Timing more important then distance ?

[BobY777]

I have heard that if two bridges are too far apart (even if the signal is strong), that there can be too much latency in the signal and the bridging of two wireless point to point devices (mainly in the 2.4 G range) may give poor results or not work at all? I guess this is true for 5.8 G also?

Most of the time I hear people talk about wanting to get a strong signal, point to point. But other talk I hear says that latency is also a critical factor in delivering a good signal, even if you have a strong signal.

This sound right?

If this is right, then it seems that there would be a distance limitation for wireless point to point installations, even if you have a strong signal.

Anyong else heard of this?

[wirelessoceans]

latency is an issue on long links, but a rf enginner with the right product can configure a long link, 30+ miles, to work reliably. There are wireless links that go 100+ miles, just not in unlicensed bands

[BobY777]

Hi again:

Thanks.

When I use to work in cellular, we had very long links. Before the 2.4 G band was taken from us, we use to have very long links. From town to town. But we didn't have to worry about latency.

We would go from one microwave transmitter to another and link them together. Hop to hop. Never had timing problems.

So maybe this latency (timing) problem I heard of in wireless Internet 2.4 G point to point system, isn't that big of a problem after all.

Anybody else with info on this is welcome to say their thoughts here. Thanks.

[ConnClark]

Latency is an issue with all 802.11 links over 4.6 miles. Its due to timing parameters defined by the spec. I have not got the spec for wi-Max yet, however in that spec it should be dealt with because it is supposed to be a long range protocol unlike wifi.

You might be able to get a wifi link to work over this range while conforming to the 802.11 spec, but it will be a pain to get it to be reliable and consistant. It is certain it won't perform as well as the equipment will be transmiting retrys on every packet. If you have the right equipment and the right driver you can toy with these timing parameters but then you are no longer conforming to the 802.11 spec and may have issues communicating with equipment that does conform.

The farthest link I have heard of using 802.11 equipment is 72 miles. This was done by changing the timings and using a 5 Watt amp and a 30+dB gain dish. Note they were breaking a few FCC rules.

Refer to my response to the thread "Long range wifi question" in the hardware section for more details

Conn Clark

[a2z]

In 802.11, distance is a problem bcos of propagation delay. If the ack timer timesout u will not be able to communicate between bridges. For ever pkt recvd the station sends an ack, but if the sender does not recv this ack frame before its ack time timesout, it will re-transmit the data. This will result in contunous retransmissions and no reply. To overcome this problem for longer distance the ack timer needs to be made configurable so that the user can increase it according to the distance between the bridges.

[BobY777]

Hmmm.

I wonder if the enterprise APs have this configuration? Maybe so?

I'll look closer at my APs. And at other APs on the market.

Hopefully they do.

Thanks.

[ConnClark]

I'm not currently aware of an AP on the market that allows you to play with the timings. We plan to add that feature to ours in a future firmware update. Right now its only possible with custom device drivers for certain chipsets. Remember anything over 1000 ft is beyond the intent of 802.11 spec and not many manufacturers are willing to push it.


Conn Clark

[FlaWireless]

We have done links from 10ft to over 10 miles with Cisco and Proxim products. The Cisco 1400 bridges work great in the long distance enviroment.

[BobY777]

Did you get a lot of retransmissions for errors?

If not, then maybe the links can go farther then we thought. Or maybe Cisco has something in them that will accept the longer latency.

[ConnClark]

If it conforms to the 802.11 spec and the link was over 4.6 miles it had retrys. Use a packet sniffer and you will see. Note some ranges beyond 4.6 miles will work better than others, success just depends on if your acknowledge packets not ariving durring the retransmittion(s).

Conn Clark