"The term “top of rack” has been coined for this design however the actual physical location of the switch does not necessarily need to be at the top of the rack. Other switch locations could be bottom of the rack or middle of rack, however top of the rack is most common due to easier accessibility and cleaner cable management. This design may also sometimes be referred to as “In-Rack”. The Ethernet top of rack switch is typically low profile (1RU-2RU) and fixed configuration. The key characteristic and appeal of the Top of Rack design is that all copper cabling for servers stays within the rack as relatively short RJ45 patch cables from the server to the rack switch. The Ethernet switch links the rack to the data center network with fiber running directly from the rack to a common aggregation area connecting to redundant “Distribution” or “Aggregation” high density modular Ethernet switches."

http://www.internetworkexpert.org/2009/04/05/top-of-rack-vs-...