User's Manual

3 Detailed Description
38 APCD-LM043-4.0
have most of the channel bandwidth to themselves. This over-subscription model is the basis
of Ethernet, DOCSIS cable networks, 802.11 radio networks, Bluetooth, and on a larger scale,
the public switched telephone network.
If a significant portion of the network traffic does not meet this typical bursty model, the Polling
MAC adjusts to maximize the user capacity. In this case, the maximum number of users is less
than the case where most of the traffic is bursty. As described in Specialized Applications on
page 155, the Polling MAC can also be optimized to support LMS4000 applications, which
have been designed, for example, to cost-effectively extend the coverage range.
Association
The Polling MAC has been designed to take advantage of the bursty, intermittent nature of
Internet usage through the concept of association. When users are transferring bursts of data,
their EUMs are associated with the CCU, and they are allocated a portion of the polling
sequence. In between bursts, the EUM is disassociated, freeing that part of the polling
sequence for other users. The determination of when to disassociate an EUM is based on the
time that has expired since any data was transferred to or from that EUM. As more and more
EUMs become associated, the bandwidth allocated to each EUM gets smaller and smaller,
consistent with the GOS constraints discussed below.
When an EUM is not associated but has data to send, it uses the random access mechanism
to send the first packet. On receiving this first packet, the CCU considers the EUM associated
and begins to poll it. The EUM remains associated as long as traffic continues to flow, but after
a short period of inactivity it is directed to disassociate.
If the CCU has data to send to a disassociated EUM, the status of the EUM changes to
associated, and the data is sent to the EUM the first time it is polled.
The maximum number of EUMs that can be associated at any one instant of time is 75. Any
EUMs trying to associate beyond this limit are denied access until the number of associated
EUMs falls below the limit.