Transcript Slide 1

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: 802.11a, 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 802.11b
– Complementary Code Keying (CCK) for
802.11b compatibility (5.5 & 11 Mbps)
– Barker Code Modulation for 802.11b
compatibility (1 & 2 Mbps)
RTS/CTS
– Optional for both 802.11 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
802.11 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”
802.11b and 802.11g 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” 802.11g 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 802.11g clients:
– Twice as many transmit opportunities as 802.11b 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)
(2) 802.11b clients:11 Mbps each
– Total Throughput: ~7.2 Mbps
(2) 802.11b 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)
(2) 802.11a clients: 54 Mbps each
– Total Throughput: ~30 Mbps
(2) 802.11a 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)
(2) 802.11g clients: 54 Mbps each
– Total Throughput: ~30 Mbps
Mixed-Mode Transmission:
– (1) 802.11g client at 54 Mbps & (1) 802.11b client
at 11 Mbps
– Total Throughput: ~11.2 Mbps
– 63% drop in throughput
Much worse if 802.11b client drops back to 1 Mbps!
Total Network Throughput in b/g Mixed-Mode
(examples from Intersil paper, by Jim Zyren, Tim Godfrey, Menzo Wentink)
10
9
8
7
6
5
4
3
2
1
0
#
802.11b
Clients
5.9
5.9
5.9
5.9
5.9
5.9
5.9
5.9
5.9
5.9
0.0
0
6.2
6.2
6.3
6.3
6.4
6.5
6.6
6.8
7.2
8.2
22.1
1
6.5
6.5
6.6
6.7
6.8
7.0
7.2
7.6
8.2
9.4
22.1
2
6.8
6.8
6.9
7.1
7.2
7.4
7.7
8.2
8.9
10.2
22.1
3
7.0
7.1
7.2
7.4
7.6
7.8
8.2
8.7
9.4
10.7
22.1
4
7.2
7.4
7.5
7.7
7.9
8.2
8.5
9.1
9.8
11.1
22.1
5
7.4
7.6
7.7
7.9
8.2
8.5
8.9
9.4
10.2
11.3
22.1
6
7.6
7.8
8.0
8.2
8.4
8.7
9.2
9.7
10.4
11.6
22.1
7
7.8
8.0
8.2
8.4
8.7
9.0
9.4
9.9
10.7
11.7
22.1
8
8.0
8.2
8.4
8.6
8.9
9.2
9.6
10.2
10.9
11.9
22.1
9
Number of 802.11g Clients
802.11g "Mixed-Mode" Estimated Throughput in Mbps
(Includes TCP/IP Overhead & 15% correction factor for network efficiency)
8.2
8.3
8.5
8.8
9.1
9.4
9.8
10.4
11.1
12.0
22.1
10
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 802.11b/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 802.11b 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