User Manual

WipLL System Description
2-8 of 2-12 Issue 03
4Mbit - Draft 05
1QDF10134AAP-SYD-FCC
From time to time (and not exceeding every 100mS) the BSR sends a "Channel
Clear" message which is an invitation for SPRs to send the score of their
requirements. It then waits for a while and receives these requirements from the
SPRs. The waiting time is called Slotted Aloha due to the fact that the BSR waits
for a time that is equivalent to 16 messages of "Request to Send" (RTS). The
messages are synchronized so that an SPR does not transmit a message before the
previous message is ended. The timing of each RTS message is represented as a
"Slot".
SPRs are independent to choose which slots to use for sending their requirements.
Occasionally a collision between SPRs can occur on a slot and then the probability
is that the request is lost. Each SPR can use more than one slot to send its request.
An SPR that was not allowed to transmit might try again during the next Slotted
Aloha process.
This system ensures that all SPRs eventually get a fair chance to transmit their
requests.
2.2.1.3. Packet Transmission
After the BSR has gathered the requests from the SPRs and decided on the priorities,
it sends a "Clear to Send" (CTS) message to the first SPR. The packet is then
transmitted from the SPR.
In the header of each packet more information about the status of the queues is
included thus avoiding the need for the SPR to participate in the next Slotted Aloha
process.
The data packet is divided into fragments and each fragment is added with CRC
(Cyclic Redundancy Check). After the packet is complete, an "Acknowledge"
(ACK) message is sent by the BSR that includes information about all fragments
that were reported as errors. These fragments can be repeated several times until the
entire message is successfully transmitted.
2.2.1.4. Polling Sequence
Each time the BSR sends a CTS (Clear to Send) message to one of the SPRs it is
considered as if the SPR is being polled.