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Extended Abstract ATLAS is one of four very large experiments being prepared for the new CERN Large Hadron Collider (LHC). The LHC is scheduled to come into operation in 2007. Experiments will involve extremely large and complex particle detectors and thousands of physicists from all over the world. Data handling from LHC experiments will be a major challenge. High energy colliding proton beams will yield interaction rates of up to 40 MHz, with an information content of 2 MB per collision. Selecting the interesting physics in such an environment necessitates a multi-level system of data collection and processing in order to reduce the initial rate of 40 MHz down to 100 Hz. This rate of events selected “on-line” in real time will be recorded on mass storage and subsequently processed further in an “off-line” environment to produce final physics results. Off-line processing will be carried out on a global basis and uses GRIDs technologies. Special purpose hardware has been designed to carry out a first level of event selection reducing the 40 MHz rate to around 100 kHz. The further two levls of processing is carried out by commodity equipment; very large farms of thousands of processors connected to thousands of data sources by Ethernet networks. Ethernet will also be used in the control and monitoring of the ATLAS experiment. The IEEE 802.3 Ethernet standard when introduced in the early 1980s was a local area network (LAN) based on CSMA-CD technology, spanning distances of up to 2.5 km and running at 10 Mbps. Over the last two decades the standard has undergone significant evolution in terms of both speed and functionality. Transmission rates of up to 10 Gbps (10 GE) are currently possible and native Ethernet connections at this speed has recently been demonstrated over Trans Atlantic distances. This talk will review the design and optimization of the Ethernet networks used in the ATLAS experiment. In particular for real time data collection and processing. Issues discussed will include, the design of networks using computer simulation, equipment requirements, congestion control through traffic shaping and the distribution of data in real time to processing farms on a world wide basis. The latter topic has led us to carry out extensive tests on the use of the 10 Gigabit Ethernet WAN PHY. Results are presented of experiments using the 10 GE WAN PHY over SONET/SDH and DWDM infrastructures in Europe, and between CERN and Canada. Traffic generators and TCP running on high end server PCs have been used to transfer Terabytes of test data at transfer rates of up to line speed (9.2 Gbps). 500 GB of physics data has been transferred a few hours between CERN and the University of Carleton, Ottawa. 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