LEP networks, implemented in 1987, are based on two Token-ring backbones using TDM as the transmission medium. The general topology is based on routers and on a distributed backbone. To avoid the instabilities introduced by the TDM and all the conversion layers it has been decided to upgrade the LEP machine network and to evaluate a new concept for the overall network topology. The new concept will also fulfil the basic requirements for the future LHC network. The new approach relies on a large infrastructure which connects all the eight underground pits of LEP with single-mode fibres from the Prevessin control room (PCR). From the bottom of the pits, the two adjacent alcoves will be cabled with multi-mode fibres. FDDI has been selected as the MAC protocol. This new concept is based on switching and routing between the PCR and the eight pits. In each pit a hub will switch between the FDDI LMA backbone and the local Ethernet segments. Two of these segments will reach the alcoves by means of a 10Base-F link. In a second phase implementation, this scheme will provide for workgroup organisation and bandwidth allocation. The technological choices make a future evolution towards ATM and 100Base Ethernet possible and allow us to preserve a large part of the investment. This paper describes the implementation of this scheme
