SSP (Supply-Side Platform)

An SSP is the seller-side counterpart to a DSP. Publishers plug their available impressions into an SSP: Magnite, PubMatic, Xandr Monetize, Index Exchange, OpenX: which exposes that inventory to every DSP simultaneously and runs real-time auctions for each available slot. The highest qualifying bid wins; the ad is served; the publisher collects revenue net of the SSP's fee.

SSPs maximize yield by creating competitive pressure. Instead of a single agency negotiating a fixed rate, every available impression is auctioned across the entire buying ecosystem. A well-configured SSP with healthy demand can increase a publisher's effective CPMs by 20–40 percent versus direct-sales-only monetization, particularly for remnant and long-tail inventory that would otherwise go unsold.