What is an SSP (Supply-Side Platform)?

An SSP (Supply-Side Platform) is software site or app owners use to sell ad impressions to many buyers at once. The SSP connects slots to exchanges and DSPs, collects bids, picks a winner, and serves the creative. It is the pair to a DSP: the DSP bids on demand, the SSP offers supply. Both sit inside programmatic advertising.

The publisher drops an ad tag or SDK. When a page or app screen loads, the SSP packs slot parameters (size, URL/bundle, GEO, consent) and sends bid requests. Buyers reply with bids; the SSP (or exchange) runs the auction and records the win. Publisher revenue is eCPM and fill rate: what share of requests became a paid impression, and how much a thousand impressions earned.

What an SSP does

Demand access. One tag instead of dozens of network pixels. A slot can take open auction, PMP, and programmatic guaranteed.
Price control. Floors (minimum bid), floors by GEO and format, priority for direct deals over the open auction.
Buyer competition. Several DSPs on one impression usually lift eCPM versus selling to one network at a fixed CPM.
Controls. Block creative categories, competitor ads, unwanted formats; report by domain, app, country.

Formats match the buy side: display, video, native, in-app (banner, interstitial, rewarded). On mobile the SSP is often paired with mediation: the SDK auctions networks and programmatic demand together. A small site often sits in AdSense or YAN and never opens a standalone SSP console; an independent SSP shows up when the publisher wants several exchanges, its own floors, and PMPs — not one network as the only middleman.

SSP, exchange, and publisher

In a clean diagram the publisher owns inventory, the SSP is the seller, and the ad exchange is the marketplace where DSP bids meet. In production the lines blur: a large SSP runs its own exchange; Google sells through AdX and through publisher products. For a buyer that means the same domain can arrive from different SSPs with different fees and spoofing risk.

Header bidding (a separate topic) changes the order: the publisher’s browser or server asks several SSPs/exchanges before the ad server, instead of a waterfall (“network A first, then B”). Waterfall left leftovers to cheaper networks; a parallel call raises competition for the same impression.

Money and quality

The publisher sees a gross bid and a net after SSP and exchange fees. The gap is the chain’s take rate. Ads.txt / app-ads.txt on the publisher side lists who may sell the domain; without it, DSPs often drop the request as unauthorized.

For an arbitrage buyer the SSP is not a buying console. It is the inventory path they see inside a DSP: app bundle, domain, SSP name in the log. A low publisher eCPM is not the same as a cheap, clean offer click: remnant and in-game slots can be both cheap and dirty. Buyers check post-click CR, bot traffic, and vertical fit.

What an SSP is not

Not a DSP. A DSP spends the advertiser’s budget. An SSP maximizes publisher revenue. If a vendor ships “DSP + SSP,” those are two products, not synonyms.
Not the publisher ad server. An ad server (Google Ad Manager and peers) decides which line item or programmatic channel fills the slot. The SSP is a sales channel, often wired into that ad server.
Not a contextual-advertising account. Direct and Google Ads buy inside their own networks (YAN, GDN); a publisher joins those via AdSense/YAN, rather than “buying an SSP.”
Not RTB. RTB is the auction for an impression. The SSP runs that auction or sends inventory into it; some SSP deals are a fixed price with no open bid.