Half/Full Duplex question

Half duplex is all there is in wireless (over the air).

5.5mbs throughput is normal for an 802.11b device. The other 5.5 is overhead (protocol, security etc.)
Reply to
Airhead
Loading thread data ...

802.11b is always HDX. If you are getting a STR of 5.5 Mb/s you are doing better than most.
Reply to
Bob Willard

I'm using IBM Thinkpad T40 with Intel Pro wireless LAN 2100 3B mini card and a Linksys WRT54G wireless router. The best transfer rate I can get is 5.5 Mbps which is half duplex. How can I make use of the full bandwidth of 11 Mbps for 802.11b? What do I have to do as far as device driver, firmware, configuration is concern to be networked using full duplex? Any suggestions would be greatly appreciated.

Robert

Reply to
Robert

Thanks for the replies. Does that mean if I switch to 802.11g I can only get at most 27 Mbps or does 802.11g uses full duplex?

Reply to
Robert

all the 802.11 protocols use the same wireless channel for traffic in both directions - so from that perspective wifi is half duplex.

but - it isnt that you get 50% of the bandwidth because the protocol is half duplex, but that there is a lot of overhead for things like encapsulation and gaps between packets.

interference, signal levels vs local noise, reflections and other usage specific issues can have a big impact on useful throughput as well. and both protocols drop to lower data rates as conditions degrade.....

so with 802.11g you will get something on the order of 25 Mbps total (under good conditions, if you are lucky).

802.11g is designed to be backward compatible with 802.11b, so the amount of overhead usually goes up since the system has to make the channel and data comprehensible to the older, slower devices if they happen to be around.

but - there can be much more data going in 1 direction or the other depending on load. the protocol is effectively demand driven, so apart from overhead singalling, the channel is only occupied by either the AP or the PC when there is actual data to send.

it sounds like you were testing the link, perhaps by moving a file or using a test utility. such tests often move nearly all data in 1 direction, so the results you saw is probably 90% or more of the total data moved.

the test you did might have included IP and MAC overhead rather than just application data (depends on what got counted) and might have included data in both directions - without knowing what you were measuring and where the thruput numbers came from - we cant tell.

Reply to
stephen

correct.

Yes, thought thats the in-air bandwidth in any one direction. Then subtract protocols, security etc. Expect to see 3-3.5 Mb/s

What????? The other 5.5 is the traffic going the other way.... thats what half-duplex means...

Reply to
Mark McIntyre

I should have added: assuming symmetrical traffic. Obviously if you're mostly transmitting then traffic is assymetric. A file copy is heavily biassed in one direction, so you tend to see slightly better throughput. If you've several things going on at once on your machine, or if you've communication between several machines going on, traffic may be much more symmetrical.

The other point to bear in mind is that not only is the above true, but ALSO all devices on a given AP share the same channel -> two devices, both busy sending data -> each sees half the bandwidth. You're then relying on the same contention model that DSL uses, ie you're hoping that not every device is simultaneously busy.

Reply to
Mark McIntyre

Mark McIntyre wrote in news: snipped-for-privacy@4ax.com:

Indeed - although as you say, it's contended rather than 'shared' in the sense of 'divided evenly'.

This contention is one of the reasons why I use multiple access points on different channels for my (fairly extensive) home/business network, and specify this solution for others.

It is very instructive exercise to measure data transfer rates across a busy wireless network :(

Hope this helps

Reply to
Richard Perkin

From one of my previous postings:

I keep getting asked "how fast can it go" type questions. Perhaps some numbers might help. This is stolen from an Atheros PDF at:

formatting link
some additions and corrections by me.

Non-overlapping Modulation Max Max Max Channels ------- | Link TCP UDP | | | | |

802.11b 3 CCK 11 5.9 7.1 802.11g (with 802.11b) 3 OFDM/CCK 54 14.4 19.5 802.11g only 3 OFDM 54 24.4 30.5 802.11g turbo 1 OFDM 108 42.9 54.8 802.11a 13 OFDM 54 24.4 30.5 802.11a turbo 6 OFDM 108 42.9 54.8

The paper claims that encryption is enabled for these calculations, but my numbers seem to indicate that these number are for encryption disabled. Dunno for sure. The Max TCP and Max UDP are the theoretical maximum thruput rates.

Reply to
Jeff Liebermann

~ On Wed, 2 Mar 2005 10:36:28 -0600, in alt.internet.wireless , "Airhead" ~ wrote: ~ ~ >Half duplex is all there is in wireless (over the air). ~ ~ correct. ~ ~ >5.5mbs throughput is normal for an 802.11b device. ~ ~ Yes, thought thats the in-air bandwidth in any one direction. Then subtract ~ protocols, security etc. Expect to see 3-3.5 Mb/s ~ ~ >The other 5.5 is overhead (protocol, security etc.) ~ ~ What????? The other 5.5 is the traffic going the other way.... thats what ~ half-duplex means...

Wrong - if you try a unidirectional transfer (say one-way UDP packet blast) over "11Mbps" 802.11b, you will see about 5.5Mbps. The 50% reduction in payload througput relative to nominal signaling rate really is due to overhead.

Matthew Gast has an excellent explanation here: When Is 54 Not Equal to 54? A Look at 802.11a, b, and g Throughput

formatting link
As far as whether 802.11 is "half duplex" - technically one should call it simplex, if you use the appropriate (radio) definition of simplex and not the telephony/serial definition. See the ATIS Glossary
formatting link
and check out the definitions for "simplex operation", "duplex operation" and "half-duplex (HDX) operation".

Aaron

Reply to
Aaron Leonard

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.