CIR V1

F.9 Example Descriptors for an IrDA Control Mouse

;/////////////////////////////////////////////////////////////////////////////// ;// Sample 3D Mouse Descriptors.

;// This is a sample only. Not all object bytes are ;// valid. (Vendor ID, et al., are set to 0), etc.

;///////////////////////////////////////////////////////////////////////////////

EXTRN bcdVersion

;=============================================================================== ; Descriptor ID Codes ;===============================================================================

;/////////////////////////////////////////////////////////////////////////////// ;// Descriptor Tables

;// 3-axis, 3-button Mouse ;///////////////////////////////////////////////////////////////////////////////

;=============================================================================== ; Copyright String ;=============================================================================== psCOPYRIGHT:

DB '© 1997 BigTime IrBus Corporation. All rights reserved.' cCOPYRIGHT EQU ($-psCOPYRIGHT)

;=============================================================================== ; Mouse (3D) Descriptors ;===============================================================================

;===============================================================================

;===============================================================================

;=============================================================================== cCONFIG_REQ EQU ($-pdCONFIGURATION) ;===============================================================================

;===============================================================================

Appendix G. Multiple Hosts

G.1 Introduction

A likely user environment for IrDA Control is multiple host systems interacting within the same user space. This could occur in many different situations, including:

∙A PC-TV and a “traditional” PC in the same room

∙Multiple Home Automation (HA) hosts in the same room

∙An office with two PCs or similar devices

∙Showroom, trade show, etc.

In any of these situations, the transmissions from any one host could potentially interfere with the transmission of one or more hosts. This appendix describes a media access scheme that allows the multiple IrDA Control hosts in a user space to share the IR medium. While media access technologies such as FDMA, CDMA, and TDMA are well known, they are not appropriate for IrDA Control; wavelength and subcarrier division are not practical with infrared technologies available in the near term, for example. For this reason, the IrDA Control multiple hosts solution implements a Packet Reservation Multiple Access (PRMA) scheme that is based on a statistical slotting algorithm.

G.2 Operational Scenario

There are a number of situations that the multi-host aspects of IrDA Control must be able to accommodate:

∙Occasions where a host can detect the presence of other hosts in the IR medium,

∙Occasions where hosts cannot detect each other, but can only infer the presence of multiple hosts because of interference at one or more peripherals.

An example is shown in Figure G.1.

The following is a list of possible connections in the IR medium:

1.Hosts cannot communicate with each other, and hosts do not interfere at peripherals (disjoint hosts - can be operated independently)

2.All the hosts in a given user space could communicate with each other either directly or indirectly (via peripherals)

3.None of the hosts in a given user space can communicate with each other, but there is interference between at least two hosts at one or more peripherals (hidden host model)

While it is possible to adopt different strategies for each of the above situations, a single multiple hosts scheme best serves the implementation of IrDA Control. Further, a multiple host scheme that addresses the hidden host model is equally effective for the other multiple host scenarios.

Figure G.1: PC-TV and PC with overlapping IrDA Control coverage areas

G.3 Dithering Scheme

The multiple host scheme for IrDA Control is based on a dithering algorithm that models a random walk in one dimension. In this scheme, a host randomly dithers the start of the host polling cycle in an attempt to create a clear slotting of frames within a period. This is a form of Packet Reservation Multiple Access (PRMA), but without confirmation of the reservation from other hosts. This scheme forces a host to dynamically adjust its frame timing in an attempt to fit within the parameters of a 13.8ms polling period.

An example of the multiple host interference for three hosts is shown in Figure G.2. This figure shows the interference caused by overlapping of the host frames (captioned Host frames before dithering), and the noninterfering slotting for the three hosts found by the dithering scheme (captioned Host frames after dithering).

The dithering algorithm is based on minimizing the errors seen at each host. The interference caused by hidden hosts may result in CRC errors or framing errors in either the poll or response packets of a host. These errors manifest themselves at the host as CRC errors or framing errors in the response packet, or, in the extreme case, as a lack of response to a host poll5. A host maintains a count of the errors seen at the host over a number of host frames. When a host detects a sufficient number of errors have occurred over several frames, the host infers that there is interference in the space. It delays the start of its next cycle by a random time chosen uniformly from the interval of 0 to 12 milliseconds, and resumes sending IrDA Control frames. Once the number of errors drops below a specified threshold, the host stops dithering if a steady state has been reached. Consequently, there is a period of time where the host will dither the start of the host polling cycle in order to search for a clear time gap in the IR space. If the sum of all the frame times from all the hosts overlapping in the space exceeds 13.8ms, this dithering process will continue indefinitely, since no optimal timing solution is possible. In this scheme, the overall IrDA Control system dithers randomly to search for a solution within the 13.8ms period constraint.

Every host in the multi-host environment whose error rate exceeds the specified threshold, the “high error threshold”, must dither its frame until the error rate observed by that host drops below another threshold, the “low error threshold”. The hysteresis of the two levels of error thresholds prevents false triggers and contributes to the stability of the algorithm.

5Note that a peripheral may not always respond to a device hail –viz, an enumeration hail or a binding hail. Hence, lack of response to a device hail is not treated as an error.

IRBus frame for Host1

Figure G.2: Multiple host interference with three IrDA Control hosts

The multiple host scheme applies only when a host is the Normal Mode (or Mode1). For a host in IrDA Coexistence Mode (or Mode 2), the dithering of the host frame caused by the multiple hosts scheme will interfere with IrDA communications, voiding the benefits of the coexistence mode. Besides, a non-interfering slotting of the hosts can be found only if the combined frame times of all the hosts in the user space does not exceed 13.8ms, an assumption that is clearly not valid in the IrDA coexistence mode.

G.4 Implementation Notes

The effectiveness of the dithering scheme is determined by the convergence time for a given combined frame time (the combined frame time for the three hosts in figure G.2, for example, is 5+3.3+3.3 = 11.6ms).

The following parameters affect the convergence time of the dithering scheme: the cycle count, the high error threshold, and the low error threshold. The effective values for these parameters are determined through simulations. In addition, the following empirical rule is used to control the dithering for a host – “For a dithering scheme with a frame count of n, if no errors are detected on the nth frame, dithering is stopped”.

Cycle Count – is the number of 13.8ms cycles over which the errors are measured by a host. A host could potentially measure the error rate after every host polling cycle. However, errors are measured over a number of cycles to overcome the effects of transient errors.

High error threshold – When the percentage of errors observed at a host exceeds this value, the host starts to dither its frame. This parameter is chosen so that dithering is not triggered due to transient errors.

Low error threshold – A host stops dithering as soon as the percentage of errors fall below the low error threshold. In absence of this parameter, the dithering scheme may become unstable due to errors varying about the high error threshold value. The absolute value of the low error threshold is less than that of the high error threshold and is chosen to optimize the ‘stable’ convergence time.

G.5 Host Algorithm

The host algorithm for implementing the multiple hosts scheme is given below:

On successful reception of a frame: responsecount++;

When a receive error (CRC error, framing error) occurs,

or no peripheral response to a host poll (except for host hails):

responsecount++;

errorcount++;

lastcycleerrorcount++;

When a 13.8ms cycle expires:

void doMultiHostProcessing()

{

cyclecount++;

if (cyclecount >= DITHER_CYCLE_COUNT) { if (dodither) {

if (responsecount * DITHER_LOW_THRESHOLD > errorcount * 100) {

dodither = FALSE;

}

} else {

if (responsecount * DITHER_HIGH_THRESHOLD < errorcount * 100) {

dodither = TRUE;

}

}

cyclecount = 0; responsecount = 0; errorcount = 0;

if (lastcycleerrorcount == 0) { dodither = FALSE;

}

if (dodither) {

/* Wait a random time (0-12ms) */

}

}

lastcycleerrorcount = 0;

}