User's Manual

3 Detailed Description
46 APCD-LM043-4.0
All of these charts illustrate that many (LMS4000) users can share the limited bandwidth of
the channel, yet most of the time, each perceives that they have most of the channel to
themselves.
Atypical Applications
The Polling MAC has been optimized for normal user applications. One basic assumption that
has been made in the design of the Polling MAC is that users are only associated for a small
fraction of the time they are sitting in front of their computers. This usage is typified, for
example, by a file transfer (Web page for example) every two minutes or so—each transfer
taking a second or two. The MAC takes advantage of this usage pattern by only associating
with active EUMs.
A second assumption is that EUMs become active independently. If many EUMs
simultaneously attempt to use the random access opportunity, they will collide multiple times
and may not get through.
If the above assumptions are reasonable, then it is also reasonable to assume that a limited
number of EUMs will be associated at any given time, as demonstrated in Performance
Modelling on page 42.
There are several computer applications where usage is not consistent with the above
assumptions. These applications, which are discussed below, can compromise the efficient
operation of the LMS4000 network and may cause the network to slow down.
Broadcast Applications
Some applications broadcast messages to which all or a large number of hosts are expected
to respond. If these applications are running over the system, not only will responses from
disassociated EUMs collide as the random access opportunities are overwhelmed, but those
that do get through will quickly use up all of the available associations. With so many
associated EUMs, polls are farther apart and throughput degrades, even if the newly
associated EUMs have no further traffic to send. As well, EUMs that are not associated are not
able to associate and are therefore be blocked for a few seconds. The following applications
can cause this type of problem:
• Broadcast pings: WaveRider recommends not using broadcast pings.
• SNMP broadcast requests: WaveRider recommends not using SNMP broadcast
requests.
• Windows Network Neighborhood: This traffic can be blocked using port filtering at
the CCU or EUM level, as discussed in Port Filtering on page 49.
Periodic Packet Sources
Some applications send individual packets at fixed, often large, intervals, expecting only a
single packet or small number of packets in response. The direct impact of these applications
is that EUMs that are sent periodic packets remain associated for a longer period of time than
that warranted by their end-user traffic level and continue to be polled unnecessarily. The
atypical applications themselves will function very well; however, they will use up a significant
amount of the channel bandwidth. This group includes the following applications: