Term Paper: Mlps Qos vs. ATM

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[. . .] One-way network elements deal with an overflow of traffic is to use a queuing algorithm to manage the traffic, determining how to prioritize it onto an output link.

The QOS service policy maintains the queue depth, marks traffic, and identifies non-critical traffic on a per-VC basis. The QOS service policy aims to achieve the following:

Employ Network-Based Application Recognition to classify non-business-critical traffic.

Apply Weighted Random Early Detection to manage queue depth.

Apply class-based marking using the set command to mark IP precedence values based on traffic type.

PRECEDENCE VALUE

TYPE OF TRAFFIC

Non-business critical

Default, used for normal traffic

Future real-time traffic, such as voice over IP (VoIP).

Reserved for network control traffic.

FIGURE 2 -- PRECEDENCE LEVELS (Paw, 2002)

Basically, the potential congestion points are the ATM VCs that enable the DSL-connected users. IP flows arrive at the Ethernet interface at high speeds and flow out of the ATM VCs. These are configured for the unspecified bit rate (UBR) ATM service category with a default peak cell rate (PCR) of the T1 interface. Therefore, the QOS service-policy tracks traffic arriving on the Ethernet interface (p. 257). The remarked values then are used by WRED to create service classes based on IP precedence and to provide differentiated service through individual drop levels per class.

Figure 3-- Priority queuing places data into four levels of queues: high, medium, normal, and low. (Pulse, 2002)

Levels of Hierarchy

There is a need to manage critical resources in the nodes of both an ATM or MPLS network (Zheng, 2001). One way of simplifying the management of the trunk capacity is through the use of aggression. ATM's fixed-format cell header allows only two levels of hierarchy: the virtual path (VP) and the virtual channel (VC).

MPLS, on the other hand, allows for an essentially unlimited level of hierarchy using label stacking. Nodes in MPLS and ATM networks employ label switching. This means that the packer header labels need only to be unique on an individual link.

Label switching involves mapping an incoming label to an outgoing label on a per-connection basis. An end-to-end connection is then a concatenation of such label-switching actions.

Label stacking occurs when a switch maps a number of connections into another aggregate connection at a higher hierarchical level. Therefore, the next higher level flow contains the aggregate of many lower-level flows.

Such flow aggregation eases the task of network routing traffic engineering by reducing the number of required connections.

Determining QOS

An ATM VP contains many VCs. VP cell relaying operates only on the VPI portion of the cell header (Zheng). Assuming that every node in a network is interconnected to every other node by a VPC, then only the total available entry-to-exit VPC bandwidth need be considered in admission control decisions. A VPC is easier to manage as a large aggregate than multiple, individual VCCs. The complexity and number of changes required when implementing routing, restoration and measurement are also required by VPCs as compared to VCCs.

It is important to note that QOS is determined by the VCC with the most stringent QOS in a VPC. One might envision a network of nodes interconnected by a VPC for each QOS class. However, this could quickly exhaust the VPI address space if there are more than a few QOS classes.

Unfortunately, a full-mesh design does not scale well (Paw, 2002). Even in partial mesh networks, allocating VPC capacity efficiently is a challenge. The principal issue is the static nature of VPC allocation in current ATM standards.

There are some analogies for traffic engineering between ATM VPCs and using label-stacked MPLS LSPs. The notion in using MPLS for IP traffic engineering is that of a traffic trunk, which is a set of IP packets between a pair of nodes.

For example, the packets offered to a traffic trunk may be completely defined by a set of destination IP addresses. AN MPLS LPS could be set up as a traffic trunk from every ingress router to the egress router that handles this set of destination IP addresses.

The notion of traffic trunks can also be done at one or more levels in the hierarchy. For example, in order to reduce the full mesh of LSPs to improve scalability, a set of traffic trunks formed by aggregate LSPs between core LSRs could be established, over which other LSPs could be label stacked.

ATM QOS

The guarantees of the connection-oriented ATM QOS enable the coexistence of delay sensitive applications, including real-time video and voice with bursty file transfers and transactional data traffic (Flannaghan, 2001).

In an ATM network, QOS play a key part in providing consistent results for all network users. ATM performs QOS by using three general approaches when servicing any connection:

When a required level of connection service quality is required, ATM guarantees and enforces that all devices providing the connections meet the required level of service consistently.

When a preferred QOS is requested for a connection, ATM tries to acquire resources available throughout the network to accommodate the requested level of service, while preserving and maintaining service guarantees for connections that require them.

When quality of service is unspecified for a connection, ATM tries to use available network resources in a "best-effort" attempt to provide a form of service similar to what is available in other transfer modes.

Combining and using defined ATM QOS parameters in a variety of ways have established ATM service categories. All ATM connections fit into one of these four service categories, which are indicated indirectly as a result of VPI/VCI information in each individual ATM cell header. Switches use the VPI/VCI to figure out priority for individual cells within the connection stream whenever connections using differing service categories are multiplexed.

To control the various types of network traffic, ATM standards have been modified to define the types of services most commonly used (Hesselbach, 1998). The four general ATM service categories are:

Constant bit rate (CBR) -- Connections that require a guaranteed continuous rate of transfer, such as real-time voice or video, and connections that will tolerate only minimal transfer delays, such as circuit emulation for leased lines and T1 or T3 carrier services.

Variable bit rate (VBR)-- Connections that require a lower bounded rate (such as their minimum rate of transfer), but can tolerate variation at their upper bounded rate (such as their maximum rate of transfer) to permit periods of burst transfer to occur.

Available bit rate (ABR)-- Connections not requiring a guaranteed rate of transfer, such as file transfer and e-mail. Connections that are generally more tolerant of highly unpredictable or burst traffic patterns, such as ATM interconnection with emulated Ethernet and Token Ring LANs.

Unspecified bit rate (UBR)-- Connections requiring route establishment but no guaranteed commitment of bandwidth, such as batch file transfers and lower priority bulk e-mail. Connections for programs that have no delivery constraints and perform their own error checking and flow control.

MPLS QOS

Both MPLS QOS and bandwidth management and DiffServ priority queuing management are important elements when making sure that multiservice network performance objectives are met under a range of network conditions (Paw, 2002). Both mechanisms work together to make sure QOS resource allocation mechanisms (bandwidth allocation, protection, and priority queuing) are achieved.

Basically, MPLS supports the same QOS protocols as IP does, which include IP Precedence, Committed Access Rate (CAR), Random Early Detection (RED), Weighted RED, Weighted Fair Queuing (WFQ), Class-based WFQ, and Priority Queuing. Proprietary and non-standard QOS mechanisms can also be supported with MPLS (p. 144). However, these are not guaranteed to interoperate with other non-proprietary vendors.

Since MPLS also supports the reservation of Layer 2 resources, MPLS is able deliver finely grained quality of service, similar to the manner of ATM. It is assumed that both MPLS and Differentiated Services (DiffServ) will both be deployed in networks.

DiffServ can support up to 64 classes while the MPLS shim label supports up to 8 classes. This shim header has a 3-bit field defined "for experimental use." This can pose a problem. This Exp field is only 3 bits long, while the DiffServ field is 6 bits. To deal with this problem, there are different ways to work around it.

The two main alternatives that address this problem are called Label-LSP and Exp-LSP models, yet both make the architecture complex (p. 177). The DiffServ model fundamentally defines the interpretation of the TOS bits. Assuming that the IP precedence bits map to the Exp bits the same interpretation as the DiffServ model, they can be applied to these bits.

If additional bits are used in the DiffServ model, the label value must be used to interpret the meaning of the remaining bits. Because three bits are enough to identify the required number of classes, the remaining bits in the DiffServ model are used for identifying the drop priority, which can be mapped into an L-LSP in which case the… [END OF PREVIEW]

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