Client-Side vs. Server-Side Ad Insertion: How CSAI and SSAI Work and When to Use Each

The video-streaming business continues its rapid expansion — the six leading streaming service providers now collectively count 777.6 million subscribers — yet growth is no longer a given. Many popular platforms are facing a plateau or outright decline in paid subscribers, which has pushed the industry toward a familiar solution: advertising. Introducing cheaper, ad-supported tiers lets platforms attract price-sensitive viewers while opening up a meaningful new revenue stream.

That shift makes understanding ad delivery in connected TV (CTV) and over-the-top (OTT) environments more important than ever. There are two primary methods for inserting ads into streaming video: client-side ad insertion (CSAI) and server-side ad insertion (SSAI). This article explains how each works, what sets them apart, and when each approach makes the most sense.

Key Points

• Client-side ad insertion (CSAI) is a video ad-serving method in which the video player, running inside an application or website, requests ads directly from an ad server via API.
• CSAI supports a wide range of ad formats — skippable, clickable, closable — and offers rich tracking and highly personalized ad delivery.
• Server-side ad insertion (SSAI) stitches ads directly into the video stream on the server side, before the content ever reaches the player.
• SSAI's primary advantages are resilience against ad blockers and a smoother viewer experience, since no buffering pause occurs at the ad break.

The Rise of Advertising in Video-Streaming Services

Video advertising has long been considered a premium format. It delivers engaging creative to a targeted audience, and it typically commands higher CPMs than display advertising — a meaningful incentive for publishers.

For viewers, it introduces another consumption model: rather than paying full price for an ad-free experience, they can watch the same content at a lower subscription fee, with ads appearing at set intervals.

The primary revenue models in streaming today are:

Subscription video on demand (SVOD) — consumers pay a recurring fee to watch content without ads.

Ad-supported subscription — viewers pay a reduced fee and accept ads at various points, such as pre-roll before content begins.

Transactional video on demand (TVOD) — built on a pay-per-view model, subdivided into electronic sell-through (EST), where a single payment grants permanent access, and download to rent (DTR), where access is time-limited and priced lower.

Advertising video on demand (AVOD) — content is freely available, funded entirely by advertising with no subscription fee required.

What Is Client-Side Ad Insertion (CSAI)?

Client-side ad insertion is a video ad-serving method in which the video player — embedded in an application or website — is responsible for requesting and displaying ads. When a defined ad break point is reached, the player stops the content, makes an API call to an ad server, receives the ad response, displays the ad, and then resumes the content.

All communication with the ad server happens on the client side. The ad selected for display is determined by campaign criteria, which can draw on data about the individual user, the publisher (e.g., the website or app), and the video content being watched.

Main Advantages of CSAI

• Supports a wide variety of ad formats: skippable, clickable, closable, overlay, survey-style, and click-through units.
• Tracking and measurement are detailed and straightforward to implement — VAST enables video players to report on completion rates, clicks, click-throughs, and other key events.
• Highly personalized ad serving is achievable because user, publisher, and contextual data are all accessible at the time of the ad request.
• Functions both with and without ad markers in the content manifest.

What Is Server-Side Ad Insertion (SSAI)?

Server-side ad insertion is a video ad-serving method in which ads are stitched directly into the video stream on the server, before the assembled stream is delivered to the client. The viewer's player receives a single, continuous stream that already contains the ads — it never makes a separate call to an ad server.

The analogy to traditional broadcast television is apt: much like a cable channel's ad break, the transition from content to ad and back is seamless, with no loading pause or buffering delay.

SSAI involves three core functional components:

Manifest manipulation — personalizes the video stream for each viewer by controlling which segments are served.

Ad server communication — manages the interaction between the video origin server and the ad server to retrieve ads and collect campaign performance data (e.g., ad views, impressions).

Ad bitrate and resolution normalization — bitrate is the volume of video data transmitted per second (expressed in kilobits/sec or megabits/sec); normalization ensures ads and content match in quality so there is no visible quality shift at the ad break.

Because all ad stitching occurs upstream on the server, SSAI works even on devices where injecting client-side code is difficult or impossible. Critically, since the client application never makes a direct call to an ad server, standard ad-blocking software has no request to intercept.

Main Advantages of SSAI

• Well-suited for linear streams, live streams, and VOD, with ads stitched directly into the stream and therefore highly resistant to ad-blocking software.
• Once ads are stitched into a piece of video content, that content-plus-ads stream can be distributed across an unlimited number of applications without re-processing.
• Ads can be inserted over a linear slate, eliminating the black-screen gap that can appear with other methods.
• No SDK is required — codeless integration means no ongoing client-side code updates.
• SSAI delivers high ad-fill rates; average ad completion rates on live TV reach 98%, a figure that reflects both the seamless experience and the difficulty of blocking the ads.
• Adaptive bitrate delivery means smooth playback even under varying bandwidth conditions.

CSAI vs. SSAI: A Direct Comparison

Ad Insertion Mechanism

• CSAI: Ads are fetched and inserted by the video player at runtime.
• SSAI: Ads are stitched into the stream on the server before delivery to the player.

Ad Targeting

• CSAI: Supports rich, highly personalized ad serving — user data, publisher context, and content metadata are all readily available at request time.
• SSAI: Targeting capabilities are more limited; the resource demands of real-time server-side stitching make the same level of granular personalization difficult to achieve at scale.

Measurement and Verification

• CSAI: Offers detailed, event-level tracking. VAST enables the video player to report completion, clicks, click-throughs, and other metrics natively, with no extra complexity.
• SSAI: Vendor-reported stitching confirmation is increasingly considered insufficient on its own. There is growing industry demand for independent, client-side verification that the ad was actually delivered and viewed — a meaningful measurement gap that the ecosystem is still addressing.

Viewer Experience

• CSAI: Ad decisions and fetching happen downstream, in the video player. This can result in a brief frame drop or buffering pause at the ad break.
• SSAI: Ad decisions and stitching are handled upstream, by server-side components, before the stream reaches the client. The result is smooth, uninterrupted playback with no visible seam between content and ad.

Choosing Between CSAI and SSAI

CSAI was the dominant model in OTT ad delivery for many years, and its strengths — rich targeting, detailed measurement, broad format support — remain genuinely valuable. SSAI addresses the friction points that CSAI introduces: ad blocking, buffering pauses, and the SDK overhead required on each client platform.

As streaming platforms scale and viewer experience expectations rise, SSAI has been steadily gaining ground in the video ad monetization landscape, particularly for live and linear content where completion rates and seamlessness matter most.

That said, both methods deliver ads that are targeted and contextually relevant to the viewer — a meaningful step forward from traditional broadcast slots, where audience data is imprecise and individual targeting is essentially impossible. The right choice depends on the specific distribution environment, the scale of personalization required, and the measurement infrastructure already in place.