Contract number: [ complete ]  Service contract conditions of December 2018  ‘Web services’ : machine-to-machine system allowing the  contractor  to receive or exchange  electronic business documents such as quotations,  orders, dispatch and receiving advices  or  invoices , either through a direct interface or a  third-party service provider. 3. Subject  3.1.  The contracting parties ( ‘ the parties ’ ) undertake to take the necessary measures to  mutually use electronic means for communication in the areas indicated in section 1. The  electronic documents will be exchanged using the  e-PRIOR  platform, using the  web services or the  supplier portal  system. Access to the  e-PRIOR  platform may also go through an access  point of an interoperable network such as the Open PEPPOL network.  3.2. The  contractor will agree with the  contracting authority on which of the above- mentioned means will be used. The  contractor  shall communicate his choice not later than 10  working days after the activation referred to in Section 1.2 of the present agreement.  4. IMPLEMENTATION AND MAINTENANCE OF THE ELECTRONIC SYSTEMS  4.1.  When using  web services , the parties undertake to take the necessary measures to  implement and maintain electronic systems that enable the effective use of electronic means  to exchange electronic documents. The  electronic systems are specified in the  interface  control document . The  contractor  must take the necessary technical measures to set it up at its  own cost. The parties undertake the necessary measures to implement and maintain safety  measures and procedures in order to adequately prevent messaging incurring any delays,  messages being corrupted as to content or form, or messages being lost. Similarly, the parties  shall strive to prevent third parties from obtaining unauthorised access to messages. The  safety measures and procedures are defined in the  interface control document .  If the use of safety measures and procedures leads to the rejection or detection of one or more  errors in an  EDI message , the receiver shall inform the sender thereof as soon as possible, but  at the latest within 2 (two) working days.  When a change in the  interface control document  requires adaptations, the  contractor  has up  to six (6) months from receipt of the notification to implement this change. This period can be  shortened upon mutual agreement of the parties. This period does not apply to urgent  measures required by the security policy of the  contracting authority aimed at ensuring  integrity, confidentiality and non-repudiation of information and availability of  e-PRIOR ,  which must be applied immediately.  4.2  When using the supplier portal  the  contracting authority  takes the necessary measures  to implement and maintain electronic systems that enable the effective use of the  supplier  portal .  4.3. If electronic communication is hindered by factors beyond the control of the parties, these  undertake to notify each other immediately and to make the necessary measures to restore this  communication. If it is impossible to restore the electronic communication within a  reasonable period of time, the  contracting authority  may notify the  contractor  that alternative  means of communication may be used, until the electronic communication is restored.  4.4. In any event, for reasons linked to business continuity, the  contracting authority  reserves  the right to use alternative means of communication at any moment.  47 