THREAD CONTENTS

This thread has grow over time and contains quite a lot of information. To make it easier to find specific information on this thread I have implemented a contents system which will allow you find information quickly. The thread has been split into sections and sub sections where appropriate. The contents uses Anchor points (thanks jimco for the tip!), so you can simply click on the section in the contents to jump to the section in the thread.

Sections:

  1. Summary Information of this entire thread
  2. What is DPC Latency?
  3. Newer Qualcomm Atheros Drivers vs OEM Driver 
  4. Important information before replying to this thread
  5. All Information and evidence on the wireless latency issue
    1. Summary
    2. Evidence
  6. How to test if you have wireless latency issues
  7. Workarounds/Fixes
    1. Recommended action
    2. Legacy Workarounds
  8. Dell's position on the situation
  9. Community Acknowledgements

1. SUMMARY OF INFORMATION:

If you haven't followed this thread through various developments since it was first published, take a second to read this summary. This thread has expanded massively overtime, so the information in its entirety might be difficult to take in without knowing all the facts.  

The original problem described in this thread is regarding latency spikes over WiFi on the XPS 15z. The latency spikes are caused by unknown factors which have never been truly pin pointed to any certain area (and still haven't to this day). There are many areas which have been highlighted including Windows 7 and its lack of process interruption protection, Intel WiFi Drivers, Realtek Sound Drivers and other areas. A lot of community activity was involved in trying to get to the bottom of this problem. It was eventually discovered that the Atheros Ethernet Adapter could be attributed to the problem, after a newer Non Dell OEM driver was found for the Atheros 815X Family Ethernet Controller on a third party website. This driver was tested by several community members and it was found that latency spikes were reduced heavily. Since this point, it has been a community recommendation to install the Qualcomm Atheros drivers rather than using the outdated Dell OEM driver which is dated May 2011 for the Ethernet card in the XPS 15z.

There is far more to the situation than this, but it is a summarized version of the events that have taken place over the months. If you don't know what DPC Latency is or are unfamilar with the problem being described, take a moment to read through this thread, it is seperated into logical headings and groupings to make it easier to pick out the key information.

It is important to note that while the use of the official Qualcomm Atheros driver over the Dell OEM driver has helped people in this situation it is not a guarantee that it will solve you problem. Sadly many factors can cause  latency spikes, the Atheros Ethernet Controller seems to be just one of them.

2. WHAT IS DPC LATENCY?

To understand DPC latency, its important to first understand what the two terms mean on there own:

Deferred Procedure Call (DPC) is a Microsoft Windows operating system mechanism which allows high-priority tasks (e.g. an interrupt handler) to defer required but lower-priority tasks for later execution. This permits device drivers and other low-level event consumers to perform the high-priority part of their processing quickly, and schedule non-critical additional processing for execution at a lower priority. http://en.wikipedia.org/wiki/Deferred_Procedure_Call

Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts. - http://en.wikipedia.org/wiki/Latency_(engineering)

When there is high latency various problems can occurs, one of the main issues that are a side effect of badly managed DPC latency is audio problems. This can be often seen with multimedia content like music or video. When you have bad latency you will experience drop outs and degraded audio. To see the problem visually you can use a DPC latency analysis program which will tell you when you have bad latency.

What causes high DPC latency?:

Here lies the million dollar question, many factors can cause high latency. In a lot of cases it is usually down to poorly written hardware drivers that don't perform quick enough and cause lag between process execution. This then creates high latency and then will interfere with running processes. Sometimes this can occur very rarely and is acceptable in some cases, however it becomes problem if the DPC latency constantly spikes and can be reproduced in certain scenarios.

For the specific case of the XPS 15z, the driver that seems to be the most problematic is the Dell OEM driver for the Atheros 8151 Ethernet Controller, it is difficult to pin point causes of high latency, but the problem has been attributed to it. That is not to say that the Ethernet controller is totally blame. Without Dell investigating there own driver/hardware implementations we can't even conclusively pin point the issue.

The bottom line of the situation is however, the XPS 15z has very high DPC latency in a lot of scenarios.

3. NEWER QUALCOMM ATHEROS DRIVERS VS DELL OEM DRIVERS:

Disclaimer: These drivers are NOT official Dell OEM drivers and hence not certified by Dell. They are the OFFICIAL drivers from Qualcomm Atheros but have not been compiled specifically for Dell OEM. It is 100% safe to use these drivers, but if you experience any problems, roll back to the Dell OEM driver. It is likely Dell won't help you if you have problems with your Ethernet Controller when using these drivers.

After community testing of a newer 2.0 release of the Non OEM Atheros Driver found by Dell Community User richsark it has been found that the non OEM drivers certainly help the situation a great deal with regards to the latency issue. So if you having problems described in this thread, try following the Qualcomm Atheros driver installation steps in this section.

Another Dell Community member JimK157 has found Qualcomm Atheros have been releasing  newer drivers for certain Atheros Ethernet Controllers on a regular basis, the AR815X Family Drivers which is what the XPS 15z has is part of these releases. The good news about this is there is now a dedicated page for these drivers which can be tracked:

http://www.qca.qualcomm.com/corporate/content.php?nav1=146

If you are already comfortable with updating drivers manually through device manager you can go ahead and download the drivers and manually install them, if you aren't sure what to do, follow these instructions:

  1. Go to the URL above
  2. Scroll to the AR813X AR815X Family Drivers section
  3. From the drop down menu select the "Windows WHQL Driver"
  4. The driver will be downloaded as a .rar archive, so you will need extraction software that can unpack .rar files. (Programs like WinRaR can do this)
  5. When you extract the archive to the specified location you will get a folder called "AR813x_AR815x_AR816x_[version_here]_WHQL". The version numbering will differ slightly for each driver release.
Updating the AR815X Driver Manually:

In the extracted driver folder there is a setup.exe, however due to this being the generic Qualcomm driver and not Dell OEM it is strongly recommended you only install the driver and nothing else. to avoid any performance issues or problems.

  1. Open Device Manager and expand the "Network adapters" menu
  2. Right click on your Atheros AR8151 Ethernet Adapter. It's name may differ depending on your driver version. Newer driver versions display the name as "Qualcomm Atheros AR8151 PCI-E Gigabit Ethernet Controller (NDIS 6.20)" older driver versions will display the name differently.
  3. Select Update Driver Software from the menu
  4. Select Browse my computer for driver software
  5. Now you will be asked to manually specify the location of the Ethernet driver. You will need to click browse to specify where you extracted the .rar archive to and navigate to this path Common_Dri\Win7_64 so device manager can locate the driver. When you have specified this path click OK and the driver will be automatically installed. If everything went OK device manager will state this in the window confirming a successful driver upgrade

To confirm the driver was updated to a newer version you can always compare the driver version/date to the previous version prior to updating. Making a note of the driver version/date is a good indicator.

If that all went well your now running the Qualcomm Atheros Driver rather than the Dell OEM driver. Test it out and hopefully you should find you get much better performance out of it. From the community testing done, it works better than the OEM driver in a lot of cases.

4. IMPORTANT INFORMATION BEFORE REPLYING IN THIS THREAD:

Before replying to this thread make sure you are reporting the right problem. It has become apparent that the Dell XPS 15z has different audio problems which have caused some confusion in multiple threads. This specific thread is for people that are having problems with audio when using the WIRELESS connection, which happens to be related to DPC LATENCY.

If you experience problems with audio when using the Ethernet connection or a different scenario, do not post in this thread, you have a different problemWe are trying to keep the different audio issues separate, so Dell and other community members can understand the different issues.

Remember, this thread is for audio problems when using the XPS 15z over wireless only where massive latency spikes occur as shown below:

Please Note: Disabling the Atheros Wired adapter does not solve this problem, this suggestion is for a different problem but will make no difference to the problem described in this thread.

5. EVIDENCE AND INFORMATION REGARDING THE WIRELESS LATENCY ISSUE

a. Summary of the wireless latency problem

To summarise previous threads, many community members from the Dell Community and other forums have reported audio issues on the Dell XPS 15z for a while. This specific audio issue is regarding the Intel Centrino 6230 N Wireless Card in the XPS 15z.

When a wireless connection is active and network traffic is generated, audio glitches will occur on the XPS 15z. These glitches range from audio crackling, degradation and even audio playback being distorted. In some cases this issue can go unnoticed, however the underlying problem is always present.

b. Evidence of latency spikes on the XPS 15z

From reading various articles and other forum threads on this issue, I am pretty certain it is something to do with Wireless. Dell has stated in the past that the Centrino 6230 N card is not at fault for the audio issues, however I believe they are wrong.

Thanks to another community member (), we have pretty hard evidence that the wireless card is causing these issues. If you observe the DPC latency on the Dell XPS 15z while on wireless you will find that you get very large spikes when an average amount of network traffic is generated. First some evidence of the problem:

EXHIBIT A: Performing a Latency test in real time.

The video below demonstrates the problem in real time. In this test I performed a simple action of the typical home user, by pulling a 720p Movie File from my NAS server. To make everything fair, here are the variables in this test:

  • Router: WNR3500L V1, running DD-WRT K.26 Build 14929
  • Wireless Speed: Wireless N 2.4 Ghz (Single Band)
  • NAS Server: HP ProLiant Microserver N36L (Integrated Broadcom Gigabit Ethernet Port, connected to a Gigabit switch)
All network related programs like uTorrent, Windows Update etc, were all inactive at the time of this test, minimum network traffic was being generated prior to the test below.