Quality of Service in Networks - Essay Example

www.allwriting.net Sumanta Sanyal Date: A Report: Quality of Service in Networks Introduction Quality of Service (QoS) in information networks allows a user to provide better service to certain flows within the network. This can be done either by raising the priority of the targeted flow or limiting the priority of another untargeted flow. This may require congestion-management tools where the priority of a flow is enhanced by placing it in queues and servicing the queues in different ways. In this process the queue management tool avoids congestion by placing high priority flows before low-priority ones. Policing and shaping by limiting the throughput of other flows shapes the priority of the targeted flow. Link-management tools also provide QoS by limiting large flows in preference of smaller flows (Quality of Service Networking, Cisco Systems, 2002). These are rough patterns of QoS management tools and the following definition of QoS will enable the reader to better understand the process in its full implications. QoS: Definition Quality of Service (QoS) refers to the capability of a network to provide better service to selected network traffic over various technologies like Frame Relay, Asynchronous Transfer Mode (ATM), Ethernet and 802.1 networks, SONET, and IP-routed networks that may use any or all of these basic technologies (Cisco Systems, 2002). QoS is maintained by raising priority that includes sustained assurance of dedicated bandwidth, controlled jitter and latency (required by certain real-time and interactive traffic) and improved loss characteristics (Cisco Systems, 2002). QoS systems inherently assure that raising the priority of one flow does not mean that the other flows will fail. Instead the queues developed sustain all flows sequentially in order of priority without totally inhibiting any, no matter how low priority it is. QoS technologies are being used for futuristic applications in order of campus, WAN and service provider networks. To best understand the full implications of applying QoS to systems discussed within this essay it is first essential to understand a little of the basic QoS architecture and management therewith. These are delineated hereafter in bulleted form. Basic QoS Architecture The three fundamental architectural components upon implementation of a QoS system are: QoS identification and marking techniques for coordinating QoS from end to end between network elements, QoS within a single network element (queuing, scheduling and traffic-shaping tools), and QoS policy, management and accounting functions to control and administer end to end traffic across a network (Cisco Systems, 2002). QoS Management QoS management enables the user to determine and assess policies and goals and essentially three steps as hereunder. Step 1: The network system should be baselined with devices such as RMON probes. This helps to determine traffic characteristics of the entire network. Targeted applications should also baselined in terms of response time. Step 2: Deploy QoS techniques after the traffic characteristics have been determined and one or more applications have been shortlisted for increased QoS. Step 3: Evaluate results by testing the response times of targeted applications to determine whether the set goals have been realised to the desired extent (Cisco Systems, 2002). QoS-Enabled Services QoS can be provided in wired networks by two broad approaches – Over-provisioning of resources, and Traffic engineering. With over-provisioning of resources, there are abundant resources to service satisfactorily all bandwidth-hungry multimedia applications. This is a simplistic approach with all users being served in the same service class and it has a tendency to become unpredictable for peak-hour traffic when uncontrolled congestion can occur (Chen, D., and Varshney, P.K.). With the traffic-engineering method, the one that has been delineated throughout this essay previous to this, users/applications are classified and each service class is assigned a priority. In this method two approaches are possible – reservation-based and reservation-less. In the reservation-based approach network resources are allocated according to an application’s QoS request and subjected to bandwidth management policy. Reservation-based approach is used in the InterServ Model in the Internet (Chen, D., and Varshney, P.K). In the reservation-less approach some strategies are applied as follows: Admission control strategy that assures that a node can assess a network and than guarantees that once the node has been granted permission it gets the QoS requested. Policy management strategies can ensure that no node will violate the type of service it is pre-assigned. Traffic classes’ strategies differentiate the priority of the data packets and they thus acquire a per-hop behaviour at each intermediate node. Queuing strategies have mechanisms that are ready to drop packets with lower priority in case of congestion. Reservation-less strategies ensure QoS is employed in the DiffServ Model in the Internet (Chen, D., and Varshney, P.K.). Two QoS-Enabled Applications Asynchronous Transfer Mode (ATM) in networks applies reservation-based QoS (Chen, D., and Varshney, P.K.). ATM is innately suited to QoS performance on an end-to-end connections basis in the following manners. It automatically defines cell loss ratio, cell transfer delay, and cell delay variation (Taylor, S., and Hettick, L., 2001). The ATM adaptation layers are designed to serve for other applications or services and all require differentiated and specific measurable QoS (Taylor, S., and Hettick, L., 2001). Tenet 11 is a reservation-based network support system for real time continuous media applications. In contrast it must be said that ATM supports both continuous and discrete data packets and can underlie IP networks on the Internet. Tenet 11 can do the same and supports the following QoS parameters as described by user requirements: upper bound on end-to-end message delay, bound delay violation probability, bound buffer overflow probability, bound (optional) delay jitter, and throughput guarantee obtained from traffic specifications (Protocol – Part 11, 2002). Non-QoS Supports The present-age Internet uses protocols that are geared to optimise connectivity. Even if the transmission quality in terms of both timely response and quality of content is poor the destination is reached somehow. That is the principal achievability of the Internet and most IP-based networks and this is a best-efforts service. A best-efforts service only guarantees that it will try its best to transmit information as quickly as possible without even guaranteeing that it will be on a timely basis and that the information will reach its destination altogether. The Internet can be made to be user-friendly by providing QoS support through the Internet Streaming Protocol, Version 11. This is a reservation-based sender-defined support system that has the following characteristics: message is sent with a flow specification, if routers and destinations accept the call the call is returned in possibly modified form to the source, and typical parameters include – bandwidth, delay, delay variance and size (Protocols – Part 11, 2002). In the context of reliability it must be stated that the heterogeneity of the Internet networks means that no one system support can ensure absolute QoS with any probable disruption possible from varied sources and destinations. Specific companies like 3Com, Marlborough, Massachusetts, USA, target a connectivity between ATM and Ethernet to enhance the QoS supportability of Ethernet, which is essentially a single bandwidth network system. The company proposes to connect Ethernet end ports over an ATM backbone with switches that have Applications Specific Integrated Circuits (ASIC). These processors will ensure a seamless QoS among the Ethernet end ports (Duffy, Jim, 1999). Here it must be noted that scalability problems arise. ATM scales from 3-48 gigabits and up while Ethernet scales at 1 gigabits (Fritz, J., 2000.). QoS Specifications Traditional categories and dimensions for QoS specifications have been defined for non-functional properties of services provided for multimedia streams in distributed systems. There are three categories that include performance, reliability and security. Dimensions are more applications-specific and may be response time, delay, latency, throughput and transactions per second (TPS) under the performance category. Under the reliability category the dimensions may be time to failure, time to repair and number of failures. Under the security category dimensions may be anonymity, encryption and authentication (Bochmann et al, 1999.). Conclusion The traditional categories and dimensions for non-functional properties of services provided under QoS may be linked to performance, time and costs but these are not sufficient to service properly the emerging applications networks are being increasingly subjected to in the present age scenario. In that context emerging applications like electronic commerce, health care applications, digital publishing and data mining integrate new types of data which are heterogeneous, large-sized and/or time dependent. High volumes of data remain in numerous sites interconnected through several differential communications network systems and are potentially accessible by mobile computing systems. Information discovery and retrieval becomes a difficult task often with procurement of poor and irrelevant data at unrealistic response times. Within such a context it has become necessary to revise the traditional concepts of category and dimension in QoS so that the futuristic concepts of data quality, quality of data source and quality of services provided can be integrated within them (Bochmann et al, 1999). References Bochmann, G.V., et al, Quality of service management issues in electronic commerce applications. Extracted on 9th January, 2006, from: http://www.csi.uottawa.ca/~bochmann/ELG5125/CourseNotes/Documents/Boch99a.pdf#search='QoS%20Management%20Issues%20Bochmann' Chen, D., and Varshney, P. K., QoS Support in Wireless Sensor Networks: A Survey. Extracted on 9th January, 2006, from: http://web.syr.edu/~dchen02/explore_qos_v4.pdf#search='Chen%20and%20Varshney%20QoS' Costas, H., QoS Protocols and Architectures. Extracted on 9th January, 2006, from: www.telecom.tuc.gr/networkcourse/2000_01/qos.ppt Duffy, Jim, 1999, 3Com to span Ethernet, ATM net with QoS, Network World. Extracted on 9th January, 2006, from: http://www.networkworld.com Fritz, J., 2000, Caught between traffic in the fast lane, ITWorld. Extracted on 9th January, 2006, from: http://www.itworld.com/Net/1748/ITWnet-fritz-0224-prod/pfindex.html Media Storage and Distribution Systems, Protocols, Part 11, 2002. Extracted on 9th January, 2006, from: www.ifi.uio.no/infserv/foiler/06-protocols-part2.ppt Quality of Service Networking, Cisco Systems, 2003. Extracted on 9th January, 2006, from: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/qos.htm#1024838 Taylor, S., and Hettick, L., 2001, Introduction to ATM QoS, Network World Convergence Newslettr. Extracted on 9th January, 2006, from: http://www.networkworld.com/newsletters/converg/2001/00991550.html Read More