Quality of Service (QoS) and its Effect on the Network

In an enterprise network with a large number of users, servers, and network devices, network service or user experience might be unacceptable. Service Level Agreement (SLA) might not be provided to customers on service providers or telecommunication networks because all network traffic types are handled equally.

Users are actively accessing both internal resources, such as locally hosted applications, and the external resources hosted somewhere on the internet. This can cause a lot of network traffic that is passing on the network each day.

Now, what if all the users are accessing all the internal and external resources simultaneously? That will consume most of the router’s resources, and for that, the router will queue or be forced to drop some of the network traffic or packets and cause network congestion.


What is Quality of Service (QoS)?

To solve the problems of network congestion due to a load of active users utilizing the resources of the router, Quality of Service (QoS) should be implemented. Quality of Service (QoS) is a technology that classifies and prioritizes network traffic types, such as sensitive and non-sensitive data traffic.

Sensitive data traffic, like voice traffic and video traffic, requires a guaranteed bandwidth as they are operating in real-time. While non-sensitive data traffic, like web browsing and email, requires non-guaranteed bandwidth as the application can retransmit the dropped packets again due to network congestion.

The applications that use User Datagram Protocol (UDP), like voice traffic and video traffic, require QoS implementation as they do not guarantee the delivery of a message, unlike Transmission Control Protocol (TCP) that can retransmit the packet loss and guarantee the message delivery. With QoS, network performance and user experience will be optimized.


QoS Queuing

All computer networking devices, such as routers, have a buffer with a limited storage capacity, and all the incoming traffic is temporarily stored until they can process and transmit. That process in QoS is called queuing.

Queuing the traffic increases delay on the network, and that will cause congestion. If there is a lot of incoming traffic, the buffer will be full, and the upcoming incoming traffic will be dropped. There should be a free space on the router’s buffer for the upcoming incoming traffic to be stored, process, and transmit. The below diagram shows the process of classifying and prioritizing network data.

Quality of Service


Characteristics of Network Traffic

Quality of Service (QoS) tools enables us to manage these four network traffic QoS characteristics:

1. Bandwidth – it is the maximum amount of data per second. It is sometimes referred to as the speed or capacity of the link. This is commonly measured as bits per second (bps). On network devices, like routers and switches, the bandwidth is associated per interface. The interface bandwidth can either be Ethernet (10 Mbps), Fast Ethernet (100 Mbps), or Gigabit Ethernet (1000 Mbps).

2. Delay – the time it takes for a message to travel from the source device to the destination device. It is sometimes referred to as latency. A high latency will cause a delay for traffic to arrive on the destination device and therefore causes a slower response time when establishing a connection to a specific device or application.

3. Jitter – it is the variation of the one-way delay of a consecutive packet that is being sent. For example, the first packet is sent and the second packet is sent after 50 milliseconds (ms), the time it takes for the second packet to be sent after the first packet was sent is the jitter, and it is 50 ms.

4. Loss – it happens when the buffer of the router is full, and new incoming packets are being dropped. Having too much packet loss will cause the receiving device to receive an incomplete message.


Quality of Service (QoS) Mechanism

Listed below are the most commonly used QoS tools to manage the QoS characteristics of network traffic:

1. Classification –  applied to the router’s interface and classifies if the packet requires QoS implementation or not.

2. Marking – it marks the packets based on classification. It puts a value on the packet header so that the packet can be easily recognized throughout the network based on its classification.

3. Congestion Management – prioritizes the transmission of each packet by queuing on each interface. 

4. Congestion Avoidance – drops packets early to avoid congestion.

5. Queuing – stores packet into the buffer and hold until it is their turn to exit on the router’s interface.

6. Policing – enforces rate limit by dropping down or marking the packets.

7. Shaping – enforces rate limit by delaying the packets and store them in the router’s buffer for a certain amount of time.


Quality of Service (QoS) Implementation and Configuration

The following are the steps on how to configure and implement QoS in our network:

1. Identify the traffic and its requirement. Check the data packets if it is voice traffic, web traffic, application traffic, or email traffic and apply QoS based on the requirement. 


2. Divide the traffic into classes. Classify the traffic and mark them by putting a value on the packet header. The configuration below shows that TCP/UDP port 80 (HTTP) is assigned to class ‘Web-Traffic’.

Router(config)#class-map Web-Traffic
Router(config-cmap)#match protocol http

3. Assign QoS policies for each class. Apply necessary action of whether to allow, drop, or rate-limit the specific class of packet. The below configuration shows that class ‘Web-Traffic’ is being rate limited into 1000kbps.

Router(config)#policy-map Web-Traffic-Policy
Router(config-pmap)#class Web-Traffic
Router(config-pmap-c)#bandwidth 1000
Router(config-pmap-c)#class class-default


4. Assign the policy into the interface. The below configuration shows that the policy ‘Web-Traffic-Policy’ is being enforced to interface gigabitEthernet 0/0. The interface gigabitEthernet 0/0 of the router now has a maximum bandwidth for HTTP traffic of 1000kbps.

Router(config)#int gigabitEthernet 0/0
Router(config-if)#service-policy output Web-Traffic-Policy

Download our Free CCNA Study Guide PDF for complete notes on all the CCNA 200-301 exam topics in one book.

We recommend the Cisco CCNA Gold Bootcamp as your main CCNA training course. It’s the highest rated Cisco course online with an average rating of 4.8 from over 30,000 public reviews and is the gold standard in CCNA training: