Quality of Service (QOS)

Quality of service

Quality of service (QoS) refers to any technology that manages data traffic to reduce packet loss, latency and jitter on a network. QoS controls and manages network resources by setting priorities for specific types of data on the network.

Organizations can reach a QoS by using certain tools and techniques, such as jitter buffer and traffic shaping. For many organizations, QoS is included in the service-level agreement (SLA) with their network service provider to guarantee a certain level of network performance.

3 categories of QOS

The five main categories of tools used to implement QoS are as follows.

1. Classification and Marking
2. Congestion Management  (Queuing)
3. Congestion Avoidance
4. Policing and Shaping
5. Link Efficiency

   Classification and Marking

These  two distinct items within the QoS mechanisms to implement QoS. In general terms they identify and split traffic into different classes and mark the traffic according to desired behaviors.

Classification tools sort packets into different traffic types, to which different policies can be applied. Classification can be done at every node in the network or be implemented at the edge of the network when the packet enters the network. 

Marking writes a field within the packet, frame, cell or label to preserve the classification decision that was reached during the classification process. Packets that are marked as high priority, such as a voice packet, will generally never be dropped by congestion avoidance mechanisms. On the other hand, if packets are marked as low priority they will be dropped when congestion occurs.

Congestion management queuing 

Congestion management queuing algorithms use the marking on each packet to determine which queue to place packets in. Each queue is given different treatment based on the class of packets in the queue. 

Congestion management tools are implemented on all output interfaces in a QoS enabled network.

 Cisco IOS uses the following congestion management queuing methods:

• FIFO (First in First Out), Priority Queuing (PQ), Custom Queuing (CQ)  ,Weighted Fair Queuing (WFQ)  , Class Based Weighted Fair Queuing (CBWFQ)  , Low Latency Queuing (LLQ)

Congestion Avoidance

Congestion Avoidance monitors network traffic loads in an effort to anticipate and avoid congestion. Congestion Avoidance is achieved through packet dropping. Typically, congestion avoidance is implemented on output interfaces where high-speed links intersect with low speed links.

It use Weighted Random Early Detection (WRED) to avoid congestion by dropping low priority packets and allowing high priority packets to continue on their path

Policing and Shaping

Policing or Shaping mechanisms are used to condition traffic before transmitting or when receiving traffic. 

Policing controls bursts and conforms traffic to ensure each traffic type gets the prescribed bandwidth. In some cases policing can help service providers maintain service level agreements (SLA). 

Shaping helps smooth out speed mismatches in the network and limits transmission rates. These mechanisms are typically used to limit the flow from high speed links to low speed links, to prevent the low speed links from becoming over run

Link Efficiency


Although not exclusively QoS tools, link efficiency tools are categorized as QoS tools because they are often used in conjunction with QoS. Both of the link efficiency tools were created outside of the realm of QoS, and were used as independent Cisco IOS Tools.

Header-compression is a tool that is used to reduce the IP overhead of a Real-Time Transport Protocol (RTP) voice packet which reduces the overall size of the IP packet. 

Link Fragmentation and Interleaving (LFI) is used to reduce delay and jitter on slower speed links by breaking the larger packets, such as FTP file transfers, into smaller packets and interleaving them in with the small voice packets. 

Wi-Fi Multimedia (WMM)

 It is an automated way of handling Quality of Service (QoS) on your Wi-Fi network,  that in theory should allow your router to provide improved video and voice performance over your network.

Normally when you use a wireless router all traffic is given the same priority. This means different devices and applications have the same opportunity to transfer packets of data.

WMM maintains the priority of audio, video and voice, over other applications which are less time critical.Using QoS (Quality of Service) Wi-Fi Multi-Media (WMM) ensures that the applications that require better throughput and performance,are inserted in queues with higher priority. 

For example, video and audio applications are given higher priority over applications such as FTP

WMM defines the following four queues, in decreasing order of priority:

• Voice.The highest priority queue with minimum delay, which makes it ideal for applications such as voice over IP(VoIP) and streaming media.
• Video. The second highest priority queue with low delay is given to this queue. Video applications are routed to this queue.
• Best Effort. The medium priority queue with medium delay is given to this queue. Most standard IP applications use this queue.
• Background. Low priority queue with high throughput. Applications, such as FTP, that are not time-sensitive,  but require high throughput,can use this queue.
• 
  

 Class of Service (CoS)

Class of Service (CoS) is a way of managing traffic in a network by grouping similar types of traffic -- such as email, streaming video, voice over IP and large document file transfer -- together and treating each type as a class with its own level of network service priority.

CoS is used at the data link layer -- Layer 2 -- of the Open Systems Interconnection (OSI) model and consists of a 3-bit field within the 802.1Q virtual local area network (VLAN) or 802.1P frame header.

We will get priority values in the range of 0 to 7

initially we have the first the 3 bit to prioritize the values

The TOS byte gets a new name and is now called the DS field (Differentiated Services) and the 8 bits have changed as well. Here’s what it looks like now:

Now we have the first 6 bits to prioritize the values

Difference between QoS and CoS

 – QoS,  Quality of Service, in the context of networking refers to a set of technologies used to manage network resources by allocating application of different network behaviors to different traffic types. It manages traffic on a network in order to reduce packet loss, latency and jitter on the network. 

CoS, Class of Service is a way to manage different types of traffic over a network by dividing similar types of traffic by classes. 

CoS prioritizes traffic by allocating different levels of priority to different groups while QoS manipulates traffic according to these set priority levels.

Control-Plane Nodes – Map System that manages Endpoint to Device relationships. The border and Control-plane nodes can be the same device.

Fabric Border Nodes – A Fabric device (e.g. Core) that connects External L3 network(s) to the SDA Fabric.

Fabric Edge Nodes – A Fabric device (e.g. Access or Distribution) that connects Wired Endpoints to the SDA Fabric.

Fabric Wireless Controller – A Fabric device (WLC) that connects APs and Wireless Endpoints to the SDA Fabric

Shaping is a QoS (Quality of Service) technique that we can use to enforce lower bitrates than what the physical interface is capable of. Most ISPs will use shaping or policing  to enforce “traffic contracts” with their customers. When we use shaping we will buffer the traffic to a certain bitrate, policing will drop the traffic when it exceeds a certain bitrate.

• Tc (time interval) is the time in milliseconds over which we can send the Bc (committed burst).
• Bc (committed burst) is the amount of traffic that we can send during the Tc (time interval) and is measured in bits.
• CIR (committed information rate) is the bitrate that is defined in the “traffic contract” that we received from the ISP.

Token bucket algorithm is one of the techniques for congestion control algorithms. When too many packets are present in the network it causes packet delay and loss of packet which degrades the performance of the system. This situation is called congestion.

The network layer and transport layer share the responsibility for handling congestions. One of the most effective ways to control congestion is trying to reduce the load that transport layer is placing on the network. To maintain this network and transport layers have to work together.

We dont drop packets, we delay packets in fact