User's Manual Part 1

42 APCD-LM043-8.0 (DRAFT C)
3: Detailed Description
In the registered/disassociated state, the EUM is still not being polled. But if the EUM has
traffic to send, it tries to associate with the CCU through the random access channel. The
EUM may also become associated if the CCU has a payload to send to the EUM. Once
associated, the state of the EUM changes to active/associated.
In the active/associated state, the EUM is polled often, at a rate consistent with its subscribed
grade of service. If there is no traffic to or from an active/associated EUM for a defined interval
(typically set around 0.5 seconds), the state of the EUM changes to inactive/associated.
An inactive/associated EUM is polled less frequently than an active/associated EUM. If traffic
is resumed, the state of the EUM changes back to active/associated. If there is no traffic for a
longer defined interval (typically set around 2 seconds), the state of the EUM changes back to
registered/associated.
If an EUM is issued a deregistration request, for any reason, or if it has no traffic for an
extended period of time, 12 hours or so, its state changes back to unregistered.
3.6.2 Basic Operation of the Polling MAC
The Media Access Control (MAC) layer determines which EUM may transmit and when it may
transmit. Through the MAC layer, the CCU determines which associated EUM may transmit
next and indicates to the EUM that it can transmit by polling it. The frequency with which an
EUM is polled is based on its assigned Grade of Service (GOS). The CCU transmits a directed
poll to the EUM, which immediately transmits a response to the CCU. After the response is
received from the EUM, the CCU transmits the next poll. In this way, the inbound (EUM-to-
CCU) and outbound (CCU-to-EUM) channels are maintained collision free.
If the CCU has data to send to an EUM, then that data is sent with the directed poll. If the EUM
has data to send to the CCU, then that data is sent with the EUM response to the poll.
EUMs that are not authorized are not polled.
To optimize polling efficiency, EUMs that no longer have traffic to send are not polled. EUMs
that are not being polled can submit a request to be polled by responding to a special random
access poll transmitted regularly by the CCU. Collisions may sometimes occur on this random
access channel; however, since only a small number of users are vying for service through the
random access channel at any one time, the effect on channel performance is negligible.
Recovery from these collisions is made possible by random back-off and retry.
Once again, if the EUM requesting service through the random access channel has data to
send to the CCU, it will be included with the request message. If the CCU has outstanding
broadcast messages to send, they will be sent to all EUMs with the random access poll.
An automatic repeat request (ARQ) scheme, using acknowledgements and retransmissions to
recover from message losses due to collisions or radio link errors, provides reliable transport.
Each transmitted data payload is numbered in the packet header. Each packet header also
contains an acknowledgement for the last correctly received payload, by number. If a CCU or
EUM does not receive an acknowledgement for a payload that it has transmitted, it retransmits
that payload with the following poll of, or response from, that EUM. A payload is transmitted a
maximum of four times, after which it is discarded. Note that contrary to the 802.11b system,
MAC-layer acknowledgements are not transmitted as separate packets, reducing overhead by
33%, on average.