Srikant Devaraj

They are:
1) Dropped packets:
There are possibilities that when some packets arrive at routers, the buffers get filled up resulting the packets getting failed to deliver or drop them. Therefore depending upon the state of the network, there are situations where some, all or none of the packets get dropped. Also we cannot determine what will happen, in advance. The end user application asks for the information to be transmitted again which results in severe overall transmission delays.
2) Delay (Latency):
When the packets get transmitted, it may have to experience waiting in long queues and gets held up there resulting in an excessive delay. These packets also take a less direct route, to avoid congestion, which again causes delay. Such problems can affect a continuous application like VoIP unusable.
3) Jitter:
There are possibilities that the packets transmitted from the source reaches the destination with different delays. Depending upon the position of packets in the queues of routers, the delay varies accordingly in an unpredicted way. This variation in delay is called Jitter and it can affect the quality of audio and/or video streaming.
4) Out-of-order delivery:
When a set of packets, which are related to each other, is routed through internet, there are possibilities of packets taking different routes which results in a different delay. The outcome of the transmission is that these packets arrive in a different order when compare to the order in which they were sent. This is applicable for quality seeking applications like video and VoIP streams, which gets affected by latency and/or out of order delivery.
5) Error:
Sometimes packets get misdirected, combined together or corrupted, when they travel from source to destination. Then the receiver needs to detect this mistake and asks the sender to repeat the transmission process again.

Quality of Service (QoS)
Quality of Service (QoS) is defined as the ability of a network which offers improved service to preferred network traffic over a variety of technologies. The major objective of QoS is to provide priority that includes dedicated bandwidth, restricted jitter, latency and improved loss characteristics. It is important to make sure that giving priority for one or more network stream of packets should not make other network streams to fail.

Some of the Applications that require QoS:
• For Streaming multimedia, a guaranteed throughput is required to ensure that there is sufficient level of quality
available during transmission of packets.
• Voice through internet phones like IP Telephony or Voice over IP (VOIP) needs firm limits on jitter and delay.
• Video Teleconferencing will need low jitter and latency.
• A significant application, such as a remote surgery will surely need a guaranteed level of availability.

Factors affecting QoS:
Quality of service is affected by various factors, which can be divided into: (QoS, 2007)
1. Human Factors which include,
• Strength of service,
• Availability of service,
• Latency,
• User details.

2. Technical factors which include
• Dependability,
• Measurability,
• Efficiency,
• Maintainability,
• Grade of Service

For mobile multimedia networking service, the most important factors are the call set up delay, possibility of end-end jamming and the effective bandwidth. These factors can be relied on both network and terminal dependent.
The call set up delay is the time interval taken from the moment user begins a network connection request to the acknowledgement showing the call nature received by the calling terminal.

The shortage of network resources either at the users end or the control end can cause failed call efforts. The possibility of end- to- end jamming (blocking) can occur at the radio links, at the interworking units between the mobile multimedia and the fixed networks or the transit networks.The effective bandwidth has been build up to determine the resource usage after considering the QoS requirements and various statistical characteristics, which effectively represents the exchange between sources of different types.

Measurement of QoS parameters:
QoS parameters cannot be always measured directly. The comparative load of their influence in the user's estimate depends on the type and nature of the service. The service requirement for users will be different for different applications like interactive applications, conversational, messaging or recovery services and distribution services.
On the other hand, Network Performance (NP) parameters are measurable. The Network Performance reflects the efficiency of the network in providing services to customers including many factors that influence QoS. The network providers combine the principles of the QoS parameters, so that the customers are fully satisfied with the resulting QoS obtained at an satisfactory cost. The essential difference between QoS and NP is that QoS is more of user oriented and NP is more of provider oriented. The terminal also has a strong control on the quality. The relationship between QoS and NP is complex to determine and it is difficult to know the range of each NP parameter generating a desired quality of service levels. When there are several sets of network performance parameters, it leads to an acceptable quality of service. Out of these sets, whichever set has low network costs is chosen.

QoS Management:
The performance properties that influence the quality of multimedia presentations are of a major concern.
• End-user preferences and requirements: The end users require excellent sound and video quality and low cost
• Terminal constraints: There are many restrictions to screen size, audio equipments, operating systems feedback, availability of decoding software, etc.
• Server constraints: Constraints in the form of number of users, overall throughput limitations etc.
•Viewed document constraints: There are concerns regarding encoded information structure, measurable encodings etc
• Network constraints: Concerns regarding available throughput, delay, jitter plays a major role in network constraints.

QoS over mobile links
When mobile multimedia transport systems are working in a location where extensive mobility and alteration of network characteristics are prevalent, these systems will not be efficient. These challenges, primarily due to large- scale mobility requirement s, limited radio resources and fluctuating network conditions, fundamentally impact on our ability to deliver multimedia flows over mobile and, in general, QoS fluctuating networks. Therefore, one of the major challenges is the delivery of multimedia flows to mobile devices with QoS constraints.

The consequence of the mobile multimedia network during the transmission of packets depends on various factors that include the load on the network and its behavior after losing the packets and during retransmissions. Due to constraint in the bandwidth and drastic bit error rate, there are problems popped up by the nature of the medium in a network. The strength of a signal shows the quantum of traffic received, the different paths in which the packets need to take and the execution of handovers taking place. All these have an impact on QoS. Different applications will increase the loads on the network thereby increasing traffics. These applications will need different quality of service requirements like delay, jitter, bandwidth etc for various streams like data, voice and video.

User Perception:
The way in which, a user differentiates a QoS is an important issue. They can be differentiated as a measure of quality with reference to a particular case. Other type of differentiation is the evaluation of the quality of a particular application like video clip depending on the actual length of the same with respect to time.

When a multimedia video clip is transmitted through a network, it experiences lot of disturbances like congestion, packet delays etc. Assume that a clip is transmitted across a network, which is affected by various types of disturbance in terms of congestion. This can be shown in the required quantum of corrupted packets and request for retransmissions. Therefore the time plays an important role when there is disturbance caused by the effects of network. There can be different cases and manners, when a user views a video clip. One case will be viewing with less and frequent disturbances to quality. Other will be a case with long and rare disturbances during the entire video clip.

This is my first technical blog and I wanted to make it as detailed as possible on this topic on QoS.

References:
(QoS, 2007) Quality of Service in broadband networks. Retrieved October 11, 2007, from Quality of Service Web site: http://cnx.org/content/m13374/latest/

(QoS Evaluation, 2002): Subjective Evaluation of QoS ; Quality of Service for Mobile Multimedia Communication http://www.een.bris.ac.uk/UMPTIDUMPTI/data/papers/rhodes/rhodes.html

(Wolisz, Adam, 2000). TKN telecommunications network group - Prof. Wolisz. Retrieved October 5, 2007, from Handover in all IP mobile networks Web site: http://www.tkn.tu-berlin.de/research/handover/handover.htm