IEEE802.11

What is IEEE ? Standard for wireless local area networks (wireless LANs) developed in 1990 by IEEE Intended for home or office use (primarily indoor) standard describes the MAC layer, while other substandards (802.11a, b) describe the physical layer Wireless version of the Ethernet (802.3) standard

Wi-Fi Standard (802.11x) Mission: promote interoperability as the global wireless LAN standard Wi-Fi Board members include AMD, Apple, Cisco, Compaq, Dell, Epson, Ericsson, Fujistu, Gateway, HP, IBM, Intel, Microsoft, NEC, Nokia, Nortel, Philips, Samsung, Sharp, Sony, TDK, Toshiba,

802.11b LAN Configuration Access Point Device

Example b Wireless LAN Equipment

Intended Use Wireless Internet access inside hotel lobbies, conference rooms, etc. Wireless with your Latte? Wireless home networking Wireless at the Airport

Wi-Fi Market in the News Wireless LAN equipment market $969 Million in 2000 to estimated $4.5 Billion in 2006 In 2001: Microsoft adds in Windows XP Major hotel chains install Wi-Fi Internet access 462 Starbucks stores offer wireless Internet Microsoft joins WECA board (the alliance) Intel Joins WECA board Most PC/Laptop manufacturers offer Wi-Fi

802.11b Interoperability The Wireless Ethernet Compatibility Alliance (WECA) certification provides a standard for wireless technology, ensuring interoperability of products from different vendors. WECA Certification Mark

Competing Short-Range Wireless Technologies Short-range wireless solutions: Bluetooth (Wi-Fi) HomeRF (not as popular) Who will prevail? more suitable for wireless LANs (office, hotel, airport,…) Bluetooth is designed for personal area networks – smart appliances, printers, scanners, etc.

Commercial Wireless Data Communications Present Near Future Cellular 19K Proprietary Systems 128K Digital Cellular 384K Data Rate Bits per second Bluetooth Technology 721K 11M 54M a Technology b Wireless LANs Home RF

IEEE b Technology The standard is believed to be very good, will be widely accepted and will allow hardware prices to decrease. Developed by consortium of major companies with focus on interoperability. Optimized for wireless LANs. Uses radio frequency signals in unlicensed 2.4GHz band to send and receive data. Uses Direct Sequence Spread Spectrum (DSSS) RF method. Equipment dynamically selects lower data rates as RF signal quality decreases: 11, 5.5, 2, 1 Mbits. Allows roaming among radio access points. Standard adopted by Institute of Electrical and Electronics Engineers (IEEE) in September 1999.

IEEE b Technology More Specifically This in itself can form a standalone network as if wired devices were connected to a hub. The Access Point sits on both the wireless network segment (space) and the wired segment, acting as a bridge from the wireless to the wired segments. A bridge forwards data packets from one side to the other at the MAC layer b defines how the RF channel is used, allowing multiple devices to communicate on the channel as if it were a wire.

Flavors of x (1 Mbps) Older standard b (11 Mbps) Current technology g (22+ Mbps) Expected future standard

Under the Hood (Wireless Ethernet) Why can’t we use regular Ethernet for wireless? Ethernet: A sees B, B sees C,  A sees C Wireless: Hidden node problem A sees B, B sees C, yet A does not see C A B C

Under the Hood (Wireless Ethernet) Why can’t we use regular Ethernet for wireless? Ethernet: B sees C, C sees D  B & C can’t send together Wireless: B can send to A while C sends to D A BC D

Under the Hood The Protocol Sender A sends Request-to-Send (RTS) Receiver B sends Clear-to-Send (CTS) Nodes who hear CTS cannot transmit concurrently with A (red region) Nodes who hear RTS but not CTS can transmit (green region) Sender A sends data frame Receiver B sends ACK Nodes who hear the ACK can now transmit RTS CTS A B

Collision Resolution Two senders might send RTS at the same time Collision will occur corrupting the data No CTS will follow Senders will time-out waiting for CTS and retry with exponential backoff

More Wireless Challenges Problem: mobility Nodes can move and get “out of touch” How to deal with intermittent connectivity? Solution: hierarchical structure A set of access points hooks into a wired distribution system Access points cover the target area Roaming nodes connect to nearest access point at any given time

Hierarchical Structure: Communication between roaming nodes is via the distribution system Distribution System AP1 AP2 AP3 A B C D E F

Choosing the Access Point Active Scanning Roaming node periodically sends probe frame All APs within range reply with a probe response frame Roaming node chooses best AP and replies to it with association request AP acknowledges request with association response. AP notifies previous AP (if any) of the established association (handover).

Choosing the Access Point Passive Scanning APs advertise themselves periodically by sending beacon frames Roaming nodes can associate themselves with an AP by sending the association request AP acknowledges request as before with an association response

Frame Format ControlDurationAddr1Addr2Addr3Addr4ControlDataCRC Distribution System AP1 AP2 AP3 A B C D E F Frame Type (RTS,CTS,…) ToDS FromDS Ultimate Destination (E) Immediate Sender (AP3) Intermediate Destination (AP1) Source (A)

Physical Properties Frequency hopping Direct sequence Diffused infrared

Range from Access Point to Computer 100 Feet Access Point Computers Range = 0 to 700 feet 11 Mbits 5.5Mb 2 Mb 1 Mb 0 Mb 5.5Mb 11 Mb

Basement 1st 3rd 2nd Electrical Engineering Building

Network Setup Basic Network Setup is Cellular Mobile Terminals (MT) connect with Access Points (AP) Standard also supports ad-hoc networking where MT’s talk directly to MT’s

IEEE Physical Layers b802.11a Modulation SchemeDSSSOFDM Spectrum (GHz)2.4 – , Data Rate (Mbps)1 – Subchannels11 overlapping8 independent InterferenceMicrowave, Cordless Phones,Bluetooth, HomeRF, Light Bulbs! HyperLAN II AvailabilityTodayLate August? Cost$250 AP, $100 PC Card??? (same)

Media Access Control- Ethernet CSMA/CD (Carrier Sense Multiple Access with Collision Detection) If media is sensed idle, transmit If media is sensed busy, wait until idle and then transmit immediately Collisions can occur if more than one user transmits at the same time If a collision is detected, stop transmitting. Reschedule transmission according to exponential backoff

Media Access Control (802.11) Would like to use CSMA Nice for bursty traffic Make for seamless replacement of wired LANs with wireless LANS Use CSMA, but can’t use CD P T /P R ratio is too high Don’t want to waste energy on mobiles Use Collision Avoidance instead Don’t always start transmitting immediately after someone else

CSMA/CA Details SIFS (Short Interframe Space) DIFS (Distributed Interframe Space) Scenario: B and C want to transmit, but A currently has control of medium B randomly selects 7 slots of backoff, C selects 4 slots C transmits first, then B

What is HIPERLAN/2? European standard developed by ETSI/BRAN (European Telecommunications Standards Institute/Broadband Radio Access Networks) Physical Layer is very similar to a (OFDM operating in the 5 GHz spectrum) Standard based on wireless ATM (Asynchronous Transfer Mode)

HIPERLAN/2 MAC BCH – Miscellaneous header FCH – Details how the DL and UL phases will be allocated ACH – Feedback on which resource requests were received RCH – Random access resource request

Performance Comparison

Conclusions IEEE is a widely accepted standard in the United States for wireless LANs Primarily a “cellular” random access scheme with provisions for ad hoc networking and contention free access b products are available now, but better to wait for a products later this year HIPERLAN/2 is being pushed in Europe Wireless ATM solution for real-time traffic Standard reflects the network topology There is an effort to agree on one world-wide standard, keep your fingers crossed