Editing List of interface bit rates (section)

==Factors limiting actual performance, criteria for real decisions==
Most of the listed rates are theoretical maximum [[throughput]] measures; in practice, the [[measuring network throughput|actual effective throughput]] is almost inevitably lower in proportion to the load from other devices ([[contention (telecommunications)|network]]/[[bus contention]]), physical or temporal distances, and other [[overhead (computing)|overhead]] in [[data link layer]] protocols etc. The maximum [[goodput]] (for example, the file transfer rate) may be even lower due to higher layer protocol overhead and data packet retransmissions caused by line [[Noise (signal processing)|noise]] or [[interference (communication)|interference]] such as [[crosstalk]], or lost packets in [[network congestion|congested]] intermediate network nodes. All protocols lose something, and the more robust ones that deal resiliently with very many failure situations tend to lose more maximum throughput to get higher total long-term rates.

Device interfaces where one bus transfers data via another will be limited to the throughput of the slowest interface, at best. For instance, [[SATA]] revision 3.0 ({{val|6|ul=Gbit/s}}) controllers on one PCI Express 2.0 ({{nowrap|5 Gbit/s}}) channel will be limited to the {{nowrap|5 Gbit/s}} rate and have to employ more channels to get around this problem. Early implementations of new protocols very often have this kind of problem. The physical phenomena on which the device relies (such as spinning platters in a hard drive) will also impose limits; for instance, no spinning platter shipping in 2009 saturates SATA revision 2.0 ({{nowrap|3 Gbit/s}}), so moving from this {{nowrap|3 Gbit/s}} interface to [[USB 3.0]] at {{nowrap|4.8 Gbit/s}} for one spinning drive will result in no increase in realized transfer rate.

Contention in a wireless or noisy spectrum, where the physical medium is entirely out of the control of those who specify the protocol, requires measures that also use up throughput. Wireless devices, [[Broadband over Power Lines|BPL]], and [[modem]]s may produce a higher [[line rate]] or [[gross bit rate]], due to [[error-correcting code]]s and other [[physical layer]] overhead. It is extremely common for throughput to be far less than half of theoretical maximum, though the more recent technologies (notably BPL) employ preemptive spectrum analysis to avoid this and so have much more potential to reach actual gigabit rates in practice than prior modems.

Another factor reducing throughput is deliberate policy decisions made by [[Internet service provider]]s that are made for contractual, risk management, aggregation saturation, or marketing reasons. Examples are [[rate limiting]], [[bandwidth throttling]], and the assignment of [[IP address]]es to groups. These practices tend to minimize the throughput available to every user, but maximize the number of users that can be supported on one backbone.

Furthermore, chips are often not available in order to implement the fastest rates. [[AMD]], for instance, does not support the 32-bit [[HyperTransport]] interface on any CPU it has shipped as of the end of 2009. Additionally, [[WiMAX]] service providers in the US typically support only up to {{val|4|ul=Mbit/s}} as of the end of 2009.

Choosing service providers or interfaces based on theoretical maxima is unwise, especially for commercial needs. A good example is large scale data centers, which should be more concerned with price per port to support the interface, wattage and heat considerations, and total cost of the solution. Because some protocols such as SCSI and Ethernet now operate many orders of magnitude faster than when originally deployed, scalability of the interface is one major factor, as it prevents costly shifts to technologies that are not backward compatible. Underscoring this is the fact that these shifts often happen involuntarily or by surprise, especially when a vendor abandons support for a proprietary system.