Video over IP - Design Considerations

Video Quality

The minimum acceptable standards for "broadcast" video applications (ie. in which the quality of the signal approaches that seen on TV) can be generally described as "VHS-quality and above":

• Minimum frame rate: 30 fps
• Minimum resolution: 240 "lines"
  • roughly equivalent to CIF format 352x288
• Minimum S/N: 42 dB

Video-on-demand (movies) applications require substantially higher resolutions to be acceptable to the viewer.

Sending video over an IP network presents inherent limitations on the quality of the video signal. Bandwith limitations and the packet nature of IP work against high-quality streamed video.

Video Streaming

Commercial solutions exist for streaming video over IP networks, including:

• Progressive Networks' RealVideo
• Xing Technology's StreamWorks
• VDOnet's VDOLive
• Vosaic
• VXtreme
• Vivoactive
• Microsoft's NetShow

The majority of these use UDP or RTP to increase network and bandwidth efficiency (ie. better quality at lower bit rates). They are all designed for broadcast of video across IP networks, but optimized for the Internet or corporate intranet (low bit rate, high collisions) and for display within (web-based) desktop applications. They are not truly scaleable (ie. they drop frames due to network congestion rather than degrade spatial resolution).

Another IP media-streaming solution is H.323. This standard is aimed at video-conferencing, but can work very well in a broadcast-video and video-on-demand application. Advantages are that it supports simultaneous audio/video/data and the stack should permit implementation of an integrated IP telephony - video services architecture. In particular, H.323 embodies call connection protocols (obviously useful for telephony) and control protocols that can be used for video channel-changing, video-on-demand program selection, etc.

Prior to implementation and before a streaming video solution is selected, it is recommended that each potential video-over-IP solution be tested on the proposed network architecture.

Bandwidth Utilization

Bandwidth will need to be shared among video-on-demand (VOD), real-time video, and IP telephony, and should therefore be allocated accordingly.

For example, assuming H.323 transmission at 64Kb/s IP for telephony and 768Kb/s for video, bandwidth can be allocated as:

Use	Bandwidth Allocation	one 10Mb/s segment (at 50% utilization) is capable of handling
IP Telephony	5%	7 simultaneous telephone calls
VOD	60%	4 simultaneous video-on-demand (movie) programs
Real-time Video	35%	2 simultaneous real-time (television) programs

This bandwidth allocation indicates that four clients may share one 10Mb/s segment without interference from one another.

Bandwidth utilization is kept to approximately 50% to ensure minimal network collisions, which will cause "stuttering" of audio and dropped video frames. Higher bandwidth utilization can be achieved through proper tuning:

• Use multicast for real-time video programming, since all clients will be receiving the same stream.
• Use large buffers in the clients to ensure that dropped packets can be re-sent and the video stream re-assembled before display.
• Use RSVP for IP telephony so that these packets are the last to be dropped. (Ensure that all switches handle RSVP traffic.)

The architecture of the International Systems project can be optimized to provide high-quality video services over IP:

• Provide 10Mb/s to each client (set-top decoder)
• Minimize contention by using switches rather than hubs
• Use 100Mb/s backbone between servers
• Use out-of-band communications to the PMS for tracking/billing
• Utilize UDP or RTP for video traffic
• Consider RSVP to maintain QoS to video applications
• Use H.323 to simplify coexistence of voice-over-IP (telephony) and video services
• Define and maintain minimum video standards, and do not let video quality degrade due to other uses of the network -- or risk the wrath of your viewers.