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Tier Based Bandwidth Calculation for devices Connected either directly or through hubs to USB 2.0 Host Controller.

IP.com Disclosure Number: IPCOM000021779D
Original Publication Date: 2004-Feb-09
Included in the Prior Art Database: 2004-Feb-09
Document File: 3 page(s) / 25K

Publishing Venue

IBM

Abstract

Disclosed is a design to improve the performance of USB 2.0 Host Controller (HC) by Tier based Bandwidth Calculation. Traditionally, for devices residing anywhere in the USB topology, the worst case delay is taken into account for Bandwidth Calculation. This becomes an overhead if the devices are connected directly to the HC Ports. A new method is disclosed here, wherein a device residing on different location in the bus topology, the bandwidth is calculated by tier based delay from the HC till the point where the device resides.

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Tier Based Bandwidth Calculation for devices Connected either directly or through hubs to USB 2.0 Host Controller.

Introduction

USB is a serial bus and the HC resides on the host computer and devices are connected to the host either directly or through external hub. All the transfers are initiated by the host computer, the data is transferred in terms of packets. USB device cannot initiate any transfers. USB support different type of transfers namely Isochronous , Interrupt, Bulk and Control.

Traditionally, irrespective of device presence in the topology the HC may not initiate any transaction for the given payload (PL) using wrost case analysis. For example, for a payload of 512 bytes, even though the transaction can take place after 54144 bit times (from SOF), the HC may not start due to worst case bandwidth calculation . So there is always a need for a better solution to overcome this problem. The analysis is based on the bit time of one or more transactions(Txn).

Calculation of Bit Time:

In general the total bit time for any type of transaction is given by, Total Bit Time(TBT) = Data Payload + tp_oh + dp_oh + hp_oh + status_oh + sr_dly + ip_dly + xact_dly

Data Payload : Transaction Length to be transferred in terms of bits. *tp_oh : Token Packet overhead.
*dp_oh : Data Packet overhead.
*hp_oh : Handshake Packet overhead(excluding Isochronous). *status_oh : Status packet overhead (Applicable to Control Transactions). *sr_dly : Start Delay.
*ip_dly : Inter Packet Delay.
*xact_dly : Transaction Delay. * - All overhead delays and xact_dly varies depending on the device presents in the topology(as shown in Fig. 1.1).

A case study with comparison of delays in current and disclosed method are discussed below. Total Bit time for 125 microsecond = 60000. Available Bit time in 125 microsecond for transaction (SOF to EOF1) = 59440.

1

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TIER 1

      TIE R 2 - D evice D ire ctly C onnected to H C P ort (Total D elay = 232 B it Tim e s)

         TIE R 3 - D evice C onnected to H U B 1 (Total D elay = 333 B it Tim es)

Device

   C able D elay(26ns)

 Hub Cross D e la y (3 6 B it tim e s + 4 n s )

         TIE R4 - D evice C onnected to H U B 2 (Total D elay = 434 B it Tim es)

Hub1

Device

 D evice Function D elay(192

Bit tim es)

         TIER 5 - D evice C onnected to H U B 3 (Total D elay = 535 B it Tim es)

Hub1

H ub2

Device

  Host C o n tr o lle r

Hub1Hub2

         TIE R 6 - D evice C onnected to H U B4 (Total D elay = 635 Bit Tim es)

H ub3

Device

Hub1H ub2

H ub3

Hub4

TIER 7 -

   D evice C onnected to HUB5 (To ta l D e la y = 736 B it Tim es)

Device

                        H ub3 Hub1H ub2 Hub5

Hub4

Device

Figure.1.1. Delay Based Topology.

Case Study - Bulk Transaction :

A case study of data payload is 512 bytes and a Bulk IN Transaction is taken into account.

Current Method TBT = 4096 + 64 + 64 + 48 + 96 +192 + 736 = 5296. Disclosed Method
Tier 2(T2) TBT = 4096 + 64 + 64 + 48 + 96 +192 + 232 = 4792. Tier 3(T3) TBT = 4096 + 64 + 64 + 48 + 96 +192 + 333 = 4893. Tier 4(T4) TBT = 4096 + 64 + 64 + 48 + 96 +192 + 4...