SSAI vs CSAI: A Technical Breakdown on VAST Tag differences

SSAI vs CSAI: How Delivery Impacts VAST Tag Behavior

In the world of programmatic advertising, the methods used for ad delivery can significantly impact the effectiveness of campaigns. Two such techniques, Server-Side Ad Insertion (SSAI) and Client-Side Ad Insertion (CSAI), play vital roles in how Video Ad Serving Template (VAST) tags behave.

This article provides a detailed technical breakdown of these two methods, explores their pros and cons, and explains how they interact with VAST tags so you can make an informed decision for your ad delivery strategy.

Pro Tip: If your campaign budget allows for it, consider testing both SSAI and CSAI to see which yields better performance for your specific audience and content type. This hands-on approach can provide invaluable insights beyond theoretical advantages.

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SSAI: A Technical Breakdown

SSAI (Server-Side Ad Insertion), often referred to as ad stitching, is a method where ads are dynamically stitched into the video stream on the server side before being sent to the user’s device. This means the viewer receives one seamless video stream that already includes the ads — no separate ad request is made by the client’s device.

This approach has several implications:

• Ad Blocking Prevention: Because the ad becomes part of the content stream, it is nearly impossible for client-side ad blockers to detect or block it.
• Reduced Client-Side Load: The server handles the VAST tag processing, media file selection, and error handling, which minimizes the risk of playback issues caused by device incompatibility.
• Consistent Experience Across Devices: SSAI ensures that even devices with limited processing power can deliver ads reliably, since the heavy lifting is done upstream.

VAST tags in SSAI are parsed server-side, which allows ad servers to pre-validate creatives, select the correct media format (MP4, HLS, DASH), and stitch it into the stream.

SSAI setups often use VAST 4.x features, such as Mezzanine files, which are high-quality master assets used to generate adaptive bitrate renditions for streaming.

Example VAST Tag for SSAI

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<VAST version="3.0">
    <Ad id="123456">
        <AdSystem>Ad Server</AdSystem>
        <Creatives>
            <Creative>
                <Linear>
                    <Duration>00:00:30</Duration>
                    <MediaFiles>
                        <MediaFile type="video/mp4" width="1920" height="1080" delivery="progressive" bitrate="1500">
                            <![CDATA[http://example.com/hd-ad.mp4]]>
                        </MediaFile>
                        <Mezzanine delivery="streaming" width="1920" height="1080" type="application/x-mpegURL">
                            <![CDATA[http://example.com/hd-ad-master.m3u8]]>
                        </Mezzanine>
                    </MediaFiles>
                </Linear>
            </Creative>
        </Creatives>
    </Ad>
</VAST>

SSAI Pros and Cons

Pros

• Bypasses ad blockers and guarantees ad delivery
• Provides a smoother, buffer-free viewing experience (ads pre-stitched)
• Reduces device-side VAST errors (parsing and selection handled server-side)
• Improves measurement accuracy with server-to-server tracking

Cons

• More complex and costly to implement (requires a stitching service or CDN support)
• Limited real-time personalization (ads are selected before playback starts)
• Interactive ad formats (e.g., clickable overlays) are harder to implement

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CSAI: A Technical Breakdown

CSAI (Client-Side Ad Insertion) is the more traditional approach, where the client's device (e.g., a browser, mobile SDK, or CTV app) makes a direct ad request, fetches the VAST response, and handles ad playback.

Because the ad is requested and inserted locally, CSAI allows for real-time personalization — for example, targeting based on user location, device, or even behavior during the current session.

However, this method comes with its own challenges:

• Susceptibility to Ad Blocking: Since the request is made by the client, browser-based ad blockers can detect and block VAST calls.
• Device & Player Dependency: Different players may implement VAST support differently, which can lead to parsing errors or unsupported creatives.
• Performance Impact: Ad playback may introduce buffering between content and ad segments, creating a less seamless experience.

Example VAST Tag for CSAI

CSAI setups often use lighter media files (SD or lower bitrate) to reduce buffering on slower devices or networks.

Copy
<VAST version="3.0">
    <Ad id="123456">
        <InLine>
            <AdSystem>Ad Server</AdSystem>
            <AdTitle>Ad Title</AdTitle>
            <Creatives>
                <Creative>
                    <Linear>
                        <Duration>00:00:30</Duration>
                        <MediaFiles>
                            <MediaFile type="video/mp4" width="360" height="640" delivery="progressive" bitrate="1500">
                                <![CDATA[http://example.com/hd-ad.mp4]]>
                            </MediaFile>
                        </MediaFiles>
                    </Linear>
                </Creative>
            </Creatives>
        </InLine>
    </Ad>
</VAST>

CSAI Pros and Cons

Pros

• Supports highly dynamic and personalized ads
• Easier to implement (no stitching infrastructure needed)
• Fully supports interactive ad formats (clickable banners, companion ads)

Cons

• Vulnerable to ad blockers
• More prone to playback errors and latency
• Can create a disjointed experience with buffering between content and ads

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Comparing SSAI and CSAI

Here’s a quick reference comparison:

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Future Trends in Ad Delivery

The line between SSAI and CSAI is gradually blurring. Hybrid approaches are emerging, where servers stitch the ads but also pass metadata to the client for tracking and limited interactivity.

• 5G & Edge Computing: Faster networks will reduce CSAI buffering, making client-side more viable even for longer ad pods.
• Advanced Personalization: SSAI platforms are starting to incorporate real-time decisioning, enabling better targeting without losing the seamless experience.
• Privacy-First Tracking: With third-party cookie deprecation, server-to-server measurement in SSAI may become the industry standard.

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Conclusion

Understanding SSAI and CSAI — and their interaction with VAST tags — is crucial for ad ops, developers, and advertisers.

• Choose SSAI if you prioritize seamless playback, ad blocker resistance, and reliable delivery.
• Choose CSAI if you need dynamic targeting, interactivity, and simpler implementation.

In many cases, a hybrid approach offers the best of both worlds, combining SSAI’s reliability with CSAI’s flexibility.