|
In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to the probability of the telecommunication network meeting a given traffic contract, or in many cases is used informally to refer to the probability of a packet succeeding in passing between two points in the network within its desired latency period. 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. This definition includes the human in the assessment and demands an appropriate subjective weighting of diverse defects such as noise and tones on the circuit, loudness levels, noticeable echos, etc., and includes grade of service. With the advent of digital telephone networks new types of impairments, such as the effects of bit errors in codecs, arose together with a tendency to express QoS in terms of engineering parameters that can be measured objectively, eliminating the uncertainty of human subjectivity.
Problems QOS, Quality of Service Network When the Internet was first deployed many years ago, it lacked the ability to provide Quality of Service guarantees because its infrastructure was too limited, consisting of data links no faster than 56 Kbps, the speed of the modern dial-up modem. 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 that can happen to packets as they travel from origin to destination and they result in the following problems, as seen from the point of view of the sender and receiver: Applications requiring QoS A defined Quality of Service may be required for certain types of network traffic, for example: 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. Obtaining QoS QoS mechanisms Quality of Service 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. 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 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 that is often ignored by the naive 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 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. QoS problems with some technologies The following properties may only be used on end ports, but not on server, backbone or other ports that mediate many concurrent flows. See also Books | ||||||||
|
| |||||||||
 |
|
| |