quaity of service? what does this mean in networking

[jenniee]

hey can some one help me with what 'QOS' actually means. i know it stands for quality of service but dont know how it works. please can some one help

thank you
jennie

[Will.Spencer]

QoS is a generic name for a set of competing algorithms which attempt to enable network administrators to treat different types of network traffic with different levels of priority.

Queuing

One method of implementing QoS is to utilize some sort of advanced queueing algorithm.

Simple networks process traffic with a FIFO (First In - First Out) queue. Packets which are received first are processed first.

QoS comes into play when the network admin wants to treat some packets differently than others. For example: e-mail packets can be delayed for several minutes with no one noticing, while VoIP packets cannot be delayed for more than a tenth of a second before users notice a problem.

In a true FIFO system, all packets are stored in one queue. An advancement over FIFO queuing is Fair Queuing (FQ). In an FQ system, each type of packet is stored in a it's own queue. FQ isn't terribly useful, but a variation of it is. That variation is Weighted Fair Queuing (WFQ). In a WFQ system, each queue can be given a different priority level. That is where QoS really begins.

Improvements to WFQ include Class Based Weighted Fair Queuing (CB-WFQ), where each type of traffic is assigned to a class and each class is given it's own queue. CB-WFQ allows for easier queue management.

Hierarchical Weighted Fair Queuing (HWFQ) is another improvement to simple WFQ. In HWFQ, the network device monitors the worst-case packet delay for each queue and adjusts to queue priorities automatically.

Random Early Detection

Random Early Detection (RED) is a an algorithm which simply drops packets if too many are being received. This causes the devices which are sending the packets to notice a problem and reduce their transmissions.

An improvement to RED is Weighted Random Early Detection (WRED). WRED is RED which utilizes the IP headers priority value to determine which packets to drop.

Traffic Shaping and Rate Limiting

Another method of implementing QoS is traffic shaping. In traffic shaping, traffic from each source is monitored for bandwidth utilization. When traffic from a specific source is too high, packets from that source are then queued (delayed).

Rate Limiting is an improvement on traffic shaping. With Rate Limiting, the packets are not simply queued, the packets can also have their IP priority levels altered or they can be dropped altogether.


Did this help to answer your question? This is actually far better than the current FAQ answer, so I will move this post into the FAQ.

[jenniee]

hello could someone please help me with another question, i am struggeling with this and have reserched the question , but i have been unsucessful.

What are the five most important things you think a standard defining QoS should cover, the dimensions used to measure them and justify their selection,

thank you
jennie

[Will.Spencer]

Originally Posted by jenniee

What are the five most important things you think a standard defining QoS should cover...

You can pull these directly from - Next Steps for the IP QoS Architecture:

All of the following responses form a part of the QoS intention:

- To control the network service response such that the response
to a specific service element is consistent and predictable.

- To control the network service response such that a service
element is provided with a level of response equal to or above a
guaranteed minimum.

- To allow a service element to establish in advance the service
response that can or will be obtained from the network.

- To control the contention for network resources such that a
service element is provided with a superior level of network
resource.

- To control the contention for network resources such that a
service element does not obtain an unfair allocation of
resources (to some definition of 'fairness').

- To allow for efficient total utilization of network resources
while servicing a spectrum of directed network service outcomes.

[Will.Spencer]

Originally Posted by jenniee

... the dimensions used to measure them ...

This gets a wee bit more tricky.

Luckily, I don't have to come up with an answer, because Arjan Durresi has already done some good work on the subject.

Read Arjan's PowerPoint presentation Measuring QoS Parameters in IP Networks.

That should get you on the right path.

[Will.Spencer]

Originally Posted by jenniee

... justify their selection ...

Justify them to whom?

This is the hardest item, because it's so dependent upon your specific environment.

Are you justifying them on a technical level or a business level?

Are you justifying them in comparison to each other, or in comparison to no measurements at all?

Are you justifying the goals, or the metrics used to measure progress towards those goals?

Tell us more about your requirement and we may be able to come up with something.

[jenniee]

Thank you so much will.Sspencer for all ur help you have been great. the infformation you posted has been very helpful. thank you.

jennie

[Will.Spencer]

Are you in Level 3 of the Mobile Communications program at the Univeristy of Salford? 8)

[jenniee]

Why has QoS as a concept become so important to our understanding of networks.

[jenniee]

no although i did study at the university of salford , whom r you?

[Will.Spencer]

Originally Posted by jenniee

What are the five most important things you think a standard defining QoS should cover...

I came across a better answer to this question in the September 2000 issue of IEEE Spectrum:

Technically, QoS refers to an aggregation of system performance metrics. The five most important of these are:

• Availability. Ideally, a network is available 100 percent of the time. Criteria are quite strict. Even so high-sounding a figure as 99.8 percent translates into about an hour and a half of down time per month,which may be unacceptable to a large enterprise. Serious carriers strive for 99.9999 percent availability, which they refer to as “six nines,” and which translates into a downtime of 2.6 seconds a month.

• Throughput. This is the effective data transfer rate measured in bits per second—it is emphatically not the same as the maximum capacity, or wire speed, ofthe network, often erroneously called the network’s bandwidth. Sharing a network lowers the throughput realizable by any user, as does the overhead imposed by the extra bits included in every packetfor identification and other purposes. A minimum rate of throughput is usually guaranteed by a service provider.

• Packet loss. Network devices, like switches androuters, sometimes have to hold data packets in buffered queues when a link gets congested. If the link remains congested for too long, the buffered queues will overflow and data will be lost. The lost packets must be retransmitted, adding, of course, to the total transmission time. In a well-managed network, packet loss will typically be less than 1 percent averaged over, say, a month.

• Latency. The time taken by data to travel from thesource to the destination is known as latency, or delay. Unless satellites are involved, the latency of a 5000-km voice call carried by a circuit-switched telephone network is about 25 ms. For the public Internet, a voice call may easily exceed 150 ms of latency because of delays, such as those caused by signal processing (digitizing and compressing the analog voice input) and congestion (queuing).

• Jitter, which is another way of saying latency variation, has many causes, including: variations in queue length; variations inthe processing time needed to reorder packets that arrived out of order because they traveled over different paths; and variations in the processing time needed to reassemble packets that were segmented by the source before being transmitted.

This answer has the added advantage that these goals are much easier to relate to technically achievable metrics.

[Will.Spencer]

Originally Posted by jenniee

Why has QoS as a concept become so important to our understanding of networks.

QoS is becoming much more critical because VoIP and Video over IP almost require QoS to operate within the boundaries of acceptable quality.

[jenniee]

hey will spencer, y did you ask if i studied at the univeristy of salford on that particular course?

thank you for ur help.

[Will.Spencer]

Originally Posted by jenniee

no although i did study at the university of salford , whom r you?

I'm Will Spencer. I'm the webmaster here.

[Will.Spencer]

Originally Posted by jenniee

hey will spencer, y did you ask if i studied at the univeristy of salford on that particular course?

I'm just wondering where all of these questions are coming from.

I worry that you won't learn what you need to learn in your courses if I/we help you too much with your homework. ops: