In 1984 the ISO defined the seven layer OSI reference model which consists of physical, datalink, network, transport, presentation, session and application layers. Each of the layers describes the services required to send information from one application to another, as well as interfaces between the layers in order to aid with the interconnection of standards. The physical layer concerns itself with the physical transmission of data over a medium; the data-link layer with the organisation of data and detection of transmission errors; the network layer with how data is routed from one application to another; the transport layer with transparent transfer of data; the session layer with organising and synchronising data exchange; the presentation layer