BY:MUHAMMAD BILAL SATTI

Doing BS ELECTRONIC ENGINEERING FROM INTERNATIONAL ISLAMIC UNIVERSITY H-10 ISLAMABAD.

Bilal.bsee1721@iiu.edu.pk

QUALITY

OF

SERVICE(Q

OS)

CO

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NE

TW

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QOS

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, bandwidth, throughput, transmission delay, availability, jitter, reliability etc.

We can formally define QoS as something a flow seeks to attain.

We will discuss these Flow Characteristics one by one.

RELIABILITY

 The term "reliable" is a synonym for assured.

In computer networking, a reliable protocol provides reliability properties with respect to the delivery of data to the intended recipient(s), as opposed to an unreliable protocol, which does not provide notifications to the sender as to the delivery of transmitted data.

Lack of reliability means losing a packet or acknowledgment, which entails retransmission.

 It has long been considered one of three related attributes that must be considered when making, buying, or using a computer product or component. Others aspects are availability and serviceability.

For example, it is important that electronic mail, file transfer, and the internet access have reliable transmissions than telephony or audio conferencing.

TRANSMISSION DELAY

Source-to-destination delay is another important aspect of a flow characteristics.

In a network based on packet switching, transmission delay  is the amount of time required to push all of the packet's bits into the wire. In other words, this is the delay caused by the data-rate of the link.

Applications can tolerate delay in different degrees.

Telephony, audio conferencing, video conferencing, and remote log-in need minimum delay, while delay in file transfer or email is less important.

It is given by the following formula:

  Dt=N/R secs

N:number of bits

R:rate of transmission

JITTER

Jitter is the variation in delay for packets belonging to the same flow. 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.

For example, if four packets departs at times 0,1,2,3 and arrives at 20,21,22,33, all have the same delay of unit 20. On the other hand, if the above four packets arrives at 21,23,21, and 28, they will have different delays:21,22,19,24.

For applications such as video and audio the first case is acceptable but the second case is not.

High jitter means difference between delays is large. Low jitter means variation is small.

BANDWIDTH

The concept of Internet bandwidth refers to the amount of data that may be transferred over an Internet connection per second. Bandwidth is measured in bits per second, and most Internet connections have a bandwidth on the scale of megabits per second. Every Internet connection has a specified maximum bandwidth, but many factors can combine to limit this for a particular device. This results in a slowness of the perceived speed of a connection. 

QoS policy should prioritize the use of bandwidth to provide quality of service to customers or users.

Different applications needs different bandwidths.

For example, in video conferencing we need to send millions of bits per second to refresh a color stream while the total number of bits in an email may not reach even a million.

TECHNIQ

UES TO IM

PROVE

Q

OS(SCHEDULIN

G)

SCHEDULING

Packets from different flows arrives at a switch or router for processing.

A good scheduling technique treats different flows in a fair and appropriate manner.

Several scheduling techniques are designed to improve the quality of service.

I will discuss three scheduling techniques here

1. FIFO queuing

2. Priority queuing

3. Weighted fair queuing

FIFO QUEUING

In First In-First out Queuing packets will wait in a buffer until the node is ready to process them.

If the average arrival rate is higher than the average processing rate , the queue will fill up and new packets will be discarded.

A FIFO queue is familiar to those who had to wait for a bus at a bus stop.

PRIORITY QUEUING

In priority queuing, packets are firstly assigned to a priority class.

Each class with its own queue.

The packets with highest priority queue will processed first and packets with lowest priority will be processed last.

The system will continue to serve as long as there is something in the queue.

WEIGHTED FAIR QUEUING

In this technique the packets are assigned to different classes and admitted to different queues.

In this case the queues are weighted on basis of their priority i.e. higher priority means higher weight.

The system processes packets in each queue in a round-robin fashion with the number of packets selected from each queue based on corresponding weight.

TECHNIQ

UES TO IM

PROVE

Q

OS(TRAFF

IC S

HAPING)

TRAFFIC SHAPING

It is the mechanism to control amount and the rate of the traffic sent to the network.

Two techniques can shape traffic

1. Leaky bucket

2. Token bucket

LEAKY BUCKET

If a bucket has a small hole at the bottom then the water will leak from the bucket at a constant rate.

The rate at which water leaks from the bucket will not depend on the rate at which water is input to the bucket.

The input rate can vary but the output remains constant.

So in networking a Technique called Leaky bucket can smooth out the bursty traffic.

DISADVANTAGE

Leaky bucket is very restrictive. It does not credit an idle host.

If a host is not sending for a while, its bucket becomes empty. Now if the host sends a bursty data, the leaky bucket allows only average rate.

The time at which the host was idle doesn’t taken into account.

TOKEN BUCKET

Token bucket takes in account the idle time of the host,.

With each clock ticks tokens are added into the bucket.

When data needs to be send, then it collects a token from the bucket and then sends the data packet, consisting of data == no. of tokens.

RESOURCE RESERVATIONS AND ADMISSION CONTROL

Flow of data needs resources such as buffer, bandwidth, CPU time , and so on.

QoS can be improved if these resources are reserved before time.

Admission control refers to a mechanism used by router, or a switch to accept or reject a flow based upon flow specifications.

Before a router accepts a flow, it checks the flow specifications to see if its capacity and its previous commitments to others flows can handle the new flow.

INTEGRATED SERVICES

Two models have been designed to provide quality of service in the Internet: Integrated Services and Differentiated Services. We discuss the first model i.e. Integrated services here.

Integrated Services sometimes called IntServ. is a flow-based QoS model designed for IP which means that user needs to create a flow, a kind of virtual circuit, from the source to the destination and inform all routers of the resource requirement.

Signaling Flow Spécification Admission

Service Classes

RSVP

Problems with Integrated Services

To implement a flow-based model over a connectionless protocol we use signaling protocol to run over IP that provides the signaling mechanism for making a reservation. This protocol is called as RSVP.

When a source makes a reservation, it needs to define a flow specification. It has two parts.

Rspec(resource spec.)

Tspec(traffic spec.)

Rspec. defines the resource that the flow needs to reserve and Tspec. defines the traffic characterization of the flow.

After a router receives the flow specification from an application, it decides to admit or deny the service. This is called as Admission.

Two service classes have been defined for integrated services.

Guaranteed service

Controlled-load service

Guaranteed service class is designed for real time traffic that needs a guaranteed minimum end-to-end delay. This type of service guarantees that the packet will arrive within a certain delivery time and are not discarded if flow traffic stays within boundary of Tspec.

Controlled-load service class is designed for applications that can accept some delays, but are sensitive to an overloaded network and to the danger of losing packets.

RSVP is a signaling protocol to help IP create a flow and consequently make a resource reservation.

Multicast Trees used to enable RSVP to provide resource reservations for all kinds of traffic including multimedia which uses multicasting.

Receiver-Based reservations used in RSVP as the receivers, not the sender, makes the reservation.

RSVP has several types of messages.

Path Messages

Resv. Messages

PATH MESSAGES

Path is needed for reservation, so RSVP used path messages that travels from sender and reaches at all receivers in the multicast path.

RESV MESSAGES

After a receiver has received a path message, it sends a Resv message. The Resv message travels towards from sender and makes a resource reservation on the router that’s support RSVP.

RESERVATION MERGING

In RSVP, the resources is not reserved for each receiver in a flow, the reservation is merged.

RESERVATION STYLES

When there is more than one flow , the router needs to make a reservation to accommodate all of them.

In wild card filter the router creates a single reservation for all senders.

In fixed card filter, the router creates a different reservation for each flow.

In shared explicit style, the router creates a single reservation that can be shared by a set of flows.

DIFFERENTIATED SERVICES

Differentiated Services (DS or Diffserv) was introduced by the IETF (Internet Engineering Task Force) to handle the shortcomings of Integrated Services.

Two major changes were made:

1. The main processing was moved from the core of the network to the edge of the network.

2. The per flow service is changed to per class service.

The Differentiated Services is a class based Qos model designed for IP.

ISSUES

As the internet grows day by day so scalability is a major issue.

Those who are opposing Integrated service model argues that applications may need more than guaranteed and control-load types of services.

So a new model is designed to overcome these issues.

DS FIELD

In Diffserv. Each packet contains a field called as DS field.The value of this field is set at the boundary of the network by the host or the first router designed as the boundary router.

The DS field contains two subfields: DSCP & CU.

DSCP is a 6-bit subfield that defines per-hop behavior (PHB).

The 2-bit CU subfield is not currently used

The DiffServ model defines PHB for each node that reveives packet .So far 3 PHB’s are defined: DE PHB,EF PHB & AF PHB.

PHB’S

The DE PHB is the same as best-effort delivery, which is compatible with TOS.

The EF PHB provides Low Loss, Low latency & Ensured Bandwidth.

The AF PHB delivers the packet with high assurance as long as the class trafficdoes not exceed the traffic profile of the node.

To implement DiffServ, the DS node used Traffic conditioners such as meters , markers, shapers and droppers.

TRAFFIC CONDITIONERS

The meters checks to see if the incoming flow matches the negotiated traffic profile. It also sends results to other components.

A marker can remark a packet that is using best-effort delivery or down-mark a packet based on information received from meter.

A shaper used the information received from meter to reshape the traffic is it is not complaint with the negotiated profile.

A dropper, which works as a shaper with no buffer, discards packets if the flow severely violates the negotiated profile