Page 165 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes The advantage of this algorithm is to save up to the maximum size of the bucket. This means that
bursts of up to number of packets can be sent at the maximum speed for a certain period of time.

Task What are some preventive measures for congestion control?

10.3 Quality of Service

Quality of Service (QoS) determines the capability of a network to provide predictable service
over various technologies including frame relay, Asynchronous Transfer Mode (ATM), Ethernet,
SONET and IP-routed networks. The networks may use any or all of these technologies. The QoS
also ensure that while providing priority for one or more flow does not make other flows fail.
A flow may be a combination of source and destination addresses, source and destination socket
numbers, and the session identifier or any packet from a certain application or from an incoming
interface. The QoS is primarily used to control over resources like bandwidth, equipment, wide-
area facilities and so on, make more efficient use of network resources, provide tailored services,
provide coexistence of mission-critical applications, etc.

The traffic to computer networks often receives equal priority and the computer networks
usually do not differentiate between non-critical browser traffic and critical business applications.
The quality of service (QoS) of computer networks is evaluated with respect to the traffic priority
to understand why QoS is desirable in an intranet and the Internet. The bandwidth is considered
an important subject for Internet and intranet services. The amount of data that is being transmitted
through the Internet has been increasing exponential and new applications like real audio and
video; VoIP, videoconferencing, etc. keep on demanding increased bandwidth. The conventional
Internet applications like WWW, FTP, telnet; etc. cannot tolerate packet loss but are less sensitive
to variable delays. However, most real-time applications can compensate for a reasonable
amount of packet loss but are usually very critical towards high variable delays. Therefore,
bandwidth plays an important role in providing a good quality of service. A QoS is defined as
a policy framework that describes the quality of a specific stream of data in terms of bandwidth,
buffer usage, priority, CPU usage, etc. However, the IP protocol stack provides only one QoS in
term of best effort in which the packets are transmitted from point to point without any guarantee
for a special bandwidth or minimum time delay. The best effort traffic model handles all Internet
requests with equal priority and serves them with the first come first serve strategy.

10.3.1 Basic QoS Architecture

QoS enables better service to certain flows in a network by either raising the priority of a flow
or limiting the priority of another flow. The queue management tool, policing and shaping,
Link efficiency tools, etc. are used for controlling the flows and congestion. Thus, QoS tools
intend to alleviate most congestion problems. The basic QoS architecture involves three
fundamental pieces for QoS implementation. They are QoS identification and marking techniques
for coordinating QoS from end to end between network elements, QoS within a single network
element, for example, queuing, scheduling, and traffic-shaping tools and QoS policy management
and accounting functions to control and administer end-to-end traffic across a network. QoS
Identification and Marking is carried out through classification and reservation. Classification
refers to the identifying and providing preferential service to a type of traffic. In classification,
the packet may or may not be marked. If the packet is identified but not marked, it is said to be
on a per-hop basis. If packets are marked, IP precedence byte is set. Common methods of
identifying flows are Access Control Lists (ACLs), Policy-based Routing, Committed Access
Rate (CAR), Network-based Application Recognition (NBAR), etc.

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