The USART's synchronous capabilities were primarily intended to support synchronous protocols like IBM's [[synchronous transmit-receive]] (STR), [[binary synchronous communications]] (BSC), [[Synchronous Data Link Control]] (SDLC), and the ISO-standard [[highHigh-levelLevel dataData kinkLink controlControl]] (HDLC) synchronous link-layer protocols, which were used with synchronous voice-frequency [[modem]]s. These protocols were designed to make the best use of bandwidth when modems were analog devices. In those times, the fastest asynchronous voice-band modem could achieve at most speeds of 300{{nbsp}}bit/s using [[frequency-shift keying modulation]], while synchronous modems could run at speeds up to 9600{{nbsp}}bit/s using [[phase-shift keying]]. Synchronous transmission used only slightly over 80% of the bandwidth of the now more-familiar asynchronous transmission, since start and stop bits were unnecessary. Those modems are obsolete, having been replaced by modems which convert asynchronous data to synchronous forms, but similar synchronous telecommunications protocols survive in numerous block-oriented technologies such as the widely used [[IEEE 802.2]] (Ethernet) link-level protocol. USARTs are still sometimes integrated with MCUs. USARTs are still used in routers that connect to external CSU/DSU devices, and they often use either Cisco's proprietary HDLC implementation or the [[Internet Engineering Task Force|IETF]] standard [[Point-to-Point Protocol]] in HDLC-like framing as defined in RFC 1662.
