IEEE A technical Overview

Transcription

1 IEEE A technical Overview Pablo Brenner Director of Engineering BreezeCom 1 BreezeNET 7/8/97

2 Introduction Overview of the Emerging Standard Technical details Special areas of interest? 2 BreezeNET 7/8/97

3 Why do we need a Standard? Multi-Vendors Compatibility Protects customers investment High Volumes reduce prices 3 BreezeNET 7/8/97

4 Why Not Just use Wireless Ethernet? First Ethernet predecessor was Radio-Based (ALOHA) Ethernet is simple, cheap, widely accepted But... 4 BreezeNET 7/8/97

5 Ethernet could not do it. Collision Detection Would require a Full-Duplex Radio Wouldn t help on recognizing collisions on the receiver end Carrier Sense The fact that a station doesn t sense traffic does not mean that the receiver side can receive (Hidden Stations) 5 BreezeNET 7/8/97

6 Ethernet could not... The Mall Problem How can you partition two collocated networks? Mobility and Roaming Security Issues Power Saving Requirements 6 BreezeNET 7/8/97

7 Wireless LAN System BSS Components STA 1 STA 2 AP DS AP STA 3 BSS 2 STA 4 7 BreezeNET 7/8/97

8 Architecture MAC MLME MAC MIB MLME_GET/SET PLCP PLME_GET/SET PHY MIB Station Management PLME_GET/SET PMD PLME 8 BreezeNET 7/8/97

9 Basic Access Mechanism CSMA/CA Stations listen before transmission If medium free for more than DIFS: transmit If not, use backoff mechanism. Immediate access when medium is free >= DIFS DIFS Busy Medium DIFS PIFS SIFS Contention Window Backoff-Window Next Frame Slot time Defer Access Select Slot and Decrement Backoff as long as medium is idle 9 BreezeNET 7/8/97

10 Other Collision Avoidance Mechanism: NAV Needed to handle Hidden Stations Problem AP SA2 SA1 10 BreezeNET 7/8/97

11 NAV Operation (RTS/CTS) Stations exchange Duration information using short frames (RTS/CTS), other stations, recognize the medium as busy for the said duration Src G3 RTS G1 G1 Data G1 G1=SIFS G3=DIFS CW=Contention Window Dest CTS Ack G3 CW Other NAV NAV (RTS) (CTS) Defer Access Next MPDU Backoff after Defer 11 BreezeNET 7/8/97

12 Collision detection mechanism Real Collision detection would require a full- duplex radio (high cost) Unable to recognize collisions at the receiving end Uses Positive Acknowledge (ACK) 12 BreezeNET 7/8/97

13 Fragmentation Long fragments: higher probability of error Microwave ovens interference (4ms noise, 4 ms clear) Collision recovery is less expensive if we use fragmentation 13 BreezeNET 7/8/97

14 Frame format FC Duration ID Addr 1 Addr2 Addr 3 Seq Nr Addr 4 Data CRC BreezeNET 7/8/97

15 Frame Control Field Protocol Version (2 bits) = 0 Type (6 bits) ToDS FromDS More Frag Retry Power Mgt More Data WEP 15 BreezeNET 7/8/97

16 Beacon Frames Send Periodically by the AP Provide information for new stations Keeps Synchronization 16 BreezeNET 7/8/97

17 Scanning Passive Active Authentication Association Init Procedure 17 BreezeNET 7/8/97

18 Roaming Procedure Not defined in the standard, left as implementors decision 18 BreezeNET 7/8/97

19 BreezeCom Patented Algorithm Designed for Rotterdam Port requirements. The AP sends periodic Neighbor Beacons when appearing as a regular station Stations receive Synch information, so learn how and when to join the new AP Stations compare Signals Strength, and make the decision 19 BreezeNET 7/8/97

20 BreezeCom Algorithm (ctd...) Station knows which AP is it roaming to so lets both APs (new and old) know about that. New AP starts accepting frames addressed to the roaming station, buffers stations as regular Power Saving mode Old AP inserts Jump Message for synch the actual roaming. 20 BreezeNET 7/8/97

21 BreezeCom Algorithm (ctd...) Stations waits for Jump message, and roams when received. Station Associates with new AP, and then receives buffered packets. 21 BreezeNET 7/8/97

22 How do APs know about Neighbors? Automatically, first station that roams the hard way lets AP Know about the neighboring relationship 22 BreezeNET 7/8/97

23 Load Balancing Currently no provisions on Need a Load parameter in beacon frames Needs special BreezeCom SNAP formatted beacon (same format as Neighbor Beacon) BreezeCom will disclose both algorithms to PCMCIA partners Use of Exponential Random Backoff procedure to prevent stations from bouncing 23 BreezeNET 7/8/97

24 Load Balancing (ctd...) AP may use the Jump frame to cause a specific station to join a different AP. 24 BreezeNET 7/8/97

25 Power Saving Mechanisms Power Saving Stations Notify the AP AP buffers frames for Power Saving Stations, and broadcast/multicast frames AP sends TIM (Traffic Information Map) on Beacon Frames Some Beacons contain DTIM (Delivery TIM) 25 BreezeNET 7/8/97

26 Power Saving Mech (ctd...) Power Saving Stations may poll the AP for the buffered frames using PS-POLL frames The AP will send mcast/bcast frames immediately after the DTIM The AP will send unicast buffered frames immediately after the mcast/bcast frames after the DTIM. 26 BreezeNET 7/8/97

27 Status Draft 5.0 in Sponsor Ballot Approval expected Q BreezeNET 7/8/97

28 Status (ctd...) First interoperability tests performed with 4 PCMCIA vendors Results very promising, beyond expectations 28 BreezeNET 7/8/97

29 Voting members 2 Highly Contributing members Naftali Chayat (BreezeCom Chief Scientist)» Author of the 2 MBit/s FH original proposal» Presented 3 MBit/s proposal» Chairman of the FH Higher Rates interest group Pablo Brenner (BreezeCom Director of Engineering)» Author of several proposals (MultiRate Support) 29 BreezeNET 7/8/97