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iscrete steps of 100 ma and superspeed devices usb 3 x allows a host or a hub to provide up to 4 5 w in six steps of 150 ma usb c allows for dual lane operation of usb 3 x with larger unit load 250 ma up to 7 5 w 103 usb c also allows for type c current as a replacement for usb bc signaling power availability in a simple way without needing any data connection 104 usb power standards specification max current voltage max power low power device up to usb 2 0 100 ma 5 v a 0 50 w low power superspeed usb 3 x device 150 ma 5 v a 0 75 w high power device up to usb 2 0 500 ma b 5 v 2 5 w high power superspeed usb 3 x single lane device 900 ma c 5 v 4 5 w high power superspeed usb 3 x dual lane device d 1 5 a e 5 v 7 5 w battery charging bc 1 5 a 5 v 7 5 w type c 1 5 a 3 a 5 v 7 5 w 15 w power delivery spr d 5 a f up to 20 v 100 w power delivery epr d 5 a f up to 48 v g 240 w 1 2 the v bus supply from a low powered hub port may drop to 4 40 v up to five unit loads with non superspeed devices one unit load is 100 ma up to six unit loads with superspeed devices one unit load is 150 ma 1 2 3 for usb c only up to six unit loads with multi lane devices one unit load is 250 ma 1 2 3 a 60 w operation requires an electronically marked cable rated at 5 a 20 v 100 w operation requires an electronically marked extended power range epr cable to recognize battery charging mode a dedicated charging port places a resistance not exceeding 200 ω across the d and d terminals shorted or near shorted data lanes with less than 200 ω of resistance across the d and d terminals signify a dedicated charging port dcp with indefinite charging rates 105 106 in addition to standard usb there is a proprietary high powered system known as poweredusb developed in the 1990s and mainly used in point of sale terminals such as cash registers signaling edit main article usb communications signaling usb phy usb signals are transmitted using differential signaling on twisted pair data wires with 90 ω 15 characteristic impedance 107 usb 2 0 and earlier specifications define a single pair in half duplex hdx usb 3 0 and later specifications define one dedicated pair for usb 2 0 compatibility and two or four pairs for data transfer two data wire pairs realising full duplex fdx for single lane 1 variants require at least superspeed ss connectors four pairs realising full duplex for two lane 2 variants require usb c connectors usb4 gen 4 requires the use of all four pairs but allow for asymmetrical pairs configuration 108 in this case one data wire pair is used for the upstream data and the other three for the downstream data or vice versa usb4 gen 4 use pulse amplitude modulation on three levels providing a trit of information every baud transmitted the transmission frequency of 12 8 ghz translate to a transmission rate of 25 6 gbd 109 and the 11 bit to 7 trit translation provides a theoretical maximum transmission speed just over 40 2 gbit s 110 usb data operation modes operation mode name introduced in lanes encoding data wires nominal signaling rate original label usb if current 53 current old marketing name logo low speed n a usb 1 0 1 hdx nrzi 2 1 5 mbit s half duplex low speed usb ls basic speed usb full speed 12 mbit s half duplex full speed usb fs high speed usb 2 0 480 mbit s half duplex hi speed usb hs usb 3 2 gen 1 1 usb 3 0 usb 3 1 gen 1 usb 3 0 1 fdx 1 hdx a 8b 10b 6 5 gbit s symmetric superspeed usb ss usb 5gbps usb 3 2 gen 2 1 usb 3 1 gen 2 usb 3 1 128b 132b 10 gbit s symmetric superspeed ss usb 10gbps usb 3 2 gen 1 2 n a usb 3 2 2 fdx 1 hdx a 8b 10b 10 10 gbit s symmetric n a usb 3 2 gen 2 2 128b 132b 20 gbit s symmetric superspeed usb 20gbps usb 20gbps usb4 gen 2 1 usb4 1 fdx 1 hdx a 64b 66b b 6 used of 10 10 gbit s symmetric usb 10gbps usb4 gen 2 2 2 fdx 1 hdx a 10 20 gbit s symmetric usb 20gbps usb4 gen 3 1 1 fdx 1 hdx a 128b 132b b 6 used of 10 20 gbit s symmetric usb4 gen 3 2 2 fdx 1 hdx a 10 40 gbit s symmetric usb 40gbps usb4 gen 4 2 usb4 2 0 2 fdx 1 hdx a pam 3 11b 7 t 10 80 gbit s symmetric usb 80gbps asymmetric 1 hdx a 40 gbit s up 120 gbit s down n a 120 gbit s up 40 gbit s down 1 2 3 4 5 6 7 8 usb 2 0 implementation 1 2 usb4 can use optional reed solomon forward error correction rs fec in this mode 12 16 b 128 bit symbols are assembled together with 2 b 12 bit 4 bit reserved synchronization bits indicating the respective symbol types and 4 b of rs fec to allow to correct up to 1 b of errors anywhere in the total 198 b block low speed ls and full speed fs modes use a single data wire pair labeled d and d in half duplex transmitted signal levels are 0 0 0 3 v for logical low and 2 8 3 6 v for logical high level the signal lines are not terminated high speed hs uses the same wire pair but with different electrical conventions lower signal voltages of 10 to 10 mv for low and 360 to 440 mv for logical high level and termination of 45 ω to ground or 90 ω differential to match the data cable impedance superspeed ss adds two additional pairs of shielded twisted data wires and new mostly compatible expanded connectors besides another grounding wire these are dedicated to full duplex superspeed operation the superspeed link operates independently from the usb 2 0 channel and takes precedence on connection link configuration is performed using lfps low frequency periodic signaling approximately at 20 mhz frequency and electrical features include voltage de emphasis at the transmitter side and adaptive linear equalization on the receiver side to combat electrical losses in transmission lines and thus the link introduces the concept of link training superspeed ss uses a new coding scheme with an increased signaling rate gen 2 1 mode and or the additional lane of usb c gen 1 2 and gen 2 2 modes a usb connection is always between an a end a downstream facing port dfp of either a host or a hub and a b end the upstream facing port ufp of either a peripheral device or a hub historically this was made clear by the fact that hosts had only type a and peripheral devices had only type b ports and every compatible cable had one type a plug and one type b plug usb c type c is a single connector that replaces all legacy type a and type b connectors so when both sides are equipment with usb type c ports normally the device s type defines which is the dfp and which is the ufp some devices e g modern smart phones can act as both consequently the connected devices negotiate which is the host and which is the peripheral device protocol layer edit main article usb communications protocol layer during usb communication data is transmitted as packets initially all packets are sent from the host via the root hub and possibly more hubs to devices some of those packets direct a device to send some packets in reply transactions edit main article usb communications transaction the basic transactions of usb are out transaction in transaction setup transaction control transfer exchange related standards edit the wireless usb logo media agnostic usb edit the usb implementers forum introduced the media agnostic usb ma usb v 1 0 wireless communication standard based on the usb protocol on 29 july 2015 wireless usb is a cable replacement technology and uses ultra wideband wireless technology for data rates of up to 480 mbit s 111 the usb if used wigig serial extension v1 2 specification as its initial foundation for the ma usb specification and is compliant with superspeed usb 3 0 and 3 1 and hi speed usb usb 2 0 devices that use ma usb will be branded as powered by ma usb provided the product qualifies its certification program 112 interchip usb edit main article interchip usb interchip usb is a chip to chip variant that eliminates the conventional transceivers found in normal usb the hsic physical layer uses about 50 less power and 75 less board area compared to usb 2 0 113 it is an alternative standard to spi and i2c usb c edit main article usb c usb c officially usb type c is a standard that defines a new connector and several new connection features among them it supports alternate mode which allows transporting other protocols via the usb c connector and cable this is commonly used to support the displayport or hdmi protocols which allows connecting a display such as a computer monitor or television set via usb c all other connectors are not capable of two lane operations gen 1 2 and gen 2 2 in usb 3 2 but can be used for one lane operations gen 1 1 and gen 2 1 114 displaylink edit main article displaylink displaylink is a technology which allows multiple displays to be connected to a computer via usb it was introduced around 2006 and before the advent of alternate mode over usb c it was the only way to connect displays via usb it is a proprietary technology not standardized by the usb implementers forum and typically requires a separate device driver on the computer comparisons with other connection methods edit firewire ieee 1394 edit at first usb was considered a complement to firewire ieee 1394 technology which was designed as a high bandwidth serial bus that efficiently interconnects peripherals such as disk drives audio interfaces and video equipment in the initial design usb operated at a far lower data rate and used less sophisticated hardware it was suitable for small peripherals such as keyboards and pointing devices the most significant technical differences between firewire and usb include usb networks use a tiered star topology while ieee 1394 networks use a tree topology usb 1 0 1 1 and 2 0 use a speak when spoken to protocol meaning that each peripheral communicates with the host when the host specifically requests communication usb 3 0 allows for device initiated communications towards the host a firewire device can communicate with any other node at any time subject to network conditions a usb network relies on a single host at the top of the tree to control the network all communications are between the host and one peripheral in a firewire network any capable node can control the network usb runs with a 5 v power line while firewire supplies 12 v and theoretically can supply up to 30 v standard usb hub ports can provide from the typical 500 ma 2 5 w of current only 100 ma from non hub ports usb 3 0 and usb on the go supply 1 8 a 9 0 w for dedicated battery charging 1 5 a 7 5 w full bandwidth or 900 ma 4 5 w high bandwidth while firewire can in theory supply up to 60 watts of power although 10 to 20 watts is more typical these and other differences reflect the differing design goals of the two buses usb was designed for simplicity and low cost while firewire was designed for high performance particularly in time sensitive applications such as audio and video although similar in theoretical maximum signaling rate firewire 400 is faster than usb 2 0 high bandwidth in real use 115 especially in high bandwidth use such as external hard drives 116 117 118 119 the newer firewire 800 standard is twice as fast as firewire 400 and faster than usb 2 0 high bandwidth both theoretically and practically 120 however firewire s speed advantages rely on low level techniques such as direct memory access dma which in turn have created opportunities for security exploits such as the dma attack the chipset and drivers used to implement usb and firewire have a crucial impact on how much of the bandwidth prescribed by the specification is achieved in the real world along with compatibility with peripherals 121 ethernet edit the ieee 802 3af 802 3at and 802 3bt power over ethernet poe standards specify more elaborate power negotiation schemes than powered usb they operate at 48 v dc and can supply more power up to 12 95 w for 802 3af 25 5 w for 802 3at a k a poe 71 w for 802 3bt a k a 4ppoe over a cable up to 100 meters compared to usb 2 0 which provides 2 5 w with a maximum cable length of 5 meters this has made poe popular for voice over ip telephones security cameras wireless access points and other networked devices within buildings however usb is cheaper than poe provided that the distance is short and power demand is low ethernet standards require electrical isolation between the networked device computer phone etc and the network cable up to 1500 v ac or 2250 v dc for 60 seconds 122 usb has no such requirement as it was designed for peripherals closely associated with a host computer and in fact it connects the peripheral and host grounds this gives ethernet a significant safety advantage over usb with peripherals such as cable and dsl modems connected to external wiring that can assume hazardous voltages under certain fault conditions 123 124 midi edit the usb device class definition for midi devices transmits music instrument digital interface midi music data over usb 125 the midi capability is extended to allow up to sixteen simultaneous virtual midi cables each of which can carry the usual midi sixteen channels and clocks usb is competitive for low cost and physically adjacent devices however power over ethernet and the midi plug standard have an advantage in high end devices that may have long cables usb can cause ground loop problems between equipment because it connects ground references on both transceivers by contrast the midi plug standard and ethernet have built in isolation to 500 v or more esata esatap edit this section s factual accuracy may be compromised due to out of date information please help update this article to reflect recent events or newly available information june 2026 the esata connector is a more robust sata connector intended for connection to external hard drives and ssds esata s transfer rate up to 6 gbit s is similar to that of usb 3 0 up to 5 gbit s and usb 3 1 up to 10 gbit s a device connected by esata appears as an ordinary sata device giving both full performance and full compatibility associated with internal drives esata does not supply power to external devices this is an increasing disadvantage compared to usb even though usb 3 0 s 4 5 w is sometimes insufficient to power external hard drives technology is advancing and external drives gradually need less power diminishing the esata advantage esatap power over esata a k a esata usb is a connector introduced in 2009 that supplies power to attached devices using a new backward compatible connector on a notebook esatap usually supplies only 5 v to power a 2 5 inch hdd ssd on a desktop workstation it can additionally supply 12 v to power larger devices including 3 5 inch hdd ssd and 5 25 inch optical drives esatap support can be added to a desktop machine in the form of a bracket connecting the motherboard sata power and usb resources esata like usb supports hot plugging although this might be limited by os drivers and device firmware thunderbolt edit main article thunderbolt interface thunderbolt combines pci express and displayport into a new serial data interface original thunderbolt implementations have two channels each with a transfer speed of 10 gbit s resulting in an aggre...
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