Tuesday, June 29, 2010

Smartphones, Fast Dormancy & more..

Ever since the birth of smartphones, they have made a huge impact in the wireless industry. The consumers hunger for bandwidth is ever on the rise. This lead to manifold increase in the mobile data traffic. Earlier this year Cisco published a report on mobile data traffic growth. According to this report the mobile data traffic handled by the mobile operators will grow from seven billion megabytes worldwide (in 2008) into 63 billion megabytes in 2013.

In this article I will talk a bit about how smartphones impact the mobile networks, how data traffic from a smartphone is handled in a 3G network and the way forward. I will keep the post pretty simple and basic. Readers who want to know more on it please feel free to contact me.

Smartphones users want uninterrupted data connectivity for their applications. Applications like weather reports, score updaters etc connect to the internet without users intervention and provide updates to the user. To provide an uninterrupted feel to the users, an Always-On context is maintained between the mobile or the User Equipment (UE) and the network (towers + controller stations). This is called AO PDP context which is just a logical context and not a physical connection over the air. The AO PDP context is established whenever the UE is switched on and remains until the UE is powered off or switched to offline or airplane mode.

UMTS QoS classes
UMTS or the Universal Mobile Telecommunication System is one of the 3rd Generation or 3G systems widely deployed around the world. UMTS supports four classes of Quality of Service namely
  1. conversational class (real time, ex. Voice)
  2. streaming class (real time, ex. video streaming)
  3. interactive class (best effort, ex. web browsing)
  4. background class (best effort, ex. emails)
Most of the applications developed require background class QoS. If we take a simple Twitter app, all it has to do is periodically send timeline request to the server to check if there are any new updates. If there are new updates found, it has to download them. keep quiet until the status update timer expiry or the user activity. So, the typical data pattern would be like sending a few packets of data in the uplink and receive a few packets in the downlink. Similar pattern can be observed in an IM app as well.

UMTS states
When the UE is idle and there is no voice call and no AO PDP active, it is said to be in idle mode. And when it makes a call or data transfer, it moves to connected mode. There are four states defined in UMTS connected mode known as CELL_DCH, CELL_FACH, CELL_PCH, URA_PCH.

  1. Cell_DCH - In this state a dedicated resource is provided by the network to the UE and the data rate that can be achieved is more compared to other states.
  2. Cell_FACH - Shared resources are provided to the UE in this state and the data rate is comparatively less
  3. Cell_PCH - In this state, UE listens to paging and broadcast messages from the network but there is no uplink data transfer
State Transition

Network places UE in one of the connected mode states based on the user activity (other reasons are also there). Network generally moves the UE to Cell_DCH state when the voice call or data transfer is initiated. For voice calls, the UE will remain in cell_DCH through out the call duration and goes to idle mode when the call is disconnected. For Packet-Switched (PS) or data calls, network will initially place in the UE in cell_DCH state and if there is an inactivity for a long time or the amount of data to be sent is less, network will move the UE to cell_FACH state. And if there is no activity even after moving to cell_FACH, UE will be moved to cell_PCH state. And, if there are more data to send, UE will be moved to cell_DCH state. If the UE still continue to be inactive in Cell_PCH state, it can be moved to idle mode and the physical connection is removed but the PDP context or the IP address is kept active.

What are the benefits of the state transitions?
UE requires a lot of energy to be in the cell_DCH state for a long time. So, it is beneficiary to move to cell_FACH or Cell_PCH state which require less energy. The current flow in Cell_DCH will be typically in the range of 200 - 300 mA. Whereas, in Cell_FACH and Cell_PCH the sleeping current values are typically in the range of 100 - 150 mA and <5ma style="font-weight: bold;">

What is the problem with this?
The state transition times are more time and the round trip delay behavior when returning from Idle to Cell_DCH will be more. It takes about 2.5seconds for the first ping to succeed. The round trip delay time from Cell_PCH to Cell_DCH is around 1 second. There is a significant difference in time between Cell_PCH and idle. Even the power consumption is more or less same in cell_PCH and Idle mode. But most of the networks around the world dont support Cell_PCH state and send the UE to idle mode and not Cell_PCH state. Also, the decision to move the UE to Idle or Cell_PCH is taken by the network.

This scheme works well for feature phones but what about smartphones? Consider the same example of a twitter app in a smartphone, it sends and receives very small amount of data but is periodic. So, if the UE is kept in Cell_DCH, it will continue to be in Cell_DCH and drain its power quickly. If it is moved to idle mode, when the app requests for updates next time, it will take a bit longer time to provide the updates to the user. Smartphones have so many such applications each having its own periodicity.

Manufacturer Operator debate
So, we can two options, 1. UE is kept in cell_DCH/Cell_Fach 2. UE is moved to idle mode.

While the UE manufacturers were interested in option 2 which is more beneficiary for the UE to save power, the network operators opted for option 1. The reason why network operators chose option 1 (mainly to cell_FACH) is that the amount of signaling involved in DCH -> IDLE -> DCH is much more than the DCH -> FACH -> DCH. Considering the periodicity of different apps in a smartphone, the signaling overhead is enormous in option 2.

Fast Dormancy scheme
A number of UE maufacturers have come up with the fast dormancy scheme in the 3GPP forum and decided to give the power to the UE. As per this scheme, whenever UE thinks that it will be inactive for some time, it will send a message to the network indicating the same and ask the network to move the UE to idle mode, instead of waiting for the network to detect the inactivity. This proved beneficiary for the UE but not for the network. Many smartphones end up consuming more bandwidth for just signaling itself and produces an oscillatory effect in the network.

Another problem with this is that even if the network supports cell_PCH state, UE will ask the network to move it to Idle mode only. Hence, while the mobile device saves the maximum amount of energy, the network can not take advantage of the signaling reduction of the Cell-/URA-PCH states. Also, the mobile device can't benefit from the fast return to service times described above. From a mobile point of view, the current fast dormancy behavior being applied when there are only background applications is still preferable to waiting for the network sending it to Cell-/URA-PCH state as a lot of energy is wasted waiting for the network to make the decision. It should be noted at this point that the network is not slow in making the decision, it's rather intentional to ensure a good user experience to only have a user plane latency due to the state switching when absolutely necessary.

Enhanced Fast Dormancy Scheme
This has been specified in the Rel 8 specifications wherein a timer is introduced. The value of the timer will be given by the network and only after the expiry of this timer UE can trigger the connection release message to the network. Also, network now has the additional decision making power to either move the UE to idle or cell_PCH by sending a confirmation message for the message sent by UE. It is even possible to have both the schemes.

Current scenario
Most of the networks around the world are Rel 6 capable and dont support Enhanced FD scheme. Most of the smartphones dont implement Fast Dormancy feature itself after getting fierce criticism from operators. The mobile phone sales model has changed and the UE manufacturers channelize their major share of sales through operators only. So, they obviously have to give up to the operators and do not have FD feature. I have come across incidents wherein operators have threatened to shoot down mobiles with FD feature. (Its easy for networks to find whether FD is implemented or not by checking the rate of connection request s made)

What is ahead?
So, is this a shortcoming or a design issue of UMTS technology? Absolutely not!!! It is just that Rel 6 does not support those features. We have discontinuos receptions or DRXs possible in Cell_DCH in Rel 7. We also have Enhanced Cell_FACH and other features to enhance the user experience . These features are generally referred to as HSPA+ and we will soon find HSPA+ capable networks and mobile in the market. The latencies are reduced a lot in LTE or Long Term Evolution (3.9G or a 4G technology) and the problem of fast dormancy is not expected to be there.

All the required features are already through the standarisation pipe and we can expect them to be in the market soon! Till then, be prepared to drain your iPhone or Nexusone or Galaxy S's battery.

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