Quality of Service

Quality of Service ( QoS) and quality of service indicator of the quality of a communication service from the perspective of the user, that is, how much the quality of service corresponds to the requirements. Formal QoS is a set of quality requirements on the collective behavior or interaction of multiple objects.

  • 3.1 implementation in IP networks

Requirements

IEEE 802.1p

  • A user wants to be reliably connected to the desired destination and can be reliably separated by the end of the communication.
  • The connection should be rapid.
  • Problems connecting (eg destination subscriber not reachable) are to be communicated to the user as quickly as possible.
  • A communication connection is maintained stable.
  • The communication nodes want to understand each other.
  • The information to be transmitted completely and without errors.
  • There are no information other communication participants and no disturbances are transmitted.
  • The communication should go right to the original as possible.
  • There are are no long waiting periods during the communication.
  • The settlement of the communication should correspond to the correct time and data volume.

In order to measure such general user requirements, quality parameters are defined to describe these requirements.

  • Quality of the connection setup: Percentage of non- established connections, percentage of false connections, percentage of double compounds, percentage of slow connection setups.
  • Quality of an existing connection: Percentage of early disconnections, percentage of non-transmitted information, the percentage of incorrectly transmitted information, delays in the information transmission, Effective bandwidth of the transmitted information, fluctuations of the signal delay, percentage of faults on the transmitted signals, echo components.

By measuring this network-related parameters and assign them to user requirements, one tries to capture the quality of service numerically.

Since the provision of a service at high quality requirements is correspondingly complex, go network operator to demonstrate high standards of quality with higher prices. Here that certain quality parameters is promised for a certain quality standard. The customer may request a specific quality of service (see Service Level Agreement).

Adherence to high quality standards requires the proper functioning of all interacting components of a telecommunications network. The operation of the equipment must be monitored to, error parameters need to be detected and recorded, and thus form the basis for any necessary maintenance.

QoS in telecommunications networks

Major categories of QoS in communication networks are the grade, which deals with the switching technology, and the transmission quality, which deals with the transfer of technology.

In UMTS mobile four QoS classes are defined, for example:

QoS in switching technology

With quality traffic (English: grade of service ) refers to the dependent and quantifiable by the respective assessment of the resources of the QoS. The grade of a communication network depends on the measurement of the number of lines and control devices. In the telephone network, the switches are control devices, the Internet, it is the router. Quantitative descriptions of the grade of service parameters using traffic theory such as waiting probability, mean waiting time or loss probabilities. In the switching technology go, for example, among others, the following factors into a QoS:

  • Double link: A compound in which the desired connection of one or more additional connections are switched
  • Incorrect connection: A connection in which, despite correctly entered phone number other than the desired party attained
  • No trip: continuation of a connection, even though the was required to "TRIP"
  • Premature separation: Disconnect calls, without having been initiated by the calling or called terminal from
  • Difficulties in measuring: false detection of toll- relevant parameters of a connection

In this example, then the measured and weighted counting of such events form a part of the transport quality of a certain switch.

Other determinants of quality of service are:

  • Blocking: the state of a network in which a connection can not be completed because the necessary technical facilities or resources are not available.
  • Transmission delay: time period that elapses between the beginning of the input at the origin and the beginning of the output at the destination
  • Delay of signals from the connection control: For example: time between the signals " Outgoing Call " ( handset off-hook ) and " choice prompt" ( dial tone) is too high.

QoS in IP networks

The quality of service of services over IP networks - networks that use the Internet Protocol - be transferred is influenced by additional IP-specific parameters. If IP is used in a telecommunication network as the transmission protocol, so it is according to the OSI model a higher layer in this network. Therefore, in this case, for the assessment of overall quality of service of a service based on IP technology, both the merit of the lower layers, and the transmission system parameters to the relevant IP level. The perceived by the user quality of service results from the interaction of all quality impairments. Is it, for example, an Internet access, which is via a public telecommunications network via a dial-up, so the whole quality of service of transmission, transport and placement quality of the transmission channel via the public telecommunications network and the quality of higher IP layers forms. If IP is not transmitted through a public telecommunications network, but for example in a LAN based on Ethernet, so only the quality impairments this LANs and IP transmission are taken into account. A special factor in the quality of service of services that can be realized with the help of the Internet Protocol, is that it is unlike services in traditional telecommunications networks significantly affected by the terminal. In conventional ( circuit-switched ) telecommunications networks, the transmission, switching and quality of service, however, was mostly based on the quality of the network; the influences of the terminals could be neglected.

In IP networks, the influence on the QoS is usually detected by means of the following parameters:

  • Latency: the delay of the end-to- end transmission
  • Jitter: The deviation of the latency of their mean
  • Packet loss rate: the probability that individual IP packets are lost during transmission ( or - for real-time services - too late to achieve its goal )
  • Throughput per unit time transferred in the central data set

Different services have different requirements on these parameters. The latency is particularly evident in applications that require short response times ( for example, Telnet), noticeable. For example, an entry appears in a larger latency only with a certain time delay on the screen. For pure file transfers usually the overall throughput of the key parameter, the individual latency and loss rate, however, are less important here. For real-time communications such as voice over IP, however, the latency, jitter and loss rate play a far greater role, because they significantly influence the speech intelligibility. IPTV has a real-time application even quite substantial demands on the overall quality of service, since even small quality defects in the transmission itself visible impact in the image on the TV. The throughput is important because videos often require a high data rate and failure to provide this rate, the video is simply stopped.

Within a channel, it may be necessary to increase the QoS, for certain data streams to the detriment of other data streams. This can be done for example by prioritizing IP packets based on certain characteristics and properties. With these mechanisms, it is possible to make certain services such as Voice -over-IP, which requires a delay constant and continuous stream of data, more preferable than downloading a file server ( FTP) or the call of websites.

It ensures through certain reservation protocols in the network, that for the entire duration of the isochronous data communication can be assured of data streams.

IPv4 and IPv6 packets default to a flag ( DSCP Differentiated Service Code Point ( RFC 2474 ); earlier Precedence (RFC 791 ) ) in the IP header that indicates the nature of the data in this package are ( Traffic Class ). Using this flag, the data packets are (ie preferred) handled with priority. The current thinking to describe (RFC 3260 ). There are a number of other methods for the management of quality of service.

Implementation in IP networks

At the theoretical level QoS can be realized by prioritizing or parameterization of the data traffic, data rates, reservation data rate limitation and optimization package. Technically speaking, there are two mechanisms for this:

  • Either you reports pending data flows, all the active network components ( routers, etc. ) and reserved the required data rate ( IntServ, Integrated Services )
  • Or you can select all data packets and the active network components handle / prefer the packets according to their marks ( DiffServ, Differentiated Services ).

Since IntServ, in practice usually means the Resource Reservation Protocol ( RSVP), has a high administrative overhead and result already a device that does not support IntServ can fail the whole mechanism, the alternative DiffServ has now become established. DiffServ is also more scalable. Often the term Type of Service (ToS ) is encountered. For ToS 1 byte in the IP header was registered, of which only 6 bits were used by ToS. However, it could be established no binding standard, comply with all the manufacturers of network equipment. Meanwhile, the six most significant bits of the previous ToS field in the IPv4 header as well as the 'Type of Service octet in the IPv6 header of the Internet Engineering Task Force ( IETF) redefined. They are now called uniformly Differentiated Services Field and can be a numerical value Differentiated Services Code Point (DSCP ) are evaluated.

Since the practical implementation of comprehensive QoS measures on the Internet hardly developed yet for years, there are critical voices, whether QoS mechanisms on the Internet are ever needed or suitable.

One reason for the stagnation is the fact that so far neither explicit numerical values ​​for QoS parameters such as jitter and packet loss rate could still prevail at all a consistent use of code points and a uniform definition for the per- hop behavior in DiffServ. The EU project MUSE of the EU's 6th Framework Programme has solved this drawback by defining four QoS classes which these values ​​are fixed. In another European research project PLANETS from a practical implementation proposal was derived, in which even the lengths of the queues are given.

QoS in ATM networks

In establishing a connection in ATM networks the end system is with the service provider of a guaranteed class of service / QoS class.

  • Constant Bit Rate (CBR)
  • Real-time variable bit rate ( rt- VBR)
  • Nonrealtime variable bit rate ( nrt - VBR)
  • Unspecified Bit Rate / Available Bit Rate ( UBR / ABR)

QoS Parameter:

  • Jitter
  • Error rate
  • Delay (latency)
  • Bit error rate
  • Packet loss
  • Echoes

The best-known queuing mechanisms are:

  • FIFO (First In - First Out) - The data packets that arrive first will be forwarded as a first
  • LIFO ( Last In - First Out) - The data packets that arrive last are first routed
  • LLC ( Low Latency Custom Queuing ) - There is a small runtime guarantees of predefined packages
  • Priorities queuing - The data packet with the highest priority will be forwarded before the low-priority packets.

For details on individual queuing algorithms, refer to Network Congestion Avoidance

Traffic engineering (telecommunications) Flag word Parameterized complexity Asynchronous Transfer Mode
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