HOW CAN A COUNTRY PROVIDE A GOLD STANDARD TELECOMMUNICATION NETWORK? Ian G. Kennedy School of Electrical and Information Engineering, University of the Witwatersrand Phone (011) 717-7228, Fax (011) 403-1929, e-mail:
[email protected] ABSTRACT - A Service Level Agreement (SLA) is a quality of service contract between a service provider and a customer. The customer may be the end user, another service provider or an interconnect agreement customer. The agreement guarantees that the customer will receive a specific level of performance. This year, at the ITC Seminar on Access Networks and Systems, Schneps-Schneppe et al surveyed the bestpractice agreements. They classified the SLAs of leased circuit operations and built a unifying quality of service (QoS) index. They show that a practical index may be built from merely three parameters —- reliability, performance and price. The work is important because it leads to schemes for optimally providing the QoS and enables telecommunication bodies to determine penalties for infringements of SLA contracts. The present paper extends some of the work to include real telecommunication networks consisting of links and nodes in tandem and parallel. It reviews how reliability is affected by combining components of different reliability requirements. An illustrative single metric of QoS and a set of SLA categories are chosen. Next, the paper develops the pragmatic and parsimonious mathematics for evaluating networks composed from elements of different SLA standards. The paper interprets the consequences of the mathematics for telecommunication service providers and warns about the dire consequences of including even a single element of baser reliability.
KEYWORDS—- Service Level Agreements, SLA, network reliability, telecommunication network components, Quality of Service, QoS, customer.
I. INTRODUCTION THIS paper builds on very recent work to include real telecommunication networks consisting of lines and nodes. Section II reviews the definition of Quality of Service (QoS). Section III emphasizes the importance of QoS guarantees. Section IV defines a Service Level Agreement (SLA) and revises the important metrics. Section V looks at the market pressures to have SLA contracts, while Section VI focuses on a single metric. Section VII defines reliability and the classes of service. Section VII combines links and
nodes in tandem, while Section IX analyses how reliable networks are constructed from varied components. Section X summarizes the algebra of SLAs.
II. WHAT IS QUALITY OF SERVICE? Generically, Quality of Service (QoS) means ensuring a consistent, predictable network to convey digital data. In other words, making sure that the requirements of the customer's application are satisfied. QoS is here formally defined as the capability of a network to define and negotiate levels of performance, reliability and predictability between the customer and the network provider. QoS is to the ability of a network element (e.g. an application, host or router) to have some level of assurance that its traffic and service requirements can be satisfied.
III. WHY DO ENTERPRISES NEED OF SERVICE?
QUALITY
Enterprises require greater reliability and bandwidth, yet their traffic is becoming less predictable. A. Availability Assurances Reliability is critical for effective running of a business. The network is increasingly relied upon for doing business, and the expectations for quality assurances are the same as for a private, controlled network. Telecommunication networks are used within the enterprise and externally for electronic commerce with business partners. As business is increasingly conducted over the networks, it becomes important to ensure that these networks deliver appropriate levels of quality. QoS measurements allow the enterprise to perform mission-critical business over the public network. B. Bandwidth Demands Applications are getting more demanding. Missioncritical applications deployed over networks increasingly require assured quality, reliability, and timeliness. In particular, applications that use voice, video streams, or multi-media must be carefully managed within a network to preserve their integrity.
Paper presented at the South African Telecommunication Networks & Applications Conference SATNAC 2001, 2 – 5 September 2001, Wild Coast Sun, South Africa Produced by: Document Transformation Technologies
ISBN Number: 0-620-27769-6 www.satnac.org.za Organised by: Global Conferences
C. Critical Applications Managing QoS becomes increasingly difficult because many applications deliver unpredictable bursts of traffic. For example, usage patterns for data applications are virtually impossible to predict, yet networks need to be able to support the enterprise's mission-critical applications even during peak periods.
architecture of the PSTN shown in Figure 1 has been engineered to achieve 99.999 percent uptime. Because users are accustomed to nearcontinuous availability of voice services over the PSTN, they will not tolerate inferior service in new networks. Thus successful deployment of telecommunications services in converged networks must deliver the same system-wide reliability levels as the PSTN does.
IV. WHAT IS A SERVICE LEVEL AGREEMENT? A Service Level Agreement (SLA) [1] is a QoS contract between a service provider and a customer. (The customer may be the end user, another service provider or an interconnect agreement customer.) The agreement guarantees that the customer will receive a specific level of performance. Schneps-Schneppe et al [1] survey the best-practice agreements. They classify the SLAs of leased circuit operations and build a unifying QoS index. They show that a practical index may be built from merely three parameters — reliability, performance and price. The work is important because it leads to schemes for optimally providing the QoS and enables telecommunication bodies to determine penalties for service level agreement infringements.
V. WHY SERVICE LEVEL AGREEMENTS? Telecommunication network operators are being forced by market pressures to provide SLA agreements: •
•
•
The key to a telecommunication network operator obtaining revenue from a network is to provide customers with a specific QoS, flexible billing options, and reasonable prices. Customers will begin to distinguish between competing telecommunication service providers by the SLAs that are offered. Thus it is becoming critical for network operators (and the ombudsman) to develop the traffic engineering expertise to ensure contractual compliance with the SLA. Telecommunication service providers are entering into SLAs with their customers to guarantee to their customers certain network metrics. This way, the customers will be assured that their applications will perform well over the network. After decades of improvement, the Public Switched Telephone Network (PSTN) has achieved an extremely high degree of network availability. This reliability is ensured, for example, through the use of independent power sources. The centralized, switch-based
Figure 1: Typical PSTN interconnections
VI. WHAT SHALL WE MEASURE? What to measure is an open question. There are a number of characteristics that may quantify QoS in a circuit switched network, including: • minimizing blocking probability • minimizing queueing delay • guaranteeing reliability. The present paper concentrates on reliability of service, as measured by the availability (uptime) of the end-to-end link.
VII. HOW IS RELIABILITY QUANTIFIED? We need practical definitions for reliability and its measure for SLAs: Reliability — Reliability is quantified here by the end-to-end availability. There are three service levels, 99.999%, 99.99% and 99.9%, known as 5 nines, 4 nines and 3 nines reliability respectively. (Because it is defined end-to-end, the number of end-to-end SLA pairs increases as the product of the number of sources and sinks!) Classes of Service —The present paper adopts just three classes of service, named after the metals of the Olympic medals: Gold, Silver and Bronze. These classes of service will be abbreviated as Au, Ag and Bz.
VIII. HOW ARE COMPONENTS COMBINED TO MAKE REAL NETWORKS? A real end-to-end connection consists of channels (arcs) and switches (nodes) in tandem (+). What is the mathematics of combination? Using Newton's method of approximation, we can draw up Table 1, showing all possible operations and their results.
Table 1: Olympic metal mathematics: Serial connections Bz + Au = Bz Bz + Ag = Au + Ag = Ag Ag + Ag = Ag Bz Au + Bz = Bz Ag + Bz = Bz Bz + Bz = Bz
In short, the algebra of olympic metal SLAs is such that the associative, commutative and distributive laws hold true.
Au + Au = Au Ag + Au = Ag
In general, X + Y = Min (X, Y), with X, Y ε {Au, Ag, Bz}. (1) In practice, the reliability of switches is very much greater than the reliability of links, so the reliability of a typical chain of 6 switches and 5 links will be dictated by the reliability of the 5 links. Because of the assumption of statistical independence of the reliability of the links, the reliability of the chain will be of the order of the reliability of a link. A communication path through such a network is only as reliable as its weakest channels and switches. Thus a gold standard network can only be constructed out of gold standard channels and switches. A country can only provide a gold standard telecommunication network if it is demonstrably constructed from components that are gold standard.
IX. HOW ARE COMPONENTS COMBINED TO MAKE REAL, RELIABLE NETWORKS? A reliable end-to-end connection consists of channels (arcs) and switches (nodes) in parallel (*). What is the mathematics of this combination? Using again Newton's method of approximation, we can draw up Table 2, showing all possible operations. Table 2: Olympic metal mathematics: Parallel connections Au * Au = Au Ag * Au = Au Bz* Au = Au Au * Ag = Au Ag *Ag = Ag Bz * Ag = Ag Au * Bz = Au Ag * Bz = Ag Bz* Bz = Bz In general, X * Y = Max (X, Y), with X, Y ε {Au, Ag, Bz}. (2) A communication path built up of redundant parallel components is as reliable as its most reliable channels and switches. Finally, because X*(B + C + D) = X*B + X*C + X*D, (3) the distributive law also holds true.
X. CONCLUSION This paper has shown that a gold standard, nationwide telecommunication network can be built to provide end-to-end Quality of Service as contracted in an end-to-end Gold Service Level Agreement with the customer. The algebra of SLAs has been developed. The interesting concluding point is that if just one customer demands (and pays for) a gold SLA, then a gold service level must be provided to every other customer he can contact!
XI. ACKNOWLEDGEMENTS The author wishes to thank Gary Agnew for his comments on an earlier version of this paper.
XII. REFERENCE [1]
Schneps-Schneppe, M., Enyukov, I., Internet Access Service Level Agreement - A Challenge to Teletraffic Engineering. ITC 14 Specialists Seminar on Access Networks and Systems, Girona, Spain. 265-275. ISBN 84-7653-777-8. April 25-27, 2001
HOW CAN A COUNTRY PROVIDE A GOLD STANDARD TELECOMMUNICATION NETWORK? Ian G. Kennedy School of Electrical and Information Engineering, University of the Witwatersrand Phone (011) 717-7228, Fax (011) 403-1929, e-mail:
[email protected] Dr. Ian Kennedy is a Senior Research Officer at the School of Electrical and Information Engineering University of the Witwatersrand His current research and lecturing interests include research on teletraffic modelling: Telecommunication network traffic, performance analysis, simulation, design and dimensioning. This year he is supervising two PhD candidates and two Masters candidates in Electrical Engineering plus two PhD candidates in the School of Commerce. This year he has also presented his research at the International Teletraffic Congress Specialists Seminar in Spain, and at the IEE Engineering Education conference in London. He will be returning to London next year to present three new papers at the IEE conference next year. He conducts postgraduate courses in Teletraffic Engineering, Research Methods and Research Writing. He also conducts courses on managing research, plagiarism and on using the Web in Research. Send E-mail to
[email protected]
