diff --git a/g3doc/sitemap.md b/g3doc/sitemap.md
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--- a/g3doc/sitemap.md
+++ b/g3doc/sitemap.md
@@ -22,6 +22,7 @@
 *   Testing
     *   Media Quality and performance
         *   [PeerConnection Framework](/test/pc/e2e/g3doc/index.md)
+            *   [Architecture](/test/pc/e2e/g3doc/architecture.md)
             *   [Video analyzer](/test/pc/e2e/g3doc/default_video_quality_analyzer.md)
         *   Call framework
         *   Video codecs test framework
diff --git a/test/pc/e2e/g3doc/architecture.md b/test/pc/e2e/g3doc/architecture.md
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@@ -0,0 +1,208 @@
+<?% config.freshness.reviewed = '2021-04-12' %?>
+
+# PeerConnection level framework fixture architecture
+
+## Overview
+
+The main implementation of
+[`webrtc::webrtc_pc_e2e::PeerConnectionE2EQualityTestFixture`][1] is
+[`webrtc::webrtc_pc_e2e::PeerConnectionE2EQualityTest`][2]. Internally it owns
+the next main pieces:
+
+*   [`MediaHelper`][3] - responsible for adding audio and video tracks to the
+    peers.
+*   [`VideoQualityAnalyzerInjectionHelper`][4] and
+    [`SingleProcessEncodedImageDataInjector`][5] - used to inject video quality
+    analysis and properly match captured and rendered video frames. You can read
+    more about it in
+    [DefaultVideoQualityAnalyzer](default_video_quality_analyzer.md) section.
+*   [`AudioQualityAnalyzerInterface`][6] - used to measure audio quality metrics
+*   [`TestActivitiesExecutor`][7] - used to support [`ExecuteAt(...)`][8] and
+    [`ExecuteEvery(...)`][9] API of `PeerConnectionE2EQualityTestFixture` to run
+    any arbitrary action during test execution timely synchronized with a test
+    call.
+*   A vector of [`QualityMetricsReporter`][10] added by the
+    `PeerConnectionE2EQualityTestFixture` user.
+*   Two peers: Alice and Bob represented by instances of [`TestPeer`][11]
+    object.
+
+Also it keeps a reference to [`webrtc::TimeController`][12], which is used to
+create all required threads, task queues, task queue factories and time related
+objects.
+
+## TestPeer
+
+Call participants are represented by instances of `TestPeer` object.
+[`TestPeerFactory`][13] is used to create them. `TestPeer` owns all instances
+related to the `webrtc::PeerConnection`, including required listeners and
+callbacks. Also it provides an API to do offer/answer exchange and ICE candidate
+exchange. For this purposes internally it uses an instance of
+[`webrtc::PeerConnectionWrapper`][14].
+
+The `TestPeer` also owns the `PeerConnection` worker thread. The signaling
+thread for all `PeerConnection`'s is owned by
+`PeerConnectionE2EQualityTestFixture` and shared between all participants in the
+call. The network thread is owned by the network layer (it maybe either emulated
+network provided by [Network Emulation Framework][24] or network thread and
+`rtc::NetworkManager` provided by user) and provided when peer is added to the
+fixture via [`AddPeer(...)`][15] API.
+
+## GetStats API based metrics reporters
+
+`PeerConnectionE2EQualityTestFixture` gives the user ability to provide
+different `QualityMetricsReporter`s which will listen for `PeerConnection`
+[`GetStats`][16] API. Then such reporters will be able to report various metrics
+that user wants to measure.
+
+`PeerConnectionE2EQualityTestFixture` itself also uses this mechanism to
+measure:
+
+*   Audio quality metrics
+*   Audio/Video sync metrics (with help of [`CrossMediaMetricsReporter`][17])
+
+Also framework provides a [`StatsBasedNetworkQualityMetricsReporter`][18] to
+measure network related WebRTC metrics and print debug raw emulated network
+statistic. This reporter should be added by user via
+[`AddQualityMetricsReporter(...)`][19] API if requried.
+
+Internally stats gathering is done by [`StatsPoller`][20]. Stats are requested
+once per second for each `PeerConnection` and then resulted object is provided
+into each stats listener.
+
+## Offer/Answer exchange
+
+`PeerConnectionE2EQualityTest` provides ability to test Simulcast and SVC for
+video. These features aren't supported by P2P call and in general requires a
+Selective Forwarding Unit (SFU). So special logic is applied to mimic SFU
+behavior in P2P call. This logic is located inside [`SignalingInterceptor`][21],
+[`QualityAnalyzingVideoEncoder`][22] and [`QualityAnalyzingVideoDecoder`][23]
+and consist of SDP modification during offer/answer exchange and special
+handling of video frames from unrelated Simulcast/SVC streams during decoding.
+
+### Simulcast
+
+In case of Simulcast we have a video track, which internally contains multiple
+video streams, for example low resolution, medium resolution and high
+resolution. WebRTC client doesn't support receiving an offer with multiple
+streams in it, because usually SFU will keep only single stream for the client.
+To bypass it framework will modify offer by converting a single track with three
+video streams into three independent video tracks. Then sender will think that
+it send simulcast, but receiver will think that it receives 3 independent
+tracks.
+
+To achieve such behavior some extra tweaks are required:
+
+*   MID RTP header extension from original offer have to be removed
+*   RID RTP header extension from original offer is replaced with MID RTP header
+    extension, so the ID that sender uses for RID on receiver will be parsed as
+    MID.
+*   Answer have to be modified in the opposite way.
+
+Described modifications are illustrated on the picture below.
+
+![VP8 Simulcast offer modification](vp8_simulcast_offer_modification.png "VP8 Simulcast offer modification")
+
+The exchange will look like this:
+
+1.  Alice creates an offer
+2.  Alice sets offer as local description
+3.  Do described offer modification
+4.  Alice sends modified offer to Bob
+5.  Bob sets modified offer as remote description
+6.  Bob creates answer
+7.  Bob sets answer as local description
+8.  Do reverse modifications on answer
+9.  Bob sends modified answer to Alice
+10. Alice sets modified answer as remote description
+
+Such mechanism put a constraint that RTX streams are not supported, because they
+don't have RID RTP header extension in their packets.
+
+### SVC
+
+In case of SVC the framework will update the sender's offer before even setting
+it as local description on the sender side. Then no changes to answer will be
+required.
+
+`ssrc` is a 32 bit random value that is generated in RTP to denote a specific
+source used to send media in an RTP connection. In original offer video track
+section will look like this:
+
+```
+m=video 9 UDP/TLS/RTP/SAVPF 98 100 99 101
+...
+a=ssrc-group:FID <primary ssrc> <retransmission ssrc>
+a=ssrc:<primary ssrc> cname:...
+....
+a=ssrc:<retransmission ssrc> cname:...
+....
+```
+
+To enable SVC for such video track framework will add extra `ssrc`s for each SVC
+stream that is required like this:
+
+```
+a=ssrc-group:FID <Low resolution primary ssrc> <Low resolution retransmission ssrc>
+a=ssrc:<Low resolution primary ssrc> cname:...
+....
+a=ssrc:<Low resolution retransmission ssrc> cname:....
+...
+a=ssrc-group:FID <Medium resolution primary ssrc> <Medium resolution retransmission ssrc>
+a=ssrc:<Medium resolution primary ssrc> cname:...
+....
+a=ssrc:<Medium resolution retransmission ssrc> cname:....
+...
+a=ssrc-group:FID <High resolution primary ssrc> <High resolution retransmission ssrc>
+a=ssrc:<High resolution primary ssrc> cname:...
+....
+a=ssrc:<High resolution retransmission ssrc> cname:....
+...
+```
+
+The next line will also be added to the video track section of the offer:
+
+```
+a=ssrc-group:SIM <Low resolution primary ssrc> <Medium resolution primary ssrc> <High resolution primary ssrc>
+```
+
+It will tell PeerConnection that this track should be configured as SVC. It
+utilize WebRTC Plan B offer structure to achieve SVC behavior, also it modifies
+offer before setting it as local description which violates WebRTC standard.
+Also it adds limitations that on lossy networks only top resolution streams can
+be analyzed, because WebRTC won't try to restore low resolution streams in case
+of loss, because it still receives higher stream.
+
+### Handling in encoder/decoder
+
+In the encoder, the framework for each encoded video frame will propagate
+information requried for the fake SFU to know if it belongs to an interesting
+simulcast stream/spatial layer of if it should be "discarded".
+
+On the decoder side frames that should be "discarded" by fake SFU will be auto
+decoded into single pixel images and only the interesting simulcast
+stream/spatial layer will go into real decoder and then will be analyzed.
+
+[1]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/test/peerconnection_quality_test_fixture.h;l=55;drc=484acf27231d931dbc99aedce85bc27e06486b96
+[2]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/peer_connection_quality_test.h;l=44;drc=6cc893ad778a0965e2b7a8e614f3c98aa81bee5b
+[3]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/media/media_helper.h;l=27;drc=d46db9f1523ae45909b4a6fdc90a140443068bc6
+[4]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/analyzer/video/video_quality_analyzer_injection_helper.h;l=38;drc=79020414fd5c71f9ec1f25445ea5f1c8001e1a49
+[5]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/analyzer/video/single_process_encoded_image_data_injector.h;l=40;drc=79020414fd5c71f9ec1f25445ea5f1c8001e1a49
+[6]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/test/audio_quality_analyzer_interface.h;l=23;drc=20f45823e37fd7272aa841831c029c21f29742c2
+[7]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/test_activities_executor.h;l=28;drc=6cc893ad778a0965e2b7a8e614f3c98aa81bee5b
+[8]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/test/peerconnection_quality_test_fixture.h;l=439;drc=484acf27231d931dbc99aedce85bc27e06486b96
+[9]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/test/peerconnection_quality_test_fixture.h;l=445;drc=484acf27231d931dbc99aedce85bc27e06486b96
+[10]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/test/peerconnection_quality_test_fixture.h;l=413;drc=9438fb3fff97c803d1ead34c0e4f223db168526f
+[11]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/test_activities_executor.h;l=28;drc=6cc893ad778a0965e2b7a8e614f3c98aa81bee5b
+[12]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/test_activities_executor.h;l=28;drc=6cc893ad778a0965e2b7a8e614f3c98aa81bee5b
+[13]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/test_peer_factory.h;l=46;drc=0ef4a2488a466a24ab97b31fdddde55440d451f9
+[14]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/pc/peer_connection_wrapper.h;l=47;drc=5ab79e62f691875a237ea28ca3975ea1f0ed62ec
+[15]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/test/peerconnection_quality_test_fixture.h;l=459;drc=484acf27231d931dbc99aedce85bc27e06486b96
+[16]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/peer_connection_interface.h;l=886;drc=9438fb3fff97c803d1ead34c0e4f223db168526f
+[17]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/cross_media_metrics_reporter.h;l=29;drc=9d777620236ec76754cfce19f6e82dd18e52d22c
+[18]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/cross_media_metrics_reporter.h;l=29;drc=9d777620236ec76754cfce19f6e82dd18e52d22c
+[19]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/test/peerconnection_quality_test_fixture.h;l=450;drc=484acf27231d931dbc99aedce85bc27e06486b96
+[20]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/stats_poller.h;l=52;drc=9b526180c9e9722d3fc7f8689da6ec094fc7fc0a
+[21]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/sdp/sdp_changer.h;l=79;drc=ee558dcca89fd8b105114ededf9e74d948da85e8
+[22]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/analyzer/video/quality_analyzing_video_encoder.h;l=54;drc=79020414fd5c71f9ec1f25445ea5f1c8001e1a49
+[23]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/test/pc/e2e/analyzer/video/quality_analyzing_video_decoder.h;l=50;drc=79020414fd5c71f9ec1f25445ea5f1c8001e1a49
+[24]: /test/network/g3doc/index.md
diff --git a/test/pc/e2e/g3doc/vp8_simulcast_offer_modification.png b/test/pc/e2e/g3doc/vp8_simulcast_offer_modification.png
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