SIEMENS 5881 Broadband Internet Router
User’s Guide
Chapter 5 Advanced Setup
QoS
SIEMENS
37
QoS
Quality of Service actively manages network resources to sustain service levels for priority applications.
Mission-critical and real-time Internet applications demand a network that provides high bandwidth and low
latency. Such applications cannot tolerate unpredictable degradations of network services. Therefore, network
services must contain features that provide adequate assurance of sustained service levels. Some of the
benefits associated with Quality of Service include:
Guaranteed available bandwidth and minimum delays to real-time Voice over IP traffic
Dynamic allocations of bandwidth to non-critical applications
User control over network traffic levels, and potential cost-efficiencies
Advanced differentiation of network services
Measurement and reporting of network service levels
Applications, such as video conference or IP telephony, must be able to communicate their service level
requirements to an infrastructure that can consistently meet those requirements. To do this, QoS control
mechanisms must be present in each network element. This router provides such QoS control mechanisms
and can interpret the service requirements indicated by network applications, fully participating in any
differentiated services architecture.
This router provides Quality of Service using two methods:
Differentiated Services Framework
(DiffServ) and
Weighted Fair Queuing
(WFQ).
Differentiated Services Framework
(DiffServ) is a facility to prioritize the requirements of each Class of Service (for example e-mail, streaming
video, voice) according to defined policies. DiffServ is suited to Metropolitan Area Networks or private networks
where control over the infrastructure is guaranteed, and differentiated services can be deployed end-to-end.
To employ DiffServ, each packet of data is tagged with a six-bit pattern known as the DiffServ CodePoint
(DSCP), replacing the three IP precedence bits in the ToS byte of the IPv4 header. This tag determines the
processing of each packet as a Pre-Hop Behavior (PHB) at each DiffServ node. Each DSCP is read and
network resources are allocated to a packet according to the Class of Service defined in its associated policy.
When DiffServ is activated on your router, data packets are read and marked according to their DiffServ
priority. The packets are then queued and processed according to the defined QoS policy.
Weighted Fair Queuing
(WFQ) is a flow-based queuing algorithm that performs two functions simultaneously:
•
It schedules priority traffic to the front of the queue to reduce response time.
•
It fairly distributes remaining bandwidth between remaining queues.
Consequently, WFQ ensures that queues are not starved for bandwidth and that traffic service levels are made
more predictable.
Weighted Fair Queuing adapts automatically to changing network conditions and requires minimal
configuration. WFQ is implemented on the router and applies to network traffic passing through it. Unlike
DiffServ, external nodes have no affect on QoS through Weighted Fair Queuing.
Weighted Fair Queuing provides a means of ensuring that high priority or mission-critical applications receive
adequate levels of bandwidth. This is accomplished by controlling two key factors in QoS policies.
Manipulation of these two factors determines the quality of service to each application.
