolution IEEE MAC IEEE QoS Future Directions 3

4 Relationship to WiSense Project WiSense Project Heterogeneous sensor networks 4 Needs standard interfaces between different technologies

5 Why Do We Need Standards? Consumers and Network Providers Mix and match 5 Equipment Vendors Embed their IPR into standards and collect fees Engineers Job security!!!

6 IEEE 802 Overview 6 IEEE Project 802 LAN/MAN Standards Committee (aka IEEE 802 or LMSC) Develop LAN and MAN standards Mainly for link and physical layers of the network stack In operation since March 1980 (30 year anniversary) WORKING GROUP/TAG CHAIRS BRIDGING/ARCH CSMA/CD WLAN WPAN BWA ResPackRing TAG Radio Regulatory TAG Coexistance MBWA Handoff DISBANDED Token Bus DQDB Broadband TAG Fiber Optic TAG ISLAN Security Demand Priority CATV WRAN HIBERNATION LLC Token Ring

7 WLAN Standards Overview 7 11z 11v 11u IEEE s 11aa IEEE n MAC PCF HCCA EDCA Common MAC independent of the PHY PHY-dependent parameters Other IEEE common (draft) standards IEEE i (security of data frames) IEEEE w (security of management frames) IEEE k (radio resource management) Common MAC (DCF) IEEE a PHY IEEE b PHY IEEE g PHY IEEE n PHY

8 Radio Spectrum 8 UHF ISM S-Band Band S-Band ISM C-Band C-Band Satellite Downlink C-Band Radar (Weather) C-Band ISM C-Band Satellite uplink X-Band X-Band Radar (police/weather) Ku-Band Ku Band Radar (police) Frequency Range MHz 2-4 GHz GHz 4-8 GHz GHz GHz GHz GHz 8-12 GHz GHz GHz GHz GHz IEEE operates in the ISM (Industrial, Scientific, and Medical) bands ISM bands are license-free, provided that devices are lowpower IEEE b and IEEE g devices operate in S-band ISM IEEE a devices operate in C-band ISM IEEE n devices operates in both bands

9 IEEE Protocol Entities 9 MSDU MAC Sublayer MAC Layer Management Frm Cntr Dur ADD 1 ADD 2 ADD 3 SEQ # ADD 4 Data FCS PLCP Sublayer PMD Sublayer PHY Layer Management Preamble Short Training Field Long Training Field Signal Field 8 ms 8 ms 4 ms MPDU Srvc Field MPDU Tail PAD PPDU

10 Why 4 Addresses are Needed? 10 BSS 1 STA 1 AP STA 2 DS AP STA 3 STA 4 BSS 2 Stations belong to different BSS communicate through the distribution system (DS) An access point (AP) is a STA that provides access to DS Data moves between a BSS and the DS via an AP The DS can be either wireline or wireless (WLAN Mesh)

11 Guard Time PHY Layer Evolution 11 IEEE IEEE a Data Rates up to 2 Mbps Data Rates up to 54 Mbps Frequency OFDM Slot/Frame IFF T IFF T S N + 1 IFF T S S S S S N S N S N S 5 S 6 S 7 S 2N S N OFDM Symbol (FTT duration) Time Bandwidth IEEE n Data Rates up to 600 Mbps MIMO AP MIMO propaga tion channel Station y y y 1 2 N h h h N Channel Matrix h h h N h h h N1 N2 NN space x x x 1 2 N n n n 1 2 N Data Rates up to 11 Mbps IEEE b N M N Tx and M Rx - multiple parallel channels frequency time

12 Common MAC Distributed Control Function (DCF) DCF is the basic access mechanism for IEEE It employs carrier sense multiple access with collision avoidance (CSMA/CA) Each successful transmission is acknowledged by the recipient. Contention window is randomly generated from a uniform distribution between 0 and CW. CW is doubled each time there is a collision up to a maximum value, CW max. Successful transmission brings CW back to its minimum value, CW min. 12 EIFS Contention Window Busy Medium DIFS Next Frame Access Deferral Back Off SIFS ACK DIFS = DCF Inter-Frame Spacing EIFS = Extended Inter-Frame Spacing SIFS = Short Inter-Frame Spacing

13 Carrier Sensing (Clear Channel Assessment- CCA) Physical Carrier Sensing 13 Detecting the absence or presence of carrier by listening to the medium. Virtual Carrier Sensing Using Network Allocation Vector (NAV) together with the Duration field in the various IEEE frames

14 RTS/CTS Mechanism RTS/CTS: Request to Send/Clear 14 to Send Deals with hidden terminals and employs virtual sensing SIFS SIFS Area reachable By MTU 1 SIFS SIFS SIFS DIFS Area reachable By MTU 1 RTS Frame Frame CTS ACK ACK NAV (RTS) NAV (CTS) NAV (Fragment 1) NAV (ACK 1)

15 MTT (Mbps) IEEE MAC Theoretical Maximum Throughput (No Collisions) 15 X Frame and PHY Overhead Preamble P-HDR M-HDR PLCP-PDU MAC-SDU MAC-PDU PLCP-SDU FCS T Data T P T Phy x h Rate frame BitStream (PMD-SDU) IFS IFS BO WLAN MTT T MSDU T DIFS T SIFS T BO T RTS T CTS T ACK T Data 40 IEEE a IEEE a Simulation IEEE g IEEE g Simulation x T MSDU Rate 2T P 2T Phy T DIFS x T SIFS T BO T RTS T Increasing the rate without reducing overhead is not sufficient to achieve high throughput CTS Frame (MSDU) Length (bytes)

16 Frame Aggregation 16 Carrier MPDU Frame Control Dur / ID Address 1 Address 2 Address 3 Seq Control Address 4 QoS Control A- MSDU FCS Subframe 1 Subframe 2... Subframe n Subframe Header 14 B MSDU Pad B 0-3 B A-MSDU Aggregation DA SA Len 6B 6B 2B Length CRC MPDU Header MPDU Payload FCS Length CRC MPDU Header MPDU Payload FCS Length CRC MPDU Delimiter MPDU A-MPDU Aggregation MPDU Header MPDU Payload FCS PSDU

17 WLAN QoS (IEEE e) 17 Allows both prioritized and parameterized QoS Prioritized QoS is offered by Enhanced Distributed Channel Access (EDCA) Parameterized QoS is offered by the HCF Controlled Channel Access (HCCA) Introduces Traffic Stream ID Introduces User Priority (UP) similar to IEEE Q priorities 8 priority levels Employs the notion of Transmission Opportunity (TXOP) TXOP is of finite length Introduces traffic parameter specification (TSPEC) Necessary for parameterized QoS Introduces block acknowledgement (BA) Improves efficiency

18 Transmission Opportunity (TXOP) TXOP is an interval of time when a particular station has the right to initiate frame exchange sequence onto the WM. TXOP can be attained by either the EDCA (contention) or the HCCA (contention free) TXOP is always of finite duration TXOP duration should allow for al least one frame exchange sequence to be initiated. After the first frame, no other exchange sequence is allowed if the time left of the TXOP is not sufficient to complete the sequence Other stations use NAV, set to TXOP duration, to refrain from accessing the WM. CF-Poll TXOP Frame Frame 18 CF-Poll +ACK TXOP Frame NAV NAV TXOP TXOP

19 Access Categories IEEE defines 4 Access Categories (AC) for use with EDCA. The priority of an AC to access the WM is determined by the Arbitration Inter-frame Spacing AIFS[AC], and congestion window, CW min [AC] and CW max [AC] One-to-one mapping between UP and AC 19 Mapping to AC Transmit Queues Per Queue Channel Access Function UP Designation AC BK BK BE BE CL VI VO NC AC_BK AC_BK AC_BE AC_BE AC_VI AC_VI AC_V0 AC_VO Background Background Best Effort Best Effort Video Video Voice Voice

20 Enhanced Distributed Control Function (EDCF) 20 AIFS[j] AIFS[i] DIFS/AIFS Busy Medium DIFS PIFS Contention Window Next Frame SIFS Each QoS station has a separate channel access function per AC. Access rules are similar to those of DCF (CSMA/CA) The TXOP duration is advertised by the AP in the EDCA Parameter Set IE. The QoS station ensures that its transmission does not exceed the TXOP limit Fragmentation may be employed A continuation TXOP is granted if there is a frame available for transmission that fits in the remaining TXOP duration A continuation TXOP is granted to the same AC that initially won the TXOP. Internal collisions are handled as if they were external collision. The higher priority AC gains access to the WM.

21 CAP CAP CAP CAP CAP CAP Controlled Channel Access 21 HCCA(Hybrid Controller Channel Access) is a polling scheme that is centrally controlled by Hybrid Coordinator (HC) HC resides in the AP. TXOP are assigned by the HC to a QSTA at a regular interval and for a specified duration TXOP duration and frequency are determined based on Traffic Specifications (TSPEC IE) Traffic Streams (TS) are locally identified using TSID (part of TID) HC may generate CFP. However it is mandatory for HC to use CFP for QoS data transfers Controlled access phase (CAP) cab be initiated at anytime by the HC Beacon CFP CP CFP Repetition Interval

22 TSPEC IE 22 Element ID Length TS INFO Nominal MSDU Size Maximum MSDU Size Minimum Service Interval Maximum Service Interval Inactivity Interval Suspension Service Interval Start Time Minimum Data Rate Mean Data Rate Peak Data Rate Maximum Burst Size Delay Bound Traffic Type TSID Direction Access Policy User Aggregation APSD TSInfo Priority Ack Policy Schedule Rsvd Minimum Physical Rate Surplus Bandwidth Allowance Medium Time All TSPEC traffic parameters are optional TS INFO field contains information about TSID, UP, and access policy (EDCA, HCCA, or both) Medium Time is used for EDCA admission control Medium Time is the amount of time (in units of 32 microseconds) an AC is admitted to access the medium per 1 sec intervals When Medium Time is exceeded, the QoS station refrains sending a particular AC Medium Time is computed using mean value analysis that may prove to be useless for bursty applications

23 TSPEC Procedure QSTA 23 ADDTS Request (TSPEC) QAP ADDTS Response (TSPEC, Schedule) Elements ID Length Schedule Info Service Start Time Service Interval Maximum TXOP Duration Specification Interval The AP uses the traffic parameters to perform admission control on the incoming request Service Interval is the time between two successive service periods (SP) Directly related to bandwidth reserved

24 WLAN Popularity and Growth From a humble start at 1990 to one of the most successful standards in the history of data communications. Addresses real needs (Mobility) Operates in unlicensed bands Deployments are everywhere Conventions Home Enterprise Municipalities The Wi-Fi Alliance estimates that over 1000,000,000 devices will be sold annually by Laptop computers Game consoles Wi-Fi Phones 24

25 Where to Go From Here? New WLAN application demands the availability of more bandwidth Video streaming at home Smart phones upload 25 STB1 STB2

26 New PHY Specifications: The Road to Gbps WLAN 26 PCF HCCA EDCA STA Common MAC (DCF) AP IEEE a PHY IEEE b PHY IEEE g PHY IEEE n PHY IEEE ac PHY IEEE ad PHY STA From MIMO to MU-MIMO and SDMA Two new PHY layers are in the work: IEEE ac operates in the < 6GHz band IEEE ad operates in the 60 GHz band Chosen by the PC World Magazine as one of technologies that will change everything: