top of rack (ToR)
Top of Rack (ToR) is a networking concept used in data centers that was developed specifically for virtualized data centers. As can be seen from the name Top of Rack, a ToR switch or rack switch is mounted on top of the racks or in the server racks, thus simplifying cabling. A ToR switch does not necessarily have to be located on the rack, it can also be located anywhere in the rack.
Alternative networking concepts include Middle of Row( MoR) and End of Row( EoR). In these concepts, the rack with the switches is located in the middle or at the end of the lined-up server racks. The aim of the various networking concepts is to minimize the cabling effort between the many servers - often several tens of thousands - with TP cables or with fiber optic cables. In this context, 10- gigabit Ethernet can be implemented over short distances with TP cables, whereas 25- and 40-gigabit Ethernet require fiber optic cables.
The ToR concept is suitable for high- performance blade servers and supports migration to 10 Gigabit Ethernet. They also simplify and shorten cabling and facilitate replication of rack configurations. By having switches built into each rack, servers can be better aggregated and connected to other data center components. The ToR architecture is implemented by a ToR switch, which implements the high-speed connections between servers, storage systems and other data center devices. Because the ToR switches are rack-mounted, cabling to individual blade servers is relatively simple.
ToR switches support Data Center Bridging( DCB) and thus Lossless Ethernet and Converged Enhanced Ethernet( CEE). For optimal and cost-effective adaptation to 10 Gigabit Ethernet or 40 Gigabit Ethernet, ToR switches have slots for transceiver modules. The port density that can be achieved in this way depends on the transceiver module in question. For example, a Tor switch can have 48 ports with SFP+(Small Form-Factor Pluggable plus) modules or 44 ports with QSFP(Quad Small Form Factor Pluggable) modules on one height unit. The non-blocking data throughput of ToR switches can be one terabit per second (Tbit/s) and higher.
The high data throughput and low latency in the nanosecond range are ideal for use in high-performance clusters( HPC), cloud computing or real-time transactions. Other applications in converged data centers include storage area networks( SAN), network attached storage( NAS), Fibre Channel over Ethernet( FCoE) and Internet Small ComputerSystem Interface over IP( iSCSI).