4.4 Standards for Mobile Streaming

4.4 Standards for Mobile Streaming

The need to have an optimized end-to-end multimedia streaming service over mobile networks has pushed the 3GPP organization to standardize such service within the Service Aspects Codec Working Group. The first standardized service is in Release 4 specifications of transparent end-to-end packet-switched streaming service (PSS). Release 5 specifications define additional capabilities to the streaming service. At the time of writing this chapter, Release 6 specifications were being written by 3GPP.

4.4.1 Release 4 PSS

The specifications for PSS in Release 4 are basically defined in two documents. ^[37], ^[38] Here we describe the main features and architecture of a mobile streaming service that is implemented according to Release 4 specifications. No details on the file formats for PSS are included in this chapter.

Figure 4.7 shows the end-to-end architecture for PSS service. ^[39] In this architecture only content servers and streaming clients are required. The dashed elements may not be present. The content server resides behind the Gi interface, possibly within the operator's network. However, streaming servers also may reside on the public Internet. Content cache servers can be used for service optimization, and may keep the replicated copies of the most-popular items streamed by the users. Portals servers are engines that allow an easier access facility to the end user, often through the use of searching and browsing capabilities. User and terminal profile servers store user preferences and terminal capabilities data that can be used to control the streaming session to the end user.

Figure 4.7: Network elements involved in a 3G packet-switched streaming service.

While a streaming server can be designed primarily for use over the Internet and also serve mobile streaming users, eventually in an nonoptimized fashion, a streaming client must be designed merely for mobile use. A system view of a Release 4 PSS client is shown in Figure 4.8, ^[40] while the protocol stack for layer 3 and above is shown in Figure 4.9. ^[41] A PSS client can be either extended or simple, depending on whether or not it implements the functional components in the dashed blocks (vector graphics and capability exchange). The features for extended PSS clients have been defined in Release 5 specifications.

Figure 4.8: System architecture of a Release 4 PSS client.

Video Audio Speech	Scence description Presentation description Still images Bitmat graphics Vector graphics Text	Presentation description
Payload formats	HTTP	RTSP
RTP
UDP	TCP		UDP
IP

Figure 4.9: Protocol stack for Release 4 PSS.

4.4.1.1 Control and Scene Description Elements

Three control elements are included in PSS (see Figure 4.8):

1. Session control (and setup) of streaming sessions between a streaming client and one or more streaming servers, with the possibility of VCR-like operations such as PAUSE, PLAY, STOP, FAST-FORWARD, and REWIND. Session control and setup is made with two protocols: HTTP (Hypertext Transfer Protocol) ^[42] for reliable (i.e., over TCP/IP) transport of discrete media (still images, bitmap graphics, text, scene or presentation description) or RTSP (Real-Time Streaming Protocol) ^[43] for reliable or unreliable transport of session setup (i.e., over TCP/IP or UDP/IP, respectively) and control of continuous media (speech, audio, or video).

2. Session establishment refers to the methods to invoke a PSS session from a browser or directly by entering the URL ^[44] in the terminal's user interface; in other words, the ways to obtain the initial session description (e.g., an SDP ^[45] presentation description, a SMIL ^[46] scene description or directly the RTSP URL of the content).

3. Capability exchange enables the adaptation of the streamed content to the user's device, depending on its characteristics and capabilities. No explicit protocol is defined for the simple PSS. This makes the assumption that the user is aware of the requirements of the content to be streamed (e.g., the screen size). Protocols for capability exchange can be specified for extended PSS.

The scene description component consists of spatial layout of the different media and the description of the temporal relationship (i.e., synchronization) of the media that is included in a media presentation.

4.4.1.2 Media Elements

3GPP PSS can support a rich set of media, either continuous or discrete. Continuous media are media flows that must be displayed/played preserving their temporal relationship. They are transported via RTP/UDP/IP packet encapsulation, using appropriate payload formats defined for each codec. Discrete media have no time element among their properties. They are transported using HTTP/TCP/IP packet encapsulation.

Continuous media in Release 4 PSS are speech, audio, and video. The following decoders can be supported in a streaming client:

• AMR (Adaptive MultiRate) narrowband is the mandatory speech decoder for PSS. ^[47] Speech is encoded at 8 kHz sampling frequency at 8 different bit rates ranging from 4.75 to 12.20 kbps. An AMR speech flow is packetized by a streaming server using the RTP payload format described in reference ^[48].

• AMR wideband decoder is mandatory for a PSS client when speech encoded at 16 kHz sampling frequency is supported. ^[49] It supports nine different bit rates ranging from 6.60 to 23.85 kbps. An AMR wideband flow is packetized using the RTP payload format described in reference ^[50].

• MPEG-4 AAC. If audio is supported, AAC Low Complexity ^[51] with a maximum sampling frequency of 48 kHz in mono (1/0) and stereo (2/0) is the decoder supported. However, the AAC Long-Term Prediction decoder can also be supported. An AAC flow is packetized using the payload format as described in reference ^[52].

• H.263 Video. Profile 0 Level 10 is the mandatory decoder for video streams. ^[53] It supports video at a maximum bit rate of 64 kbps at QCIF picture size (176 x 144 pixels). Optionally, Profile 3 Level 10 can also be supported ^[54] to provide better error resilience. An H.263 video stream is packetized using the payload format defined in reference ^[55].

• MPEG-4 Visual is an optional decoder that can be supported at Simple Profile Level 0. ^[56], ^[57] An MPEG-4 video stream is packetized using the payload format described in reference ^[58].

Discrete media in Release 4 PSS are still images, bitmap graphics, vector graphics, and text. The following discrete media decoders can be supported in a streaming client:

• JPEG is the mandatory format for still images. ^[59] It is supported in the baseline and progressive DCT mode (nondifferential Huffman encoding).

• GIF is the format supported for bitmap graphics. Two formats can be decoded by a PSS client: GIF87a and GIF89a. ^[60], ^[61]

• Vector graphics. No decoders are specified for a simple PSS client. However, decoders can be specified for extended PSS clients.

• The text decoder is supposed to be used in a SMIL presentation with the text formatted following XHTML Mobile Profile. ^[62] The supported character coding formats are UTF-8 ^[63] and UCS-2. ^[64]

4.4.2 Release 5 PSS

The specifications for Release 5 PSS are defined in three documents. ^[65], ^[66], ^[67] Release 5 enhances PSS with new features. The major features are described here (file format issues will not be covered here).

4.4.2.1 Control Elements

The most important enhancement of the application control plane is a new mechanism for capability exchange, which means the functionality of PSS servers to provide content for a wide set of mobile devices. The device capabilities and preferences are described in a device profile using attributes into an RDF ^[68] document that follows the structure of the CC/PP ^[69] framework and the CC/PP application, UAProf. ^[70] Following are some of the possible attributes that can characterize a mobile device:

• Audio channels

• Max polyphony capabilities when supporting scalable polyphony MIDI sounds

• List of MIME types that the PSS device accepts

• Screen size

• SMIL modules supported

• Decoding video byte rate

• Size of predecoder buffer

• Size of postdecoder buffer

• Number of bits per pixel

• Color capability

• Pixel aspect ratio

• List of supported character sets

Whenever this mechanism for capability negotiation is used, device capability profiles are stored on a device profile server that is inquired by a PSS server (via HTTP or RTSP) before delivering multimedia content to the mobile device.

4.4.2.2 Media Elements

The new media formats that are introduced in Release 5 of PSS specifications are all of the discrete type (i.e., transport over HTTP/TCP/IP):

• Scalable polyphony MIDI (SP-MIDI) is the format to support synthetic audio. ^[71]

• PNG (Portable Network Graphics) is yet another format to support bitmap graphics. ^[72]

• SVG (Scalable Vector Graphics) is the mandatory format used to support vector graphics. Optionally, the Basic profile also can be supported. ^[73], ^[74]

• Timed text is supported as downloadable text (not streamed via RTP/UDP/IP). Timed text can define color, font, writing direction (i.e., left to right, or other directions), Karaoke highlighting, and other attributes. ^[75]

This section discussed the main features of PSS in Release 4 and Release 5 specifications. The next section covers some QoS and performance issues of mobile multimedia streaming.

^[37]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). General description (Release 4), TS 26.233, v.4.2.0 (2002–03).

^[38]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). Protocols and codecs (Release 4), TS 26.234, v.4.5.0 (2002–12).

^[39]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). General description (Release 4), TS 26.233, v.4.2.0 (2002–03).

^[40]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). Protocols and codecs (Release 4), TS 26.234, v.4.5.0 (2002–12).

^[41]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). Protocols and codecs (Release 4), TS 26.234, v.4.5.0 (2002–12).

^[42]Fielding, R. et al., Hypertext transfer protocol - HTTP/1.1, IETF (Internet Engineering Task Force) RFC 2616, June 1999.

^[43]Schulzrinne, H., Rao, A., and Lanphier, R., Real-time streaming protocol (RTSP), IETF (Internet Engineering Task Force) RFC 2326, April 1998.

^[44]Berners-Lee, T., Masinter, L., and McCahill, M., Uniform resource locators (URL), IETF (Internet Engineering Task Force) RFC 1738, December 1994.

^[45]Handley, M. and Jacobson, V., SDP: Session description protocol, IETF (Internet Engineering Task Force) RFC 2327, April 1998.

^[46]W3C, Synchronized multimedia integration language (SMIL 2.0), Recommendation, August 2001.

^[47]3GPP TSGS-SA, Mandatory speech codec speech processing functions. AMR speech codec. General description (Release 5), TS 26.071, v.5.0.0 (2002–06).

^[48]Sjoberg, J. et al., RTP payload format and file storage format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) audio codecs, IETF (Internet Engineering Task Force) RFC 3267, March 2002.

^[49]ITU-T, Wideband coding of speech at around 16 kbits/s using Adaptive Multi-Rate Wideband (AMR-WB), Recommendation G.722.2, January 2002.

^[50]Sjoberg, J. et al., RTP payload format and file storage format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) audio codecs, IETF (Internet Engineering Task Force) RFC 3267, March 2002.

^[51]ISO/IEC, Information technology - Coding of audio-visual objects - Part 3: Audio, 14496-3, 2001.

^[52]Kikuchi, Y. et al., RTP payload format for MPEG-4 audio/visual streams, IETF (Internet Engineering Task Force) RFC 3016, November 2000.

^[53]ITU-T, Video coding for low bit rate communication, Recommendation H.263, February 1998.

^[54]ITU-T, Video coding for low bit rate communication. Profiles and levels definition, Recommendation H.263 Annex X, April 2001.

^[55]Bormann, C. et al., RTP payload format for the 1998 version of ITU-T Rec. H.263 (H.263+), IETF (Internet Engineering Task Force) RFC 2429, October 1998.

^[56]ISO/IEC, Information technology - Coding of audio-visual objects - Part 2: Visual, 14496-2, 2001.

^[57]ISO/IEC, Streaming video profile, 14496-2, 2001/Amd 2, 2002.

^[58]Kikuchi, Y. et al., RTP payload format for MPEG-4 audio/visual streams, IETF (Internet Engineering Task Force) RFC 3016, November 2000.

^[59]ISO/IEC, Information technology - Digital compression and coding of continuoustone still images. Requirements and guidelines, 10918-1, 1992.

^[60]Compuserve Incorporated, GIF graphics interchange format: a standard defining a mechanism for the storage and transmission of raster-based graphics information, Columbus, OH, 1987.

^[61]Compuserve Incorporated, Graphics interchange format: version 89a, Columbus, OH, 1990.

^[62]WAP Forum Specification, XHTML Mobile Profile, October 2001.

^[63]The Unicode Consortium, The Unicode standard, Version 3.0, Addison-Wesley, Reading, MA, 2000.

^[64]ISO/IEC, Information technology - Universal multiple-octet coded character set (UCS) - Part 1: architecture and basic multilingual plane, 10646-1, 2000.

^[65]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). General description (Release 5), TS 26.233, v.5.0.0 (2002–03).

^[66]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service. Service aspects. Stage 1 (Release 5), TS 22.233, v.5.0.0 (2002–03).

^[67]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). Protocols and codecs (Release 5), TS 26.234, v.5.3.0 (2002–12).

^[68]W3C, Resource description framework (RDF) schema specification 1.0, Candidate Recommendation, March 2000.

^[69]W3C, CC/PP structure and vocabularies, Working Draft Recommendation, June 2001.

^[70]WAP Forum, WAP UAProf specification, Specification, October 2001.

^[71]MIDI Manufacturers Association, Scalable polyphony MIDI specification version 1.0, RP-34, Los Angeles, February 2002.

^[72]Boutell, T., et al., PNG (Portable Network Graphics) specification version 1.0, IETF (Internet Engineering Task Force) RFC 2083, March 1997.

^[73]W3C, Scalable vector graphics (SVG) 1.1 specification, Working Draft Recommendation, February 2002.

^[74]W3C, SVG mobile specification, Working Draft Recommendation, February 2002.

^[75]3GPP TSGS-SA, Transparent end-to-end packet-switched streaming service (PSS). Protocols and codecs (Release 5), TS 26.234, v.5.3.0 (2002–12).