A Guide to Client-Side and Server-Side Ad Insertion

Ad insertion refers to the process of integrating advertisements into a video stream, much like commercial breaks on traditional TV. In the streaming world, ads can be stitched into on-demand videos or live broadcasts in real-time, allowing platforms to monetize their content by selling ad slots to advertisers. This practice is vital for generating revenue, funding content production, and even enabling free or lower-cost streaming options for consumers. In other words, ad insertion is what makes models like free ad-supported streaming (think Pluto TV or YouTube) and hybrid subscription tiers (like Netflix or Disney+ with ads) financially viable.

There are two approaches to inserting ads; Client-Side Ad Insertion (CSAI) and Server-Side Ad Insertion (SSAI). At a high level, CSAI handles ad placement in the user’s playback device or app, whereas SSAI handles it on the server before the content ever reaches the viewer. Each approach has its own workflow, advantages, and challenges. In this guide, I will explore both CSAI and SSAI in depth, covering what “ad stitching” means, how each method works end-to-end, their pros and cons, industry standards involved, business use cases, the impact of ad blockers, and future trends.

What is Ad Stitching?

Ad stitching refers to the manner in which ads are combined with video content, and it is a foundational concept in both CSAI and SSAI monetization. In simple terms, stitching is how and where the ad gets “spliced” into the stream of video. The key difference between CSAI and SSAI is where this stitching takes place in the workflow:

In Client-Side ad stitching, the integration of ads happens on the viewer’s device. The video player itself is responsible for requesting, loading, and inserting the ad into the content stream at the designated break (e.g. a pre-roll before the content, a mid-roll at a cue point, or a post-roll after content). The player fetches ad content from an ad server and then plays it back in sequence with the main video, making the transition appear seamless. Because this happens on the client, CSAI can leverage detailed viewer data (cookies, device IDs, etc.) in real time to request highly targeted ads, and it can even adjust which ads to play on the fly.
In Server-Side ad stitching, the ad is inserted upstream on the server before the stream is delivered to the viewer. A server-side “stitcher” or manifest manipulator fetches the ad from the ad server and merges it with the content stream (transcoding if necessary) into one continuous video feed. The viewer’s device then receives a single, smooth stream that already contains both content and ads, as if it were one video. This approach ensures a consistent, TV-like experience. The viewer isn’t directly aware of separate ad files, and the transitions are handled by the server.

How Does CSAI Work?

CSAI typically relies on the capabilities of the video player and the integration of an ad SDK (Software Development Kit). For example, a common setup is a web player using the IAB’s VAST standard (explained later) or Google’s IMA SDK to request ads. When an ad break is reached, the player will pause the content, make a call to the ad server (passing along whatever targeting data it has, such as user demographics or behavior signals), retrieve an ad video file or stream, and then play that ad in the player’s viewport. After the ad finishes, the player returns to the content stream. The actual ad shown to each user can be highly customized based on real-time data and ad decisioning logic, since the client can tell the ad server who the user is (to the extent privacy rules allow) and request a tailored ad.

CSAI is widely used in web-based video players and mobile streaming apps. It’s the traditional approach to online video ads, dating back to early YouTube and Hulu, where the player itself handles ad playback.

The end-to-end CSAI workflow involves several steps working in concert on the viewer’s device and the ad server side:

Ad Request Trigger: As the content plays, when it’s time for an ad (determined either by a pre-defined cue point or a dynamic decision), the client triggers an ad request. This could be at a pre-roll (start of video), a mid-roll break (e.g., a timestamp or chapter break), or a post-roll (end of video).
Ad Server Call: The video player sends a request to an ad server (e.g., Google Ad Manager, FreeWheel, etc.) typically using a URL that contains information about the content, the ad slot, and targeting data. This is often done via a VAST (an XML or JSON manifest) tag URL or an API call. For example, the player might pause the content and call out: “give me an ad now for user X watching content Y”.
Ad Decision & Response: The ad server, upon receiving the request, runs through its decisioning logic – possibly involving real-time bidding if programmatic demand is involved – and selects an appropriate ad. It then responds to the player with a payload describing the ad to play. In many cases this is a VAST response that includes the ad media URL, duration, tracking URLs for impressions and clicks, etc. The ad might be a progressive video URL or a streaming manifest (HLS/DASH) for the ad creative.
Ad Playback on Client: The player then loads the ad media and begins playback. If the ad is a simple linear video, the player plays it just like it would play any video segment. If interactive elements are present (e.g., via VPAID, which is an API for interactive ads), the player’s ad SDK would enable things like clickable overlays or skip buttons. Throughout playback, the player fires tracking pings (to count impressions, quartiles, clicks, etc.) as specified by the ad server’s response.
Seamless Transition Back to Content: Once the ad is done, the player rejoins the content stream. Ideally, this transition is smooth, resuming the content exactly where it left off. The viewer then continues watching their content until the next ad break triggers another cycle.

Throughout this process, CSAI leverages the client’s resources. The advantage is that it allows for real-time ad personalization – the ad decision can consider very granular, up-to-the-moment data about the user or session. It also means the client has direct insight into ad playback, enabling robust tracking. However, it also means the viewer’s device does a lot of work (pausing content, fetching ads, decoding and playing ads), which can introduce latency or playback hiccups if not handled well.

Pros and Cons of CSAI

Implementing CSAI offers several notable benefits, especially in terms of flexibility and targeting, but it also comes with challenges in user experience and ad avoidance. Below are the key pros and cons:

Advantages of CSAI:

Rich Personalization & Targeting: Because ads are fetched by the client, CSAI can leverage detailed user data to serve highly targeted ads in real time. Advertisers can deliver ads tailored to the viewer’s demographics, behavior, or location on a per-user basis. This real-time ad control means if a user skips or interacts with an ad, the system can adjust subsequent ad choices on the fly.
Interactive Ad Formats: CSAI supports a wide variety of ad formats and interactions since the player has full control. Skippable ads, clickable overlays, companion banners, interactive quizzes or surveys within ads – all these are possible on the client side. Standards like VPAID (for interactive/video ads) or newer frameworks can run in the client, enabling rich ad experiences that simply aren’t possible if the ad is “baked into” the video. For example, a client-side player can show a “Click here to learn more” button on top of a video ad or allow the user to skip after 5 seconds.
Detailed Analytics & Tracking: With CSAI, the platform can get very granular data about ad performance. The player can report when the ad started, how many seconds were watched, if the ad was skipped, if it was clicked, etc., essentially every interaction. This means more accurate metrics for advertisers (impressions, quartile completions, click-through rates, etc.) and easier debugging of issues.
Supports Complex Ad Logic: Client-side logic can handle ad scheduling (e.g., a VMAP playlist of multiple ads in a break), waterfalling (trying a backup ad if one fails), competitive separation (ensuring two ads from rival brands don’t play back-to-back), frequency capping per user, and so on. These complex rules can be enforced by the player or ad SDK since it has knowledge of what’s been shown to the user.

Drawbacks of CSAI:

Vulnerability to Ad Blockers: CSAI’s biggest weakness is that ad content is delivered separately to the client, making it detectable by ad-blocking software. Browser extensions or device-level ad blockers can intercept the ad calls or the ad content and prevent it from displaying. Because the ads are not hardwired into the video stream, blockers can identify ad URLs or known ad-serving domains and simply block those requests. This means users with ad blockers may see a blank where the ad would be, or the content might just skip the ads entirely, undermining monetization. For ad-supported platforms, this is a significant threat to revenue if not mitigated.
Latency and Playback Interruptions: Since CSAI requires pausing the main content to fetch and play ads, viewers might experience a slight delay or buffering at each ad break. If an ad server is slow to respond or the ad file is large, the user sits waiting while nothing happens. “Pausing content to load ads can lead to buffering or latency,” which degrades the viewer experience. On slower devices or connections, the transitions might not be seamless, causing visible buffering spinners or sudden quality shifts that can frustrate users.
Device and Browser Compatibility: CSAI relies on the device’s player capabilities. Not all devices or platforms handle client-side ads equally well. For instance, a smart TV app or game console might have a limited or older player that doesn’t support interactive ads or certain codecs. This can lead to inconsistent ad behavior across devices. Each client platform (iOS, Android, web, Roku, etc.) may require its own integration and testing. Maintaining an ad SDK across numerous app versions and devices can be complex.
Client Resource Consumption: In CSAI, the viewer’s device does the heavy lifting of retrieving and stitching ads in real time. On lower-end devices or in cases of poor optimization, this can tax the CPU/memory and potentially cause video playback to stutter or the app to crash. Essentially, CSAI offloads work to potentially thousands or millions of clients, which can introduce variability in performance.
Tracking Integrity Issues: While CSAI allows detailed tracking, ironically that tracking can also be hampered by ad blockers or if the user’s environment restricts certain scripts. Also, discrepancies can arise (for example, if the player fails to report an impression due to a network drop). In contrast, SSAI (as we’ll see) can centralize tracking. Nonetheless, this is a minor con relative to the others, since CSAI tracking is usually considered more straightforward than SSAI.

What is Server-Side Ad Insertion (SSAI)?

Server-Side Ad Insertion (SSAI) – also known as server-side ad stitching or dynamic ad insertion (DAI) in many contexts is the method of inserting ads into video streams on the server or cloud side, before the content reaches the viewer’s device. In SSAI, the ad is not an afterthought at the player; instead, the streaming server combines content and ads into one continuous stream and delivers it as a single piece to the client.

With SSAI, when a viewer requests a video or tunes into a live stream, the server (or a dedicated ad stitching service) will communicate with the ad server, fetch the ad creatives, and merge them into the content stream (often by manipulating the manifest in HLS/DASH and preparing segments accordingly). The user’s video player then just plays the stream as if it were any other video, oblivious to the fact that some segments are ads. From the viewer’s perspective, ads and content are part of one smooth sequence - much like a traditional TV broadcast where the viewer doesn’t “do” anything special when a commercial comes on.

A hallmark of SSAI is that it often requires transcoding or format normalization on the server. The server-side stitcher will usually ensure that the ad transcoding matches the content stream’s format and bitrate. For example, if you have a 1080p HLS stream at several bitrates, the SSAI system will retrieve the ad and either select or transcode it into the same bitrate profiles so that switching into and out of the ad is as seamless as possible. Many SSAI workflows use manifest manipulation: inserting the ad’s URL segments into the playlist manifest for that user’s session, so the player naturally requests and plays those segments in line with the content.

Because all the ad stitching logic resides server-side, the client’s video player typically does not need special ad-handling code (it just plays a continuous video). This makes SSAI especially attractive for platforms where client capabilities are limited (smart TVs, older devices, streaming sticks). You can deliver ads without requiring a complex ad SDK on the device. SSAI essentially outsources the heavy lifting to the cloud.

How Does SSAI Work?

The SSAI process can be visualized as an assembly line on the server that constructs a personalized stream for each viewer. Here’s an overview of the workflow in a typical SSAI setup:

Stream Request & Manifest Generation: A viewer starts a video (on-demand) or joins a live stream. Rather than giving the same manifest/stream to everyone, the server or CDN edge uses an SSAI service to generate a unique manifest for that session. This manifest includes slots or markers where ads should appear (for live, these might come from SCTE-35 markers in the feed, for VOD from predefined cue points).
Ad Decision on Server: When an ad slot is encountered, the SSAI service (server-side) makes a call to the ad server on behalf of the user. It sends the necessary targeting info (which could be viewer profile data passed from the app, or contextual info like content metadata or geo-IP). Because this is server-initiated, it might have less granular user info than CSAI (unless the app forwarded it), but it can still do household-level or session-based targeting. The ad server returns an ad or a pod of ads to play, typically in the form of a VAST response.
Fetching and Preparing Ads: The SSAI server then retrieves the actual ad video files/streams specified by the ad server. It may need to transcode these ads to match the streaming format. This step is crucial to avoid a noticeable quality or volume change when the stream switches to the ad. In modern implementations, ads are often encoded into multiple bitrates for adaptive streaming, just like the main content, so the user’s adaptive player can continue to adapt during ads.
Stitching Ads into Content Stream: The server inserts the ad content into the stream. For HLS/DASH, this means inserting the ad segments and adjusting the manifest timeline. For example, if the user is watching a live event and an ad break comes, the manifest they receive will include the ad segments next, then return to the live feed segments after the ad break, all time-synchronized. For VOD, the server might serve a single continuous media file or playlist that simply has the ad frames where needed. Essentially, the output is one continuous ABR stream that contains both the video content and the ads, back-to-back.
Delivery to the Client: The viewer’s player receives this stitched stream (often via the CDN). Since it’s a single stream, the player doesn’t pause or make additional requests when an ad comes, it’s just playing what it was given. The transition is seamless: one moment you’re watching content, then the next segment is an ad, then back to content, without extra buffering. This results in an experience where “the transition between the video content and the ads” is smooth and TV-like.
Tracking and Beacons: One complexity with SSAI is how to record that an ad was viewed. Typically, the SSAI service will handle firing the tracking beacons either on behalf of the client or by instructing the client’s player to fire them. Some SSAI solutions send tracking calls from the client side as the ad plays (which can expose them to ad blockers, unless mitigated), while others now do server-side beaconing – meaning the server triggers the impression events so that ad blockers have no client-side call to intercept. We’ll discuss this more in the tracking section, but suffice it to say that SSAI requires some engineering to ensure ads are properly counted.
Playback Continues: After the ad break, the content continues. In a live scenario, the server might have to adjust the stream slightly if the ad break content length didn’t exactly match the slot (e.g., by speeding up catch-up or skipping some non-critical content) so that the user doesn’t fall too far behind “live.” Many SSAI systems manage timing to keep streams in sync.

An example of SSAI in action is a live sports stream where millions of viewers get the same global feed, but each viewer’s stream has ads inserted tailored to their region or profile. The server-side system ensures a UK viewer sees a British ad while a US viewer sees an American ad, all while watching the same event. The viewers experience no additional buffering; the ads feel like they are part of the broadcast.

SSAI solutions are offered by various vendors and often come as part of streaming infrastructure. They can be used for both live and on-demand; live is particularly common because of the need for a TV-like experience at scale.

CSAI vs SSAI: A Comprehensive Comparison

Both CSAI and SSAI ultimately aim to serve ads to viewers, but they do so in very different ways. Choosing one over the other can have significant implications for a streaming service’s performance, revenue, and viewer satisfaction. Let’s compare CSAI and SSAI across key dimensions:

Ad Personalization & Targeting: CSAI allows highly granular targeting on a per-user basis, leveraging cookies or device IDs to serve personalized ads in real time. SSAI can also target individuals, but fine-grained personalization may be limited by what data the server has and the need to scale. As noted, SSAI vendors can personalize streams but face heavier resource demands to do so for every single viewer. In short, CSAI excels at per-user tailoring (e.g. two people in the same household might see different ads), whereas SSAI often targets at a segment level (e.g. all users in a region or subscription tier see the same ad) unless substantial infrastructure is in place for one-to-one personalization.
User Experience (Playback Quality): SSAI is generally better for seamless playback. It minimizes buffering and avoids the jarring transitions that can occur in CSAI. Viewers often can’t tell when content ended and an ad began in an SSAI stream, which is ideal for immersion. CSAI, if not perfectly optimized, can lead to noticeable pauses or even slight “blips” in the video when switching to ads, and on weaker devices it might drop frames. Thus, for pure viewing quality, SSAI has the edge, especially for live events where continuity is key.
Resistance to Ad Blockers: This is a stark difference; SSAI is inherently ad-blocker resistant, while CSAI is easily targeted by them. Many ad-blockers are optimized to block CSAI ads, but are not sophisticated enough to detect SSAI ads. If your audience heavily uses ad blockers (as is common on desktop web viewing), SSAI ensures ads still show up. CSAI might result in a large portion of your ad impressions being nullified by blockers. So for maximizing reach, SSAI wins hands down. (However, note that in app environments like CTV or mobile apps, ad blockers are less of an issue to begin with, so CSAI can suffice there. It’s mainly web streaming where this matters most.)
Ad Formats and Interactivity: If you need interactive ads (clickable, gamified, surveys, choose-your-own-adventure, etc.), CSAI is the way to go. SSAI, as discussed, generally supports only linear video ads without viewer interaction (except maybe simple QR codes or things baked into the video). CSAI can display companion banners, allow click-through on ads, support skippable ads with countdowns, and other rich media experiences. SSAI currently can’t replicate these well because the ad is just a video in the stream. So, the choice might depend on your advertising clients: if they demand interactive VPAID ads or detailed engagement features, CSAI might be required.
Scalability and Infrastructure: CSAI leverages the distributed power of user devices – which generally makes it easier on the service’s own infrastructure (no heavy server processing for ads; you just serve the ad files and content normally). It scales naturally as long as your ad server can handle the requests. SSAI requires significant server-side resources to generate unique streams. That said, modern cloud-based SSAI solutions are quite scalable, but it’s an additional layer that must be managed. Smaller providers might find implementing CSAI via a player SDK simpler than integrating a full SSAI pipeline. On the other hand, large broadcasters who already have robust cloud streaming workflows may handle SSAI at scale using their encoding and packaging infrastructure.
Measurement and Reporting: CSAI provides rich, immediate analytics from the client-side as each event happens. SSAI has to simulate or gather those stats, often with some delay or approximation. There is industry momentum towards better SSAI measurement (such as using the IAB’s Open Measurement SDK in players purely to measure ads even if inserted server-side. Still, advertisers sometimes trust CSAI metrics more because they come directly from the user’s device. There’s also the issue of verification. With CSAI, third-party measurement companies can easily integrate at the client to verify ads served, while with SSAI they often need special accommodations. If precise analytics are a top priority, CSAI has an edge in out-of-the-box granularity.
Content Types and Use Cases: CSAI was traditionally more popular for on-demand streaming and desktop/mobile environments, where personalization and interactivity were valued. SSAI has been prevalent in live streaming and connected TV use cases, where a TV-like experience and reliability are key. We’re now seeing convergence, but if you have a live linear FAST channel with millions of concurrent viewers, SSAI is usually the go-to. For an interactive web series with choose-your-own-ad breaks, CSAI would be necessary.

Ultimately, many streaming platforms evaluate all these factors; user base, device distribution, ad products, and technical resources to decide which insertion method (or combination) is best. There is no one-size-fits-all answer; some have even implemented both, using CSAI in certain apps and SSAI in others, or migrating from one to the other as their needs change. But CSAI remains very much in use, especially where personalization and interactive ad capabilities are paramount. In fact, as we’ll discuss in the Future section, hybrid approaches are emerging to get the best of both worlds.

Ad Standards and Placements Delivered with CSAI and SSAI

When implementing ad insertion, certain industry standards and formats ensure compatibility between ad servers, video players, and stitching services. Likewise, best practices have emerged for where to place ads in content. Here we outline key ad serving standards and typical ad placements that apply to both CSAI and SSAI workflows:

Common Video Ad Serving Standards:

VAST (Video Ad Serving Template): VAST is an XML/JSON schema defined by the IAB that standardizes how video ads are delivered from an ad server to a video player. A VAST response essentially tells the player (or server, in SSAI’s case) what ad to play, the media file URL or streaming URIs, the duration, and tracking URLs for various events. VAST is crucial because it ensures any player can understand any ad server’s response as long as both follow the standard. Both CSAI and SSAI rely on VAST as the lingua franca in CSAI the player parses VAST, in SSAI the stitching server parses it to know which ad to fetch and how to report on it. Modern VAST (version 4.x as of 2025) also supports things like ready signals for interactive ads, mezzanine files for quality, etc. It’s the baseline for virtually all video ad integrations.
VMAP (Video Multiple Ad Playlist): VMAP is another IAB spec, used to define a playlist or schedule of multiple ads within a video stream. Think of VMAP as a way to say: “this video will have an ad at 00:05:00 (5 minutes in) and another at 00:10:00, etc.” It’s essentially a timeline for ads. VMAP is often used for VOD content by CSAI implementations. The player reads a VMAP and knows when to cue ads. In SSAI, the server could use VMAP to plan out where to stitch ads in a long video. VMAP may work in conjunction with VAST (each ad break entry in VMAP points to a VAST tag for what to play at that break). It gives content owners control over ad placement in their content while still deferring to ad servers for the actual ad decision.
VPAID (Video Player Ad-Interface Definition): VPAID is a (now somewhat legacy) standard for enabling interactive and scriptable ads in video players. With VPAID, an ad can be a little piece of code (often JavaScript for web players) that runs in the player to create interactive elements or advanced tracking. This was widely used for rich media ads (like overlaying products, mini-games in ads, etc.). In CSAI, a player can support VPAID to allow these fancy ads to run. However, VPAID has downsides (security issues, not mobile-friendly) and importantly SSAI cannot effectively use VPAID because SSAI doesn’t run a player for the ad, it just treats it as video. Most SSAI workflows will skip or ignore VPAID ads, or they have to “burn in” a simpler version of the ad. The industry has been moving away from VPAID, introducing newer specs like SIMID (Secure Interactive Media Interface Definition) to handle interactivity in a safer way. But as of 2025, many ad buyers still use VPAID for certain campaigns. If a platform uses SSAI exclusively, they often have to tell advertisers not to use VPAID creatives (or the ads will be served as linear video without the interactive part).
SIMID and OMID: SIMID is the successor to VPAID, focusing only on the interactive element and not controlling the video playback. It’s designed to be friendlier to both CSAI and SSAI (the idea is SIMID code could run in parallel with a video even if it’s stitched). OMID (Open Measurement Interface Definition) is a standard for measurement – ensuring that things like viewability and verification can be done consistently whether ads are client or server. Many SSAI providers integrate OMID signals so that third-party measurement is possible even with server-stitched ads. These are part of the evolving ad tech standards that complement VAST/VMAP.

Typical Ad Placement Types in Video Streams:

Pre-Roll Ads: These are ads that play before the viewer’s selected content starts. Pre-rolls are ubiquitous in both CSAI and SSAI. The viewer hasn’t seen any content yet, so attention is usually high – making pre-rolls valuable. However, if made too long or frequent, they could annoy users before they even get to the content. Technically, pre-rolls are straightforward: the player knows to request an ad as soon as play is initiated (CSAI) or the server prepends an ad to the content (SSAI). Pre-roll ads are typically short (15-30 seconds, sometimes up to a minute).
Mid-Roll Ads: These ads play in the middle of content, during a natural break. For on-demand videos, mid-roll break points might be set by the content publisher (e.g., after scene transitions or chapters) or dynamically inserted every X minutes for longer content. For live streams, mid-rolls occur during timeouts or intermissions (like half-time in a game, or between program segments). Mid-rolls are often inserted in clusters (ad pods) especially for long-form content (e.g., a 3-minute ad break composed of several spots). They tend to have higher completion since the user is already invested in the content, but if overdone, they can cause viewers to drop off. From an insertion standpoint, mid-rolls require coordination: CSAI players either monitor timestamps or listen for server cues (like SCTE-35 markers) to know when to trigger an ad; SSAI systems often rely on those same cues or pre-set markers to splice ads in. Mid-rolls are powerful for monetization but the most sensitive for user experience because they interrupt content. That’s why ensuring they start quickly and transition smoothly (especially with SSAI) is key.
Post-Roll Ads: Ads that play after the main content has finished. Post-rolls capture the viewer at the end of their viewing session. Not all users stick around for post-roll ads (many might close the player once the show or clip ends), so post-roll impressions are generally lower. However, they’re less intrusive to the experience since they don’t interrupt content at all, the story is over, and then an ad plays. Some platforms auto-play another video after the current one, in which case a “post-roll” might effectively become a pre-roll to the next video. In terms of insertion, post-roll in CSAI is simply triggered when content playback ends; in SSAI, the server can append an ad to the end of the content stream. Post-rolls are common on short-form content and in dedicated video players (like after a news clip, you see a post-roll ad). For long content or binge-watching, their use is a bit less, many prefer to serve another pre-roll for the next video instead.
Overlay Ads and Companions: These are not “in-stream” video ads but related placement types. Overlay ads are those semi-transparent banners that might appear at the bottom of the video (often in the first 1/3 of content in some free services). Companion ads are banners or text that show up next to the video (on web) in sync with the video ad. These are typically only relevant in CSAI (since SSAI doesn’t have a concept of an overlay image on the client). They require player support to display while the content continues playing. I mention them because they’re part of a holistic ad strategy and a reason one might favor CSAI in a desktop web scenario. You can show a companion banner on the page alongside the video ad for extra monetization.

In both CSAI and SSAI, the ad server and content publisher coordinate to ensure the right number and timing of ads. Too many mid-rolls, and viewers leave; too few, and you leave money on the table. Industry standards like VMAP help orchestrate this, and analytics help tune the frequency. From a tech perspective, CSAI players must handle pausing and resuming around these placements, and SSAI servers must handle seamlessly splicing at those exact points. Both methods use the same taxonomy of ad slots (pre, mid, post), but implement them differently under the hood.

The Impact of Ad Blockers on CSAI and SSAI

Online viewers increasingly use ad blockers to avoid advertisements, and this has direct implications for CSAI and SSAI:

For Client-Side Ad Insertion (CSAI), ad blockers are a well-known Achilles’ heel. Since CSAI involves the player reaching out to an ad server or loading ad content, those network calls and the ad assets can be identified by blocking software. Browser extensions like uBlock Origin or Adblock Plus maintain lists of known ad-serving domains and can intercept requests to those domains, preventing the ad from ever loading on the client (at least before Chrome updates began limiting the effectiveness of these extensions). They can also hide or remove HTML elements (like an iframe or video element) that are identified as ads. In practice, this means if a user has an ad blocker, when the CSAI player tries to get an ad, it might receive nothing – resulting in either a blank slot or the player immediately jumping back to content. From the user’s perspective, they see no ad; from the platform’s perspective, that’s lost revenue. Ad blockers can also block tracking beacons, so even if an ad plays, the impressions might not be recorded if those tracking URLs are filtered.

Modern ad blockers are quite sophisticated on web browsers, and they specifically target CSAI patterns (such as calls to well-known ad server endpoints, or the insertion of VAST XML). They generally do not target the video content itself, only the separate ad content. As a result, CSAI strategies have had to evolve: some publishers use domain sharding or proxy techniques to try to disguise ad calls, but it’s a cat-and-mouse game. There’s also the approach of detection – if the player detects an ad blocker, it might prompt the user or switch tactics. One interesting hybrid solution from a provider (Brightcove) is to automatically fall back to SSAI if a blocker is detected. When the player detects an ad blocker, it automatically requests ads from the server instead. This way, even a user with an ad blocker ends up getting the ad via SSAI (because the ad blocker can’t easily distinguish it). However, implementing such dual logic can be complex and is not universally available.

For Server-Side Ad Insertion (SSAI), ad blockers are far less effective. Because the ad is part of the continuous stream, a blocker would have to block the entire video stream to stop the ad, which of course means the user can’t watch the content either. In most cases, ad-blocking software doesn’t do that, because it can’t easily tell which video segments contain ads versus content. Thus, SSAI is often touted as “ad blocker-proof.”

Ad Servers and Their Role in Ad Insertion

An ad server is the brain of the ad delivery process. Whether you’re using CSAI or SSAI, an ad server is typically the component that decides which ad to show, delivers the ad creative or tag, and tracks the delivery. In the context of video streaming ad insertion, the ad server’s role is pivotal as it bridges the content platform and the advertiser demands.

Key roles of an ad server in the insertion workflow include:

Ad Decisioning: When a video player (CSAI) or server (SSAI) requests an ad, the ad server is responsible for selecting the best ad to fill that request. It looks at the targeting parameters (e.g., user location, device, content genre, time of day) and available campaigns in its system. It may run through direct sold ads first, then maybe call out to programmatic exchanges if needed (this is often called an “ad waterfall” or header bidding in digital ads). Ultimately it returns a decision: e.g., serve a 30-second car ad from Campaign A. This decision could involve complex logic, frequency capping (making sure the user hasn’t seen it too many times), pacing for the campaign, etc. In other words, the ad server matches the right ad to the right viewer at the right time – a process sometimes called yield optimization or ad targeting.
Storage and Delivery of Ad Assets: Ad servers either host the ad creatives (videos) or provide URLs to where they are stored (an ad content delivery network). For instance, if the chosen ad is a nationwide commercial, the ad server either directly provides the media file URL in the VAST response or a redirect to another server that has it. In SSAI, the stitching service will fetch from that URL; in CSAI, the player will fetch it. Modern ad servers often integrate with CDNs to ensure the creative can be delivered quickly to the user once chosen.
Ad Formatting and Compatibility: The ad server ensures the response is in a format that the insertion system expects – typically a VAST or a VMAP. It may also indicate if a VPAID (interactive) component is available or not. Essentially, it packages the ad decision into a standardized reply. If multiple ads need to play in sequence (ad pod), the ad server might supply a VAST with a or multiple entries, or use VMAP to outline them.
Ad Tracking and Measurement: Ad servers are also responsible for counting and tracking the delivery of ads. They provide tracking URLs for impressions, quartile milestones, clicks, etc., which either the client or the SSAI service will call when those events happen. The ad server logs those to produce metrics for advertisers. In CSAI, the player pings those URLs (which often go back to the ad server or a third-party tracker); in SSAI, either the server pings them or it passes them to the client to ping. This reporting is crucial for billing and analytics. Ad servers often aggregate this data to show how many impressions were delivered, completion rates, click-throughs, etc. If an advertiser is using a third-party ad tracking service (like an ad verification company), the ad server will include those third-party trackers in the VAST as well.
Frequency Capping and Sequence Management: The ad server ensures users don’t see the same ad too frequently (if the advertiser set a limit) and can also deliver sequenced ads (e.g., Part 1 of a story in one break, Part 2 in the next). To do this, it keeps track of user exposure via cookies or device IDs. In CSAI, this is straightforward (the ad server reads a cookie or identifier from the ad request). In SSAI, often a user ID has to be passed to the ad server, since all requests come from the server’s IP otherwise the ad server might not distinguish users. Many SSAI setups use a unique session or viewer ID in the ad call to allow the ad server to apply frequency capping and targeting properly.
Integration with Content Management: The ad server usually interfaces with the video platform’s CMS or scheduling system. For live, an ad server might be triggered by SCTE-35 markers (via a middleware that translates those into ad calls). For VOD, the content system might instruct the player or SSAI service where ads should go and provide corresponding ad tags for those slots. The ad server’s logic can depend on content metadata. For example, not showing competitor ads during certain content, or adjusting ads based on content ratings (no alcohol ads on kids content, etc.). These rules can be configured in the ad server’s campaign settings.

Examples of popular ad servers include Google Ad Manager (formerly DFP), FreeWheel, SpringServe, SpotX (as an SSP with ad serving capabilities), Smart AdServer, etc. These are companies focusing on CSAI/ads management. On the SSAI side, some providers like Google have a specific “DAI” server, and others like AWS MediaTailor integrate with your ad server by acting as a client to it.

Ad Tracking and Analytics for Ad Insertion

Measuring the performance of ads is just as important as delivering them. Ad tracking and analytics provide the data that shows whether an ad campaign is effective, how viewers are interacting with ads, and how much revenue is being generated. Both CSAI and SSAI approaches need robust tracking, but they implement it somewhat differently.

Key metrics and data points collected via ad tracking include:

Ad Impressions: The basic count of how many times an ad was viewed. Typically counted when an ad starts (or after a few seconds to ensure it wasn’t immediately skipped). This is usually triggered by an “impression beacon” - a URL that gets pinged when the ad begins playback. Every VAST contains an <Impression> tag URL for this purpose.
Quartile Completions: It’s standard to track 25%, 50%, 75%, and 100% of the ad viewed. These quartile events indicate how much of the ad the viewer watched. If a lot of viewers drop off before completion, that might indicate ad length or placement issues. VAST provides <FirstQuartile>, <Midpoint>, <ThirdQuartile>, and <Complete> tracking URLs which the player or SSAI service triggers accordingly.
Clicks or Engagements: If the ad is clickable (usually CSAI only, since SSAI ads may not allow click-through), tracking how many click-throughs occurred is crucial for direct-response advertisers. A <ClickTracking> URL in VAST is pinged when the user clicks, and usually the ad will open a landing page.
Viewability and Audibility: Advertisers often care if the ad was actually viewable (on screen) and with volume on. In video, viewability means the video was in the viewport and played for at least 2 seconds with 50% of pixels in view (per MRC standard). Some analytics can capture this, often via the Open Measurement SDK or player callbacks. Audibility metrics (was sound on?) are also tracked in some cases.
Error reports: If an ad fails to play (maybe due to file error or timeout), the ad tech will log an error code. This helps diagnose issues with ad delivery.

The way these metrics are collected:

In CSAI, the video player’s ad plugin triggers these tracking URLs at the appropriate times. The data then is aggregated by the ad server or third-party. Many players also integrate with analytics platforms (like Conviva, Mux, or Google Analytics) to send custom events for ads (like “Ad started at 10:05:23 for user X”). This allows correlating ad performance with user behavior (did an ad cause drop-off? etc.). Content providers can see these stats sometimes live - e.g., number of concurrent ad views at this minute, etc.
In SSAI, tracking is a bit trickier. If the client isn’t aware of ad boundaries, how to know when to ping the trackers? SSAI solutions handle this by either instructing the client with markers or by doing server-side tracking:
- Server-side beaconing: The SSAI service itself fires the impression and quartile beacons on behalf of the user. Since it knows when it stitched the ad and how far the user likely watched (based on segment requests or playback polling), it can simulate those events. This is invisible to the client and unblockable by the user.
- Client-side tracking with cues: Some SSAI implementations insert timeline metadata (ID3 tags in HLS or DASH Event Messages) to signal ad start/stop to the player. The player (if it supports it) then can trigger trackers like a CSAI player would. This requires the player to be a bit SSAI-aware, but many modern players do support basic SSAI cue ingestion.
- Many SSAI providers also integrate with the Open Measurement SDK (OMSDK) on client, which can detect when an ad is playing in the video even if it’s stitched, by recognizing patterns or watermarks. This allows third-party verification of viewability.

The analytics from SSAI might be aggregated by the SSAI platform and shared with the ad server. There can be minor discrepancies between what an SSAI service reports and what the ad server sees, if, say, a client didn’t actually play an ad to completion (maybe stream was closed) but the server assumed it did. To mitigate this, some SSAI feed real playhead data back (if the player provides it).

Apart from ad-specific metrics, platforms also look at:

Ad Break Conversion: Did the viewer stay through the ad break or did they leave the stream when an ad started? This is important for user experience evaluation.
Ad Duration and Completion Rate: If using skippable ads (CSAI scenario like YouTube-style), what percentage skip vs. complete?
Revenue Metrics: e.g., ad revenue per user or per hour of content (especially in AVOD, how much $ do we make per streaming hour – a metric known as “Ad ARPU”). This is derived from ad impressions * CPMs (cost per thousand) of those ads.

Looking ahead, tracking is getting even more sophisticated with cross-device attribution (did seeing an ad on TV lead you to search on your phone later?), but that’s beyond the scope of insertion itself. For our purposes, a strong analytics foundation is a must-have for any CSAI/SSAI deployment to iterate and succeed.

The Importance of Video Players and Why Their Capabilities Matter

The video player is the software component that actually presents content (and ads) to the viewer. Its capabilities set the ceiling for what can be achieved in ad insertion on the client side. A robust video player is especially crucial for CSAI, but even for SSAI it plays a role in handling streams and tracking.

Key player capabilities and features that impact ad insertion include:

Ad Tag Support (VAST/VMAP/IMA): The player (often via a plugin or SDK) needs to understand ad tags. In CSAI, that means parsing VAST responses, handling VMAP schedules, and possibly working with specific SDKs like Google IMA (Interactive Media Ads SDK) which simplifies some of this. If a player can’t parse a VAST, it won’t know how to play the ad. Most modern players (Video.js, JW Player, Bitmovin Player, THEOplayer, etc.) have built-in support or extensions for VAST/VMAP. Some native platforms (like some smart TV environments) might not, requiring custom development.
Handling Different Ad Formats: A capable player should support linear video ads, nonlinear ads (like overlays), and if needed companion banners. For example, if a VMAP says show a banner ad while the video plays, does the player have a UI element for that? For interactive (VPAID/SIMID) ads, can the player execute the ad’s code in a secure way? Not all players can, which might restrict what campaigns you can accept. The player should also handle skippable ads (display a skip button at the right time).
Ad Scheduling and Pod Management: The player often needs to manage a sequence of ads (an ad pod) within a break. This includes playing multiple ads back-to-back seamlessly. It should also gracefully handle when an ad fails (e.g., if one ad file 404s, skip to the next rather than break the whole stream). Scheduling is often driven by VMAP or business rules, and a good player can either read those or be configured to call ads every X minutes if needed.
Seamless Switching Between Content and Ads: The player’s ability to switch media streams or insert ads without hiccups is crucial. In HTML5 environments, this might mean using Source Buffers for content and ads, or using a single Media Source and splicing. Some players use two video tag elements (one for content, one for ads) and swap, which can cause flicker if not done well. High-quality players have optimized this transition to minimize or eliminate any flicker or audio pop.
Adaptive Bitrate (ABR) Support for Ads: The player should handle adaptive streaming for ads just like content. Many CSAI implementations leverage the same ABR logic - meaning the player doesn’t care if a segment is ad or content, it just sees the next URL. But if using something like a Google IMA SDK, the SDK might take over during ads and manage its own ABR. Ensuring the player/SDK picks the right quality is important. Poor ABR handling might lock the ad at one quality (e.g., always start at lowest quality every ad, which looks bad).
Integrations for Analytics and Beacons: The player should be able to fire tracking events at precise times (quartiles etc.). A well-designed player will emit events or callbacks for “ad started”, “ad first quartile”, “ad completed” which can be tied into analytics scripts. Many players offer an API for these. For example, video.js has plugins that trigger VAST event callbacks.
Ad Skipping and Controls: If the service allows skipping ads after X seconds, the player must display a skip button and enforce the countdown correctly. If it allows pausing ads or not (some do, some don’t), the player’s controls might need to be disabled during ads or changed. For interactive ads, it might show a clickable area or allow clicking the video. All these UI/UX aspects are implemented at the player level.
Fallback Behaviors: Good players handle scenarios where ads can’t be filled. For instance, if the ad server returns an empty response (no ad), the player should simply continue the content after a minimal delay, so the user isn’t left hanging. Or if an ad fails mid-way, perhaps resume content early. These behaviors affect user experience significantly.

For SSAI, the requirements on the player are lighter:

The player just needs to be a solid HLS/DASH player that can play the combined stream without choking on transitions. SSAI streams sometimes have discontinuities or timeline jumps (like after an ad, PTS timestamps might jump if using server-side stitching). Players should handle those, though most do handle HLS discontinuity tags and DASH period transitions these days.
The player should still ideally be able to report some information for tracking (maybe through ID3 tags). If the SSAI relies on the player to do beaconing, the player must parse those cue tags and act on them.
In SSAI, a player doesn’t need a full ad SDK; a basic streaming player works. That’s why SSAI is great for platforms where you can’t embed an ad SDK. But if you want advanced features like skipping an ad, the player would need custom logic to, say, fast-forward the stream - which usually isn’t allowed by the service.

Many companies provide unified player SDKs that handle a lot of these complexities out-of-the-box. For example, the Bitmovin Player or THEOplayer or JW Player’s SDK can do both CSAI and SSAI, and come with built-in support for the IAB standards, ad pods, etc. They advertise features like “ad preloading” (to fetch the next ad in background to avoid delay) and “ad error handling”.

Streaming services often invest in a good player SDK or develop one in-house if needed. On some platforms, they might be constrained (e.g., YouTube on some smart TVs might rely on the TV’s built-in player API which could be limited), but generally, control over the player equates to control over the ad experience. It’s the last mile that can make or break ad monetization - after all, if the player fails to play an ad correctly, all the backend magic is for naught.

Companies Specializing in CSAI and SSAI Ad Technologies

The ad tech ecosystem for video is populated by various vendors and platforms. Some companies focus on client-side ad delivery and management, while others specialize in server-side stitching solutions. Here are some key players (as of 2025) in each category:

Companies focused on Client-Side Ad Insertion (CSAI):

Google Ad Manager (DFP/IMA): Google’s ad server (formerly DFP) is ubiquitous for CSAI, and their IMA SDK is widely used in players to handle VAST ads on client-side. Google also offers a DAI solution, but historically they are strong in CSAI for web/mobile.
FreeWheel: Now part of Comcast, FreeWheel is an ad server and suite often used by broadcasters and premium video publishers. It’s known for robust decisioning and is often used in CSAI setups for OTT (e.g., many cable channels’ apps).
SpotX (now part of Magnite): SpotX started as a video ad network/SSP but offers client-side SDKs and ad serving primarily for connected TV and OTT. They often work in CSAI contexts by providing ad decisions and facilitating programmatic demand.
SpringServe: A leading ad server specifically in the OTT/CTV space (recently acquired by Magnite as well). SpringServe is used by many streaming platforms to manage CSAI on CTV devices, known for its lightweight and fast decisioning geared to video.
Publica: Another ad platform (acquired by Integral Ad Science) that provides header bidding and ad decisioning for CTV. It’s often used to maximize yield in client-side inserted ads on smart TVs by running auctions among multiple demand sources.
EPOM: A smaller ad tech company providing ad serving solutions which can be used for video. Not as industry-common as the above but serves niche clients.

These CSAI-focused companies empower publishers to serve targeted ads with detailed control and integrate with rich media. They might not physically insert the ad (the player does that), but they provide the tooling and SDKs to make CSAI work. For instance, Google’s IMA SDK is a piece of client software to request and play ads (in fact it can also do server-side stitching now in some contexts, but it’s mostly known for client use).

Companies (and products) focused on Server-Side Ad Insertion (SSAI):

Yospace: A pioneer in SSAI, Yospace (owned by RTL Group) provides a server-side stitching platform used by many major broadcasters, especially for live sports and simulcast of TV channels. Yospace is known for its robust scaling and having deployed SSAI for events like the World Cup, with strong personalized ad capabilities and ad tracking features.
AWS Elemental MediaTailor: Amazon Web Services offers MediaTailor as a cloud service for SSAI. It can take in a stream and a VAST tag and output a stitched stream with ads. It’s fairly easy to integrate and works well with AWS’s broader video services. MediaTailor can also do client-side, but its main purpose is server-side personalization at scale.
Google DAI (Dynamic Ad Insertion): Google offers a server-side ad insertion service, often used in conjunction with Google Ad Manager. It’s commonly used by broadcasters (e.g., those who use Google Ad Manager for linear streams). Google DAI handles stitching in the cloud and was one of the earlier solutions for server-side insertion in big events like Olympics streaming.
MediaKind (Prisma): MediaKind (formerly Ericsson Media Solutions) has SSAI technology under the Prisma suite. They target telecom and cable providers moving to streaming, offering a server-side ad solution that integrates with broadcast systems.
Nowtilus: A provider of SSAI solutions (with their product “AI.band”) focusing on personalized ad insertion. They often highlight advanced targeting and seamless integration with various OTT platforms.
Broadpeak: A company specializing in video delivery networks, Broadpeak has an SSAI solution and has been innovating in targeted ad insertion, especially in Europe. They often integrate SSAI with their CDN and multicast offerings, aiming at efficient ad delivery.

Additionally, some video platform companies provide integrated SSAI:

Harmonic: With its VOS platform, Harmonic enables SSAI for stream personalization.
Brightcove: They have SSAI services (having acquired Ooyala’s SSAI tech and integrating it).
Azure (Microsoft): Azure Media Services had an SSAI component as well.

Between these lists, some companies do both CSAI and SSAI: for example, Google straddles both, as does FreeWheel (it has an SSAI product too, called FreeWheel Hybrid Ads). But generally the ones listed under SSAI are known primarily for stitching tech, while those under CSAI are more about the ad decisioning and client player integration.

It’s also worth noting video player companies are not ad insertion companies per se, they partner closely. They provide the player tech that works with CSAI (via any ad server) or SSAI (by playing stitched streams). Some have their own SSAI demos or even lightweight SSAI solutions (e.g., THEOplayer had server-side ad insertion in some docs). But typically, they integrate with the above companies.

Finally, “platform” companies (e.g., Roku, Hulu) have their own internal ad tech. Roku has a Video Ads SDK for CSAI on their platform. Hulu developed SSAI for live TV. But those aren’t available as services to others, so I have focused on vendors.

The Future of Ad Insertion in Video Streaming

As we look towards the future (the coming years beyond 2025), ad insertion technology is poised to evolve in several exciting ways. Streaming services, advertisers, and technology providers are all innovating to improve monetization while maintaining viewer satisfaction. Here are some key trends and predictions for the future of CSAI and SSAI in video streaming:

AI-Driven Personalization: Artificial intelligence and machine learning will play a growing role in ad tech. AI can analyze vast amounts of user data (within privacy limits) to improve targeting decisions - picking not just an ad relevant to a user, but the type of ad format and when to show it for maximum impact. AI might also optimize ad creatives on the fly (e.g., choosing different ad lengths for different viewers or even editing ads to be more relevant). In insertion terms, AI could help decide the optimal moments for dynamic ad breaks in content where breaks aren’t fixed, keeping viewers engaged. It can also predict if a certain viewer is likely to disengage at an ad and adjust frequency accordingly. We can imagine a future where two people watching the same show not only get different ads, but completely different ad experiences (like one gets a single 60-second ad, another gets two 30s, based on their tolerance), orchestrated by AI.
Contextual Ad Insertion & Scene Recognition: With privacy concerns growing, contextual targeting is making a comeback. Future ad insertion might leverage content analysis using AI to understand what’s happening in the video (Is it an action scene? Who is the actor? What’s the genre?) to serve contextually relevant ads without needing personal data. For instance, an SSAI service might dynamically insert a sports equipment ad when it detects a sports scene in a movie. This could also help determine when to place the ad: e.g., finding a low-tension moment to break in. Such scene-aware ad insertion would make ads feel less intrusive. Some companies are already exploring this, analyzing frames in real-time for brand safety and targeting signals.

As we head into the future of streaming, ad insertion technology will continue to evolve towards being more intelligent, transparent, and user-friendly. The end goal for streaming providers is to sustainably generate revenue without driving away viewers, and that means the ads we see will be increasingly well-integrated and relevant. In the coming years, expect your streaming apps to know when and how to serve you ads in a way that you barely notice the “insertion” at all, whether it’s powered by CSAI, SSAI, or innovations yet to come. The competition for audience attention is fierce, and the services that strike the right balance in ad insertion will be the ones to thrive in the streaming economy.