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I've seen some routers with the option of configuring the DTIM interval, most of them are set to 1 or 2.

What is the purpose of the setting?

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I don't know if it will improve performance, but here's explanation what it does: en.wikipedia.org/wiki/Delivery_Traffic_Indication_Message Another article wi-fiplanet.com/tutorials/article.php/3433451/… –  AndrejaKo Aug 16 '10 at 0:54

2 Answers 2

up vote 6 down vote accepted

The DTIM is how the AP (wireless router) warns its clients that it is about to transmit the multicast (and broadcast*) frames it queued up since the previous DTIM.

This queueing and scheduled delivery is done to allow power-conscious devices to save power by turning off their receivers for brief stretches of time, only waking up their receivers when the AP indicates is has traffic for them. This procedure is only performed if one or more of the currently-associated clients on the AP is in power-save mode. If no clients are in power save mode, the AP will transmit multicasts as soon as they come in.

The DTIM interval is essentially meaningless and can be safely ignored. A shorter DTIM interval could theoretically make your wireless devices use up their batteries faster, because they'd have to wake up more often for multicasts. A longer DTIM interval could theoretically hinder multicast performance on your network. But either way, it is extremely unlikely that you'll be able to notice any difference that the DTIM interval setting makes. You'd have to set up a special, sensitive test to look for any difference, and I'm still not sure you'd be able to reliably measure one.

*in 802.11, broadcasts are a subset of multicasts, so wherever I wrote "multicasts", you can read it as "multicasts or broadcasts".

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Curiously, I just found out that changing DTIM from 1 to 3 on my wifi router, I practically doubled the battery lifetime of my cellphone (while the wifi is on). So far I cannot see any adverse effects. I wonder if I can increase it even further? The router's limit is 255, and since my home network is NAT'ted and only contains two other computers (which are mostly offline), I cannot imagine that there could be much (if indeed any!) multicast traffic. –  Vilx- Dec 3 '11 at 21:45
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This also explains why the wifi at my workplace used to drain much less battery than at my home, even though the signal was weaker and the channels overlapped with other nearby wifi networks. –  Vilx- Dec 3 '11 at 21:49
    
@Vilx I've seen some Wi-Fi clients choke on DTIM interval values that were too high, so if you find something that works for now, remember to try setting it back lower if you see problems the next time a friend or guest can't get his phone onto your network. –  Spiff Dec 3 '11 at 23:35
    
OK. Do you have any idea - what services could there be that would use multicast/broadcast in my private network, even though the only two online devices are my router (DD-WRT) and my cellphone (Android)? Also, Android device is idle, so no streaming music or anything. –  Vilx- Dec 4 '11 at 13:25
    
@Vilx ARP and DHCP both use broadcast. –  Spiff Dec 4 '11 at 16:57

For some environments the DTIM can be critical.

We use IPods (IOS 6) as Nurse Call Alerting devices, via push e-mail.

Apple was very aggressive about saving battery life when the changed from IOS 5 to 6.

Push mail being sent when DTIM set to lower value cause unreliable mail delivery.

By changing the DTIM setting on our wireless controller (AP manager) from 3 to 1, we improved mail delievery from 95% to 99%.

P.S. We initially changed the IPods to not "auto lock" which turns off battery saving, however battery life was reduced to 4 hours maximum, which was not acceptable.

Bottom line is for some deployments the DTIM is very important.

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