WaveRider LMS4000 User Manual page 55

3 Detailed Description
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.
Basic Operation of the Polling MAC
The Media Access Control (MAC) layer determines which unit (CCU or EUM) gets to transmit
and when it gets to transmit. Through the MAC layer, the CCU determines which associated
EUM gets to 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.
Network Usage
The design of the Polling MAC has been optimized to allow maximized user capacity for
typical patterns found in Internet usage, which include browsing the world wide web,
accessing email, transferring files, and streaming audio and video. The common characteristic
of these uses is that they are bursty—data is transferred in bursts, with time in between the
bursts when no data is transferred. As a result, not all users will be transferring data at the
same time. In fact, the number of users that are actually transferring data at any one time is
generally much smaller than the number sitting in front of their computers which, in turn, is
much smaller than the total number of end users. As a result, many users can share the radio
link and, for the short time they need it, use a significant portion of the link bandwidth. In other
words, many users share the limited bandwidth of the channel, yet each perceives that they
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