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We currently have an IoT device, that connect to our servers using a raw TCP connection over 2G, and from times to times the device send a "keep-alive"-ish message , as we've seen it's consume less battery rather than to reopen the connection everytime we need to send a message (~once every 1 to 5 minutes)

We're thinking about switching to LTE-M, especially as we've seen the eDRX mode would permit us to save potentially a lot on the battery life, however I have the following question:

When in eDRX mode, is the TCP connection still open, i.e if I send from the server some data, will the client receive it when it awakes?

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  • Why not try it and see?
    – Mawg says reinstate Monica
    Mar 9, 2018 at 7:01
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    @Mawg unfortunately I'm currently in china and there's no LTE-M network deployed here yet :(
    – sysko
    Mar 10, 2018 at 10:20
  • @sysko what did you find? Will the network actually create a SYN packet? Or will it just wake up the device? And the application code has to then handle the polling the server?
    – Ashish
    Dec 14, 2018 at 13:05

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Yes, when using LTE-M's eDRX mode, the TCP connection is still open, but there are some complications depending on which carrier you are using.

First of all, sending data from the server to the mobile device will never wake up the mobile device instantly. The mobile device will wake up at the next eDRX interval (although some other non-cellular event could also cause it to wake sooner).

Now, the confusing part is whether or not your mobile device will receive the data from the server when it wakes at that next eDRX interval. The GSMA LTE-M Deployment Guide actually discusses this in sections 6.2 and 6.4. The troubling line is in section 6.4:

Thus, it means that in case when a LTE-M device is in either PSM or eDRX, mobile terminating messages, depending on MNO choice the messages will either be buffered or discarded

To put it in different terms, the cell service provider (e.g. Verizon, AT&T, etc) gets to decide whether they want to buffer the TCP packets to forward your device when it wakes up later, or just discard them.

Even on carriers which do buffer data during eDRX sleep periods, they tend to have an upper time limit on the order of minutes.

However, your server is likely unaware of the intermittent nature of the eDRX connection, and will be doing it's normal TCP retries, and there is always the chance that one of those retries will be sent during the eDRX window and get through to the mobile device. When this happens, it looks like it "just works" even on networks which do discard data, but it's more luck than anything else. (Shortening the eDRX cycle time and using faster TCP retry rates on the server will both improve your luck in this setup.)

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    Just now, nearly a year later, I found official documentation from AT&T (US) clearly detailing their eDRX/PSM buffering rules. For anyone who cares, packets are dropped if your device is in PSM mode, and only the first 4KB of data are buffered in eDRX mode for up to 4x EDRX timing cycles. Verizon still buffers nothing, ever.
    – JustinB
    Nov 10, 2020 at 15:16

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