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

19-2 Industrial Communication Systems
at specific moments and other applications that impose no more than a certain delay for its signal.
If these conditions are not reached simply, the client side of these applications will not work properly
and we will be losing quality.
Technically speaking, network QoS can be defined as the application of functionality and features
to actively manage and satisfy networking requirements of applications sensitive to delay, loss, or delay
variation (jitter), guarantying also the availability of bandwidth for critical application flows.
Using QoS methods and technologies [2,3] several benefits are reached:
• Control over resources: It can be determined which network resources (bandwidth, equipment,
etc.) are being used. For example, critical traffic as voice or video over IP may consume a link and
QoS helps control the use of the link by dropping low-priority packets.
• More efficient use of network resources: Using network analysis management and accounting
tools, it can be determined how traffic is handled, and which traffic shows latency, jitter, or packet
loss. QoS tools [1] can be used to tune the handling of the traffic.
• Coexistence of critical applications: By using QoS technologies, it is possible that most critical
applications receive the most efficient use of the network. It is possible, for example, that time-Â
sensitive multimedia, which requires bandwidth and minimized delay, get them while other applications
in the same link receive fair service without interfering with critical applications traffic.
These needs are directly linked with the idea of QoS, seen as a sum of value and warranty, being only
a part of the framework of services management for best practices IT recommendations, as ITIL or
CobIT [4]. The value of the network is what we search using a particular network, the particular communication
between different kind of devices. For the case of an industrial network, the value is going to
be what we want to get with communication between servers and clients in our industrial network. The
warranty is much more linked with the quality, meaning a defined set of features for a concrete network
and can be expressed as the level of service of the network for a number of features:
• Availability or how much time is guaranteed that the network is up continuously
• Capacity or how many resources (dedicated bandwidth, delay, etc.) are available for each of the
applications and how is the capacity to react to a change in the need of resources
• Network security or the ability to protect any piece of information, following a security policy
that include facts like the confidentiality, integrity, or authentication for any kind of access events
in the network
• Continuity as a sum of the other features, but specially thought for disaster incidents such as a
fire or a flood
Once the network management staff has built a Network Policy for all these features, it is necessary to
know the tools for implementing each of the directives in the policy and, fortunately, we have now real
technical solutions in place for these kinds of situations, and we have enough reliable standards.
The rest of this chapter presents an introduction on how to manage all these features to get the best
QoS for IP industrial networks, although practically every single piece of information in the chapter
can be applied to any IP network. The real difference between industrial IP networks and nonindustrial
IP networks are the applications we run on them and, for IP QoS, we only need to know some general
characteristic of these applications. The next section shows the important connections for QoS, previously
defined, with the basics of information security topics and the efforts developed in the last years
to build some standards for managing information security topics related with industrial networks.
Then, we go in depth of the classical terminology, techniques, and architectures used today for getting
a good QoS in any IP network, showing the two most important models: IntServ and DiffServ. Also, we
analyze how the communication devices in the network can classify and mark any IP packet and how to
manage the possible traffic congestion we can see in the same devices. To end, we analyze two possible
situations we must manage only if our IP industrial network uses routers: the congestion avoidance and
high availability solutions for routers.
© 2011 by Taylor and Francis Group, LLC