What is QoS in Networking?

Quick definition: Quality of Service (QoS) is a network feature that sorts and prioritizes data, ensuring that essential activities like video calls get the necessary bandwidth and speed. This process helps maintain efficient and reliable network performance for high-priority tasks.

The network admin job is more challenging than it used to be.  At one time, data and voice applications operated on separate independent networks. Nowadays, enterprise networks routinely carry mission-critical transactional systems alongside streaming video, teleconferencing, and voice applications. The explosion of the Internet of Things (IoT) adds more complexity to the job.

Qualified networking professionals are in great demand, and certifications like the CompTIA Network+ are widely used to assess candidates’ networking skills and knowledge. Keith Barker's Network+ Certification Training course covers topics such as fault tolerance, traffic shaping, and quality of service that network administrators must master to earn Network+ certification. Today, we'll explore quality of service in more detail. 

What is QoS (Quality of Service)?

Today’s network traffic is digitized, meaning information packets must be delivered on time and sequenced appropriately. If they are not, then video and voice applications, in particular, will be disrupted with the jitter, delays, and dropouts that can make those applications unusable! It’s not enough that the computer network is ‘up and running’; it must also deliver the required level or quality of service (QoS.)

Now, most organizations don't have unlimited network resources, so they must make the best use of those they have. They can do that by employing QoS technologies to allocate available resources so that traffic for high-priority applications gets differentiated handling and enjoys higher levels of network capacity than lower-priority systems.  

QoS Measurements

QoS measurements are essential for ensuring network resources are allocated appropriately and that the quality of services meets the requirements of different applications and users. The following parameters are related to quality of service in computer networks:

1. Bandwidth or link speed: The maximum data transfer rate for the link 

2. Throughput: The actual transfer rate that can be achieved in practice

3. Latency or delay: the time for a packet to go from source to its destination

4. Jitter: The arrival of packets late and/or out of sequence due to network congestion  

5. Loss: Where packets are lost in transit, typically due to network congestion

Different applications have different tolerance levels to these network situations. They may require a minimum bandwidth and not tolerate packet delay or loss above certain thresholds. These applications – think streaming video, Voice over IP (VoIP), mission-critical transaction processing, etc. – are called ‘inelastic.’

The Key Components of QoS and How They Work

As a network administrator, you have several quality of service (QoS) technologies available to ensure applications get the required bandwidth and traffic handling to keep packet jitter, delay, and loss within designated tolerances. The main components of QoS in computer networks are queuing, bandwidth management, and classification. 

• Queuing: Queuing refers to the process of managing and prioritizing network traffic when there is congestion or limited bandwidth. Often, algorithms help determine the order in which packets are processed and transmitted based on their assigned priority or class of service (CoS). The primary goal of queuing in QoS is to ensure that critical or high-priority traffic receives preferential treatment over lower-priority traffic. 

• Bandwidth management: Similar to queuing, bandwidth management prioritizes network traffic based on the priority or class of service. However, rather than ordering different packets, this step determines how much bandwidth a user or application can use. This helps prevent network congestion. 

• Classification: Each application is differentiated according to its operational characteristics and importance to the business. So, a VoIP telephony application serving the sales department needs real-time performance and thus expects low levels of jitter and latency and minimal packet delay and loss! While perhaps just as important, the email system is less time-sensitive and can tolerate higher levels of delay and loss. Operational and business needs drive traffic classification and will be reflected in “Service Level Agreements (SLA)” that IT negotiates with the business department heads.  

QoS Models in Computer Networks

Generally, there are three QoS models that reflect increasing levels of service management in computer networks:

1. Best Effort: All packets receive the same priority, with no guarantee that packets will be delivered. In essence, no quality of service is implemented.

2. Differentiated Services: The DiffServ QoS model allows traffic to be managed according to differing network requirements – for example, low latency for VoIP and best-effort service for non-critical file transfer or website traffic. DiffServ does not provide a guarantee of network resource availability. DiffServ is implemented using the 6-bit differentiated services code point (DSCP), which is part of the differentiated services field (DS field) in the IPv6 header.

3. Integrated Services: Like DiffServ, the IntServ QoS model allows traffic to be managed according to application operating requirements. However, IntServ guarantees quality of service by using the Resource Reservation Protocol (RSVP) to reserve network resources before dispatching packets. Integrated Services promises the highest level of QoS, but it requires IntServ-compatible routers throughout the network.

Each packet header is tagged with the appropriate traffic classification. This can be done by IP address or port, as well as by application or user. In addition to its classification, each packet header contains information such as its source and destination, sequence, etc. The QoS tool examines the packet and assigns the appropriate priority.

The next step is for the queuing tool to sequence the packets and for bandwidth management to allocate the appropriate bandwidth. They do this based on rules set up by network administrators. The queuing mechanism stores and sequences packets in queues until the network is ready to transmit them.

The mechanism will also ensure that appropriate priority (Priority Queuing – PQ) is applied to each sequence of packets. Traffic in priority queues is always processed before traffic in lower priority queues. So, latency is kept to a minimum for high-priority traffic, and there is less likelihood of jitter.

Combined with queuing, bandwidth management ensures that high-priority traffic will always get available bandwidth over lower-priority traffic. By managing bandwidth allocation, QoS helps avoid network congestion!

Techniques used to manage bandwidth include traffic shaping and scheduling algorithms.

How to Implement QoS

QoS implementation should follow these high-level steps:

1.  Planning: Start by understanding the service requirements for each department’s applications and agree on application priorities and classification.

2. Design: Based on business service level needs, select a QoS model for each application and assign them to policies that reflect the defined traffic classes.

3.  Network Setup and Test: Implement the QoS policies in computer network tables and test the design in a controlled test network.

4. Rollout: Deploy the QoS design onto the production network in phases, for example, by network segment or QoS policy. 

5. Monitor and Refine: Monitor and evaluate the performance of QoS design, making modifications as necessary.

Conclusion

Implementing and monitoring network quality of service in the computer network is an important aspect of the network administrator’s job. A well-designed and managed QoS implementation makes the most of an organization’s network resources and ensures user satisfaction.

Considering a transition into network administration? Begin with obtaining a CompTIA Network+ certification. To help you get up to speed with networking concepts such as quality of service, learn with Keith Barker in his CompTIA Network+ Certification Training.