Topology

USB is described by the standard as a tiered star bus topology. The bus controller in the host computer provides a root hub, which is considered the first tier. From that root hub, each cable hop connects at the second tier to either a function (a peripheral device such as a keyboard or mouse) or another hub, which can connect at the next tier to additional peripherals and hubs. Six tiers are allowed in addition to the tier represented by the root hub, and hubs are not allowed in the seventh tier. Consequently, a maximum of five hubs are allowed between the root hub and a peripheral device. Note that a physical device can include more than a single function, as in the case of printer/scanner/fax machines, and can also include a hub, as in the case of many Apple keyboards.

Protocol

The USB host controller is considered the bus master; it manages data transfers between a source and a destination using a pipe, a logical construct that comprises the configuration, control, and status parameters that define the connection between that source and destination. A special type of pipe, referred to as a default control pipe, is established for the zero endpoint in each device when the device is connected to the bus. Default control pipes are managed by the host controller for bus control purposes and are not visible to client software. Each device reports its configuration information when the host detects the device connected and powered up on the bus. Other pipes can then be created, as required by the class and features of that device. These other pipes are also managed by the host controller but can be used by client applications to access device features. For more information about device classes, see the USB Class Specification documents available from the USB Implementers Forum website http://www.usb.org/developers/devclass/.

The host controller manages all transactions, and most transactions involve transmission of three packets. To initiate each transaction, it polls devices on the bus (at a scheduled interval) by sending a token packet that identifies the address of the device, the endpoint number that specifies the target function, the transfer type, and whether the transfer is directed toward the root hub (upstream or in) or away from the root hub (downstream or out). Then the source (depending on the transfer direction) sends to the destination either a data packet or an indication that there's no data to send. The destination typically responds with a handshake packet that indicates success or failure of the transfer.

Concentration of control functions in the host allows simpler devices, but imposes limits on bus throughput. This protocol is effective for minimizing bus control overhead.

Transfer Speeds

USB defines three transfer speeds:

• Low-speed, 1.5 Mbps, is used typically for a limited number of devices such as keyboards, mice, and similar human interface devices (HIDs). This limitation helps to avoid degradation of bus utilization.

• Full-speed, 12 Mbps, is used for more data-intensive devices, such as printers and digital cameras.

• High-speed, 480 Mbps, supports more data-intensive devices such as audio and video devices.

To maintain backward compatibility, USB hubs include a transaction translator that can convert high-speed transmissions to low-speed and full-speed rates as needed by downstream devices in the next tier, and, conversely, can convert upstream transmissions from low-speed and full-speed devices to the high-speed rate for transfer to the upstream hub.

Transfer Types

USB defines four types of data transfers:

• Control transfers are used to establish the configuration of a device when it is connected to the bus. Can be used for device-specific purposes.

• Interrupt transfers are used to transmit time-critical data as needed, while preserving its integrity. Typically used to avoid perceptible degradation of the user experience.

• Isochronous transfers are used to transmit (or stream) data in real time while tolerating some possible data loss. Typically used for audio streams and video streams.

• Bulk transfers are used to transmit large amounts data reliably with minimal concern for speed. Typically used for transfers to and from devices such as printers and scanners.

USB 1.1

USB 1.1 ports, such as those on Apple keyboards, comply with the Universal Serial Bus Specification 1.1 Final Draft Revision. For low-speed (1.5 Mbps) and full-speed (12 Mbps) devices, the USB register set complies with either the Open Host Controller Interface (OHCI) specification or the Universal Host Controller Interface (UHCI) specification.

USB 2.0

USB 2.0 ports on Macintosh computers include support for high-speed (480 Mbps) devices using an Enhanced Host Controller Interface (EHCI) or a Universal Host Controller Interface (UHCI). These ports support the use of cables and connectors made to meet earlier versions of the specification, except that high-speed operation requires the use of shielded cables.

Keyboard and USB

Each external Apple USB keyboard (not integrated into a product, as in laptop computers) has a captive cable with a USB Type A connector. The keyboard includes a bus-powered USB hub with two USB Type A ports.

Apple provides an HID class driver for the Apple Keyboard, which supports the USB boot protocol. Other keyboards intended for use on the Macintosh platform must support the HID boot protocol, as defined in the USB Device Class Definition for Human Interface Devices (HIDs).

External Modem and USB

An external Apple USB modem is available as a configure-to-order option on some Macintosh computers.

The modem has the following features:

• modem bit rates up to 56 Kbps (supports V.90, V.92, and K56flex modem standards)

• Group 3 fax modem bit rates up to 14.4 Kbps

The modem appears to the system as a USB device controlled by drivers. The modem responds to the typical AT commands and provides the audio stream for call progress monitoring.

USB Connectors

USB ports on current Macintosh computers use USB Type A connectors, which have four pins each. Two of the pins are used for power and two for data. Figure 1 is an illustration of a Type A port; Table 1 shows the signals and pin assignments.