What is Quality of Service? QoS Types and What Affects QoS? – To sum it up, it is the overall performance of a service. Now we have loads of services that we use every day. These include telephone service or a computer network or your cloud computing service. QoS is also used within the network, particularly the performance seen by the users of the network. Before you master the topic of QoS, we will go through the basics of Quality of Service , and the first step in doing so is knowing the differences between the different QoS types. If you are studying for the CISCO CCNP the QoS is one of the topics that you must learn. Although I would say that we are only lightly touching the subject here, as it is just too large, for one post.
The full list of different QoS types is long, so we are going to go through a few of them in more detail. But below is the full list of Protocols.
The type of service (ToS) field in the IP(v4) header (now superseded by DiffServ)
Differentiated services (DiffServ)
Integrated services (IntServ)
Resource Reservation Protocol (RSVP)
Multiprotocol Label Switching (MPLS) provides eight QoS classes
Some ADSL modems
Asynchronous Transfer Mode (ATM)
HomePNA Home networking over coax and phone wires
The ITU-T G.hn standard provides QoS by means of “Contention-Free Transmission Opportunities” (CFTXOPs).
Audio Video Bridging
There is one that is not listed in the above as it is more default QoS then anything, although as default it is still used. This is the best-effort delivery. We will also look at Integrated Services and Differentiated Services. Although we will start with the best-effort delivery.
Best-effort is just what it sounds like – routers and switches making their “best effort” to deliver data. This is considered QoS, but it’s kind of a “default QoS”. Best effort is strictly “first in, first out” (FIFO) . Most everyday Internet routers and LAN switches operate on a best effort basis.
Next, we have Integrated Services
This is where the entire path from Point A to Point B will be defined in advance when Integrated Services (IntServ) are in effect. Integrated Services is much like the High-Occupancy Vehicle lanes found in many larger cities. If your car has three or more people in it, you’re considered a “priority vehicle” and you can drive in a special lane with much less congestion than regular lanes. Integrated Services will create this lane in advance for “priority traffic”, and when that traffic comes along, the path already exists. Integrated Services uses the Resource Reservation Protocol (RSVP) to create these paths. RSVP guarantees a quality rate of service, since this “priority path” is created in advance.
Integrated Services is defined in RFC 1633, released June 1994 . Use your favourite search engine to find a copy online and read more about this topic. Besides end-to-end signalling, IntServ requires several functions on routers and switches along the path:
Admission Control: determine whether a new flow can be granted the requested QoS without impacting existing reservations.
Classification: recognize packets that need certain levels of QoS.
Policing: taking action, including possibly dropping packets, when traffic does not conform to its specified characteristics.
Queuing and Scheduling: forward packets according to those QoS requests that have been granted.
Differentiated Services (DiffServ) model
Of course, if you’ve got a lot of different dedicated paths being created that may or not be used very often, that’s a lot of wasted bandwidth. That leads us to the third QoS model, the Differentiated Services (DiffServ) model.
Generally referred to as DiffServ, there are no advance path reservations and there’s no RSVP. The QoS policies are written on the routers and switches, and they take action dynamically as needed. In the DiffServ model, packets are classified and marked to receive a particular forwarding treatment (per-hop behavior or PHB) on nodes along their path. Since each router and switch can have a different QoS policy, DiffServ takes effect on a per-hop basis rather than the per-flow basis of Integrated Services. A packet can be considered “high priority” by one router and “normal priority” by the next. DiffServ architecture model can be found in RFC 2475, released in December 1998 .
What affects Quality of service?
Well that’s the types, but what affects Quality of service. Quality of service is affected by lots of factors, which can be divided into two main areas human and technical factors. Human factors include: stability of service quality, availability of service, waiting times and user information.
The technical factors include reliability, scalability, effectiveness, maintainability and network congestion. Now let’s get a bit technical on what affects QoS .
Due to varying load from disparate users sharing the same network resources, the bit rate (the maximum throughput) that can be provided to a certain data stream may be too low for real-time multimedia services if all data streams are get the same priority.
The routers might fail to deliver (drop) some packets if their data loads are corrupted, or the packets arrive when the router buffers are already full. The receiving application may ask for this information to be sent again. This will cause delays.
Sometimes packets are corrupted due to bit errors caused by noise and interference, especially in wireless communications and long copper wires. Which results in the packet being dropped.
It might take a long time for each packet to reach its destination, because it gets held up in long queues, or it takes a less direct route to avoid congestion. This is different from throughput, as the delay can build up over time, even if the throughput is almost normal. In some cases, excessive latency can render an application such as VoIP or online gaming unusable.
Packets from the source will reach the destination with different delays. A packet’s delay varies with its position in the queues of the routers along the path between source and destination and this position can vary unpredictably. This variation in delay is known as jitter and can seriously affect the quality of streaming audio and/or video.
When a collection of related packets is routed through a network, different packets may take different routes, each resulting in a different delay. The result is that the packets arrive in a different order than they were sent. This problem requires special additional protocols responsible for rearranging out-of-order packets to an isochronous state once they reach their destination. This is especially important for video and VoIP streams where quality is dramatically affected by both latency and lack of sequence.
Believe me, this is just the beginning when it comes to Quality of Service. It’s a huge topic and in the real world’s production networks.