How mediation works

Learn the life cycle of a mediation request

For each mediation request from a publisher's mobile app, Ad Manager uses yield groups targeting to determine which ad networks are eligible to compete and then assembles a list, or chain, of ad networks to receive that request in sequence.

1. Publisher's mobile app sends an ad request to Ad Manager

An ad request is sent to the Ad Manager ad server using the Google Mobile Ads SDK and special third-party ad network adapters. Learn more about getting started with mediation.

2. Ad Manager determines which line items and yield groups are eligible for the request

Ad Manager assembles a list of eligible line items and yield groups based on inventory targeting. Learn more about the creation of lists for matching line items and yield groups.

3. Ad Manager creates a chain of ad networks to call in mediation

If one or more yield groups matches the ad request, a single, sequential list, or "chain," of Mediation networks contained in the eligible yield groups is compiled, ordered from highest to lowest expected yield using historical performance data or user-entered CPM values. Ad Exchange uses real-time auction-winning bids.

Mediation sorts ad network tags by CPM in decreasing order, using the estimated CPM for each ad network tag and the real time Ad Exchange CPM.


Ad Network 1 has the highest CPM, and is therefore at the top of the chain.

  1. Ad Network 1 CPM $5.75
  2. Ad Exchange CPM $4.00
  3. Ad Network 2 CPM $3.50
  4. Ad Network 3 CPM $3.25
  5. Ad Network 4 CPM $2.00

The estimated CPM reflects either the "default CPM" entered for each mediation network, or the "dynamic CPM" if automatic data collection is activated. If Ad Manager collects data automatically, the estimated CPM reflects the historical CPM observed for that ad network tag adjusted for impression discrepancies with Ad Manager recorded impressions.

  • Fill rate is only used to calculate the expected value of the entire mediation chain for competition in dynamic allocation. Fill rate is not used to discount estimated CPM for an ad network tag.
  • If the number of competing network tags in a mediation chain exceeds the system limit, Ad Manager uses ad network tag fill rate, CPM and calculated order to discard ad tags and meet the limit with a maximum possible expected value.
  • If Ad Exchange has an eligible bid, the Ad Exchange creative will appear at the end of the mediation chain.
  • Ad network tags with a lower expected yield than Ad Exchange are excluded from the chain.
  • If the Ad Exchange ad’s CPM is higher than all network tags, only the Ad Exchange ad is returned. 
  • If there are no Ad Exchange eligible bids, the mediation chain will only contain ad network tags.

If you’ve setup fallback ads, we recommend you read an overview of mobile fallback ads to understand how it fits in the mediation chain.

4. Ad Manager compares the mediation chain's expected yield with other Ad Manager line items

The mediation chain competes against other Ad Manager line items in dynamic allocation based on its expected yield. Ad Exchange competes with mediation networks as part of a mediation chain. However, if Ad Exchange or another Ad Manager line item returns a higher bid than all mediation networks, the Ad Manager or Ad Exchange creative is returned directly to the publisher and no mediation chain is returned. Ad Manager line items are not included in the mediation chain.

The expected yield of the mediation chain is calculated using each mediation network tag’s CPM and the historical fill rate of all mediation network tags in the chain, including the real-time CPM from Ad Exchange.

This expected yield value is used by Ad Manager when the mediation chain competes with other demand channels, such as Ad Manager line items, in dynamic allocation.


Several yield partners have submitted bids with CPMs of different values, along with Ad Exchange and Ad Manager backfill:

  1. Ad Network 1 CPM $5.75
  2. Ad Exchange CPM $4.00
  3. Ad Network 2 CPM $3.50
  4. Ad Network 3 CPM $3.25
  5. Ad Network 4 CPM $2.00
  6. Ad Manager line item 1 CPM $5.00
  7. Ad Manager line item 2 CPM $4.00

Because Ad Network 1 has the highest CPM Ad Manager would return this mediation chain:

  1. Ad Network 1 CPM $5.75
  2. Ad Exchange CPM $4.00

5. Ad Manager sends the mediation chain to publisher's mobile app

The mediation chain, with Ad Exchange at the end, is returned to the publisher's mobile app.

6. Google Mobile Ads SDK sends a request to each ad network until an ad is returned

Using the returned mediation chain, the mobile app's SDK can then use the network adapters to send a request, or "callout," to each ad network until an ad is returned.

If none of the ad networks in the mediation chain are able to fill the ad request, the request will be unfilled and no automatic passback to the ad server is triggered.

Activate automatic data collection or ensure that each mediation network’s CPM is adjusted periodically according to the bids actually received by each partner.

7. Google Mobile Ads SDK displays the returned ad

The Google Mobile Ads SDK displays the final returned ad in the publisher's mobile app.

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