Frequently Asked Questions

  1. What is PSS?
  2. What is the purpose of PSS?
  3. What is PSS-AG?
  4. What standards does PSS-AG test?
  5. What is the relationship and position of PSS-AG to standardization bodies?
  6. How does the PSS standard differ from terminal standards such as H.323 and H.324?
  7. What are H.324M and 3G-324M, and how do they compare to PSS?
  8. Why do I need to be interoperable with other vendors?
  9. What components are being tested?
  10. What types of members are suitable for the PSS?
  11. Can I only be a tester or do I have to be a content generator too (or vice versa)?
  12. What responsibilities do members have?
  13. What are the benefits of the PSS AG to the mobile industry?
  14. How can a company join the PSS AG?
  15. Where can PSS-AG test procedures be found?
  16. How are the tests being monitored?

Q.  What is PSS?

A.  Packet-Switched Streaming Services (PSS) is a standard defined by the Third Generation Partnership Project (3GPP). The PSS specification covers the protocols and codecs necessary for the delivery of streaming content to multimedia terminals in Third Generation (3G) wireless networks. PSS can be used in many ways, for example:

  • Viewing on-demand audiovisual clips (e.g. movie trailers) from anywhere using a multimedia-equipped wireless phone.
  • Viewing subscription content (e.g. personalized news, business info, sports highlights, etc).
  • Watching content from a live camera (e.g. webcam or security camera).
  • Streaming content received via 3GPP’s Multimedia Messaging Service (MMS).

The main focus of PSS is the communication between server and terminal. The PSS specification tells operators and equipment manufacturers how the server and terminal must behave in order to enable the streaming service. The relevant specifications are TS26.233 and TS26.234, which may be found on the 3GPP FTP site. Versions are maintained for 3GPP Release-4 and Release-5.

Q.  What is the purpose of PSS?

A.  The PSS specification defines a multimedia streaming service within 3G wireless networks, and so enables the application scenarios mentioned above, as well as others. An important point is that PSS standardizes the communication used in the service. This makes it possible for vendors to design equipment that will interoperate (i.e. communicate properly with equipment from other vendors). For example, one company could provide a streaming server, and another company could provide the client solution.

While the PSS specification is defined to enable interoperable implementations, the specification itself cannot guarantee interoperability. Implementations from different companies must be tested together to ensure that the implementations are correct, and that companies have a consistent interpretation of the specification. Testing is also needed to identify and correct parts of the specification which are unclear or inconsistent.

Q.  What is PSS-AG?

A.  The Packet-Switched Streaming Services Activity Group (PSS-AG) is a subgroup of the IMTC which designs and implements interoperability tests based on the 3GPP PSS specification. The group has been actively testing since 2001. Remote testing activities operate nearly continuously, and in addition the group meets for face-to-face testing events typically 2 or 3 times per year. Currently there are 27 companies participating in PSS-AG.

The objectives of PSS-AG can be listed as follows:

  • Design and improve test procedures and technical testing specifications.
  • Execute tests based on file exchange, virtual connections, and live face-to-face testing events.
  • Validate and improve products based on PSS.
  • Test new and emerging PSS features.
  • Increase awareness of the current state of interoperability testing in order to expand the market for PSS-based streaming products.
  • Communicate with 3GPP SA4 and other standards organizations so that knowledge gained in the tests can be used to improve PSS and other relevant specifications.

Q.  What standards does PSS-AG test?

A.  The tests are based on 3GPP’s Packet-Switched Streaming Services (PSS) specification, which mainly defines a server-client protocol for streaming in 3G networks. This specification is in turn built on many others, including:

  • Signaling and control protocols (RTSP and SDP from IETF)
  • Transport protocols (RTP, RTCP, and various payload formats from IETF)
  • Audio codecs (AMR and AMR-WB from 3GPP, AAC from MPEG-4)
  • Visual codecs (H.263 from ITU, Simple Visual Profile from MPEG-4)
  • File format (MP4 file format from MPEG-4 Systems)
  • Scene description (SMIL from W3C)

Thus specifications from many standards bodies are tested in PSS-AG. Each dependent specification has parts which could be interpreted differently by different companies. Identifying such problems, whether due to incorrect implementation or due to problems in the specification, is the main goal of the tests.

Q.  What is the relationship and position of PSS-AG to standardization bodies?

A.  PSS-AG is a subgroup of IMTC, an industry consortium dedicated to promoting multimedia communication standards. IMTC is focused on interoperability. It is important to note that PSS-AG and IMTC are not standards bodies. However, IMTC and its subgroups can interact with standards bodies through liaison relationships and through the actions of individual member companies.

A liaison relationship is an official channel of communication with another body. For example, IMTC recently established a liaison relationship with 3GPP. This relationship allows PSS-AG to make official contributions to 3GPP working group meetings, and so improves communication between PSS-AG and 3GPP SA4. Liaison statements and other documents resulting from such communication may be found here.

Some standards bodies (e.g. IETF) are more suited to individual participation rather than inter-group liaisons. In such cases, PSS-AG may interact through individual PSS-AG members who also attend the other body. Information on the current status of other standards may be gathered and shared in PSS-AG, and PSS-AG companies who attend a standards development organizations may make contributions to such standards.

Finally, PSS-AG is interested in joint testing relationships with other interoperability test groups. A cross testing policy has been developed for this purpose.

Q.  How does the PSS standard differ from terminal standards such as H.323 and H.324?

A.  The main difference is that H.323 and H.324 are used for 2-way communication, typically in a videoconferencing scenario. The 3GPP Packet-Switched streaming standard is instead aimed at one-way video streaming applications, such as Video on Demand (VoD) or viewing a live camera source (e.g. web-cam). Note that some companies have developed gateways which can bridge between the standards (e.g. to allow a standard H.324 terminal to receive audiovisual content originating from a PSS server).

Q.  What are H.324M and 3G-324M, and how do they compare to PSS?

A.  These are mobile variants of the circuit-switched H.324 standard. H.324M is H.324 with the addition of a mobile annex, which improves error robustness. 3G-324M is H.324M with some additional restrictions and guidelines for use in 3G mobile phone networks. In particular, 3G-324M operates at similar bandwidth and screen size to PSS. The main difference is that PSS is a one-way streaming application over packet-switched networks, while H.324M and 3G-324M are two-way terminal standards running over circuit-switched networks.

Q.  Why do I need to be interoperable with other vendors?

A.  Multimedia communication in 3G mobile phone networks will be built upon solutions from many vendors. Terminals, in particular, must interact with components not only in the user’s home network, but also in other networks while roaming. For this reason, content, servers and terminals from different vendors must work seamlessly together.

Q.  What components are being tested?

A.  PSS-AG companies bring three types of components to the tests:

  • Content authors
  • Streaming servers
  • Multimedia clients

These components are tested via an established test procedure consisting of five stages:

Stage-1:      Raw codec bitstream (encode / decode)

Stage-2:      3GPP/MP4 File Format (author / play )

Stage-3:      Packetized bitstream (packetize / depacketize)

Stage-4:      Virtual testing (full session testing via public internet)

Stage-5:      Face-to-face testing

Note that Stage-3 has been deprecated and is no longer used by the group.

Q.  What types of members are suitable for the PSS?

A.  PSS-AG membership has only two requirements:

  1. You must bring at least one PSS-based component to actively test within the group. “Observers” are not allowed.
  2. You must be a current member of IMTC. Details on how to become a member may be found here.

Q.  Can I only be a tester or do I have to be a content generator too (or vice versa)?

A.  The testing is based on three separate tracks:

  1. Content creation tools
  2. Multimedia delivery tools
  3. Multimedia terminals

One must test a component from at least one of these tracks in order to join the group.

Q.  What responsibilities do members have?

A.  The main responsibility is to actively participate in planning and execution of the interop tests. PSS-AG members must also attend a weekly conference call, which is used for test planning, status updates, and improvement of the procedures. The testing will occasionally involve travel to live test events such as the IMTC SuperOp!

PSS-AG members are also responsible for protecting the confidentiality of other

members. Details may be found in the IMTC Rules

of Engagement.

Q.  What are the benefits of the PSS AG to the mobile industry?

A.  PSS-AG provides a place where PSS solution vendors may test their implementations in an open, accessible environment. The product interoperability, which results from these tests, is necessary in order to enable and expand the market for wireless multimedia products based on the PSS standard. In addition, knowledge gained through testing in PSS is actively fed back into the standardization process in order to clarify and improve the relevant specifications.

Q.  How can a company join the PSS AG?

A.  For information on how to join, please contact , one of the PSS-AG co-chairs. Note that each new PSS-AG member company is assigned an experienced “escort member” who will answer your questions and help introduce you to PSS-AG procedures.

Q.  Where can PSS-AG test procedures be found?

A.  The current test procedures document may be found here.

Q.  How are the tests being monitored?

A.  The PSS-AG is using a web-based test-tracking tool in order to facilitate the tests and monitor test progress. The tool is based on BugZilla, the Mozilla bug tracker. The tool keeps track of test profiles (i.e. what components and features each company is testing), and tracks each test point to a successful resolution. Reports on current test status may also be generated.