If your streaming service runs ads, you have already chosen - or are about to choose - between server-side ad insertion (SSAI) and client-side ad insertion (CSAI). The decision affects your fill rate, your ability to bypass ad blockers, your latency, and ultimately your revenue per session. Here is the comparison.
How each method works
CSAI keeps content and ads as separate streams. When the player hits an ad break, it pauses content, requests a VAST tag, fetches the ad video from the ad server's CDN, plays it, then resumes content. The user sees buffering between content and ads.
SSAI stitches ads into the content manifest server-side. The player sees one continuous stream where ads are indistinguishable from content. Ad decisioning happens at the SSAI server when the manifest is built or refreshed.
Ad-blocker bypass
Browser ad blockers identify ad requests by URL pattern and block them. CSAI requests are easy to block; SSAI requests look exactly like content requests and are nearly impossible to block without breaking playback.
On CTV, ad blockers are rare. On web and mobile, SSAI lifts effective ad impression delivery by 15–35% compared to CSAI.
Latency and viewer experience
CSAI breaks introduce 200ms–1.5s of buffering as the player swaps players or video sources. SSAI is seamless.
On older Roku devices and lower-end Fire TV sticks, CSAI buffer pauses can be jarring. SSAI is the more reliable choice for low-end CTV hardware.
Measurement, viewability, and interactivity
CSAI handles interactive ads (VPAID, IMA SDK overlays) natively. SSAI struggles with VPAID - most SSAI vendors strip VPAID and play only the linear VAST creative.
For viewability measurement (IAS, DV, MOAT), both work - modern SSAI vendors implement Open Measurement (OMID) so verification SDKs can fire from inside the stitched stream.
Cost
SSAI is more expensive to operate. SSAI vendors typically charge $0.05–$0.30 CPM on every ad-supported impression. CSAI is essentially free at the infrastructure level - you only pay your ad server.
But SSAI lifts ad delivery by 15–35% on web/mobile and improves CTV completion rates by 5–10%. For most ad-supported services the math favours SSAI clearly.
When to pick which
- CTV-only service → SSAI. Premium DSPs require it; seamless playback matters more on TVs.
- Mobile-first AVOD with ad-blocker exposure → SSAI for the bypass alone.
- Interactive overlay-heavy ads (gaming, retail) → CSAI for VPAID support.
- Very early-stage, low ad volume → CSAI initially, migrate to SSAI when volume justifies the per-impression fee.
The bottom line
For any serious CTV publisher in 2026, SSAI is the default. It earns more, plays more smoothly, and unlocks the premium DSP demand pool. OTT Engine integrates with leading SSAI vendors (AWS Elemental MediaTailor, Yospace, Broadpeak, Brightcove SSAI) out of the box. Book a demo to compare options for your inventory.
Frequently Asked Questions
What does SSAI stand for?
Server-Side Ad Insertion. The ad server stitches ads into the content manifest before delivery, producing one continuous stream.
Is SSAI worth the cost?
For most CTV and ad-blocker-exposed publishers, yes. SSAI lifts ad delivery 15–35% over CSAI and improves completion rates.
Can ad blockers stop SSAI?
No. SSAI requests are indistinguishable from content requests, making them effectively unblockable.
Does SSAI support VPAID interactive ads?
Most SSAI vendors strip VPAID and serve only the linear VAST creative. If interactive overlays are critical, CSAI is the better fit.
Who are the leading SSAI vendors in 2026?
AWS Elemental MediaTailor, Yospace, Broadpeak, Brightcove SSAI, and Harmonic VOS360 lead the market.