An Overview of Quality of Service (QoS) and QoS Routing ... - CiteSeerX

An Overview of Quality of Service (QoS) and QoS Routing in Communication Networks Abdullah M. S. Alkahtani, M. E. Woodward and K. Al-Begain Department of Computing, University of Bradford, UK {a.m.alkahtani, m.e.woodward, K.Begain}@bradford.ac.uk

Abstract- Since communication networks have become a very essential part of our life, many efforts have been done towards improving their quality of service (QoS) in order to achieve more customer satisfaction which leads to strong loyalty and therefore to more profit to service providers and to achieve global efficiency in resource utilisation. Moreover, recent advances in high speed networking technology have created opportunities for the development of multimedia applications which are characterised by multiple QoS requirements. In this paper, we presented an overview of: a) the concept of QoS, its importance and specification issues in communication networks and b) the QoS routing algorithms and their classifications.

1

INTRODUCTION

The term quality of service (QoS) has become a very dominant and attractive notion in many aspects of our daily life. Frequently, we hear or use it or some other related terms like customer satisfaction, first class, golden class, good or bad service, Total Quality Management (TQM), quality rather than quantity, etc. Since communication networks become a very essential part of our life, many efforts have been made towards improving their quality of service in order to achieve more and more customer satisfaction which leads to strong loyalty and therefore to more profit to the service providers (win-win process) and to achieve global efficiency in resource utilisation. Moreover, recent advances in high speed networking technology have created opportunities for the development of multimedia applications. These applications integrate several media such as text, graphics, audio and video. In addition, they are characterised by multiple QoS requirements. Two of the key issues in supporting QoS in communication networks are QoS specifications and QoS routing. QoS specifications aim to investigate and specify what requirements needed for QoS are and to quantify them accurately. QoS routing is not only selecting a path for transmitting data from source to destination, but doing this to satisfy constraint(s) or to optimise requirement(s). Because of the diverse QoS requirements, QoS based routing is considered to be an NP-complete problem and cannot be solved by a simple and efficient algorithm [1].

ISBN: 1-9025-6009-4 © 2003 PGNet

In this paper, we present an overview of QoS specifications and QoS routing in communication networks. 2 QOS SPECIFICATIONS IN COMMUNICATION NETWORKS One of the most important steps in requesting QoS in communication networks is to specify what these requirements are and to quantify them accurately (QoS specifications). These QoS parameters have to be specified for all the architectural layers. However, most average users would not be familiar with the QoS parameters of the lower layers. In view of this situation, it is more convenient to obtain sufficient inputs from a user (expressed in user terms) and then to map these user QoS specifications to QoS parameters for the lower layers. It is very important to determine the correct set of accurate QoS parameters for the particular media being transported, otherwise QoS guarantees cannot be obtained despite the level of sophistication of the mechanisms used for negotiation and enforcement of QoS [2]. In telecommunications, QoS is defined by ITU-T [3] as “a collective effect of service performance which determines the degree of satisfaction of a user of the service” [2](referring to [4]). In [5], it is defined as the capability of a network to provide better services to selected network traffic over various (heterogeneous) technologies. In [6], Transmitted traffic through communication networks is characterised, in a very general way, by four primary parameters (metrics): loss (unreliability), delay, jitter (delay variation), bandwidth. However, we suggest that security is another important and primary parameter for certain traffics such as money transactions in ecommerce, confidential or extremely-private applications. Together, these determine the QoS the traffic requires. Several common applications and the stringency of their QoS requirements are listed in Table 1 ([6] with alterations).

Table 1 Examples of common applications and the sensitivity of their QoS requirements. E-mail Confidential email

Loss High High

Delay Low Low

File transfer

High

Low

Money transactions Audio on demand (AOD) Video on demand (VOD) Telephony Videoconferencing Confidential Videoconferencing

High

Low

Sensitivity Jitter Bandwidth Low Low Low Low Low, Low Medium, High Low Low

Low

Low

High

Medium

Low

Low

Low

High

High

Low

Low Low

High High

High High

Low High

Low Low

Low

High

High

High

High

Application

Data traffic

Real time traffic

Security Low High Low High

LK

In general, Table 1 shows that different traffics can be classified as follows: • Delay-sensitive traffic. Most real time applications (video, audio or voice) fall into this category. Some real time traffics are more sensitive to both end-to-end delay and delay variations (jitter) like telephony and video conferencing while others are tolerate the former but not the later like audio and video on demand. • Loss-sensitive traffic. File transfer and email fall into this category. In this type of traffic, no bits may be delivered incorrectly. If a packet is damaged during transmission, it is not acknowledged and will be retransmitted. • Security-sensitive traffic. Examples are money transactions and confidential applications. • Bandwidth-sensitive traffic. Examples are video on demand (VOD). Multi-sensitive traffic. This type of traffic is associated with certain multimedia applications or when certain traffic is sensitive to more than one metric. For example, confidential email can be considered as loss-security- sensitive, confidential video conferencing can be considered as delay-security sensitive.

m( p) = ∑ m(lk i )

Where m(p) is the total of metric m of path (p),

lk i is a link in the path (p), LK is the number of links in path (p) and i = 1,...., LK . Delay, delay variation (jitter), and cost are examples of this type of composition. Various factors that determine the delay in communication networks are reviewed in [8]. •

Concave metrics: It can be represented mathematically as follows

m( p) = min(m(lk i ))

( 2)

Bandwidth is an example of this type of composition. The bandwidth we are interested in here is the residual bandwidth that is available for new traffic. It can be defined as the minimum of the residual bandwidth of all links on the path or the bottleneck bandwidth. •

2.1

( 1)

i =1

Multiplicative metrics:

Composition rules of the QOS metrics

Although multiple metrics can certainly model both networks and applications more accurately, the problem is that finding a path subject to multiple metrics is inherently difficult and in many cases is considered as NP-complete [1]. It means that it can not be solved in a real time scale which is very crucial for the majority of applications in general and for delay-sensitive applications in particular. The value of a metric over the entire path can be one of the following compositions [1][7]: •

Additive metrics: It can be represented mathematically as follows

It can be represented mathematically as follows LK

m( p ) =

∏ m(lk ) i

( 3)

i =1

Loss probability (l), indirectly, is an example of this type of composition. Why indirectly? The loss probability metric can be easily transformed into an equivalent metric that follows the multiplicative composition rule [1] (the probability of successful transmission (st)). Successful transmission (st) can be expressed as follows

st (lk ) = 1 − l (lk ) .

( 4)

st ( p ) =

LK

∏ st (lk i )

( 5)

i =1

Therefore, the loss probability metric (l) can be represented mathematically as follows l ( p ) = 1 − {[1 − l ( lk1 )] [1 − l (lk2 )]...[1 − l ( lk LK )]} ( 6)

2.2

QOS Metric Selection Criteria

As can be seen from the previous sections, QoS metrics play a very important role in providing better QoS. Therefore, they have to be selected carefully. There are a number of factors that have to be considered when selecting metrics. Metrics must be able to model the source (application) accurately. They must reflect the basic characteristics of a network. They must be quantified in order to help designing call admission protocols that map the user QoS requirement, which might be a combination of objective (e.g. image size) and subjective (e.g. image quality) terms, to network characteristics. [1, 2] 3

QOS ROUTING IN COMMUNICATION NETWORKS

Routing can be defined as the act of moving information across a network from a source to a destination(s) [9]. Therefore, QoS routing can be defined as moving information across a network from a source to a destination(s) while considering QoS requirements in order to achieve more satisfaction for customers and more optimisation of network resource usage. As mentioned earlier, one of the key issues in supporting QoS in communication networks is QoS routing, which is not only selecting a path for transmitting data from source to destination(s), but doing this to satisfy constraints or to optimise requirements. QoS routing often has one or more of the following design goals [9-11]: • Correctness • Optimality • Simplicity and low overhead • Stability and robustness • Rapid convergence • Flexibility In considering the QoS routing, it is important to distinguish two concepts [11-13]: • Routing information; Information about the topology and link states of the network. • Routing algorithm; the algorithm used to make a routing decision in order to find a feasible path

for a new connection (or an existing one in case of handover or link failure problems) based on the collected information. In this section, we present an overview of these two important concepts [12, 13]. 3.1

Routing Information

In order to enable a routing algorithm to make an optimal routing decision, it is important to make this decision based on correct and updated information about the topology and states of the links of the network (routing information). This routing information (state) can be collected in local, global or aggregated global levels [12, 13]. For the local state, each node is assumed to maintain its up-to-date local information including queuing and propagation delay, residual bandwidth of the outgoing links, security loss probability and the availability of other resources. For the global state, the combination of the local states of all nodes required. Every node is able to maintain the global state by either a link-state protocol or a distancevector protocol [6, 12, 14], which exchanges the local states among the nodes periodically. Link-state protocols broadcast the local state of every node to every other node so that each node knows the topology of the network and the state of every link. Distance-vector protocols periodically exchange distance vectors among adjacent nodes. For the aggregated global state, a common approach to achieving scalability is to reduce the size of the global state by aggregating information according to the hierarchical structure of large networks. Obviously, routing information has to be updated frequently. The more frequent updating the more accurate the routing decision. However, there has to be a trade-off between accuracy and overhead. In general, network information update timing can be [11]: • Continuous • Periodic And can be dictated by: • Major load change • Topology change 3.2

Routing Algorithms

The second important concept related to QoS routing are the routing algorithms which make the routing decisions. They can be classified based on many different criteria. In the following, we present some of these criteria. Most of these categories overlap or are dependent on one another. Nevertheless, they may clarify classifications of routing algorithms.

• 3.2.1

Information Source

Based on this criterion, routing algorithms can be classified as follows [13, 15]: • Local state routing • Global state routing. This can be divided in to two main classes [6, 13]: Link State routing (Dijkstra) [6, 11, 1620]. Distance vector routing (Bellman-Ford) [6, 11, 21]. • Aggregated (or Hierarchical) state routing.

[22-24] 3.2.2

Number Of Sources And Destinations

Based on this criterion, routing algorithms can be classified as follows: • Point-to-point (unicast or one-to-one) routing [25-30]. • Point-to-all (broadcast or one-to-all) routing

[18, 31, 32] • •

3.2.3

Point-to-multipoint (multicast or one-tomany) routing [33-35]. Multipoint-to-multipoint (many-to many) routing [29] Decision Place

Based on this criterion, routing algorithms can be classified as follows [11, 15]: • Source routing. [1, 22] • Distributed routing (each node) [1, 25, 26, 28, 36]. • Centralised routing. All routing decisions are made by some designated node, such as a network control centre. 3.2.4

Number Of Qos Metrics Considered

Based on this criterion, routing algorithms can be classified as follows: • Single metric routing. It includes routing algorithms that consider only one metric like delay, bandwidth or number of hops. Dijkstra’s or Bellman-Ford shortest path algorithms are good examples for this type of routing algorithm. • Multiple metric routing [1, 25, 26].

3.2.5

Single compound metric (SCM) or single mixed metric (SMM) routing [1, 37]. Required Qos Guarantee Levels

Although we generally state that QoS should be guaranteed, in practice the user should be able to specify the degree (or level) of guarantees. Based on this criterion, routing algorithms can be classified as follows: • Constrained QoS routing [38]: user-specified QoS should be met 100%. This level of guarantee is also known as a Hard or deterministic guarantee [39, 40]. • Best effort routing: [41-43] no guarantee is provided and the application is executed with whatever resources are available. The traditional Internet operates with this class of routing. • Constrained-Best effort routing: a routing algorithm from this class meets some QoS constraints and optimises other QoS metrics. For example, delay-constrained maximumbandwidth [44], delay-constrained least-cost [26, 45, 46] or Bandwidth-delayconstrained least-cost [47]. • Soft or statistical guarantee [39, 40]: the QoS should meet a certain user-specified percentage. This is appropriate for continuous media because continuous media normally do not need 100% accuracy in playback. In addition, this type of guarantee uses system resources more efficiently. Resource usage can be based on statistical multiplexing: resources unused by one application can be used by other applications. Different levels of guarantee are used for different types of traffic. In some cases, one connection may use different levels of guarantee for different QoS parameters. For example, the user may request that the specified bit error rate should be met 100% (hard guarantee) but the specified delay jitter value should be met 90% (soft guarantee). 3.2.6

Wired Or Wireless (Fixed Or Mobile)

Although, most of routing algorithms have been designed for traditional wired (fixed) networks like ATM and IP networks, there is a big revolution to enhance those traditional networks or to design new

network architectures in order to support mobility (anytime-anywhere approach) while considering QoS issues. Based on this criterion, routing algorithms can be classified as follows: • Wired (fixed) routing. • Wireless (mobile) routing. Most of routing algorithms in this class concentrate on finding a new route for an existing connection during the handover process between cells (rerouting) [48, 49]. 3.2.7

Connection Mode

Based on this criterion, routing algorithms can be classified as follows: • Connectionless routing. Internet routing protocols are well known examples of this class of routing. • Connection oriented routing. ATM routing protocols are well known examples of this class of routing. 3.3

QOS ROUTING AND THE COMPLEXITY PROBLEM

Wang and Crowcroft in [1] stated that “The problem of finding a path subject to constraints on two or more additive and multiplicative metrics in any possible combination is NP-complete. The results are applicable to any metric that follows additive or multiplicative composition rules and to any metrics that can be transformed to equivalent metrics that follow the additive or multiplicative composition rule". Therefore, they suggested that the only feasible combinations are one concave metric, bandwidth, and one metric of additive or multiplicative types [1] if exact results are to be obtained. However, we think that it is still feasible to combine more than one concave metric and one metric of additive or multiplicative metrics. 4

SUMMARY

In this paper, we presented an overview of: a) the concept of QoS, its importance and specification issues in communication networks and b) the QoS routing and their classifications. Since communication networks have become a very essential part of our life, many efforts have

been done towards improving their quality of service in order to achieve more customer satisfaction which leads to strong loyalty and therefore to more profit to the service providers and to achieve global efficiency in resource utilisation. Moreover, recent advances in high speed networking technology have created opportunities for the development of multimedia applications which are characterised by multiple QoS requirements. Two of the key issues in supporting QoS in communication networks are QoS specifications and QoS routing. QoS specifications aim to investigate and specify what requirements needed for QoS are and to quantify them accurately. QoS routing is not only selecting a path for transmitting data from source to destination, but doing this to satisfy constraint(s) or to optimise requirement(s). Routing Algorithms often have one or more of the following design goals: correctness, Optimality, simplicity and low overhead, stability and robustness, rapid convergence and flexibility. In considering the QoS routing, it is important to distinguish two concepts: routing information (Information about the topology and link states of the network) and routing algorithm (the algorithm used to make a routing decision in order to find a feasible path for a new connection (or an existing one in case of handover or link failure problems) based on the collected information. Routing algorithms can be classified based on different criteria: information source, number of destinations, decision place, number of considered QoS metrics, required QoS guarantee levels, wired or wireless (fixed or mobile), Connection mode. Most oh these categories overlap or dependent on one another. Nevertheless, they may clarify classifications of routing algorithms. REFERENCES [1] Z. Wang and J. Crowcroft, "Quality-of-Service Routing for Supporting Multimedia Applications", IEEE Journal on Selected Areas in Communications, vol. 14, n. 7, pp. 12281234, 1996. [2] F. Cheong and R. Lai, "QoS Specification and Mapping for Distributed Multimedia Systems: A Survey of Issues", The Journal of Systems and Software, vol. 45, n. 2, pp. 127-139, 1999. [3] ITU-TS, http://www.itu.int/home/index.html [4] ITU-TS, "Terms and Definitions Related to the Quality of Telecommunication Services (Recommendation E.800)", 1988,

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