Issue
139 February 2002
What
Good is IrD, Eh?
Part
2: Wireless Communication
byJeff
Bachiochi
Who Am I/Who
Are You
During link
start up, IrLAP enters the nonoperational mode and generates
its own 32-bit device address. This address is used in
the address discovery procedure and exchanged as identification
of each device within range. IrDA devices have two possible
modes of operation. Secondary devices respond to IrDA
communications and primary devices initiate IrDA communications.
Primary devices
can act as primary or secondary devices, whereas secondary
devices are only passive. Primary devices initiate the
discovery process by polling for devices (within range)
by dividing time into slots (see Figure 4). A primary
device will make a slot inquiry and then listen during
the remainder of the time slot for a device to reply.
All devices should randomly choose a time slot to reply
to. If the primary device hears a legal reply, it logs
the device’s address in its device log. The primary device’s
log of potential connections is continually updated until
the upper layer asks for a connection to be established.
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(Click here to enlarge)
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Figure
4—This figure shows the primary polling and secondary
response from an IR device within range. If the blue
area is left off, the figure demonstrates the primary’s
end of the poll frame, which reveals facts about itself. |
After all the
time slots have been polled, the primary device sends
out an information packet indicating what kind of device
it is and its nickname. The information frame is similar
to the response of the secondary device (see Figure 4).
The polling cycle will then begin again.
If by chance
devices come up with the same 32-bit address, an address
conflict arises. To solve the problem, those devices can
be instructed to choose new 32-bit addresses via bit 2
of the discovery flags.
