LTE Air Interface DRX Settings in Practice

In a previous post I’ve given an introduction to how LTE Discontinuous Reception (DRX) for power saving works in theory and which parameters can be configured by the network in practice. In this post I have put together a number of typical configurations I have seen in practice from Portugal, Spain, France, Germany and the US.

Network 1:

Time until DRX is enabled: 100 ms
DRX Cycle Time Short: 80 ms
DRX Cycle Time Long: 200 ms
On Duration: 10 ms
Time Alignment: 10.2 seconds
Time until idle: ?

Network 2:

Time until DRX is enabled: 200 ms
DRX Cycle Time Short: 40 ms
DRX Cycle Time Long: 320 ms
On Duration: 10 ms
Time Alignment: infinity
Time until idle: ?

The two networks above are configured very similarly. DRX mode is entered very quickly after only a fraction of a second and devices have to listen to downlink assignments for 10 milliseconds during each cycle. Both networks have very long time alignment timers, the first one over 10 seconds and the other one has even set it to infinity. During those times the mobile has to keep transmitting status and measurement information in the uplink direction so power saving is significantly reduced.  What I didn’t note at the time was how long it takes for the network to set the RRC state from RRC-CONNECTED to RRC-IDLE to completely remove the physical air interface connectivity.

Network 3:

Time until DRX is enabled: 200 ms
DRX Cycle Time Short: none
DRX Cycle Time Long:80 ms
On Duration: 4 ms
Time Alignment: 1.92 seconds
Time until idle: 30 seconds

Network 3 is setup quite differently. While it also enters DRX mode in a fraction of a second, the DRX cycle time is, with 80 ms, much shorter than in the networks 1 and 2. Equally, the on-duration is much shorter. And finally, the Time Alignment can be considered lost after only 1.92 seconds which means power saving is much higher than in the two examples above. After no data transmission of 30 seconds, the network sets the connection to RRC-Idle.

Network 4 and 5:

No DRX configured, time until idle 5 seconds

And finally, I also saw two networks that didn’t have any DRX configured at all. Instead, the networks go from RRC-CONNECTED to RRC-IDLE state of the air interface after an inactivity period of only 5 seconds. In other words, a new air interface connection and a new context in the MME and S-GW has to be created for every web page that is loaded. From a signaling point of view and also from a core network point of view this setting probably leaves a lot to be desired. At least, setting up a new RRC connection takes much less time in LTE than in UMTS so the user will most likely not notice the extra time required (100 – 150 ms) for setting up an air interface connection when he clicks on a link in the web browser.

Which Setting Is The Best?

My favorite would be network 3 as the 80ms cycle time ensures that the response times to new IP packets are much quicker than in networks 1 and 2 when the air interface link is in DRX mode. At the same time a much shorter on-duration timer ensures power savings despite the shorter cycle times. And finally, the 1.92 seconds until no further uplink signaling information has to be sent makes more sense to me than 10 seconds or even infinity from a power saving point of view. But then I’m sure network one and two have their reasons for setting their timers as they are. So if you have a different opinion, please leave a comment below.