Loss Priority Research Articles

Cell loss analysis of an ATM multiplexer with loss priority control for VBR bursty traffic

In this paper, we study the cell loss performances of an asynchronous transfer mode (ATM) multiplexer with loss priority control, whose input consists of variable bit rate (VBR) homogeneous bursty traffic. Among a variety of traffic models, we consider an on-off model and a high-low activity model, which are modeled by a Markov-modulated deterministic process (MMDP). We assume that the parameter values of the on-off model are given, but the parameter values of the high-low activity model are obtained from those of a stepwise VBR video traffic model. Here, we present a procedure of approximating to the two-state MMDP stepwise VBR traffic of the Markov process of a non-birth-death process type having more than two states. For the cell loss performance, we use some approximations to make analysis tractable. Also, for loss priority control we use the statistics of the priority ratio as a traffic parameter, and consider two different methods of assigning cell priority for the high-low activity model. By numerical examples, we show that the performance of the on-off model is different from that of our high-low activity model, because the high-low activity model chosen here has less bursty characteristics, resulting from the parameter values of the stepwise VBR video traffic model. Selective cell discarding by the partial buffer scheme increases the admissible load for the on-off model, while it increases or does not affect the admissible load for our approximated high-low activity model for different priority assignment methods and performance requirements.

Performance management issues in ATM networks: traffic and congestion control

The goal is first to introduce performance monitoring aspects of asynchronous transfer mode (ATM) networks and then to focus on traffic and congestion control schemes. To deal with this performance monitoring management, a framework for defining a generic intelligent and integrated model for network management is described. As an example of the efficiency of this intelligent management architecture, we measure the performance of a new congestion control scheme. This scheme uses the cell loss priority (CLP) bit, the explicit forward congestion indicator and the explicit backward congestion indicator. The intelligent management uses different parameters and builds a complex but efficient control scheme. We show that this new control scheme allows performance to be increased by an order of magnitude.

UPC/NPC algorithm for guaranteed QoS in ATM networks

In Broadband Integrated Services Digital Networks (B-ISDN) based on the Asynchrous Transfer Mode (ATM) technology, the required Quality of Service (QoS) must be guaranteed for connection. Cell loss ratio, which is one of the QoS parameters specified by the user in an ATM traffic contract of connection, may depend on the traffic enforcement mechanism known as the Usage Parameter Control/Network Parameter Control (UPC/NPC) or traffic policing. If two UPC/NPC algorithms are not equivalent with respect to cell discarding/tagging for a given traffic configuration, a common understanding about legality between the user and the network operator does not exist. To cope with this legality problem, the concept of cell conformance has been developed by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). However, the Cell Loss Priority (CLP) bit, which ITU-T introduced in the ATM cell header to discriminate between a high priority cell (CLP = 0) and a low priority cell (CLP = 1), may cause the measurement phasing problem in cell conformance testing in ATM networks. This makes it difficult for a network provider to define and commit cell loss ratio as a QoS parameter. In this paper, we investigate measurement phasing problems and difficulties in commitment of the cell loss ratio in ATM networks. We propose a QoS guaranteed UPC/NPC algorithm to commit the cell loss ratio, and show an example of applications of the proposed UPC/NPC algorithm.

An analysis of a variable bit rate multiplexer using loss priorities

The network of the future will carry a variety of video applications, which are characterized by high bandwidth and stringent requirements on delay and loss. In this paper, we develop a new analytic model for a variable bit rate video source, which can model a variety of video applications by a suitable choice of model parameters. Using the model we study the performance of a video multiplexer that multiplexes several full motion video sources are assumed to use a layered coding technique. We propose a service strategy that guarantees delivery of packets marked essential, whereas non-essential packets may be dropped in the event of congestion. The study shows that with layered coding that classifies packets as essential and non-essential, one can get good tail probabilities for a given number of sources at the expense of some loss for non-essential packets. Alternatively, for an allowed loss rate of non-essential packets (i.e., for a given picture quality), one can multiplex a larger number of such video sources. We also investigate the performance of the video multiplexer when: (a) the video sources are correlated, and (b) when independent video sources with different thresholds for essential packet rate are multiplexed together. We suggest that the coding scheme should distribute the bits generated by a scene change over several successive frames to prevent deterioration of picture quality when sources are correlated. Since the perceived quality of the picture at the receiver depends on the throughput of non-essential packets, we suggest a loss or service priority structure within the non-essential class to guarantee a contracted picture quality.

Estimates of loss probabilities for delay-sensitive traffic in ATM networks

Simple estimates of loss probabilities for heterogeneous delay-sensitive traffic in ATM networks are presented. Cells of different connections can have different loss priorities. Numerical results show that the estimates, which are derived from the bufferless fluid flow model, are close to the actual cell loss probabilities.

Analysis of preemptive loss priority queues with preemption distance

In a queueing system with preemptive loss priority discipline, customers disappear from the system immediately when their service is preempted by the arrival of another customer with higher priority. Such a system can model a case in which old requests of low priority are not worthy of deferred service. This paper is concerned with preemptive loss priority queues in which customers of each priority class arrive in a Poisson process and have general service time distribution. The strict preemption in the existing model is extended by allowing the preemption distance parameterd such that arriving customers of only class 1 throughp — d can preempt the service of a customer of classp. We obtain closed-form expressions for the mean waiting time, sojourn time, and queue size from their distributions for each class, together with numerical examples. We also consider similar systems with server vacations.

Revenue maximization in ATM networks using the CLP capability and buffer priority management

The cell loss priority (CLP) bit in the header of the ATM cell may be used either by the network to tag noncompliant cells, or by the application to declare two levels of quality-of-service (QoS) within the same virtual circuit (VC). We study the possibility of the use of this bit by the application alone. An application can offer two types of traffic streams to the network, namely, a precious traffic stream (with stringent QoS requirements, e.g., cell loss ratio (CLR) <10/sup -9/ and identified by the CLP bit=0) and a less precious stream (CLP=1 and less stringent QoS requirements, e.g., CLR <10/sup -4/). We study the performance of an ATM multiplexer with two traffic classes with different QoS requirements. The buffer priority schemes adopted are partial buffer sharing (PBS) and PBS+push-out (PO). We first obtain the engineering trade-off curves, between CLP=0 and CLP=1 traffic. To identify an operating point, we formulate a revenue optimization problem in which the constraints are the engineering trade-off curve and a simple model of the variation of CLP=1 demand with its price.

A unified solution to mixed loss and delay systems with partial preemptive priority

AbstractThis paper proposes a new priority scheme called partial preemptive priority (PPP) for mixed loss and delay system to satisfy very different individual performance requirements of loss (delay‐sensitive) traffic and delay (loss‐sensitive) traffic. the PPP scheme is a generic priority scheme which includes all the conventional priority schemes, e.g., preemptive priority (PP) and movable boundary (MB), as well as nonpriority (NP), schemes as special cases. By using the matrix‐analytic method, a unified matrix‐geometric solution to the mixed loss and delay priority systems with phase‐type Markov renewal process (PH‐MRP) inputs is derived. Numerical examples also are presented to demonstrate the effectiveness of the PPP scheme.

An ATM queue manager handling multiple delay and loss priorities

The asynchronous transfer mode (ATM) technique has been widely accepted as a flexible and effective scheme to transport various traffic over the future broadband network. To fully utilize network resources while still providing satisfactory quality of service (QOS) to all network users, prioritizing the user's traffic according to their service requirements becomes necessary. During call setup or service provisioning, each service can be assigned a service class determined by a delay priority and a loss priority. A queue manager in ATM network nodes will schedule ATM cells departing and discarding sequence based on their delay and loss priorities. Most queue management schemes proposed so far only consider either one of these two priority types. The queue manager handles multiple delay and loss priorities simultaneously. Moreover, a cell discarding strategy, called push-out, that allows the buffer to be completely shared by all service classes, has been adopted in the queue manager. We propose a practical architecture to implement the queue manager by using available VLSI sequencer chips.

On protective buffer policies

Studies buffering policies which provide different loss priorities to packets/cells, while preserving packet ordering (space priority disciplines). These policies are motivated by the possible presence, within the same connection, of packets with different loss probability requirements or guarantees, e.g., voice and video coders or rate control mechanisms. The main contribution of the paper is the identification and evaluation of buffering policies which preserve packet ordering and guarantee high priority packets performance (loss probability), irrespective of the traffic intensity and arrival patterns of low priority packets. Such policies are termed protective policies. The need for such policies arises from the difficulty to accurately characterize and size low priority traffic, which can generate large and unpredictable traffic variations over short periods of time. The authors review previously proposed buffer admission policies and determine if they satisfy such "protection" requirements. Furthermore, they also identify and design new policies, which for a given level of protection maximize low priority throughput.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Statistical multiplexing with loss priorities in rate-based congestion control of high-speed networks

Statistical multiplexing with loss priorities is a central element in ATM-based B-ISDN. Cell priorities arise from the marking schemes employed by the access regulators to identify excess cells, which are dropped during periods of congestion, Also, in real time applications, such as hierarchically coded voice and video, cells are assigned priorities which correspond to their importance to service quality, so that when congestion occurs only the least important are dropped. The authors present a stochastic fluid model of statistical multiplexing with loss priorities. Each Markov modulated fluid source generates streams of different priorities. The burstiness of each stream and the correlation between the priority streams are captured in the mode. The loss priority is implemented by selectively discarding cells of certain priority classes when the buffer content exceeds a corresponding threshold. To handle high dimensional source models, the authors develop an algebraic theory for the efficient computation of the spectrum of the statistical multiplexing system, which generalizes previous results for on-off sources. It is shown that to obtain the solution of the statistical multiplexing problem with J priority classes, J different 1-class problems need to be solved, together with a system of linear equations which describe the behavior of the stationary distribution at the thresholds. The numerical results demonstrate the manner in which i) the threshold level controls the tradeoff between delay of higher priority cells and the loss probability of lower priority cells, and ii) the buffer size controls the loss probability of higher priority cells. >

Analysis of a Markov-Modulated Fluid Model for Multimedia Traffic with Loss and Delay Priorities

This paper presents a complete fluid-flow analysis of a traffic control mechanism in an ATM multiplexer, known as Nested Threshold Cell Discarding with Multiple Buffers. NTCD/MB is a buffer management scheme that implements loss and delay priority queueing strategies in order to satisfy a wide range of quality-of-service requirements. We consider a heterogeneous mix of traffic which consists of real-time traffic with a stringent cell delay requirement, and nonreal-time traffic with a stringent cell loss requirement. Moreover, a proportion of real-time cells has an additional stringent loss requirement. Cell arrivals are mode led using a fluid-flow modeling approach. The system we analyze consists of two buffers interacting via a common server. The steady-state probability distribution for the buffers' content as well as other performance statistics are obtained. Our approach is based on computing the conditional probability distributions of the buffers and using this information to find the desired unconditional stationary distributions. This approach can be used, in general, to analyze multiple dependent systems of buffers. Numerical examples based on the analysis are used along with simulations to study the performance of NTCD/MB. It is shown that NTCD/MB can be used effectively to provide performance guarantees on different cell loss and delay requirements of the traffic.

A traffic control method for service quality assurance in an ATM network

The authors describe an ATM traffic control method to avoid both long-term and short-term congestion. Call admission control and policing based on user-specified traffic parameters have mainly been studied as ways to avoid long-term congestion. However, these control methods are not practical since it is difficult for users to specify accurate values of traffic parameters that include the effects of cell collision controllers and multiplexers in the customer premises network. In order to solve the problem, they propose an intelligent call admission control method. This method consists of a function to determine user's traffic parameters by monitoring the cell traffic flow of each service and a function to statistically predict long-term congestion by using Gaussian and Poisson distributions. They show that congestion can be predicted by deriving the peak, average, and variation. They also describe how to derive traffic parameters and determine their values. To avoid short-term congestion, we propose reactive control, which drops cells with lower cell loss priority when congestion occurs, and we show the buffer construction and buffer control algorithm used to achieve this. Computer simulations prove that their control strategy is effective for cell toss sensitive services such as VBR (variable bit rate) video service. >

A novel congestion control strategy in ATM networks

In order to guarantee a committed Quality of Service (QoS) to the users of a Broadband Integrated Services Digital Network (B-ISDN), preventive congestion control becomes critical, and is implemented through Call Acceptance Control (CAC), Usage Parameter Control (UPC), and Space Priority functions. Currently, Asynchronous Transfer Mode (ATM) cells are equipped with a 1-bit Cell Loss Priority (CLP) field, which can be used for service-oriented and/or UPC marking. This creates a conflict, since these two marking approaches may have contradicting objectives, and are designed to operate independently. Moreover, by admitting excessive cells as marked traffic, this group is allowed to grow uncontrollably, thereby jeopardizing the QoS committed to other (low priority) marked cells. This paper presents a solution to these problems based on a new 4-class priority strategy that unifies the two marking approaches, by utilizing a 2-bit CLP field. This is implemented by expanding the CLP field with the adjacent 1-bit RES field in the ATM cell header. A new variant of the “marking Leaky Bucket” UPC mechanism, called the Forgiving Leaky Bucket (FLB) will then be positioned at the Network-Node-Interface (NNI) of interworking ATM-based Broadband ISDN subnetworks. The FLB additionally has the power of un-marking ( forgiving) previously marked ATM cells, whenever the network conditions are appropriate. As a result, the end-to-end survival probability of a cell is improved significantly. Furthermore, “forgiving” corrects access-point bias, a phenomenon observed in interworked ATM subnetworks of different congestion conditions. Through simulations it is shown that this new priority strategy provides a significant improvement over the traditional marking Leaky Bucket UPC mechanism.

Internetting LAN's and MAN's to B-ISDN's for connectionless traffic support

One of the challenges in the interconnection of LANs and MANs to asynchronous transfer mode (ATM) networks is the support of connectionless traffic in the ATM network. A commonly proposed strategy consists of maintaining a thin, low bandwidth virtual path (VP) between each pair of gateways and requesting more bandwidth whenever a burst comes in from the LAN or MAN. Unfortunately, this strategy places a heavy burden on the ATM control processors; furthermore, it is overly conservative in the use of bandwidth. An alternative solution is proposed: on the-fly tranmission of bursts without prior bandwidth reservation. The key conditions for this scheme to work are the use of the cell loss priority (CLP) bit in the ATM cell header and the knowledge of available bandwidth on the path through bandwidth advertising. An on-the-fly strategy for connectionless traffic is described and is compared with a bandwidth renegotiation strategy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An efficient solution method for Markov models of ATM links with loss priorities

Algorithms for solving for the cell loss rates in an asynchronous transfer mode (ATM) network using cell loss priorities are presented. With the loss priority scheme, cells of low-priority classes are accepted only if the instantaneous buffer queue length at the cell arrival epoch is below a given threshold. The input is modeled by Markov-modulated Bernoulli processes. The effect of the loss priority scheme on data, voice, and video traffic is investigated. >

A patient who has lost a great deal of blood is brought into the emergency room. How do you estimate the loss and set your priorities?

Maureen McConnac, RN, MS, CCKN, CEN, is a trauma registry/research nurse, clinical nurse specialist, University Hospital, Newark, NJ.

Toward a broadband congestion control strategy

The congestion control problem in asynchronous transfer mode (ATM) based broadband networks is defined. In general, a suitable set of congestion controls will include features for admission control, buffer and queue management, traffic enforcement, and reactive control. The leading alternatives for each of these congestion control features are summarized. An approach for choosing the best of these alternatives is presented, and a reasonable set of such alternatives that captures the increased utilization due to statistical multiplexing is suggested. It uses separate and static bandwidth pools for each service category; a statistical multiplexing gain determined for each bandwidth pool that supports a variable-bit-rate (VBR) service category; traffic enforcement on a virtual circuit basis using a leaky bucket algorithm with parameters set to accommodate anticipated levels of cell transfer delay variation; and multilevel loss priorities as well as a reactive control for appropriate VBR service categories based on multithreshold traffic enforcement and explicity congestion notification.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Priority cell discarding for overload control in BISDN/ATM networks: An analysis framework

AbstractThe broadband integrated services digital networks (BISDNs) of the future will use asynchronous transfer mode (ATM) for switching and transport of basic transport units called cells. These networks are likely to exhibit extremely bursty traffic characteristics which could cause significant queue overload conditions. This paper suggests that priority cell discarding, in which a delivery priority (also known as a loss priority) is assigned to each cell, is a viable alternative for controlling this overload. We also present a framework for the analysis and comparison of the performance of such priority discarding systems. The framework consists of a general analysis model appropriate for BISDNs using ATM, a two‐dimensional structural classification of priority discarding algorithms and suggestions for performance measures, one of which is a generalization of the often‐used throughput measure. We illustrate how this framework can be useful in systems analysis by introducing a specific discarding optimization problem.