TANDBERG D11624 User Manual

Video on Frame Relay

Tandberg

D11624 rev 01

Tandberg Video on Frame Relay

Table of Contents

1. Introduction ...............................................................................................................................................3

2. Background on transmission of video over frame relay.............................................................................4

3. Technical issues.........................................................................................................................................5

4. Equipment for Video over Frame Relay....................................................................................................7

4.1 Framing the Picture..............................................................................................................................7

5. About the Product VFX-250S....................................................................................................................8

6. Application Overview................................................................................................................................9

7. Technical Specification of VFX-250S.....................................................................................................10

7.1 Network & User Interface..................................................................................................................10

7.2 User Interface.....................................................................................................................................10

7.3 Network Interface..............................................................................................................................10

7.4 Enhanced Buffer Management...........................................................................................................10

7.5 Serial Management Interface.............................................................................................................10

7.6 Mechanical/Environmental................................................................................................................11

7.7 Power Supply.....................................................................................................................................11

8. Approvals.................................................................................................................................................12

9. Cable VFX-250S .....................................................................................................................................12

10. Economics of Video over Frame Relay.................................................................................................13

11. Appendix A: Cables and Pinouts...........................................................................................................14

11.1 Standard Console Cable...................................................................................................................14

11.2 Standard EIA-449/RS449 Cables ....................................................................................................15

11.2.1 Network Cable..........................................................................................................................15

11.2.2 User Cable ................................................................................................................................15

11.3 Standard EIA-530/RS530 Cables ....................................................................................................16

11.3.1 Network Cable..........................................................................................................................16

11.3.2 User Cable ................................................................................................................................16

11.4 Standard V.35 Cables ......................................................................................................................17

11.4.1 Network Cable..........................................................................................................................17

11.4.2 User Cable ................................................................................................................................17

12. Appendix C: Glossary and abbreviations...............................................................................................18

13. Appendix D: Information about the manufacturer.................................................................................19

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1. Introduction

This document is designed as an 'eye-opener' to video over frame relay and the idea is to show a solution that works. The equipment described in this document is the VFX-250S, a framer unit from Science Dynamics Corporation. Tandberg video conferencing codecs are being used over frame relay together with this equipment and according to the setup described here. The setup has been tested and found reliable and it works well. For abbreviations throughout the document a glossary is provided in the back.

The costs and savings of using frame relay network for video transmissions are taken from calculations by Science Dynamics and should be considered examples only. Tandberg disclaims any responsibility for inaccuracies or subsequent changes in the tariffs used in developing the above costs and savings.

Here's what it takes to make videoconferencing work on frame relays today:

• Unfailing QoS (Quality of Service). The single most important factor in the delivery

of acceptable videoconferencing over frame relay is protection of the video stream from frame drops.

• Adequate bandwidth. You'll need 384 Kbps or more for room-based systems, 128 to

256 Kbps for desktop systems and up to 56 Kbps for surveillance systems. This is comparable to the requirements for circuit-switched connections such as ISDN (Integrated Services Digital Network).

• CIR(Committed Information Rate) in the frame relay WAN(Wide Area Network). It

must be 1 percent to 3 percent higher than selected bandwidth; you'll need additional transmission space to carry frame relay packet overhead without impeding delivery of the payload.

Tests have shown that even when flooding the concurrently running Ethernet with so much traffic that 98 percent of data packets were thrown away, the video stream rolled merrily along at 30 frames per second with no evidence of tiling faults or frame drops.

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2. Background on transmission of video over frame relay

The successful transmission of digitised voice over public frame relay data services the past few years has drawn attention to the question of whether video services can be transmitted over the same link.

Voice, data & video over frame relay

Digitised Video is not new. It has been used for several years by a myriad of users on ISDN or leased line connections. The common international standard for the compression of video and accompanying voice is H.320. Frame relay is not new either. It has been in existence for several years, and is now one of the most widely deployed data transmission means in the world. What is new, and revolutionary, is the ability to take a standard H.320 video stream, "packetise it", and route it over a frame relay network.

Historically, frame relay has been developed and sold primarily as a data transport technology and service solution. This should be viewed primarily as a marketing and positioning technique, not a fundamental technical limitation. All of the technical challenges of using frame relay to transport video have been met. The marketing challenge is to expand the perceived scope of frame relay beyond a "data only" image.

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3. Technical issues

There are two potential technical issues, which may affect the quality of packetised, digitised video. One is delay, or more properly jitter. Jitter is the variation in delay from one frame to the next. This is critical for video, as video requires a constant bit stream in order to maintain an image. The second is dropped frames. If a video frame is lost, it may cause a click or pop in the audio and some pixelation on the video. Too many lost frames and the video quality is impaired.

In leased line applications using TDM (Time Division Multiplexing) jitter is not an issue, as video frames arrive at known, predictable intervals. Concurrently, there is little likelihood of dropped frames unless the line itself malfunctions. However, public frame relay networks introduce issues that do not occur when running the frame relay protocol over private leased lines. Customers who wish to run digitised video over public frame relay services need to understand these issues.

Jitter can occur in public frame networks when an intermediate switch is processing someone else's frame when your frame arrives.

Jitter is created by differences in packet size

The second incoming frame is held in a buffer at the switch until the transmission of the first frame is completed. The delay that results is dependent on the length of the first frame. Since frame relay allows variable length frames, this delay is variable and unpredictable, resulting in jitter. If this jitter exceeds the ability of the receiving device to compensate by buffering, video quality will be degraded.

However, for the majority of public frame relay networks, jitter is more a theoretical problem than a real problem. Public services run on high-speed backbones. Since delay is inversely proportional to speed, this means that delay at intermediate nodes is highly unlikely. Also, many of today's Public frame relay networks use a cell (fixed frame length) based architecture between nodes, which also reduces the likelihood of jitter.

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Cell based systems cut packet into fixed sizes

Dropped frames are potentially a more serious problem. The frame relay standard allows the network service provider to control congestion by simply disposing of any frames which exceed the users CIR. In other words, if you contract for a CIR of 128 kbps but send a burst at 192 kbps, frames which exceed the 128 kbps CIR will have a DE (Discard Eligible) bit set. If some intermediate switch on the network becomes congested, these frames may be discarded. While an occasional lost frame will not seriously degrade video quality, too many will cause a noticeable loss of video quality.

In most networks, dropped frames are unlikely to occur. This depends, of course, on the capacity of the network, the actual traffic load at any given time, how the load varies, and other factors beyond the control of the end user.

The only certain way is to have enough CIR to cover all usage.

This is unnecessary in most cases, as the majority of installed public networks are not oversubscribed. Most carriers are now offering QoS (Quality of Service) or SLA (Service Level Agreement) guarantees, which categorically provide an end user with confidence that more than 99% of frames will arrive at their destination. For the Video over Frame Relay user, there are other ways of reducing the threat of frame loss.

1. The first is the configuration of the frame size. Frame relay allows the payload portion of the frame to be adjusted to carry larger or smaller amounts of information. This allows network administrators to adjust the frame size for optimal network performance. If a small frame packet is lost, it is not carrying too much information as to critically impair video function.

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