Quality of service
From Wikipedia, the
free encyclopedia
Quality of service (QoS) is the
overall performance of a telephony or computer network,
particularly the performance seen by the users of the network.
To quantitatively measure quality of
service, several related aspects of the network service are often considered,
such as error rates, bit rate, throughput,
transmission delay,availability, jitter, etc.
Quality of service is particularly
important for the transport of traffic with special requirements. In
particular, much technology has been developed to allow computer networks to
become as useful as telephone networks for audio conversations, as well as
supporting new applications with even stricter service demands.
Contents
In the field of telephony, quality
of service was defined by the ITU in 1994.[1] Quality of service
comprises requirements on all the aspects of a connection, such as service
response time, loss, signal-to-noise ratio, crosstalk, echo, interrupts, frequency response,
loudness levels, and so on. A subset of telephony QoS is grade of service (GoS) requirements, which
comprises aspects of a connection relating to capacity and coverage of a
network, for example guaranteed maximum blocking probability and outage
probability.[2]
In the field of computer
networking and other packet-switched telecommunication networks,
the traffic engineering term refers to resource
reservation control mechanisms rather than the achieved service quality.
Quality of service is the ability to provide different priority to different
applications, users, or data flows, or to guarantee a certain level of performance
to a data flow. For example, a required bit rate, delay, jitter, packet dropping probability and/or bit error
rate may be guaranteed. Quality of service guarantees are important if the
network capacity is insufficient, especially for real-time streaming
multimedia applications such as voice over IP,
online games and IP-TV, since these often require fixed bit
rate and are delay sensitive, and in networks where the capacity is a limited
resource, for example in cellular data communication.
A network or protocol that supports QoS 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.
A best-effort network or service does
not support quality of service. An alternative to complex QoS control
mechanisms is to provide high quality communication over a best-effort network
by over-provisioning the capacity so that it is sufficient for the expected
peak traffic load. The resulting absence of network
congestion eliminates the need for
QoS mechanisms.
QoS 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.[3] 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.
An alternative and disputable definition of
QoS, used especially in application layer services such as telephony and streaming video, is
requirements on a metric that reflects or predicts the subjectively experienced
quality. In this context, QoS is the acceptable cumulative effect on subscriber
satisfaction of all imperfections affecting the service. Other terms with
similar meaning are the quality of experience (QoE) subjective business
concept, the required “user perceived performance”,[4] the required “degree of
satisfaction of the user” or the targeted “number of happy customers”. Examples
of measures and measurement methods are mean
opinion score (MOS), perceptual speech quality
measure(PSQM) and perceptual evaluation of video
quality (PEVQ). See also Subjective video quality.
Conventional Internet
routers and LAN switches operate on a best effort basis. This equipment is
less expensive, less complex and faster and thus more popular than competing
more complex technologies that provided QoS mechanisms. There were four “Type of service”
bits and three “Precedence” bits provided in each IP packet header,
but they were not generally respected. These bits were later re-defined as Differentiated services code points (DSCP).
With the advent of IPTV and IP telephony, QoS
mechanisms are increasingly available to the end user.[citation needed]
A number of attempts for layer 2 technologies that add QoS
tags to the data have gained popularity in the past. Examples are frame relay, asynchronous transfer mode (ATM) and multiprotocol label switching (MPLS) (a technique
between layer 2 and 3). Despite these network technologies remaining in use
today, this kind of network lost attention after the advent of Ethernet
networks. Today Ethernet is, by far, the most popular layer 2 technology. Ethernet uses 802.1p to signal the priority of
a frame.
In packet-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.[5]
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:
Low throughput
Due to varying load from disparate users
sharing the same network resources, the bit rate (the maximum throughput) that
can be provided to a certain data stream may be too low for realtime multimedia
services if all data streams get the same scheduling priority.
Dropped packets
The routers might fail to deliver (drop)
some packets if their data loads are corrupted, or the packets arrive when the
router buffers are already full. The receiving application may ask for this
information to be retransmitted, possibly causing severe delays in the overall
transmission.
Errors
Sometimes packets are corrupted due to bit errors caused by noise and
interference, especially in wireless communications and long copper wires. The
receiver has to detect this and, just as if the packet was dropped, may ask for
this information to be retransmitted.
Latency
It might take a long time for each packet
to reach its destination, because it gets held up in long queues, or it takes a
less direct route to avoid congestion. This is different from throughput, as
the delay can build up over time, even if the throughput is almost normal. In
some cases, excessive latency can render an application such as VoIP or online
gaming unusable.
Jitter
Packets from the 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 unpredictably. 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 a network, 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 requires 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 and lack of sequence.
A defined quality of service may be desired
or required for certain types of network traffic, for example:
·
Network operations support systems either for the network
itself, or for customers' business critical needs
·
Industrial control systems protocols such as Ethernet/IP which are used for
real-time control of machinery
These types of service are called inelastic, meaning that they
require a certain minimum bit rate and a certain maximum latency to function.
By contrast, elastic applications can take
advantage of however much or little bandwidth is available. Bulk file
transfer applications that rely on TCP are generally elastic.
Circuit switched networks, especially those
intended for voice transmission, such as Asynchronous Transfer Mode (ATM) or GSM, have QoS in the core protocol and do not
need additional procedures to achieve it. Shorter data units and built-in QoS
were some of the unique
selling points of ATM for applications
such as video on demand.
When the expense of mechanisms to provide
QoS is justified, network customers and providers can enter into a contractual
agreement termed a service level agreement (SLA) 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. In other approaches, resources are reserved at
each step on the network for the call as it is set up.
An alternative to complex QoS control
mechanisms is to provide high quality communication by generously
over-provisioning a network so that capacity is based on peak traffic load
estimates. This approach is simple for networks with predictable peak loads.
The performance is reasonable for many applications. This might include
demanding applications that can compensate for variations in bandwidth and
delay with large receive buffers, which is often possible for example in video
streaming. Over-provisioning can be of limited use, however, in the face of
transport protocols (such as TCP) that over time exponentially increase the amount
of data placed on the network until all available bandwidth is consumed and
packets are dropped. Such greedy protocols tend to increase latency and packet
loss for all users.
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 their
ISP and the VoIP provider's connection to a different ISP. Under high load
conditions, however, VoIP may degrade to cell-phone quality or worse. The
mathematics of packet traffic indicate that network requires just 60% more raw
capacity under conservative assumptions.[6]
The amount of over-provisioning in interior
links required to replace QoS depends on the number of users and their traffic
demands. This limits usability of over-provisioning. Newer more bandwidth intensive
applications and the addition of more users results in the loss of
over-provisioned networks. This then requires a physical update of the relevant
network links which is an expensive process. Thus over-provisioning cannot be
blindly assumed on the Internet.
Unlike single-owner networks, the Internet is a series of exchange
points interconnecting private networks.[7] Hence the Internet's core
is owned and managed by a number of different network service providers, not a single entity. Its
behavior is much more stochastic or unpredictable.
Therefore, research continues on QoS procedures that are deployable in large,
diverse networks.
There are two principal approaches to QoS
in modern packet-switched IP networks, a parameterized system based on an
exchange of application requirements with the network, and a prioritized system
where each packet identifies a desired service level to the network.
·
Integrated
services ("IntServ")
implements the parameterized approach. In this model, applications use the Resource Reservation Protocol (RSVP) to request and
reserve resources through a network.
·
Differentiated services ("DiffServ")
implements the prioritized model. DiffServ marks packets according to the type
of service they desire. In response to these markings, routers and switches use
various queueing
strategies to tailor performance to
expectations. Differentiated services code point (DSCP) markings use the first
6 bits in the ToSfield (now
renamed as the DS Byte) of the IP(v4) packet header.
Early work used the integrated services
(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 response to these markings, routers and
switches use various queuing strategies to tailor performance to requirements.
At the IP layer, DSCP markings use the 6 bits in the IP packet header. At the
MAC layer, VLAN IEEE 802.1Q and IEEE 802.1p can be used to carry
essentially the same information.
Routers supporting DiffServ configure their network scheduler to 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 videoconferencing)
are given priority over packets in other queues. Typically, some bandwidth is
allocated by default to network control packets (such as Internet Control Message Protocol and routing protocols),
while best effort traffic might simply be given whatever bandwidth is left over.
At the Media
Access Control (MAC) layer, VLAN IEEE 802.1Q and IEEE 802.1p can be used to
distinguish between Ethernet frames and classify them. Queueing theory models
have been developed on performance analysis and QoS for MAC layer protocols.[8][9]
Cisco IOS NetFlow and the Cisco
Class Based QoS (CBQoS) Management Information Base (MIB) are marketed by Cisco Systems. [10]
One compelling example of the need for QoS
on the Internet relates to congestion
collapse. The Internet relies on congestion avoidance protocols, as
built into Transmission Control Protocol (TCP), to reduce traffic
under conditions that would otherwise lead to "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 congestion. QoS contracts limit traffic that can
be offered to the Internet and thereby enforce traffic shaping that can prevent
it from becoming overloaded, and are hence an indispensable part of the
Internet's ability to handle a mix of real-time and non-real-time traffic
without meltdown.
·
RSVP-TE
·
X.25
·
The ITU-T G.hn standard provides QoS by
means of "Contention-Free Transmission Opportunities" (CFTXOPs) which
are allocated to flows which require QoS and which have negotiated a
"contract" with the network controller. G.hn also supports non-QoS
operation by means of "Contention-based Time Slots".
End-to-end quality of service can require a
method of coordinating resource allocation between one autonomous system and another. The Internet Engineering Task Force(IETF) defined the Resource Reservation Protocol (RSVP) for bandwidth
reservation, as a proposed standard in 1997.[12] RSVP is an end-to-end bandwidth reservation
protocol. The traffic engineering version, RSVP-TE, is used in
many networks to establish traffic-engineered Multiprotocol Label Switching (MPLS) label-switched
paths.[citation needed] The IETF also defined Next Steps in Signaling (NSIS)[13] with QoS signalling as a
target. NSIS is a development and simplification of RSVP.
Research consortia such as "end-to-end
quality of service support over heterogeneous networks" (EuQoS, from 2004
through 2007)[14] and fora such as the IPsphere Forum[15]developed more mechanisms
for handshaking QoS invocation from one domain to the next. IPsphere defined
the Service Structuring Stratum (SSS) signaling bus in
order to establish, invoke and (attempt to) assure network services. EuQoS
conducted experiments to integrate Session Initiation Protocol, Next Steps in Signaling and IPsphere's SSS with
an estimated cost of about 15.6 million Euro and published a book.[16][17]
A research project Multi Service Access
Everywhere (MUSE) defined another QoS concept in a first phase from January
2004 through February 2006, and a second phase from January 2006 through 2007.[18][19][20] Another research project
named PlaNetS was proposed for European funding circa 2005.[21] A broader European
project called "Architecture and design for the future Internet"
known as 4WARD had a budgest estimated at 23.4 million Euro and was funded from
January 2008 through June 2010.[22] It included a
"Quality of Service Theme" and published a book.[23][24] Another European project,
called WIDENS (Wireless Deployable Network System) [25] proposed a bandwidth
reservation approach for mobile wireless multirate adhoc networks.[26]
In the services domain, end-to-end Quality of Service has also been discussed in the case of composite services (consisting of atomic services) or applications (consisting of application components).[27][28] Moreover, in cloud computing end-to-end QoS has been the focus of various research efforts aiming at the provision of QoS guarantees across the cloud service models.[29]
In the services domain, end-to-end Quality of Service has also been discussed in the case of composite services (consisting of atomic services) or applications (consisting of application components).[27][28] Moreover, in cloud computing end-to-end QoS has been the focus of various research efforts aiming at the provision of QoS guarantees across the cloud service models.[29]
Strong
cryptography network protocols such as Secure Sockets Layer, I2P, and virtual private networks obscure the data
transferred using them. As all electronic
commerce on the Internet requires
the use of such strong cryptography protocols, unilaterally downgrading the
performance of encrypted traffic creates an unacceptable hazard for customers.
Yet, encrypted traffic is otherwise unable to undergo deep packet inspection for QoS.
The Internet2 project found, in 2001,
that the QoS protocols were probably not deployable inside its Abilene Network with equipment available
at that time.[30] Equipment available at
the time relied on software to implement QoS. The group also predicted that
“logistical, financial, and organizational barriers will block the way toward
any bandwidth guarantees” by protocol modifications aimed at QoS.[31] They believed that the
economics would encourage network providers to deliberately erode the quality
of best effort traffic as a way to push customers to higher priced QoS
services. Instead they proposed over-provisioning of capacity as more
cost-effective at the time.[30][31]
The Abilene network study was the basis for
the testimony of Gary Bachula to the US Senate Commerce Committee's
hearing on Network
Neutrality in early 2006. He
expressed the opinion that adding more bandwidth was more effective than any of
the various schemes for accomplishing QoS they examined.[32]
Bachula's testimony has been cited by
proponents of a law banning quality of service as proof that no legitimate
purpose is served by such an offering. This argument is dependent on the
assumption that over-provisioning isn't a form of QoS and that it is always
possible. Cost and other factors affect the ability of carriers to build and
maintain permanently over-provisioned networks.
Mobile cellular service providers may offer mobile QoS to customers just as the
fixed line PSTN services providers and
Internet Service Providers (ISP) may offer QoS. QoS mechanisms are always
provided for circuit switched services, and are
essential for non-elastic services, for example streaming
multimedia.
Mobility adds complication to the QoS
mechanisms, for several reasons:
·
A phone call or other
session may be interrupted after a handover, if the new base station is overloaded.
Unpredictable handovers make it impossible to give an absolute QoS guarantee
during a session initiation phase.
·
The pricing structure is
often based on per-minute or per-megabyte fee rather than flat rate, and may
be different for different content services.
·
A crucial part of QoS in
mobile communications is Grade of Service,
involving outage probability (the probability that the mobile station is
outside the service coverage area, or affected by co-channel interference, i.e.
crosstalk), blocking
probability (the probability that the
required level of QoS can not be offered) and scheduling starvation. These performance measures are
affected by mechanisms such as mobility
management, radio resource management, admission control, fair scheduling, channel-dependent scheduling etc.
Quality of service in the field of telephony, was first
defined in 1994 in the ITU-T Recommendation E.800.
This definition is very broad, listing 6 primary components: Support,
Operability, Accessibility, Retainability, Integrity and Security.[1] A 1995 recommendation
X.902 included a definition is the OSI
reference model.[33] In 1998 the ITU published
a document discussing QoS in the field of data networking. X.641 offers a means
of developing or enhancing standards related to QoS and provide concepts and
terminology that will assist in maintaining the consistency of related
standards.[34]
Some QoS-related IETF Request
For Comments (RFC)s are Definition of the Differentiated services Field (DS
Field) in the IPv4 and IPv6 Headers (RFC 2474), andResource ReSerVation Protocol (RSVP) (RFC 2205); both these are discussed above. The
IETF has also published two RFCs giving background on QoS: RFC 2990: Next Steps for the IP QoS Architecture, and RFC 3714: IAB Concerns Regarding Congestion Control for Voice
Traffic in the Internet.
The IETF has also published RFC 4594 Configuration Guidelines for DiffServ Service Classes as an informative or
"best practices" document about the practical aspects of designing a
QoS solution for a DiffServ network. They try to
identify which types of applications are commonly run over an IP network to
group them into traffic classes, study what treatment do each of these classes
need from the network, and suggest which of the QoS mechanisms commonly
available in routers can be used to implement those treatments.
·
BSSGP
·
LEDBAT
·
QPPB
1.
^ Jump up to:a b "E.800:
Terms and definitions related to quality of service and network performance
including dependability". ITU-T Recommendation.
August 1994. Retrieved October 14,2011. Updated September 2008
as Definitions of terms related to quality of service
2.
Jump up^ Teletraffic Engineering Handbook ITU-T
Study Group 2 (350 pages, 4·48MiB)(It uses abbreviation GoS
instead of QoS)
3.
Jump up^ Real-time
reconfiguration for guaranteeing QoS provisioning levels in Grid environments Future
Generation Computer Systems, Volume 25, Issue 7, July 2009, Pages 779–784,
Elsevier
4.
Jump up^ Leonard Franken.
Quality of Service Management: A Model-Based Approach. PhD thesis, Centre for
Telematics and Information Technology, 1996.
5.
Jump up^ Peuhkuri M., IP
Quality of Service, Helsinki University of Technology, Laboratory of
Telecommunications Technology, 1999.
6.
Jump up^ Yuksel, M.;
Ramakrishnan, K. K.; Kalyanaraman, S.; Houle, J. D.; Sadhvani, R. (2007).
"IEEE International Workshop on Quality of Service (IWQoS'07)" (PDF).
Evanston, IL, USA. pp. 109–112. doi:10.1109/IWQOS.2007.376555. ISBN 1-4244-1185-8. |chapter=ignored
(help)
8.
Jump up^ Bianchi, Giuseppe
(2000). "Performance analysis of the IEEE 802.11 distributed
coordination function". IEEE Journal on Selected Areas
in Communications 18 (3): 535.doi:10.1109/49.840210.
9.
Jump up^ Shi, Zhefu; Beard,
Cory; Mitchell, Ken (2009). "Analytical Models for Understanding Misbehavior and
MAC Friendliness in CSMA Networks". Performance
Evaluation 66 (9–10): 469. doi:10.1016/j.peva.2009.02.002.
10.
Jump up^ Ben
Erwin (December 16, 2008). "How To Manage QoS In Your Environment, Part 1 of
3". Network Performance Daily video. NetQoS. Retrieved October 15, 2011.
12.
Jump up^ Bob Braden ed. L. Zhang, S. Berson, S.
Herzog, S. Jamin (September 1997). "Resource
ReSerVation Protocol (RSVP)". RFC 2205.
IETF. Retrieved October 12, 2011.
14.
Jump up^ "EuQoS - End-to-end Quality of Service support over
heterogeneous networks". Project website.
2004–2006. Archived from the original on April 30, 2007.
Retrieved October 12,2011.
16.
Jump up^ "End-to-end quality of service support over
heterogeneous networks". Project description.
European Community Research and Development Information Service.
Retrieved October 12, 2011.
17.
Jump up^ Torsten Braun;
Thomas Staub (2008). End-to-end quality of service over heterogeneous networks.
Springer. ISBN 978-3-540-79119-5.
19.
Jump up^ "Multi Service Access Everywhere". Project
description. European Community Research and Development Information
Service. Retrieved October 12, 2011.
20.
Jump up^ "Multi Service Access Everywhere". Project
description. European Community Research and Development Information
Service. Retrieved October 12, 2011.
21.
Jump up^ "PlaNetS QoS Solution". Project
website. Archived from the
original on November 12, 2009. Retrieved October
12, 2011.
22.
Jump up^ "4WARD: Architecture and design for the future
Internet". Project description. European Community
Research and Development Information Service. RetrievedOctober 15, 2011.
24.
Jump up^ Luís M. Correia;
Joao Schwarz (FRW) da Silva (January 30, 2011). Architecture and Design for the Future Internet: 4WARD EU Project.
Springer. ISBN 978-90-481-9345-5.
25.
Jump up^ "Wireless
Deployable Network System". Project description.
European Union. RetrievedMay 23, 2012.
26.
Jump up^ R. Guimaraes, L.
Cerdà, J. M. Barcelo-Ordinas, J. Garcia-Vidal, M. Voorhaen, C. Blondia (March
2009). "Quality of Service through Bandwidth Reservation on Multirate
Ad-doc Wireless Networks". Ad Hoc Networks Journal (Elsevier),
Vol. 7, Issue 2 7 (2): 388–400.doi:10.1016/j.adhoc.2008.04.002.
27.
Jump up^ D. Kyriazis, K.
Tserpes, A. Menychtas, A. Litke, T. Varvarigou, An
innovative Workflow Mapping Mechanism for Grids in the frame of Quality of
Service, Elsevier Future Generation Computer Systems, Vol. 24,
Iss. 6, pp. 498-511, 2008
28.
Jump up^ Q. Sun, S. Wang, H.
Zou, F. Yang, QSSA: A QoS-aware Service Selection Approach,
International Journal of Web and Grid Services, pp.147 - 169, 2011
29.
Jump up^ D Kyriazis, A
Menychtas, G Kousiouris, K Oberle, T Voith, M Boniface, E Oliveros, T
Cucinotta, S Berger, A Real-time Service Oriented Infrastructure,
International Conference on Real-Time and Embedded Systems (RTES 2010),
Singapore, November 2010
30.
^ Jump up to:a b Benjamin
Teitelbaum, Stanislav Shalunov (May
3, 2002). "Why Premium IP Service Has Not Deployed (and
Probably Never Will)". Draft Informational Document.
Internet2 QoS Working Group. Archived from the original on September
12, 2010. RetrievedOctober 15, 2011.
31.
^ Jump up to:a b Andy
Oram (June 11, 2002). "A Nice Way to Get Network Quality of Service?".Platform
Independent column. O'Reilly. Archived from
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32.
Jump up^ Gary Bachula
(February 7, 2006). "Testimony of Gary R. Bachula, Vice President,
Internet2" (PDF).
pp. 2–3. Retrieved October 15, 2011.
33.
Jump up^ "X.902:Information
technology – Open Distributed Processing – Reference model: Foundations". ITU-T
Recommendation. November 1995. Retrieved October 14, 2011.Updated
October 2009.
34.
Jump up^ "X.641:
Information technology - Quality of service: framework". ITU-T
Recommendation. December 1997.
35.
Jump up^ "Advanced Routing
& Traffic Control HOWTO". August 21, 2005. RetrievedOctober
14, 2011.
37.
Jump up^ Fulvio
Ricciardi. "QoS and Traffic Shaping in Transparent Bridge
mode". Router/Bridge Linux Firewall website.
ZeroShell Net Services. Retrieved October 15, 2011.
·
Deploying IP and MPLS QoS
for Multiservice Networks: Theory and Practice by John Evans, Clarence
Filsfils (Morgan Kaufmann, 2007, ISBN 0-12-370549-5)
·
Lelli, F. Maron, G.
Orlando, S. Client Side Estimation of a
Remote Service Execution. 15th International Symposium on Modeling,
Analysis, and Simulation of Computer and Telecommunication Systems, 2007.
MASCOTS '07.
·
XiPeng
Xiao (September 8, 2008). Technical, Commercial and Regulatory Challenges of QoS:
An Internet Service Model Perspective. Morgan Kaufmann. ISBN 978-0-12-373693-2.
This audio file was created
from a revision of the "Quality of service" article dated
2008-07-18, and does not reflect subsequent edits to the article. (Audio
help)
·
Henning Schulzrinne (January
9, 2008). "Network Quality of Service". Columbia
University faculty website. Retrieved October 14,2011.
Penjelasan QoS
Quality of Service (QoS) merupakan mekanisme jaringan yang
memungkinkan aplikasi-aplikasi atau layanan dapat beroperasi sesuai dengan yang
diharapkan.
Kinerja jaringan komputer dapat bervariasi akibat beberapa
masalah, seperti halnya masalahbandwidth, latency dan jitter, yang dapat membuat efek yang cukup besar bagi
banyak aplikasi. Sebagai contoh, komunikasi suara (seperti VoIP atau IP Telephony) serta video streaming dapat
membuat pengguna frustrasi ketika paket data aplikasi tersebut dialirkan di
atas jaringan dengan bandwidth yang tidak cukup, dengan latency yang tidak
dapat diprediksi, atau jitter yang berlebih. Fitur Quality of Service (QoS) ini
dapat menjadikan bandwidth, latency, dan jitter dapat diprediksi dan dicocokkan
dengan kebutuhan aplikasi yang digunakan di dalam jaringan tersebut yang ada.
Dalam jaringan packet-switched, Qos
dipengaruhi oleh beberapa factor yang dibagi menjadi factor manusia dan factor
teknis. Faktor manusia terdiri dari : stabilitas service, ketersediaanservice,
waktu tenggang, dan informasi pengguna. Faktor teknis terdiri dari keandalan,
skalabilitas, efektifitas, dan maintanabilitas.
Banyak hal dapat
terjadi pada paket saat paket – paket itu dikirimkan dari asal ke tujuannya.
Hal itu dipengaruhi beberapa faktor sebagai berikut:
Low
throughput
Oleh karina muatan
yang bervariasi dari pengguna yang menggunakan sumber daya jaringan yang sama,
bit rate dapat disediakan pada data stream terkait. Namun, hal itu terlalu
rendah untuk realtime
multimedia service jika
seluruh data stream mendapat prioritas penjadwalan yang sama.
Dropped
packets
Router mungkin akan
menjatuhkan beberapa paket jika data – datanya korup atau paket – paket itu
tiba pada saat elemen penyangganya sudah penuh. Aplikasi yang menerimanya
mungkin akan meminta agar informasi ditransmisikan. Hal ini mengakibatkan waktu
tenggang yang cukup lama pada transmisi secara umum.
Errors
Kadang – kadang
paket mengalami korup akibat kesalahan yang disebabkan oleh noise daninterference, khususnya
pada komunikasi nirkabel dan pada kabel copper yang panjang. Receiver harus mendeteksi
hal ini dan jika paket dijatuhkan, receiver meminta agar informasi yang terkait
hal itu ditransmisikan.
Latency
Akan terjadi waktu
tenggang yang cukup lama bagi sebuah paket untuk meraih tujuannya, karena paket
itu ditahan di dalam antrian atau mengambil rute yang agak jauh untuk
menghindari kemacetan. Pada beberapa kasus waktu tenggang yang berlebihan dapat
menjadikan aplikasi tidak berguna.
Jitter
Paket – paket yang
berasal dari sumber akan mencapai tujuan dengan waktu tenggang yang berbeda.
Sebuah paket delay akan memiliki posisi yang bervariasi dalam antrian router
sepanjang jalur antara asal dan tujuan. Posisinya dapat bervariasi secara tidak
terduga. Variasi dari waktu tenggang ini disebut dengan istilah jitter. Jitter
dapat mempengaruhi kualitas dari streaming audio / video.
Out-of-order
delivery
Ketika beberapa
paket – paket yang terkait dikirimkan melalui sebuah jaringan, paket – paket
yang berbeda bias saja mengambil rute yang berbeda. Hal ini mengakibatkan waktu
tenggang yang berbeda pula. Hasil dari proses ini mengakibatkan paket – paket
tiba pada urutan yang berbeda dari urutan ketika mereka dikirimkan. Masalah ini
membutuhkan protocol tambahan khusus yang bertanggung jawab untuk mengatur
ulang paket yang sudah terlambat. Pengaturan dilakukan dalam tahaoan
isochronous sessat paket tersebut mencapai tujuannya. Secara khusus, hal ini
penting diperlukan untuk video dan VoIP ketika kualitas dipengaruhi secara
dramatis oleh waktu tenggang dan urutan yang berkurang.
QoS pada Beberapa Aplikasi
Qos pada aplikasi pengiriman bisa saja berbeda. Aspek penilaian QoS dari
media – media yang terkait adalah pada factor yang mempengaruhi QoS. Secara umum aspek –
aspek penilaian itu dibedakan menjadi Bandwidht, Delay, Jitter, dan Loss.
Aplikasi – aplikasi
berbeda dalam aspek kebutuhan bandwidth. Misalnya, email, audio streaming, dan remote
login tidak membutuhkan terlalu banyak. Sedangkan file sharing dan video
streaming membutuhkan banyak bandwidth.
Pada aspek kebutuhan
waktu tenggang, aplikasi file transfer : email dan video, tidak seberapa
mengharuskan waktu tenggang yang rendah. Jika seluruh paket ditunda secara
bertahap tiap detik, maka tidak ada masalah yang berarti. Aplikasi interaktif
seperti web browser, remote login, mengharuskan waktu tenggang yang rendah.
Aplikasi yang realtime seperti video conference, telepon sangat membutuhkan
waktu tenggang yang rendah. Jika setiap perkataan dalam aplikasi
telepon ditunda lama, maka penguna tidak akan menerima koneksinya dengan baik.
Variasi waktu
tenggang pada aplikasi – aplikasi pengiriman data disebut jitter. Tiga aplikasi
pertama : Email, File sharing, dan Web access membutuhkan interval pengiriman
yang tidak terlalu teratur. Remote login membutuhkan interval pengiriman data
yang lebih teratur karena layar akan menampilkan bintik – bintik jika koneksi
mengalami gangguan pada waktu pengirimannya. Video dan audio streaming sangat
sensitif terhadap interval pengiriman yang tidak teratur. Jika pengguna sedang
menonton video dan frame mengalami penundaan misalnya 2000 detik, tidak ada
masalah yang terjadi. Namun, jika waktu penundaan bervariasi secara acak antara
1 atau 2 detik, tayangan videonya akan sangat buruk dan menjengkelkan. Begitu
juga pada transmisi audio streaming. Audio streaming membutuhkan jitter yang
tidak lebih dari 1 milisecond.
Kesimpulan
Tiap aplikasi yang
melakukan pengiriman data membutuhkan quality of service yang berbeda – beda. Hal ini dibedakan berdasarkan karakteristik data yang ditransmisikan dan
berdasarkan fungsi dari aplikasi tersebut. Quality of service pada suatu aspek
penilaian bisa rendah jika aspek tersebut tidak dibutuhkan oleh aplikasi.
Sebaliknya, aplikasi tertentu menuntut QoS pada suatu aspek yang tinggi jika
aspek tersebut sangat penting pada fungsionalitas aplikasi.
Traffic Management Quality Of Service
·
Traffic
Manajemen
Pasti terpikir bagi
anda, seandainya ada polisi yang mengatur lalu lintas, sehingga semua jalur
terpakai dengan rapi. Seandainya semua tertib, pasti tidak ada saling serobot
sehingga menyebabkan jalanan macet. Sekarang kita kembali ke lalu lintas di jaringan komputer
kita. Pernahkan anda sadar bahwa banyak sekali tipe aplikasi/data yang lewat di
jaringan kita? Apakah anda tahu, bahwa selain aplikasi web browser yang sedang
aktif, ada juga yang sedang download lagu MP3 di jaringan. Tahukah selain file
laporan keuangan yang lewat di jaringan, saat itu juga ada worm/virus yang lagi
aktif broadcast kesana kemari di jaringan? Hal-hal seperti inilah yang sering
dilupakan oleh para pengelola jaringan di suatu institusi/perusahaan.
Konsep Traffic Management adalah bagaimana mengatur semua peralatan jaringan (switch, router, firewall dll) bisa mengenali lalulintas yang sedang lewat sehingga bisa dibuat aturan tersendiri. Umpamakan saat tertentu direktur pada suatu perusahaan sedang mendownload artikel/paper dari suatu server, kemudian pada ssat yang bersamaan ada karyawan sedang mendownload film/musik, saat itu juga peralatan jaringan bisa meng-alokasi-kan bandwith secara otomatis pada akses direktur tadi sehingga tidak terganggu oleh file musik/film. Bahkan utk kondisi yang khusus, katakan dimana suatu akses ke server database keuangan sedang aktif, saat itu juga semua lalulintas yang tidak perlu bisa dihentikan sementara.
Konsep Traffic Management adalah bagaimana mengatur semua peralatan jaringan (switch, router, firewall dll) bisa mengenali lalulintas yang sedang lewat sehingga bisa dibuat aturan tersendiri. Umpamakan saat tertentu direktur pada suatu perusahaan sedang mendownload artikel/paper dari suatu server, kemudian pada ssat yang bersamaan ada karyawan sedang mendownload film/musik, saat itu juga peralatan jaringan bisa meng-alokasi-kan bandwith secara otomatis pada akses direktur tadi sehingga tidak terganggu oleh file musik/film. Bahkan utk kondisi yang khusus, katakan dimana suatu akses ke server database keuangan sedang aktif, saat itu juga semua lalulintas yang tidak perlu bisa dihentikan sementara.
·
Quality
Of Service
Ada 4 elemen QoS (kuartet QoS):
1. Service Quality :
Kualitas layanan yang diharapkan oleh user
application dapat dipahami sebagai :
ü Kemudahan untuk
mengekspresikan dan memahami dari sudut pandang user
ü Dapat diinterpretasikan
secara kuantitatif dan dipetakan pada alokasi network resources.
2. Usage Control:
Usage control adalah mekanisme untuk
meningkatkan predictability dari user behaviour.
3. Service Quality
Kualitas service diterjemahkan ke dalam
resource requirement yang direalisasikan oleh QoS oriented connection
management protocol seperti ATM user –network interface (ATM UNI) dan
Resource Reservation Protocol (RSVP).
4. QoS
Monitoring
Suatu
jaminan QoS tidak akan mungkin dilakukan tanpa monitoring performansi jaringan.
Sistem jaringan dapat bersifat dijamin atau
tidak dijamin kualitasnya. Dijamin berarti bila terjadi gangguan atau tidak
berfungsi sebagaimanayang telah dijanjikan maka provider tidak menarik biaya
dari pelanggannya atau bahkan membayar kompensasi, sedangkan bila tidak dijamin
maka kualitas layanan yang ditawarkan juga tidak sebagus yang pertama.
·
Teknik
QoS
Terdapat 3
teknik/metode QoS yang umum dipakai, yaitu: best-effort service, integrated
service, dan differentiated service. Ketiga level tersebut akan diuraikan lebih
detail dibawah ini:
- Best-Effort Service
Best-effort service digunakan untuk melakukan semua usaha agar dapat mengirimkan sebuah paket ke suatu tujuan. Penggunaan best-effort service tidak akan memberikan jaminan agar paket dapat sampai ke tujuan yang dikehendaki. Untuk aplikasi yang sensitif terhadap network delay, fluktuasi bandwidth, dan perubahan kondisi jaringan, penerapan best-effort service tidak dapat dilakukan. Sebagai contohnya aplikasi telepon pada jaringan yang membutuhkan bandwidth yang tetap, agar dapat berfungsi dengan baik dalam hal ini penerapan best-effort akan mengakibatkan panggilan telepon gagal atau terputus.
- Integrated Service (IntServ)
Model Integrated service (IntServ) menyediakan aplikasi dengan tingkat jaminan layanan melalui negosiasi parameter jaringan secara end-to-end. Aplikasi akan meminta tingkat layanan yang dibutuhkan untuk dapat beroperasi dan bergantung pada mekanisme QoS untuk menyediakan sumber daya jaringan yang dimulai sejak permulaan transmisi dari aplikasi tersebut. Aplikasi tidak akan mengirimkan trafik, sebelum menerima tanda bahwa jaringan mampu menerima beban yang akan dikirimkan aplikasi dan juga mampu menyediakan QoS yang
diminta secara end-to-end. Untuk itulah suatu jaringan akan melakukan suatu proses yang disebut admission control. Admission control adalah suatu mekanisme yang mencegah jaringan mengalami over-loaded. Jika QoS yang diminta tidak dapat disediakan, maka jaringan tidak akan mengirimkan tanda ke aplikasi agar dapat memulai untuk mengirimkan data. Jika aplikasi telah memulai pengiriman data, maka sumber daya pada jaringan yang sudah dipesan aplikasi tersebut akan terus dikelola secara end-to-end sampai aplikasi tersebut selesai. IntServ terutama ditujukan untuk aplikasi yang peka terhadap delay dan keterbatasan bandwidth, seperti pada video conference dan VoIP. Arsitekturnya berdasar pada sistem pencadangan sumber daya per aliran trafik. Setiap aplikasi harus mengajukan permintaan bandwidth, baru kemudian melakukan transmisi data.
Dua model layanan IntServ adalah:
• Guaranteed-service, layanan dengan batas bandwidth dan delay yang jelas.
• Controlled-load service, layanan dengan persentase delay statistik yang terjaga.
Sistem pemesanan sumber daya memerlukan protokol tersendiri. Salah satu protokol yang sering digunakan adalah RSVP. Masalah dalam IntServ adalah skalabilitas. IntServ hanya menjadi baik untuk voice dan video, tetapi sangat tidak tepat untuk aplikasi semacam web yang aliran trafik datanya banyak tetapi datanya kecil.
- Differentiated Service (DiffServ)
Model terakhir dari QoS adalah model differentiated service. Differentiated service menyediakan suatu set perangkat klasifikasi dan mekanisme antrian terhadap protokol atau aplikasi dengan prioritas tertentu di atas jaringan yang berbeda. Differentiated service bergantung kepada kemampuan edge router untuk memberikan klasifikasi dari paket-paket yang berbeda tipenya yang melewati jaringan. Trafik jaringan dapat diklasifikasikan berdasarkan alamat jaringan, protocol dan port, ingress interface, atau klasifikasi lainnya selama masih didukung oleh standard access list atau extended access list.
How do I configure
Traffic Control (IP QoS) on TD-W8960N
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Why need Traffic
Control?
As default, the Internet bandwidth
is shared by all clients which means that the bandwidth of each client is
uncontrollable while they are sharing the Internet. When someone is streaming
on Internet or downloading with P2P software, it will affect the performance
of other clients. Traffic Control can help avoid the situation.
How to configure
Traffic Control?
Note: You´d better set a static IP Address for your each network
client in order to manage it easily.
Step 1 Open the web browser and type the LAN IP address of the
router in the address bar (the default is 192.168.1.1), and then press Enter.
Step 2 Type the username and password in the login page,
the default username and password are both admin.
Step 3 Go to Advanced Setup-> Traffic Control page,
Set the total value of egress and ingress. They should less than the line
rate. The line rate is usually the rate provided by your ISP. Check the Enable box and click Save/Apply button.
Step 4 Go to Traffic
Control ->Traffic Control Rule page, click Add button to add a new rule.
Step 5 Fill in the parameters.
·
Rule Status--Set
it as Enable.
·
IP Range--Type
in IP Addresses of your clients in this bar. If you would like to
manage all clients in your network, please type IP Address Range of these
computers.
·
Port Range--Type
in Port numbers in this bar. If you want to manage all the ports, please
leave it as blank.
·
Protocol--TCP/UDP/ALL.
·
Precedence--Set
the specific priority for this traffic control rule.
·
It
can be Highest/High/Medium/Low/Lowest.
·
Egress Rate/Ingress Rate--You
may set Min Bandwidth and Max Bandwidth for Egress Rate and Ingress Rate. You should configure them
based your network demand.
·
(The
Egress Rate/Ingress Rate is the independent bandwidth for each computer in
the IP range list.)
Example:
Note:
Min Egress/Ingress Rate must not be empty and no less than 10 Kbps.
Max Egress/Ingress Rate should not be greater than the Egress/Ingress Total Rate, if empty or greater than the Egress/Ingress Total Rate,
the actual value will be set with Egress/Ingress Total Rate.
Step 7 Click on Save/Apply to save
the settings.
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