In some DSL variations (for example, High-bit-rate digital subscriber line (HDSL)), the modem is directly connected to the computer via a serial interface, using communication protocols such as RS-232 or V.35. In other cases (particularly ADSL), it is common for the customer equipment to be integrated with other functions, such as routing, firewalling, or other application-specific hardware and software. In this case, the entire equipment is usually referred to as a DSL router or DSL gateway.