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Quality of service, or QoS, in the field of telephony, was defined in the ITU standard X.902 as "A set of quality requirements on the collective behavior of one or more objects." In telecommunication, Telephony encompasses the general use of equipment to provide voice communication over distances. ... This article is about the location. ...


In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service refers to resource reservation control mechanisms. Quality of Service can provide different priority to different users or data flows, or guarantee a certain level of performance to a data flow in accordance with requests from the application program or the internet service provider policy. Quality of Service guarantees are important if the network capacity is limited, for example in cellular data communication, especially for real-time streaming multimedia applications, for example voice over IP and IP-TV, since these often require fixed bit rate and are delay sensitive. In computer networking and telecommunications, packet switching is a communications paradigm in which packets (messages or fragments of messages) are individually routed between nodes, with no previously established communication path. ... For the scientific and engineering discipline studying computer networks, see Computer networking. ... For another meaning of the term traffic engineering, please see transport traffic engineering. ... Streaming media is media that is consumed (heard or viewed) while it is being delivered. ... An overview of how VoIP works A typical analog telephone adapter for connecting an ordinary phone to a VoIP network Ciscos implementation of VoIP - IP Phone Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of... It has been suggested that IP Media be merged into this article or section. ...


A network or protocol that supports Quality of Service may agree on a traffic contract with the application software and reserve capacity in the network nodes, for example during a session establishment phase. During the session it may monitor the achieved level of performance, for example the data rate and delay, and dynamically control scheduling priorities in the network nodes. It may release the reserved capacity during a tear down phase. If a service (or application) wishes to use a broadband network (an ATM network in particular) to transport a particular kind of traffic, it must first inform the network about what kind of traffic is to be transported, and the performance requirements of that traffic[1]. The application presents this...


Quality of Service comprises all the aspects of a connection, such as time to provide service, voice quality, echo, loss, reliability and so on. A subset of telephony QoS is Grade of Service (GOS), which comprises aspects of a connection relating to the capacity of a network.[1] // Introduction In telecommunication, the quality of voice service is specified by two measures: The GOS (grade of service) and the QoS (quality of service). ...


A best-effort network or service does not support Quality of Service. Best effort delivery describes a network service in which the network does not provide any special features that recover lost or corrupted data. ...


The term Quality of Service is sometimes used as a quality measure, with many alternative definitions, rather than referring to the ability to reserve resources. Quality of Service sometimes refers to the level of Quality of service, i.e. the guaranteed service quality. High QoS is often confused with a high level of performance or achieved service quality, for example high bit rate, low latency and low bit error probability. See also Relation to subjective quality measures below. In telecommunications and computing, bit rate (sometimes written bitrate) is the frequency at which bits are passing a given (physical or metaphorical) point. It is quantified using the bit per second (bit/s) unit. ... Lag is a common term used to describe a symptom often encountered in computing and especially networked systems, where results of actions appear much later than expected. ... In telecommunication, an error ratio is the ratio of the number of bits, elements, characters, or blocks incorrectly received to the total number of bits, elements, characters, or blocks sent during a specified time interval. ...

Contents

Relation to subjective quality measures

An alternative and disputable definition of QoS used especially in telephony and streaming video is a metric that reflects or predicts the subjectively experienced quality, for example the "user perceived performance" [2], the "degree of satisfaction of the user", the "number of happy customers", the Mean Opinion Score (MOS) value, or the Quality of Experience (QoE) subjective business concept. In this context, QoS is the cumulative effect on subscriber satisfaction of all imperfections affecting the service. This definition includes the application and the human in the assessment, and demands an appropriate weighting of diverse objective measures such as response time, interrupts, noise, cross-talk, loudness levels, frequency response, noticeable echos, etc., and also includes grade of service. Streaming media is just-in-time delivery of multimedia information. ... [info from searchnetworking. ... Quality of Experience (QoE), also known as Quality of User Experience, is a subjective measure of a customers experiences with a vendor. ... // Introduction In telecommunication, the quality of voice service is specified by two measures: The GOS (grade of service) and the QoS (quality of service). ...


Problems

When the Internet was first deployed many years ago, it lacked the ability to provide Quality of Service guarantees due to limits in router computing power. It therefore ran at default QoS level, or "best effort". There were four "Type of Service" bits and three "Precedence" bits provided in each message, but they were ignored. These bits were later re-defined as DiffServ Code Points (DSCP) and are largely honored in peered links on the modern Internet. DiffServ or differentiated services is a method of trying to guarantee quality of service on large networks such as the Internet. ...


When looking at circuit-switched networks, Quality of service is affected by various factors, which can be divided into "human" and "technical" factors. Human factors include: stability of service, availability of service, delays, user information. Technical factors include: reliability, scalability, effectiveness, maintainability, Grade of Service, etc.[3]


Many things can happen to packets as they travel from origin to destination, resulting in the following problems as seen from the point of view of the sender and receiver:

Dropped packets 
The routers might fail to deliver (drop) some packets if they arrive when their buffers are already full. Some, none, or all of the packets might be dropped, depending on the state of the network, and it is impossible to determine what will happen in advance. The receiving application may ask for this information to be retransmitted, possibly causing severe delays in the overall transmission.
Delay 
It might take a long time for a packet to reach its destination, because it gets held up in long queues, or takes a less direct route to avoid congestion. In some cases, excessive delay can render an application, such as VoIP, unusable.
Jitter 
Packets from 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 unpredicably. This variation in delay is known as jitter and can seriously affect the quality of streaming audio and/or video.
Out-of-order delivery 
When a collection of related packets is routed through the Internet, 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 necessitates 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 or lack of isochronicity.
Error 
Sometimes packets are misdirected, or combined together, or corrupted, while en route. The receiver has to detect this and, just as if the packet was dropped, ask the sender to repeat itself.

In telecommunication, jitter is an abrupt and unwanted variation of one or more signal characteristics, such as the interval between successive pulses, the amplitude of successive cycles, or the frequency or phase of successive cycles. ... Isochronous means having an equal time difference or occurring simultaneously. ...

Applications requiring QoS

A defined Quality of Service may be required for certain types of network traffic, for example:

  • streaming multimedia may require guaranteed throughput to insure that a minimum level of quality is maintained.
  • IP telephony or Voice over IP (VOIP) may require strict limits on jitter and delay
  • Video Teleconferencing (VTC) requires low jitter and latency
  • Alarm signalling (eg. Burglar alarm)
  • dedicated link emulation requires both guaranteed throughput and imposes limits on maximum delay and jitter
  • a safety-critical application, such as remote surgery may require a guaranteed level of availability (this is also called hard QoS).
  • a remote system administrator may want to prioritize variable, and usually small, amounts of SSH traffic to ensure a responsive session even over a heavily-laden link.

These types of service are called inelastic, meaning that they require a certain level of bandwidth and a certain maximum latency to function. This article does not cite any references or sources. ... IP Telephony, also called Internet telephony, is the technology that makes it possible to have a telephone conversation over the Internet or a dedicated Internet Protocol (IP) network instead of dedicated voice transmission lines. ... Burglar (or intrusion), fire and safety alarms are found in electronic form today. ... A life-critical system or safety-critical system is a system whose failure or malfunction may result in death or serious injury. ... Remote surgery (also known as telesurgery) is the ability for a doctor to perform surgery on a patient even though they are not physically in the same location. ... In telecommunications and reliability theory, the term availability has the following meanings: 1. ...


By contrast, elastic applications can take advantage of however much or little bandwidth is available.


Obtaining QoS

  • Per call
  • In call
  • In advance: When the expense of mechanisms to provide QoS is justified, network customers and providers typically enter into a contractual agreement termed an SLA (Service Level Agreement) which specifies guarantees for the ability of a network/protocol to give guaranteed performance/throughput/latency bounds based on mutually agreed measures, usually by prioritizing traffic.
  • Reserving resources: Resources are reserved at each step on the network for the call as it is set up. An example is RSVP, Resource Reservation Protocol.

Service Level Agreement (SLA) is that part of a service contract where the level of service is formally defined. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a transport layer protocol designed to reserve resources across a network for an integrated services Internet. ...

QoS mechanisms

Quality of Service (QoS) can be provided by generously over-provisioning a network so that interior links are considerably faster than access links. This approach is relatively simple, and may be economically feasible for broadband networks with predictable and light traffic loads. The performance is reasonable for many applications, particularly those capable of tolerating high jitter, such as deeply-buffered video downloads.


Commercial VoIP services are often competitive with traditional telephone service in terms of call quality even though QoS mechanisms are usually not in use on the user's connection to his ISP and the VoIP provider's connection to a different ISP. Under high load conditions, however, VoIP quality degrades to cell-phone quality or worse. The mathematics of packet traffic indicate that a network with QoS can handle four times as many calls with tight jitter requirements as one without QoS. The amount of over-provisioning in interior links required to replace QoS depends on the number of users and their traffic demands. As the Internet now services close to a billion users, there is little possibility that over-provisioning can eliminate the need for QoS when VoIP becomes more commonplace.


For narrowband networks more typical of enterprises and local governments, however, the costs of bandwidth can be substantial and over provisioning is hard to justify.[4] In these situations, two distinctly different philosophies were developed to engineer preferential treatment for packets which require it.


Early work used the "IntServ" philosophy of reserving network resources. In this model, applications used the Resource reservation protocol (RSVP) to request and reserve resources through a network. While IntServ mechanisms do work, it was realized that in a broadband network typical of a larger service provider, Core routers would be required to accept, maintain, and tear down thousands or possibly tens of thousands of reservations. It was believed that this approach would not scale with the growth of the Internet, and in any event was antithetical to the notion of designing networks so that Core routers do little more than simply switch packets at the highest possible rates. In computer networking, IntServ or integrated services is an architecture, which specifies the elements to guarantee quality of service (QoS) on networks. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a transport layer protocol designed to reserve resources across a network for an integrated services Internet. ...


The second and currently accepted approach is "DiffServ" or differentiated services. In the DiffServ model, packets are marked according to the type of service they need. In response to these markings, routers and switches use various queuing strategies to tailor performance to requirements. (At the IP layer, differentiated services code point (DSCP) markings use the 6 bits in the IP packet header. At the MAC layer, VLAN IEEE 802.1Q and IEEE 802.1D can be used to carry essentially the same information) This article does not cite any references or sources. ... DiffServ Code Point (DSCP) is a field in packets transferred on DiffServ networks for packet classification purposes. ... A virtual LAN, commonly known as a VLAN, is a logically segmented network mapped over physical hardware. ... IEEE 802. ... Links ANSI/IEEE Std 802. ...


Routers supporting DiffServ use multiple queues for packets awaiting transmission from bandwidth constrained (e.g., wide area) interfaces. Router vendors provide different capabilities for configuring this behavior, to include the number of queues supported, the relative priorities of queues, and bandwidth reserved for each queue.


In practice, when a packet must be forwarded from an interface with queuing, packets requiring low jitter (e.g., VoIP or VTC) are given priority over packets in other queues. Typically, some bandwidth is allocated by default to network control packets (e.g., ICMP and routing protocols), while best effort traffic might simply be given whatever bandwidth is left over. IP Telephony, also called Internet telephony, is the technology that makes it possible to have a telephone conversation over the Internet or a dedicated Internet Protocol (IP) network instead of dedicated voice transmission lines. ... VTC = The Venerable Tagging Crew ... The (ICMP) is one of the core protocols of the Internet protocol suite. ...


Additional bandwidth management mechanisms may be used to further engineer performance, to include: The tone or style of this article or section may not be appropriate for Wikipedia. ...

As mentioned, while DiffServ is used in many sophisticated enterprise networks, it has not been widely deployed in the Internet. Internet peering arrangements are already complex, and there appears to be no enthusiasm among providers for supporting QoS across peering connections, or agreement about what policies should be supported in order to do so. Traffic shaping (also known as packet shaping) is an attempt to control computer network traffic in order to optimize or guarantee performance, low latency, and/or bandwidth by delaying packets[1]. Traffic shaping deals with concepts of classification, queue disciplines, enforcing policies, congestion management, quality of service (QoS), and fairness. ... In computer networks, rate limiting is the function of controlling the maximum rate of traffic sent or received on a network interface. ... Although the token bucket algorithm has several uses, it is best understood in the context of network traffic shaping or rate limiting. ... Although the leaky bucket algorithm has several uses, it is best understood in the context of network traffic shaping or rate limiting. ... It has been suggested that this section be split into a new article entitled Scheduling (communications). ... Weighted Fair Queuing (WFQ) is a packet scheduling technique allowing guaranteed bandwidth services. ... A network router queueing method that allows traffic to share bandwidth equally, after being grouped by classes. ... Weighted round robin (WRR) is a best-effort connection scheduling discipline. ... Deficit round robin (DRR), also deficit weighted round robin (DWRR), is a modified weighted round robin scheduling discipline. ... Network congestion avoidance is a process used in computer networks to avoid congestion. ... Network congestion avoidance is a process used in computer networks to avoid congestion. ... It has been suggested that TCP and UDP port be merged into this article or section. ... Network congestion avoidance is a process used in computer networks to avoid congestion. ... TCP global synchronization in data networking can happen to TCP/IP flows during periods of congestion because each sender will reduce their transmission rate at the same time when packet loss occurs. ... Network congestion avoidance is a process used in computer networks to avoid congestion. ... DiffServ or differentiated services is a method of trying to guarantee quality of service on large networks such as the Internet. ... Peering is voluntary interconnection of administratively separate Internet networks for the purpose of exchanging traffic between the customers of each network. ...


One compelling example of the need for QoS on the Internet relates to this issue of congestion collapse. The Internet relies on congestion avoidance protocols, as built in to TCP, to reduce traffic load under conditions that would otherwise lead to Internet Meltdown. QoS applications such as VoIP and IPTV, because they require largely constant bitrates and low latency cannot use TCP, and cannot otherwise reduce their traffic rate to help prevent meltdown either. QoS contracts limit traffic that can be offered to the Internet and thereby enforce traffic shaping that can prevent it from becoming overloaded, hence they're an indispensable part of the Internet's ability to handle a mix of real-time and non-real-time traffic without meltdown. In data networking and queueing theory, network congestion occurs when incremental increases in offered load lead either only to small increases in network throughput, or to an actual reduction in network throughput. ... IP Telephony, also called Internet telephony, is the technology that makes it possible to have a telephone conversation over the Internet or a dedicated Internet Protocol (IP) network instead of dedicated voice transmission lines. ... This article is about internet protocol television. ... The Transmission Control Protocol (TCP) is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data using Stream Sockets. ...


Asynchronous Transfer Mode (ATM) network protocol has an elaborate framework to plug in QoS mechanisms of choice. Shorter data units and built-in QoS were some of the unique selling points of ATM in the telecommunications applications such as video on demand, voice over IP. The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... In networking, a communications protocol or network protocol is the specification of a set of rules for a particular type of communication. ... The Unique Selling Proposition was proposed as a theory to explain a pattern among successful advertising campaigns of the early 1940s. ... Telecommunication involves the transmission of signals over a distance for the purpose of communication. ... Video on demand (VOD) systems allow users to select and watch video and clip content over a network as part of an interactive television system. ... An overview of how VoIP works A typical analog telephone adapter for connecting an ordinary phone to a VoIP network Ciscos implementation of VoIP - IP Phone Voice over Internet Protocol, also called VoIP, IP Telephony, Internet telephony, Broadband telephony, Broadband Phone and Voice over Broadband is the routing of...


QoS Priority Levels

Priority Level Traffic Type
0 Best Effort
1 Background
2 Standard (Spare)
3 Excellent Load

(Business Critical)

4 Controlled Load

(Streaming Multimedia)

5 Video

(Interactive Media)


[Less than 100ms latency and jitter]

6 Voice

(Interactive Voice)


[Less than 10ms latency and jitter]

7 Network Control Reserved Traffic

[Lowest latency and jitter]

QoS problems

  • Internet2 QoS Working Group concluded that increasing bandwidth is probably more practical than implementing QoS.[1][2]

Protocols that provide Quality of Service

This article does not cite any references or sources. ... In the context of computer networking, frame relay consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... X.25 is an ITU-T standard protocol suite for wide area networks using leased lines, the phone or ISDN system as the networking hardware. ... Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide. ... In computer networking, IntServ or integrated services is an architecture, which specifies the elements to guarantee quality of service (QoS) on networks. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a transport layer protocol designed to reserve resources across a network for an integrated services Internet. ... Resource Reservation Protocol Protocol that supports the reservation of resources across an IP network. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... In computer networking and telecommunications, Multi Protocol Label Switching (MPLS) is a data-carrying mechanism that belongs to the family of packet-switched networks. ... IEEE 802. ... IEEE 802. ... 802. ... RFC 1349, Type of Service in the Internet Protocol Suite The TOS byte in the IPv4 header has had various purposes over the years, and has been defined in different ways by five different RFCs. ...

QoS Solutions

The research project MUSE defined a QoS concept which was further worked out in another research project PLANETS. The new idea of this solution is to agree on a discrete jitter value per QoS class which is imposed on network nodes. Including best effort four QoS classes were defined, two elastic and two inelastic. The solution has several benefits:

  • End-to-end delay and packet loss rate can be predicted
  • It is easy to implement with simple scheduler and queue length given in PLANETS
  • Nodes can be easily verified for compliance
  • End users do notice the difference in quality

See also

The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... Best effort delivery describes a network service in which the network does not provide any special features that recover lost or corrupted data. ... Class of Service (CoS) is a 3 bit field within a layer two Ethernet frame header when using IEEE 802. ... // Introduction In telecommunication, the quality of voice service is specified by two measures: The GOS (grade of service) and the QoS (quality of service). ... [info from searchnetworking. ... Network neutrality (equivalently net neutrality, Internet neutrality or NN) refers to a principle that is applied to residential broadband networks, and potentially to all networks. ... Quality of Experience (QoE), also known as Quality of User Experience, is a subjective measure of a customers experiences with a vendor. ... Senator Ted Stevens of Alaska, who referred to the Internet as a series of tubes. ... Streaming media is multimedia that is continuously received by, and normally displayed to, the end-user while it is being delivered by the provider. ... Subjective video quality is a subjective characteristic of video quality. ... It has been suggested that Two-tiered Internet be merged into this article or section. ... Traffic shaping (also known as packet shaping) is an attempt to control computer network traffic in order to optimize or guarantee performance, low latency, and/or bandwidth by delaying packets[1]. Traffic shaping deals with concepts of classification, queue disciplines, enforcing policies, congestion management, quality of service (QoS), and fairness. ... QPPB, an acronym for QoS Policy Propagation via BGP, is a mechanism that allows propagation of QoS policy and classification by the sending party based on access lists, community lists and AS paths, thus helping to classify based on destination instead of source address. ...

Books

  • "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by John Evans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-12-370549-5)

External links

  • QoS Tutorial
  • Quality of Service Networking
  • Network QoS

Notes

  1. ^ ITU-T Study Group 2, Teletraffic Engineering Handbook (350 pages, 2.7 MB)(It uses abbreviation GoS instead of GOS) http://www.com.dtu.dk/teletraffic/handbook/telenook.pdf
  2. ^ Leonard Franken. Quality of Service Management: A Model-Based Approach. PhD thesis, Centre for Telematics and Information Technology, 1996.
  3. ^ Peuhkuri M., IP Quality of Service, Helsinki University of Technology, Laboratory of Telecommunications Technology, 1999.
  4. ^ http://bennett.com/blog/index.php/archives/2006/08/17/how-much-bandwidth-is-enough/

  Results from FactBites:
 
RFC 2990 (rfc2990) - Next Steps for the IP QoS Architecture (8004 words)
QoS architectures add a somewhat different constraint, in that the network is placed in an active role within the task of resource allocation and service delivery, rather than being a passive object that requires end systems to adapt.
QoS signaling protocols which are intended to undertake resource management and admission control require the use of identity authentication and integrity protection in order to mitigate this potential for theft of resources.
Both forms of QoS architecture require the internal elements of the network to be able to undertake classification of traffic based on some form of identification that is carried in the packet header in the clear.
Quality of service - Wikipedia, the free encyclopedia (1607 words)
In the field of telephony, telephony quality of service QoS was defined by the ITU as the cumulative effect on subscriber satisfaction of all imperfections affecting a telephone conversation.
QoS skeptics further point out that if you are dropping many packets on elastic low-QoS connections, you are already dangerously close to the point of congestion collapse on your inelastic high-QoS applications, without any way of further dropping traffic without violating traffic contracts.
QoS contracts limit traffic that can be offered to the Internet and thereby enforce traffic shaping that can prevent it from becoming overloaded, hence they're an indispensable part of the Internet's ability to handle a mix of real-time and non-real-time traffic without meltdown.
  More results at FactBites »

 
 

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