QoS ITS ALL ABOUT QUALITY !!!

What Is QoS?

Quality of Service (QoS) for networks is an industry-wide set of standards and mechanisms 

for ensuring high-quality performance for critical applications. By using QoS mechanisms,

 network administrators can use existing resources efficiently and ensure the required 

level of service without reactively expanding or over-provisioning their networks.

Traditionally, the concept of quality in networks meant that all network traffic was 

treated equally. The result was that all network traffic received the network’s best 

effort, with no guarantees for reliability, delay, variation in delay, or other performance

 characteristics. With best-effort delivery service, however, a single bandwidth-intensive 

application can result in poor or unacceptable performance for all applications. The QoS 

concept of quality is one in which the requirements of some applications and users are more

 critical than others, which means that some traffic needs preferential treatment.

QoS Concepts
The goal of QoS is to provide preferential delivery service for the applications that need
it by ensuring sufficient bandwidth, controlling latency and jitter, and reducing data loss.
 The following table describes these network characteristics.

Network Characteristics Managed by QoS

Network Characteristic Description

Bandwidth The rate at which traffic is carried by the network.
Latency The delay in data transmission from source to destination.
Jitter The variation in latency.
Reliability The percentage of packets discarded by a router.

The Internet Engineering Task Force (IETF) defines two major models for QoS on IP-based
networks: Integrated Services (Intserv) and Differentiated Services (Diffserv).
These models encompass several categories of mechanisms that provide preferential
 treatment to specified traffic. The following table describes the three general
categories of QoS mechanisms.

Category of QoS Mechanisms      Description

Admission control                    Determine which applications and users are entitled to
                                     network resources. These mechanisms specify how, when,
                                     and by whom network resources on a network segment
                                     (subnet) can be used.

Traffic control                      Regulate data flows by classifying, scheduling, and
     marking packets based on priority and by shaping traffic
     (smoothing bursts of traffic by limiting the rate of flow).
     Traffic control mechanisms segregate traffic into service
     classes and control delivery to the network. The service
     class assigned to a traffic flow determines the QoS
     treatment the traffic receives.

Note: The Intserv model integrates resource reservation and traffic control mechanisms to support special
 handling of individual traffic flows. The Diffserv model uses traffic control to support special
handling of aggregated traffic flows.

Uses and Benefits of QoS

Network administrators can use QoS to guarantee throughput for mission-critical applications so that

 their transactions can be processed in an acceptable amount of time. Network administrators can also

 use QoS to manage User Data Protocol (UDP) traffic. Unlike Transmission Control Protocol (TCP), 

UDP  is an inherently unreliable protocol that does not receive feedback from the network and, therefore, 

cannot detect network congestion. Network administrators can use QoS to manage the priority of 

applications that rely on UDP, such as multimedia applications, so that they have the required bandwidth

 even in times of network congestion, but do not overwhelm the network.

Network Technologies and Support for QoS

QoS depends on support throughout the network. To achieve QoS from sender to receiver, all of the network  elements through which a traffic flow passes — such as network interface cards, switches, routers, and  bridges — must support QoS. If a network device along this path does not support QoS, the traffic flow  receives the standard first-come, first-served treatment on that network segment.

Network technologies such as Frame Relay, asynchronous transfer mode (ATM), and the more traditional 

local area network (LAN) technologies (including Ethernet, Token Ring, and 802.11 wireless LAN) support

 QoS mechanisms. ATM, in particular, offers a high degree of support for QoS. Because ATM is a 

connection-oriented networking technology, it can establish a service contract that guarantees a 

specific quality of service and can allocate network resources. ATM enforces the service contract

 and allocates bandwidth at the hardware level. For more information about ATM QoS

Topics that i cover in this QOS series are

1. Traffic Classification
2. Ways to Mark traffic on the LAN/WAN
3. Congestion Management (Queuing strategies)
4. Congestion Avoidance
5. Traffic Policing
6. Traffic Shaping
7. Link Efficiency
8. Calculating bandwidth Voice packets

Summary of qos: 

references:
http://packetlife.net/wiki/cisco-qos-notes-and-theory-made-easy/
http://technet.microsoft.com/en-us/library/cc757120(v=ws.10).aspx