There are three unlicensed bands - 900 MHz, 2.4 GHz, and 5.7 GHz within the Industry, Medical and Scientific Frequency This presentation focuses on 2.4 GHz because our products use those bands today and it adheres to the IEEE 802.11b standard. The 5.7 GHz band is promising for future products and we are actively pursuing projects in that area. Recently, the FCC also opened up the 5.2 GHz band for unlicensed use by high speed data communications devices. 5.2 GHz is the same band that is used for the ETSI HYPERLAN specification in Europe. A nearby neighbor of the 900 MHz band is the cellular phone system. This helped the early development of the WLAN industry in the 900 MHz band because of the availability of low cost small RF components in that band. 2.4GHz has a neighbor in the PCS system. That helps with component costs too. There are no such neighbors for the 5 GHz band. The WLAN industry will have to driver the development of low cost components for 5GHz on our own. We think this means practical, cost effective, PCMCIA products in the 5 GHz band are a few years away. The other downside to 5GHz is the poor range performance as compared to 2.4GHz.
900Mhz is a band that is becoming overcrowded due to consumer products. It does offer longer range (for the same gain antennas) than 2.4GHz, but it has limitations on the maximum size of antennas that limits its overall range. At 900MHz the highest datarate that can reliably be obtained is under 1Mb, due to the limited frequency range. At 2.4GHz, the lower power transmitter, allows very high gain antennas, which allows long distance communication (up to 25 miles). The frequency range is also much wider than 900Mhz, allowing higher datarate with a reliable range. 5GHz offers more bandwidth, allowing higher datarates, however the nature of the higher frequency limits range. Typical range for 5GHz indoors is about 50 Feet, and outdoors is limited to about 2500 feet.
With Frequency Hopping, the FCC requires the use of 75 different channels before repeating the use of any one channel. The maximum time on any one frequency is 400mS in any 30 second period. 802.11 has defined 26 hopping patterns in three different sets. These 26 patterns are designed to have minimum interference with each other. These patterns are called orthogonal patterns. If interference appears on a frequency, any data for that frequency is impaired, and will be retransmitted on the next frequency.
By using these codes, the receiver could actually miss several bits and the software would be able to still identify that the code was intended to be a 1 or a 0. If there was an interfering signal, the unit would still be able to get the data through without loss of data or reduction in throughput or performance. If a bit was received that was a 01111011011, when compared to a 1, there would be two bits different. Comparing it to a 0, there would be 9 bits different. More than 5 databits would have to be inverted to change the value, which translates to over half of the signal lost before the original message cannot be reconstructed.
When a Client comes on line, it will broadcast a Probe Request. An AP that hears this will respond with details. The client makes a decision who to associate with based on the information returned from the AP. Next the Client will send an authentication request to the desired AP. The AP authenticates the client, and sends an acknowledge back. Next the client sends up an association request to that AP. The AP then puts the client into the table, and sends back an association response. From that point forward, the network acts like the client is located at the AP. The AP acts like an Ethernet hub. The AP’s broadcast a beacon at predetermined (and programmable) intervals. This broadcast contains information about the AP such as RF hops to the backbone, load, hopping pattern, etc. The client listens to ALL AP’s it can hear. It builds an information table about each one and enters in the information the AP sends during beacons, including the signal strength of the AP.
As the client is moving out of range of his associated AP, the signal strength will start to drop off. At the same time, the strength of anther AP will begin to increase. At some point in time, BEFORE communication is lost, the client will notify AP A that he is going to move to AP B. B and A will also communicate to assure any information buffered in A get to B over the backbone. This eliminates retransmitting packets over the air, and over the backbone. The same handoff can occur if the load on A become large, and the client can communicate with someone other than A.
The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication.
ISM Unlicensed Frequency Bands Extremely Low Very Low Low Medium High Very High Ultra High Super High Infrared Visible Light Ultra- violet X-Rays Audio AM Broadcast Short Wave Radio FM Broadcast Television Infrared wireless LAN Cellular (840MHz) NPCS (1.9GHz) 10 22 10 16 10 14 10 12 902-928 MHz 26 MHz 5 GHz (IEEE 802.11) HyperLAN HyperLAN2 2.4 – 2.4835 GHz 83.5 MHz (IEEE 802.11)
900 MHz vs. 2.4 GHz vs. 5GHz 900 MHz 2.4 GHz PROs CONs Greater Range than 2.4 GHz ( For in- Building LANs) Maximum Data Rate 1 Mbps Limited Bandwidth Crowded Band Global Market IEEE 802.11b/g Higher Data Rates (10+ Mbps) Less Range than 900 MHz (For In-Building LANs) 5 GHz Global Market IEEE 802.11a Higher Data Rates (20+ Mbps, up to 50M) Much Less Range than 900 or 2.4GHz Higher Cost RF Components Large Antenna required
The 802.11 Standard 1997 1–2 Mbps 2.4G 1999 54 Mbps 5G Orthogonal FDM 1999 11 Mbps 2.4G 2001 54 Mbps 2.4G Power Management ( awake, doze ) Timing Management ( beacon, sync within 4usec )
It uses fixed network access points (AP) with which mobile nodes can communicate. These network APs are connected to wired network to widen the LAN's capability by bridging wireless nodes to other wired nodes.
All communications between mobiles and wired network clients go through the AP.
Mobiles can roam between APs and seamless wide area coverage is possible.
success rate for full Ethernet frame (12,144 bit) <30%, (1-p)**n
Error rate p = 10 -6 , 1% will be damaged.
Medium Access Control P oint C oordination F unction ( PCF )
Base polls other stations
Broadcast a beacon frame periodically (10ms to 100ms) with system parameters (hopping sequence, dwell time, clock synchronization)
Base determines the transmission priority
Can Coexist with DCF
Short InterFrame Spacing RTS/CTS/ACK Fragment burst
The 802.11 Data Frame Structure WEP More Frames Ordered frames Data ControlMgmt To AP RTS CTS ACK Intercell traffic addresses Sleep / Awake Frame length plus ack (used for NAV) Intercell traffic addresses Fragment sequence
Association Process -- Passive Scanning Steps to Association: Access Point A Access Point B Initial connection to an Access Point Client evaluates AP response, selects best AP. AP sends Probe Response Client sends probe Client sends authentication request to selected AP (A). AP A confirms authentication and registers client. Client sends association request to selected AP (A). AP A confirms association and registers client.
Re-association Process Steps to Re-association: Adapter listens for beacons from APs. Adapter evaluates AP beacons, selects best AP. Adapter sends association request to selected AP (B). AP B confirms association and registers adapter. Access Point A Access Point B Roaming from Access Point A to Access Point B AP B informs AP A of re-association with AP B. AP A forwards buffered packets to AP B and de-registers adapter.