in pooling systems  the counterparty makes a pool of sufficient underlying assets available ( 58 ) owing to operational differences across member states in respect of the procedures for counterparties » delivery of underlying assets to the national central banks ( in the form of a pool of collateral pledged with the national central bank or as repurchase agreements based on individual assets specified for each transaction )  minor differences may occur with regard to the timing of the valuation and other operational features of the margin framework 