Understanding the Android "Radio State Machine" for better battery life

Question

On the Android documentation page Optimizing Downloads for Efficient Network Access, the gist is that "waking up the radio is bad", "batch your transfers" or "piggyback on GCM". That article leaves some inner working principles for the curious:

1. It said "Every time you create a new network connection, the radio transitions to the full power state." What does connection mean here? Is that a TCP connection? Does that mean sending a UDP packet will not wake up the radio?

2. In Standby it said: "Standby: The minimal energy state during which no network connection is active or required." Does that mean the network module is completely shut off? If so how can GCM even work even when the device is in sleep mode? If not, roughly how much battery does it use compared to full power mode?

3. "Every time you create a new network connection, the radio transitions to the full power state." How does that gibe with long lived TCP connections? If I create a TCP connection and then just keep receiving packets, then I won't be creating new network connections or sending out any data. Does that allow the network module to go to Standby mode?

4. Do iPhones work pretty much the same way?

Answer 1

@minhaz's link got me started in a long quest to understand cellular networking in the Network Layer side of things. So here goes:

This question turns out to be mainly about how 3G networking is implemented. Answering the central question: "So how can GCM receive messages while in the 'Idle State'" can answer all the questions above.

Short Answer

Yes, while in Idle model the radio can still receive limited "control signals". Basically the network operator will ask the device to switch energy states to be able to receive the actual payloads. The mechanism they use to do that is similar to how you receive phone calls or SMS.

Long Answer

It turns out that the state transition is controlled by the network operator, not the cell phone itself. From Resource Radio Control (RRC); Protocol specification:

8.6.3.3 Generic state transition rules depending on received information elements

The IE (Information Element) "RRC State Indicator" indicates the state the UE (User Equipment) shall enter. The UE shall enter the state indicated by the IE "RRC State Indicator...

And how does the network operator do that when the RRC is in Idle Mode? From the book 3G, 4G and Beyond: Bringing Networks, Devices and the Web Together:

2.2.3.3 Radio Resource Control States

... Idle state -- devices not actively communicating with the network are in this state. Here, they periodically listen to the paging channel for incoming voice or video calls and SMS messages.

From LTE in Bullets:

23.1 The RRC connection establishment procedure is always initiated by the UE but can be triggered by either the UE or the network. ... The network triggers the RRC connection establishment procedure by sending a Paging message. ...

So there we have it. Now it is fairly obvious to see how all these tie together. To answer the original questions:

1. Connection means an RRC connection. Since an RRC connection is Layer 3, all (normal) sorts of network activity including TCP and UDP, will create an RRC connection (i.e. "wake up the radio").

2. Since the radio still needs to listen to the paging channel it is not completely shut off. Empirically it still uses energy as experimented by the XMPP link provided by @minhaz. In the result it is 2 orders of magnitude less energy consumption compared to the other states.

3. As mentioned in various places on Stack Overflow and apparently TCP connections are maintained in memory and doesn't really care if the underlying layers have gone through a RRC reconnection procedure. If the TCP connection is idle the RRC connection can be released (i.e. UE can become idle). If it keeps receiving data the network will not instruct the UE to release RRC connection so it won't go Idle.

4. Since RRC belongs to UMTS WCDMA which is underlying 3G it is likely that iPhone also operates similarly.

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Notes:

• The resources I linked to mixes 3G and 4G. I suspect 4G is an incremental improvement so the main concepts can be mixed between them.

Resource dump for more indepth understanding (i.e. sources for the above digest):

• Radio Resource Control - Wikipedia
• 3G, 4G and Beyond: Bringing Networks, Devices and the Web Together
• Introduction to 3G Mobile Communications
• WCDMA Design Handbook
• LTE for 4G Mobile Broadband: Air Interface Technologies and Performance
• Convergence Technologies for 3G Networks: IP, UMTS, EGPRS and ATM
• LTE in Bullets
• Let's Learn LTE: RRC States in LTE
• Cell PCH State
• 3GPP 25.331 Radio Resource Control (RRC); Protocol specification
• 3GPP 36.201: Evolved Universal Terrestrial Radio Access (E-UTRA); LTE physical layer; General description
• High Performance Browser Networking - Chapter 7. Mobile Networks

Answer 2

I am not an SOC expert but here what i understand by reading same documentation.

It said "Every time you create a new network connection, the radio transitions to the full power state." What does connection mean here? Is that a TCP connection? Does that mean sending a UDP packet will not wake up the radio?

Any sort of data transfer over the network will wake the radio, it dosn't matter if it is UDP or TCP 1 byte or 1 MB.

In Standby it said: "Standby: The minimal energy state during which no network connection is active or required." Does that mean the network module is completely shut off? If so how can GCM even work even when the device is in sleep mode? If not, roughly how much battery does it use compared to full power mode?

Without waking up the entire radio, still you can keep connection to a wireless tower. I believe this design has done long time ago to save mobile battery and it has nothing to do with Android. It might uses a different chip for this job only but i will leave this answer for a SOC expert.

"Every time you create a new network connection, the radio transitions to the full power state." How does that gibe with long lived TCP connections? If I create a TCP connection and then just keep receiving packets, then I won't be creating new network connections or sending out any data. Does that allow the network module to go to Standby mode?

for opening a TCP connection you always need some degree of Handshaking and that will keep radio transmitter wake. TCP is a reliable protocol that means transfer will be validated on both end for avoiding transmission error. And, in our case it means no matter what, system will always send, receive something over network. A TCP keep alive also does network handshaking (keep_alive) and that means if no data has been transmitted still it will keep the transmitter wake.