Quality of service

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In the field of telephony, telephony quality of service QoS was defined in the ITU standard X.902 as "A set of quality requirements on the collective behavior of one or more objects."

In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to control mechanisms that 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. Quality of Service guarantees are important if the network capacity is limited, especially for real-time streaming multimedia applications, for example voice over IP and IP-TV, since these often require fixed bit rate and may be delay sensitive.

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 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.

A best-effort network or service does not support Quality of Service.

The term Quality of Service is sometimed used as a quality measure with many alternative definitions, rather than referring to control mechanisms. In computer networking, a good QoS may mean advanced QoS mechanisms, or high probability that the network is able to provide the requested level of performance. High QoS is often confused with a high level of performance, for example high bit rate, low latency and low bit error probability.

Another widespread definition used especially in telephony and streaming video is "user perceived performance" [1] or "degree of satisfaction of the user". In this context, QoS is the cumulative effect on subscriber satisfaction of all imperfections affecting the service. This definition includes the human in the assessment and demands an appropriate subjective weighting of diverse defects such as response time, interrupts, noise, cross-talk, loudness levels, frequency response, noticeable echos, etc., and also includes grade of service. This definition resembles to the Mean Opinion Score (MOS) value, which is a subjective quality measure that can be predicted based on objective performance measures.

Contents

[edit] 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.

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 must 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. Alternatively, it might follow a fast, direct route. Thus delay is very unpredictable.
Jitter 
Packets from source will reach the destination with different delays. 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 to the one with which 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 impacted 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.

[edit] 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
  • IP telephony or Voice over IP (VOIP) may require strict limits on jitter and delay
  • Video Teleconferencing (VTC) requires low jitter
  • 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).

These types of service are called inelastic, meaning that they require a certain level of bandwidth to function - any more than required is unused, and any less will render the service non-functioning.

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

[edit] 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 prioritising traffic.
  • Reserving resources: Resources are being reserved at each step on the network for the call as it is set up. An example is RSVP, Resource Reservation Protocol.

[edit] 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.[2] 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.

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)

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.

Additional bandwidth management mechanisms may be used to further engineer performance, to include:

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.

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.

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 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.

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.

[edit] 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]

[edit] QoS problems

QoS has not been generally deployed in the public Internet. The main reasons for this can summarized as follows:
1. QoS mechanisms only need to go into action when there is scarce capacity and various communications flows are competing. So if there is enough capacity, QoS is not necessary.
2. QoS mechanisms only seem to work when the network is almost full and not fully full. The result is QoS only adds a couple of percentage points of extra capacity under the right circumstances.
3. Qos only works if the switches can derive an order in which to treat applications. If all-streams have top-priority there is no way to determine which ones should get priority over other ones.
4. There is a glut in backbone capacity
5. It is cheaper to upgrade to higher capacity lines or equipment.
6. Traffic growth of IP-traffic is up to 100% per year in the western world. AMS-IX StatisticsThis has as an effect that the benefits of QoS are often short-lived. It gets an operator through the month, not through to next year.
7. QoS needs management and constant attention to determine that shortages don't occur. Overengineering the network results in less attention.
8. It's hard for telecommunications companies to determine what their customers find important applications. The telecommunications company might give higher priority to different application then the customer.
9. Getting QoS to work over the edges of multiple networks requires management and agreements of how to prioritise traffic between those networks. It's often easier to add extra capacity to interconnects.
10. Where it is possible to make guarantees on capacity it's generally easier not to determine priority per flow, but to give the customer a guaranteed amount of capacity between two points. This allows the customer to make decisions on priority. This is the basis of most MPLS implementations.

Papers discussing the technical and commercial reasons for this include:

  • Internet2 QoS Working Group determined that "it doesn't scale" to large networks (such as the Internet).[1][2]
  • Why We Don't Need QOS: Trains, Cars, and Internet Quality of Service by Dan Bricklin.[3]

[edit] Protocols that provide Quality of Service

[edit] See also

[edit] 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)

[edit] External links

Counter view: QoS is not about moving highest priority traffic first and slowing down low priority traffic as this article states. Rather it is about the network reserving resources to insure applications (requiring different packet treatment) equitably share available resources such that user QoE is maximized.

[edit] Notes

  1. ^ Leonard Franken. Quality of Service Management: A Model-Based Approach. PhD thesis, Centre for Telematics and Information Technology, 1996.
  2. ^ http://bennett.com/blog/index.php/archives/2006/08/17/how-much-bandwidth-is-enough/
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