A method, an interconnect and a system for processing data is disclosed. The method comprises the steps of: a) receiving a request to perform a data transaction between a master unit and a slave unit, b) receiving an indication of a quality of service requirement associated with said data transaction; c) determining an interconnect quality of service level achievable when transmitting said data transaction over the interconnect logic having regard to any other pending data transactions which are yet to be issued; d) determining a slave quality of service level achievable when responding to said data transaction once received by said slave unit from said interconnect logic; and e) determining whether the combined interconnect quality of service level and the slave quality of service level fails to achieve the quality of service requirement and, if so, reordering the pending data transactions to enable the quality of service requirement of each data transaction to be achieved. Hence, arbitration between data transactions occurs prior to those transactions being provided to the interconnect. It will be appreciated that this enables pending data transactions to be systematically reordered and the quality of service level for each of these reordered data transactions to be accurately calculated to ensure that the quality of service requirement for each of those data transactions is achieved. Accordingly, this enables all aspects of quality of service to be budgeted together and true end-to-end quality of service may be determined for each data transaction.