Continuing our interview with Stephen Clee, managing director of Datmedia, he explained that mobile video systems use protocols to setup, control and terminate communication systems. Protocols are a precise set of rules, timings and syntaxes that govern the accurate transfer of information between devices or software applications. There are several types of protocols that are used in media streaming systems including link control protocols, network routing protocols (IP), transmission control protocols (TCP or UDP), session control protocols (RTP) and application control protocols (RTSP). Protocols that are used to control media streaming sessions are called media streaming protocols.

Media streaming protocols are used to setup, control and end media streaming sessions. Streaming protocols provide familiar media controls such as start, stop and pause the playing of media programs. Some of the common protocols that are used by mobile devices to obtain video services include Microsoft Media Streaming (MMS), real time streaming protocol (RTSP), hypertext transfer protocol (HTTP) and 3rd Generation Partnership Project (3GPP).

MMS protocol is a proprietary protocol that was developed by Microsoft. RTSP is an industry standard protocol that was designed for control of media streaming sessions. HTTP is a more general media control protocol that can also be used to control media streaming sessions. 3GPP is the only open standard protocol dedicated to wireless communication.

Figure 8 shows how media streaming protocols are used to provide media flow control capabilities. This example shows how RTSP control messages are sent via a web browser to a streaming media server requesting that media streaming be setup, initiated and paused to a media viewer.

Streaming media originates from a media server which passes through a variety of network devices such as routers, switches and firewalls before it is received by a media player. Streaming media may have difficulty getting through firewalls as some firewalls block protocols that are related to streaming video. Firewalls may be setup to disable (block) specific protocols and their related industry standard ports (logical channel numbers).

Mobile devices generally do not suffer from firewall issues because the streaming service is broadcast as a radio message available to everyone. Content can be protected and available to chosen subscribers via a DRM (Digital Rights Management) system. The DRM provides authentication for the subscriber to receive content. Because there are several media streaming protocol options, and a media server and browser may be able to use more than one media protocol, the selection of media protocol must be generated by the server originating the stream.

When the media session begins, the device receives a link (URL) to the streamed content. This link is prefaced with a SMS(text) or MMS(graphic icon) message. The user must accept the link before content is delivered.

Figure 8, Real Time Streaming Protocol Operation

This is a necessary step to comply with government law and it prevents automatic video spam. The mobile device decides which streaming protocol to use through the use of a moniker. A moniker is a preface to the URL (e.g. RTSP://<URL>) that indicates which protocol will be used during the requested communication session. The operator of the streaming service must usually purchase SMS or MMS credits from another company in order to send these messages. This business model allows small operators to enter the arena of streamed content delivery. The handset users pay the mobile network operators for the data bandwidth that is used for streamed media as part of their on-going subscription charges.

When a media streaming session is first established, the mobile video system may not be able to play video although other functions behave normally. From diagram 8, normal traffic occurs on channel A and streamed media is delivered on channel B. The telecom company responsible for the mobile network may choose to block channel B, whilst allowing transmission of channel A. This creates a 'walled garden' and is intended to drive users towards the telecom's own media services. Walled gardens are weakening and it is usually possible to contact the mobile network operator and request that permission for streamed media is granted to a given handset.

Media broadcasters invest in the creation of content that is distributed to viewers. Viewers may use display devices that are attached or communicate with different types of systems. Some of the different types of systems that may be used to distribute media include IPTV, mobile video and internet television. In order to maximise their audiences, broadcasters try to make media content available to viewers regardless of the type of system that is being used. An application and delivery mechanism that operates within a variety of environments is called a cross platform system.

Cross platform systems must be capable of adapting media formats with different data transmission rates and alternate encoding formats. Cross platform systems use a variety of protocols along with other media and control information.

Figure 9 demonstrates how cross platform capability can be achieved. This relies on the application software complying with open standards to be independent from web browser and media type. The importance of browser and media type independence cannot be stressed enough. It is this capability that allows the same software core to service PC, mobile device and set top box environments.

The web server senses the destination platform via the browser and delivers the correct graphical environment to suit. The example below shows how media files can be setup to be made available on a mobile video, IPTV and broadband television formats.

Mobile devices typically use a browser to graphically navigate the internet to find and select media for viewing. The browser acts as a host for a software program or module (called a client) that is used to convert static information, available on the Web, into usable forms, i.e. text and graphics. Mobile devices typically use a special limited version of a browser called a micro-browser. Micro-browsers are used to display web page documents in wireless devices (usually using WML formats).

The capabilities of micro-browsers include the ability to graphically display web pages and some image formats such as icons and pictures, but not Flash information. Their handling of JavaScript also tends to be limited. A point worth noting is that Flash is a proprietary web technology from software vendor Adobe. It is used for animated graphics and video streaming but has virtually no support on mobile phones. Therefore web designers need to take this into account if they want to reach the widest possible audience. JavaScript on the other hand is being adopted on mobile devices and is designed to enable interactivity for web pages. However the limitations of micro-browsers are disappearing. In 2006 mobile browser technology was introduced with JavaScript support that can match and even better the performance of PC browsers displaying standard graphics. This will usher in a new era for web applications on mobile handsets.

When a browser is presented with a streamed media format that it does not have the capability to decode and process, it must find and launch (initiate) a media player software program that can process the media. As of 2006, most micro-browsers did not have the capability to receive and process streaming media directly.

Streaming media must be converted into a format that can be displayed (rendered) to the viewing device. Streaming media is converted to audio and visual formats by a media player. A media player is a software application and/or device that can convert media such as video, audio or images into a form that can be experienced by humans. Media players may contain support for different media formats and compression (codec) systems, as well as being able to communicate using multiple network streaming protocols. The common streaming media formats used for mobile video include Real Media, Window

Figure 9, Cross Platforms Window

Figure 10 Choice of WiMax or 3G Delivery Based on Location

Media and 3GPP. 3GPP actually uses a format called MPEG-4. This format was developed to be resilient to transmission errors. The visually most important part of a video frame is given transmission priority so that if video data is lost a picture can still be displayed.

Media player programs may be already available in the mobile device or they may be downloaded. As mobile devices suffer physical restrictions that conventional PC does not, this means that a single media player program can be used. This has resulted in mobile devices falling into two main camps: those with Microsoft Windows Media player and those with Real Media player.

Mobile video providers may offer different stream format options which may be selected by the user. The media player for a specific device may have a limited number of media format capabilities for that type of device. Datmedia is developing a 'sniffer' program that will detect which type of device is being used and automatically select an appropriate media stream type.

Figure 10 shows that mobile devices may communicate with more than one type of radio access such as GPRS, 3G, WiFi or WiMax, and there may be more than one connection available for the streaming selection. The choice of a connection type can impact the capabilities of the media streaming session and the type of access point can have dramatic differences in the service costs. For example, free Wi-Fi compared to usage-based GPRS or 3G. WiMax cellular structures are gaining in popularity and offer communities and business the opportunity to minimize costs for high bandwidth mobile activities.

The availability of multiple access points requires selection of the appropriate access connection type (access point selection). The access point selection may be a manual choice or it may be automatically selected through the use of a short script file. Currently, mobile video devices require the user to manually select the access point. This is done in a device configuration mode for the media player which selects the access point used for communication.

CStephen Clee is the managing director of Datmedia, Ltd. Stephen has considerable expertise in the areas of interactive online multi-media. He was part the UK's first internet radio station launched in 1996 (Interface Radio) and his

Datmedia is a leader in developing technology and software for online presentation and IPTV systems. Datmedia offers a range of complimentary services in the following areas: audio-video production, encoding, skin design, software development and hosting.