![]() See the link below for protocol comparisons. The NCM protocol has elaborate provisions for this. ![]() These new devices are still much lower in power than desktop PCs, thus the issue of careful data handling arises, to maximize use of DMA resources on the device and minimize (or eliminate) copying of data (zero-copy). A gap every 23 frames is not noticeable, but a gap every three frames can be viewed as very costly to throughput.Īs USB has become faster, devices utilize more data and hence there is now demand for sending large amounts of data-either to be stored on the device, or be relayed over wireless links (see 3GPP Long Term Evolution). Such reduces bus occupancy, meaning that nothing is sent for considerable fractions of bus time. After this, there is bus latency, where nothing is sent until another transfer can be initiated. The USB system works by each packet being sent as a transfer, a series of maximum-length packets terminated by a short packet or a special ZLP (zero-length packet). One significant problem is, the Ethernet frames are about 1500 bytes in size-about 3 USB 2.0 packets, and 23 USB 1.1 packets. This was a satisfactory strategy for USB 1.1 systems (current when the protocol was issued) with 64 byte packets but not for USB 2.0 systems which use 512 byte packets. Of these protocols ECM could be classified the simplest-frames are simply sent and received without modification one at a time. Remarkably, even some of the most modern platforms need minor accommodations and therefore support for these subsets is still needed. However, minor modifications of the standard subsets make practical implementations possible on such platforms. Regarding de facto standards, some standards, such as ECM, specify use of USB resources that early systems did not have. The RNDIS specification is available from Microsoft's web site. They are available for download from the USB-IF (see below). The latter three are part of the larger Communications Device Class (CDC) group of protocols of the USB Implementers Forum (USB-IF). ![]() The main industry protocols are (in chronological order): Remote NDIS (RNDIS, a Microsoft vendor protocol), Ethernet Control Model (ECM), Ethernet Emulation Model (EEM), and Network Control Model (NCM). Even though the USB is not a physical Ethernet, the networking stacks of all major operating systems are set up to transport IEEE 802.3 frames, without needing a particular underlying transport. The use of these protocols is to allow application-independent exchange of data with USB devices, instead of specialized protocols such as video or MTP (Media Transfer Protocol). There are numerous protocols for Ethernet-style networking over USB.
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