802.11g Update Byron Early & Chad Burnham University Technology Services Westnet Meetings: July 8-10, 2003

? "The wonderful thing about standards is there are so many to choose from.” - Prof. Andrew Tennenbaum “The subverting of standards and claiming compliance is obviously wrong and evil. It is also sadly traditional”. - L. Victor Marks The Hebrew alphabet begins with the letters: Aleph, Bet, Gimel –this is merely coincidental and has nothing to do with the wireless standards: a, b, and g.

802.11g Ratified June 2003 by IEEE Standards Board –3-year ratification process Incorporates 4 standards: –Compromises to component makers include: Intersil: CCK-OFDM (33 Mbps) TI: Packet Binary Convolutional Coding (PBCC-22, 6-54 Mbps)

802.11g Ratification (cont.) ERP: “extended rate” PHY Layer for MAC: –Orthogonal Frequency Division Multiplexing (OFDM) coding in the 2.4-GHz ISM band –Data Speeds: 6, 12, 24 Mbps (mandatory) Optional Data Speeds: 18, 36, 48, 54 Mbps Wi-Fi Alliance: will only certify products supporting 54 Mbps –ERP, yet another acronym conflict

802.11g Ratification (cont.) Backward Compatibility with b –Complementary Code Keying (CCK) for b compatibility (5.5 & 11 Mbps) –Barker Code Modulation for b compatibility (1 & 2 Mbps) RTS/CTS –Optional for both b & g –Essential in “mixed-mode” (b/g) networks

“Mixed-Speed” Transmissions Effects on Total Network Throughput Low speed clients significantly reduce total network throughput for all variants of IEEE WLAN standards –Client connect speeds “fall-back” as distance from Access Point increases –Slower packet delivery to nodes at cell edge dramatically reduces network throughput –Worst Mix: medium supporting both “Legacy” b and g clients

“Mixed-Mode” Throughput (“b” & “g” clients on same Access Point) Data Throughput Parameters: –Ratio of “b” to “g” clients “b” clients decrease performance –Client Transmission Rates: 1 to 54 Mbps –Network Efficiency (error rates, interference, noise) Bluetooth devices, cell phones, microwave ovens, etc. Multi-Path problems –“ACK” packets –RTS/CTS packets (4-Way “protection Mechanism”)

Throughput Parameters (cont.) –Between-Packet-Idle-Time (mandatory for CSMA/CA) 50 micro seconds for “mixed-mode” g networks –“Back-Off Counter” waiting period Internal timer on wireless node Random selection of 1-32 slot times Slot Times: 20 micro-seconds (“mixed-mode”) Favors g clients: –Twice as many transmit opportunities as b clients –Protocol Overhead (TCP/IP) –Application Overhead

802.11b Throughput Example: (examples from Intersil paper, by Jim Zyren, Tim Godfrey, Menzo Wentink) “Downstream” transmissions (AP to Client) “Downstream” transmissions (AP to Client) (2) b clients:11 Mbps each –Total Throughput: ~7.2 Mbps (2) b clients: 1 & 11 Mbps –Total Throughput: ~1.6 Mbps –77% drop in throughput!

802.11a Throughput Example: (examples from Intersil paper, by Jim Zyren, Tim Godfrey, Menzo Wentink) “Downstream” transmissions (AP to Client) “Downstream” transmissions (AP to Client) (2) a clients: 54 Mbps each –Total Throughput: ~30 Mbps (2) a clients: 6 & 54 Mbps –Total Throughput: ~9.2 Mbps –70% drop in throughput!

802.11g Throughput Example: (examples from Intersil paper, by Jim Zyren, Tim Godfrey, Menzo Wentink) “Downstream” transmissions (AP to Client) “Downstream” transmissions (AP to Client) (2) g clients: 54 Mbps each –Total Throughput: ~30 Mbps Mixed-Mode Transmission: –(1) g client at 54 Mbps & (1) b client at 11 Mbps –Total Throughput: ~11.2 Mbps –63% drop in throughput Much worse if b client drops back to 1 Mbps!

Mixed-Mode Practices: Cell Hand-Off Design: –Design network for highest possible throughput Keep S/N ratio high throughout entire cell coverage Keep Cell-Size small –Site Survey (document coverage results) Base design on least sensitive radio cards 3-D coverage must be considered Only 3 of 11 channels do not overlap –FCC: Additional plans for b/g Spectrum? FCC has not specified Power Output Levels for entire ISM band

Mixed-Mode Practices (cont.) RTS/CTS: –An extension to CSMA/CA protocol –“Protection Mechanism” for Mixed-Mode: CTS packet coded for legacy b clients (Barker Code) must precede any OFDM transmission. –Bandwidth Allocation: Station broadcasts frame advertising network time needed for data-frame & ACK Network Allocation Vector field (NAV) Reduce Collisions: Hidden Node Problem

Dual-Card Equipment: Dual-Card Access Points do not (yet?) support two “b/g” radio cards –Cisco & Proxim can’t... Can any? Proxim Only Allows: b & b/g, b/g & a, b & a –Proxim working on code problems for mixed-mode –Cisco has not yet released “g” code Means doubling number of AP’s in areas with high user density for complete b/g coverage (DU Business School Example)

Other Issues: Site Survey Problems with “g” –Link Test not yet supported Must use “b” card for site surveys Can’t determine where “g” speeds “fall-back” “b/g” card receiver appears to be less sensitive –> 5 dB difference from “b”

Other Issues: Point-to-Multi-Point (PtMP): –Proxim not yet supporting “g” –Cisco Aironet 1400 does support “g” Some “g” features still in development –AP: Load Balancing (can’t have 2 b/g cards in AP) –Client: Medium Density Distribution Client ability to attach to weaker signal AP with lower traffic Upgrade Costs: –Proxim’s “b/g-upgrade kit”: ~$125 Battery Life? – unknown at this point