The plug and play requirement has many implications

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Discrete Mathematics With Applications
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Chapter 9 / Exercise 4
Discrete Mathematics With Applications
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The plug-and-play requirement has many implications at all levels in the system, from the hardware to the operating system and the applications software. One of the primary objectives of the design of the USB has been to provide a plug-and-play capability. USB Architecture:- The discussion above points to the need for an interconnection system that combines low cost, flexibility, and high data-transfer bandwidth. Also, I/O devices may be located at some distance from the computer to which they are connected. The requirement for high bandwidth would normally suggest a wide bus that carries 8, 16, or more bits in parallel. However, a large number of wires increases cost and complexity and is inconvenient to the user. Also, it is difficult to design a wide bus that carries data for a long distance because of the data skew problem discussed. The amount of skew increases with distance and limits the data that can be used. A serial transmission format has been chosen for the USB because a serial bus satisfies the low-cost and flexibility requirements. Clock and data information are encoded together and transmitted as a single signal. Hence, there are no limitations on clock frequency or distance arising from data skew. Therefore, it is possible to provide a high data transfer bandwidth by using a high clock frequency. As pointed out earlier, the USB offers three bit rates, ranging from 1.5 to 480 megabits/s, to suit the needs of different I/O devices. Figure 23 Universal Serial Bus tree structure. Host computer Root hub Hu b Hub Hub I/O device I/O device I/O device I/O device
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Discrete Mathematics With Applications
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Chapter 9 / Exercise 4
Discrete Mathematics With Applications
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K I T TIPTUR COMPUTER ORGANISATION 2014 Prepared By Sharu Gangadhar [[email protected]] Page 113 To accommodate a large number of devices that can be added or removed at any time, the USB has the tree structure shown in figure 23. Each node of the tree has a device called a hub, which acts as an intermediate control point between the host and the I/O devices. At the root of the tree, a root hub connects the entire tree to the host computer. The leaves of the tree are the I/O devices being served (for example, keyboard, Internet connection, speaker, or digital TV), which are called functions in USB terminology. For consistency with the rest of the discussion in the book, we will refer to these devices as I/O devices. The tree structure enables many devices to be connected while using only simple point-to-point serial links. Each hub has a number of ports where devices may be connected, including other hubs. In normal operation, a hub copies a message that it receives from its upstream connection to all its downstream ports. As a result, a message sent by the host computer is broadcast to all I/O devices, but only the addressed device will respond to that message. In this respect, the USB functions in the same way as the bus in figure 4.1. However, unlike the bus in figure 4.1, a message from an I/O device is sent only upstream towards the root of the tree and is not seen by other devices. Hence,

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