wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

15-10 Industrial Communication Systems
Synchronization
(NTP or GPS)
VAN-AP
VAN-AP
to
Subdomain
QoS
monitoring
Free bandwidth
Test data stream
Process data stream
QoS
monitoring
to
Subdomain
VAN subdomain
Public network
VAN subdomain
FIGURE 15.9
VAN QoS monitoring.
In both cases, this means the network can be “not available” for several days per year. During runtime,
no online measuring data on the current state of the connection is available to the user. The provider
does, however, carry out measurements within his network, but these are only made available to the
user after the corresponding accounting period at the earliest.
To be able to react on QoS failure scenarios and to have verification against the SLA, VAN defines
its own QoS monitoring for the runtime quality of the connection. The VAN QoS monitoring focuses
on the public wide network, since it is the most uncertain part of an end-to-end connection. The public
network is considered as a black box.
Figure 15.9 gives the general topology structure of the VAN QoS monitoring. The main approach
focuses on an active measurement by generating an additional data stream directly between the involved
VAN-APs as the entrance points to the public network part of a connection. The data stream will be produced
and analyzed by a special application distributed on the producing and receiving VAN-AP. This
means the entire public network path is considered as black box. Important is the time synchronization
between them to get analyzable data. The time synchronization will be realized using a precise GPS
receiver at each location or by using NTP. Each packet of the measurement data stream will be time
stamped as base of further calculation (e.g., latency, jitter).
Most important point is that the process data stream will not be disturbed by the measurement.
Therefore, it is necessary to know the bandwidth of the line contracted with the provider and the bandwidth
needed by the entire process data stream (sum of the bandwidth of all running connections).
Since for tunneling, a UDP-based openVPN tunnel is used, the generated stream for monitoring is also
a UDP stream.
Also, the behavior and status of the tunnel has to be investigated according to the single
defined QoS classes (if more than one is used). Therefore, the monitoring traffic also has to be
generated and analyzed for the single priority classes. Figure 15.10 depicts a tunnel with different
prioritized QoS channels with pre-allocated bandwidth (realized as different queues in the
network routers).
Priority 1
Packet with
priority 3
Packet with
priority 1
Packet with
priority 2
Priority 2
Priority 3
FIGURE 15.10
Different prioritized QoS channels.
© 2011 by Taylor and Francis Group, LLC