However, due to the possibility of changing line quality, delays in processing of data, etc., there is a need for symmetric, bidirectional flow control. A symmetric alternative providing flow control in both directions was developed and marketed in the late 1980s by various equipment manufacturers. It redefined the RTS signal to mean that the DTE is ready to receive data from the DCE. This scheme was eventually codified in version RS-232-E (actually TIA-232-E by that time) by defining a new signal, "RTR (Ready to Receive)", which is CCITT V.24 circuit 133. TIA-232-E and the corresponding international standards were updated to show that circuit 133, when implemented, shares the same pin as RTS (Request to Send), and that when 133 is in use, RTS is assumed by the DCE to be asserted at all times. In this scheme, commonly called "RTS/CTS flow control" or "RTS/CTS handshaking" (though the technically correct name would be "RTR/CTS"), the DTE asserts RTR whenever it is ready to receive data from the DCE, and the DCE asserts CTS whenever it is ready to receive data from the DTE. Unlike the original use of RTS and CTS with half-duplex modems, these two signals operate independently from one another. This is an example of hardware flow control. However, "hardware flow control" in the description of the options available on an RS-232-equipped device does not always mean RTS/CTS handshaking. Equipment using this protocol must be prepared to buffer some extra data, since the remote system may have begun transmitting just before the local system deasserts RTR. The EIA-232 standard specifies connections for several features that are not used in most implementations. Their use requires 25-pin connectors and cables. The DTE or DCE can specify use of a "high" or "low" signaling rate. The rates, as well as which device will select the rate, must be configured in both the DTE and DCE. The prearranged device selects the high rate by setting pin 23 to ON. Many DCE devices have a loopback capability used for testing. When enabled, signals are echoed back to the sender rather than being sent on to the receiver. If supported, the DTE can signal the local DCE (the one it is connected to) to enter loopback mode by setting pin 18 to ON, or the remote DCE (the one the local DCE is connected to) to enter loopback mode by setting pin 21 to ON.