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

19-6 Industrial Communication Systems
• Queuing is used to evaluate the priority value and the configured police and to determine in
which of the output queues the packet must be put.
• Scheduling is the process that services the transmit queues, based on the sharing and shaping
configuration of the transmit port.
• Dropping is used to drop packets, to take advantage of Transmission Control Protocol (TCP) and
windowing mechanisms that drop selectively some packets in an attempt to avoid dropping much
more packets. This does not work with applications using User Datagram Protocol (UDP) as the
transport protocol, only for the TCP ones.
By the other side, the QoS features must be configured throughout the network for providing end-toend
QoS delivery. And there are three components needed to get this consistency:
• QoS components within a single network element, including queuing, scheduling, and traffic
shaping.
• QoS signaling techniques to coordinate QoS from end to end between network elements.
• QoS policing and management functions to control and administer end-to-end traffic across
a network.
There are nowadays only two QoS architectures for IP networks: the Integrated Services model,
(IntServ) [13], and the Differentiated Services model, (DiffServ) [14]. They are different in the way they
enable applications to send data and in how the network attempts to deliver the data within the specified
level of service.
19.3.1 Integrated Services (IntServ) Model
Integrated Services (IntServ) model is defined in RFC 1633 [13]. It provides multiple services that can
accommodate multiple QoS requirements. It can be used through the introduction of Resource
reSerVation Protocol (RSVP, RFC 2205 and 3936) [16–18], enabled at both the end points and the networks
devices in between. Each RSVP-enabled application requests a specific kind of service from the
RSVP-enabled network before sending the data. Explicit signaling via RSVP must occur to facilitate the
request. Afterward, the application informs the network of its traffic profile and requests a particular
kind of service that can contains its bandwidth and delay needs. The idea is that the application is only
going to send data after it gets a confirmation from the network.
This kind of RSVP-enabled network does also admission control, based on information from the
requesting host application and available network resources, admitting or rejecting the application
request for bandwidth. The network commits to meet the QoS requirements of the application while the
specific traffic flow for which the request was made remains within the profile specifications.
It is also possible to centralize admission control by using the Common Open Policy Service (COPS,
RFC 2748) [19] protocol at a policy decision point. When used with RSVP, COPS provides several benefits:
• Centralized management of services
• Centralized admission control and authorization of RSVP flows
• Increased scalability of RSVP-based QoS solutions
The two main drawbacks of RSVP and IntServ are the need of continuous signaling due to the stateless
architecture and the lack of scalability and, although there is an IETF working group devoted to its
study, it is not the most commonly used.
19.3.2 Differentiated Services (DiffServ) Model
The DiffServ model [14,15] is a multiple service-level model that can satisfy differing QoS needs.
However, unlike in the IntServ model, an application that uses DiffServ does not signal explicitly the
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