On some networks that support the legacy SVLTE (Simultaneous Voice and LTE)
instead of the more modern VoLTE (Voice Over LTE) standard, or even CSFB
(Circuit-switched fallback) this number would (presumably) be even higher.
</p>
<p>
XMPP server and client implementers, bearing this increased power usage in
mind, and knowing a bit about how LTE radios work, can optimize their
traffic to minimize network usage. For the downlink, LTE user equipment
(UE) utilizes Orthogonal Frequency Division Multiplexing (OFDM), which is
somewhat inefficient
<note>A Close Examination of Performance and Power Characteristics of 4G LTE Networks <<linkurl='http://www.cs.columbia.edu/~lierranli/coms6998-7Spring2014/papers/rrclte_mobisys2012.pdf'>http://www.cs.columbia.edu/~lierranli/coms6998-7Spring2014/papers/rrclte_mobisys2012.pdf</link>></note>.
On the uplink side a different technology, Single-carrier frequency
division multiple access (SC-FDMA) is used, which is slightly more
efficient than traditional (non linearly-precoded) OFDM, slightly
offsetting the fact that broadcasting requires more power than receiving.
LTE UE also implements a Discontinuous reception (DRX) mode in which the
hardware can sleep until it is woken by a paging message or is needed to
perform some task. LTE radios have two power modes: RRC_CONNECTED and
RRC_IDLE. DRX is supported in both of these power modes. By attempting to
minimize the time which the LTE UE state machine spends in the
RCC_CONNECTED state, and maximize the time it stays in the DRX state (for
RCC_CONNECTED and RRC_IDLE), we can increase battery life without degrading
the XMPP experience. To do so, the following rules should be observed:
</p>
<section2topic='Transmit no data'>
<p>
Whenever possible, data that is not strictly needed should not be
transmitted (by the server or client). Supporting &xep0352; is highly
recommended. Most importantly, XMPP pings should be kept as far apart as
possible and only used when necessary. Server operators are encouraged to
set high ping timeouts, and client implementors are advised to only send
pings when absolutely necessary to prevent the server from closing the
socket.
</p>
</section2>
<section2topic='Transmit as much data as you can at once'>
<p>
If one is on 3G, transmitting a small amount of data will cause the radio
to enter FACH mode which is significantly cheaper than its high power
mode. On LTE radios, however, transmitting small amounts of data is
vastly more expensive per bit due to the significantly higher tail-times
(the time it takes for the radio to change state). On LTE radios, one
should transmit as much data as possible when the radio is already on
(eg. by placing messages in a send queue and executing the queue as a
batch). Similarly, when data is being received the radio is already in a
high power state and therefore any data that needs to be sent should be.
</p>
<p>
These rules also apply to server operators: If you receive data, the
phones radio is already on therefore you should send anything you have.
Otherwise, batching data to be sent and sending it all at once (and as
much as possible) will help reduce power consumption.
