Quality of Service (QOS) is best defined as the performance attributes of an end-to-end flow of data (Zheng, 2001). The particular elements of QOS depend on the information that is being transported. For example, QOS for voice defines limits on specific parameters such as delay, delay variation, packet loss, and availability.
In the past, networks were engineered based on providing fixed bandwidth for relatively short duration voice calls. Today, the traffic on networks is based on statistical or bursty data. Therefore, it has become necessary to develop new statistical models to build new networks.
Functions of QOS
QOS applications are used in networks for many reasons, including to:
Guarantee a fixed amount of bandwidth for various applications.
Control latency and jitter, and ensure bandwidth for voice.
Provide specific, guaranteed and quantifiable service level agreements (SLAs).
Configure varying degrees of quality of service for multiple network customers.
However, today's connectionless networks cannot provide absolute, hard QOS, only "relative" class-of-service transport (p. 211). For services like voice and video, which need a network with high predictability, this is unacceptable. MPLS adds a connection-oriented behavior to IP, making it connection-oriented so that hard QOS can be delivered.
Differences Between ATM and MPLS
Simply put, multi-label switching (MPLS) brings the traffic engineering capabilities of asynchronous transfer mode (ATM) to packet-based network by tagging IP packets with "labels" that specify a route and priority (Flannaghan, 2001). MPLS unites the scalability and flexibility of routing with the performance and traffic management of layer 2 switching. MPLS can run over nearly any transport medium, including ATM and Ethernet, rather than being tied to a specific layer-2 encapsulation. Because it uses Internet protocol (IP) for addressing, it uses common routing and signaling protocols.
MPLS was not designed to replace ATM but rather to compliment it. MPLS eases complexity by mapping IP addressing and routing information directly into ATM switching tables. The MPLS label-swapping paradigm employs the same mechanism that ATM switches use to forward ATM cells. In the case of ATM-LSR, the ATM forwarding component performs the label swapping function. Label information is carried in the ATM Header.
MPLS has the ability to run over routers in addition to ATM switches, while providing the control component for IP on both the ATM switches and routers. For ATM switches PNNI, ATM ARP Server, and NHRP Server are replaced with MPLS for IP services yet the ATM control plane remains preserved (Zheng, 2001). PNNI is still used on ATM switches to provide ATM services. Therefore, an IP+ATM switch delivers both ATM for fast switching and IP protocols for IP services in a single switch.
In the past, at a specific performance level, the price of a router was usually higher than the equivalent ATM switch. With IP+ATM LSRs, the forwarding performance is determined by the capabilities of the ATM switches, whereas the functionality is comparable to a router. Moreover, IP+ATM switches may also have similar price and performance characteristics to ATM switches.
Values of MPLS QOS and ATM QOS
As a means to join otherwise parallel IP and ATM networks without requiring essential changes to the characteristics of either, MPLS holds value to major carriers. Maintaining two separate networks (IP and ATM) has obvious disadvantages in terms of cost, whereas running IP over ATM fails to scale over time (Paw, 2002). These issues do not explain why carriers would want to converge all their traffic over an all-IP backbone. Often the main draw of MPLS is how it supplements IP through quality of service (QOS) and traffic engineering (TE).
MPLS is often referred to as a "QOS protocol," because, on its own, the standard does not have a complete means of quality assurance or traffic differentiation (p. 178). However, it does provide is an opportunity for mapping DiffServ fields onto an MPLS label, d for conveying this information through the core of the network in a way that is more efficient and easy to use with other protocols than a pure IP/DiffServ implementation would be.
MPLS also allows users to recognize and prioritize different types of applications, while reserving network resources to support of them and defining explicit routes to carry them out. MPLS standards rely on other protocols to achieve this result, defining only a generic need for an LDP to communicate between the label router nodes.
Still, the two main options that have been suggested for the role of LDP - CR-LDP (Constraint-based Routing) and RSVP-TE (an extended version of the Resource ReSerVation Protocol) - are similar in that they describe methods for allocating bandwidth and establishing dedicated virtual routes based on QOS rules or application-specific limitations (p. 196).
End-to-end QOS in an IP network has been of utmost importance for service providers, and it is probably the area where MPLS has had its biggest impact. The potential of highly differentiated and guarantee-able IP QOS is something that both enterprise users and service providers would like to see come from MPLS deployment. However, this goal has yet to be realized. However, MPLS has proven itself to be beneficial in traffic engineering.
The ATM world has a rich feature set that is used for traffic engineering, which is a process by which traffic is optimized to follow certain paths based on specific requirements.
The Internet Protocol (IP) world also has features, although not as extensive as ATM, to provide for traffic engineering (Swallow, 1999). The problem experienced by service providers is how to combine the traffic engineering of IP with the traffic engineering of ATM. Since they are two entirely different technologies, it is difficult to implement combined end-to-end traffic engineering.
Both IP and ATM have QOS capabilities. The difference between the two has to do with their operation. IP is connectionless and ATM is connection-oriented. Again, the problem experienced by a service provider is how to combine these two different ways of implementing QOS into a firm end-to-end solution.
MPLS, as a technology, evolved from early attempts to glue the IP and ATM worlds together. Today, MPLS, for the most part, is a standardized version of Cisco's proprietary tag switching. The MPLS label, or label stack, is made up of four octets (32 bits). The label is the core of MPLS.
Traffic engineering deals with the performance of a network in supporting the network's customers and their QOS needs. The focus of traffic engineering for MPLS networks is:
the measurement of traffic, and the control of traffic.
Control of traffic deals with operations that ensure that the network hs the resources to support the user's QOS requirements.
Traffic engineering in an MPLS environment establishes objectives with regard to two performance functions:
traffic oriented objectives, and resource-oriented objectives.
Traffic oriented performance supports the QOS operations of user traffic. In a single-class, best-effort Internet service model, the key traffic-oriented performance objectives include minimizing traffic loss and delay; maximizing thoroughput; and enforcing service level agreements.
Resource-oriented performance objectives deal with the network resources, such as communication links, router and services -- those entities that contribute to the realization of traffic-oriented objectives.
Efficient management of these resources is vital to the attainment of resource-oriented performance objectives. Available bandwidth is the bottom line; without bandwidth, any number of traffic engineering operations is worthless, and the efficient management of the available bandwidth is the essence of traffic engineering.
Traffic engineering is a major aspect of QOS (Zheng, 2001). The ability of MPLS to do traffic engineering lays the groundwork for its QOS potential, even though carriers see immediate benefits from traffic engineering in the internal operation of their network, not in their customer-facing service offerings.
Still, carriers hope that the infrastructure they are investing in today will enable revenue-generating services tomorrow. And MPLS offers a rather straightforward way of solving traffic management problems such as network bottlenecks, caused by congestion on certain router paths. Here, the flexibility of MPLS in choosing the "best" path for a certain traffic flow can be leveraged to more evenly distribute traffic throughout the network.
Any network that admits traffic and users on demand (such as the Internet) must deal with the problem of congestion (Paw, 2002). The management of all user traffic to prevent congestion is an important aspect of the QOS picture. Simply stated, congestion translates into reduced thoroughput and increased delays. Congestion is a huge problem for effective QOS.
Many networks provide transmission rules for their users, including agreements on how much traffic can be sent to the network before the traffic flow is regulated (flow controlled). Flow control is an essential ingredient in preventing congestion.
MPLS defines two major network elements, a Label Edge Router (LER) and a Label Switch Router (LSR). These entities are functional descriptions, not system-level definitions. Therefore, the types of systems that can fulfill the LER or LSR functions are not limited, and, while…