Quality of Service (QoS)

Telecommunication networks, at the beginning, were implemented using technologies based on circuit switching techniques. In this type of networks, since a circuit dedicated for each communication is reserved, there is a minimum and fixed delay. It is also ensured that no congestion can exist for the communication already established (any attempt of connection is blocked if the network does not have the necessary resources). The networks based on circuit switching technologies have high quality of service since they were designed to support services for applications sensitive to losses and delays.

The packet switched networks are designed using statistic multipliplexation. These networks are the most efficient and they are adapted to grant services to most of the applications, but the variable delay and the possible loss of information can generate problems to the sensitive traffic in these parameters. In these networks, instead of blocking new attempts of communication when a lack of resources exists, they are crossed, saving the exceeding traffic in buffers during the utilization peaks. This technique generates a variable delay, correspondent to the time the packets are saved in the buffers. Additionally, as the buffers have a limit, during periods of congestion, it is possible to discard packets.

The convergent networks that support multiple services with an only infrastructure are the best option today for the operators because of their versatility and efficiency respecting its cost. However, it is possible that they don’t support a specific application with the efficiency that a constituted network for such application. In order to mitigate this behavior, the application of quality of service schemes gains an important role in convergent networks, since they must be flexible and support many types of applications and services.

The most elementary function in a quality of service scheme is to recognize the different traffic flows coming from different applications. Once it is recognized, it can continue to specify in which way each stream of traffic is treated in the network. This way, one can prioritize a type of traffic over another using the resources of the network, like, for example, prioritize the traffic sensible to delays and losses. The quality of service scheme allows to control the access to available resources.

The main parameters which are used for a quality of service scheme are:

  • Bandwidth: It defines the capacity to transfer information from end to end. When there is plenty bandwidth, there are no problems as regards quality, since the quality of service scheme is applied when there is congestion. This way, to increment the bandwidth is a way to solve the congestion problem.
  • Delay: it defines the delay there is in the communications between endpoints. Its cause is the delay associated to lines and technologies of transmission and the delay of buffering associated to the traffic queues in networks that use statistic multiplexation.
  • Jitter: This parameter indicates the different values of delay that can be presented in the packets of a communication. Even if a delay value is undesirable, having a variable delay value is more disruptive for the applications, since it is not predictable and it is difficult for the applications to adjust to it.
  • Loss: This parameter refers to the loss of packets in a conversation. For certain applications, like the ones that require transmission in real time, this parameter can be very disruptive, since the cost of a retransmission is generally higher and more disruptive than the loss of information. The retransmission implies an increase in the normal transmission time. The loss of packets can be due to errors in them, failures in the equipment or exceeded buffers.

The quality of service scheme has an important role in the communication networks of today and it influences directly in the perception that the users have of the network service. Each manufacturer applies a quality of service scheme in their own equipment, and when making a design, it is very important to take into account the compatibility between the standards. In Auben, we have a wide experience in the design, implementation and operation of quality of service schemes, and we apply those concepts to the networks of our clients to achieve an infrastructure that provides the best possible service.

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