Unspecified Bit Rate Service Research Articles

A study of a new selective cell discard scheme with two threshold control on ATM networks

AbstractWhen UBR (Unspecified Bit Rate) service is used on an ATM network, cells are discarded in congestion and imperfect packets are generated that consume network resources in a wasteful manner. As a remedy, the EPD (Early Packet Discard) control is proposed, in which the cells are discarded on a packet basis in congestion by providing a threshold to the input buffer. However, with EDP control, there is a possibility that the EOP (End Of Packet) cell in AAL5 may be discarded. Thus, two successive packets become invalid. For this reason, the TT‐EPD (Two Threshold‐EPD) that prevents discarding of the EOP cell has been proposed. This scheme sets two threshold values (TH1 and TH2) and the fixed TH1 works similarly to EPD control. TH2 carries out dynamic control and secures the input region for the EOP cell in the buffer. In this scheme, the amount of buffer between TH1 and TH2 decreases and there is an increase in the number of acceptance failures in which the cells composing a packet initially accepted are discarded by TH2. In this paper, a method that keeps the amount of buffer between TH1 and TH2 constant to reduce the acceptance missing packets is proposed. The effectiveness of the proposed scheme is demonstrated by simulation. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(8): 50– 57, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10039

TCP Explicit Congestion Notification over ATM-UBR: A Simulation Study

The enhancement of Transmission Control Protocol’s (TCP’s) congestion control mechanisms using Explicit Congestion Notification (ECN) over Asynchronous Transfer Mode (ATM) networks is overviewed. TCP’s congestion control is enhanced so that congestion is indicated by not only packet losses as is currently the case but an agent implemented at the ATM network’s edge as well. The novel idea uses EFCI (Explicit Forward Congestion Indication) bits (available in every ATM cell header) to generalize the ECN response to the UBR (Unspecified Bit Rate) service, notify congestion, and adjust the credit-based window size of the TCP. The authors’ simulation experiments show that TCP ECN achieves significantly lower cell loss, packet retransmissions, and buffer utilization, and exhibits better throughput than (non-ECN) TCP Reno.

Improving goodputs of IP packets under ATM UBR traffic with port-based queueing schemes

ATM networks are widely adopted as backbone networks. TCP/IP are the most popular protocols today and it is quite possible that packets are forwarded by ATM networks. In ATM networks, UBR (unspecified bit rate) service and ABR (available bit rate) service are common in transmitting IP packets. Because TCP has its own flow control mechanism, the flow control of ABR service could become redundant. Moreover, it could interfere with the TCP flow control. In this paper, we will focus on buffer management schemes of ATM switches with UBR service. Researchers have proposed several buffer management schemes for UBR service. However, all schemes do not consider the influence of output rates. In addition, most researchers assume that each port has its own buffers. A basic scheme called dedicated buffers per port is compared with four proposed schemes. The paper considers the situation that switch buffers are shared among all output ports and proposes a scheme called shared buffers among all ports. Then, two schemes, dedicated buffers with early packet discarding (EPD) and shared buffers with EPD, are proposed. The EPD scheme is modified. Instead of using a threshold for each flow, only one threshold is applied to buffers of each output port. The fifth proposed scheme is called shared buffers with EPD + differential buffer allocation (DBA), which also considers the effects of various output rates. Results show that goodputs are increased apparently by applying the DBA scheme. Results also show that for goodputs, when packets pass two or more switches, packet discarding schemes are far more important than whether we adopt schemes of shared buffers or schemes of dedicated buffers. Hence, we can simply adopt schemes of dedicated buffers, which are easy to implement, and incorporate a packet-discarding scheme in most cases.

Providing rate guarantees for internet application traffic across ATM networks

The TCP/IP protocol suite is the standard requirement for all applications that need to communicate over the Internet. As TCP/IP applications are unable to specify the QoS parameters needed for most asynchronous transfer mode (ATM) services, they tend to use the unspecified bit rate (UBR) service category when running across ATM networks. The UBR service utilizes any bandwidth that is left unused by the rest of the ATM services. This has led the ATM forum's traffic management group to define a new service category called guaranteed frame rate (GFR). GFR is intended to provide minimum cell rate guarantees and fair access to excess bandwidth left over from higher-priority services. This article first presents a tutorial overview of GFR and then presents a survey of the research work that has been carried out toward the design and implementation of associated ATM switch mechanisms.

Performance of TCP in IP/ATM internetworks

The Asynchronous Transfer Mode (ATM) level flow control schemes and their impact on Transmission Control Protocol (TCP) over ATM have been extensively studied (Fang et al., A simulation study of TCP performance in ATM networks, in: Proceedings of IEEE GLOBECOM'94, San Francisco, 28 November–2 December 1994, pp. 1217–1223; S. Kalyanaraman et al., Performance of TCP/IP over ABR, in: Proceedings of Globecom'96, London, November 1996; H. Li et al., A simulation study of TCP performance in ATM networks with ABR and UBR services, in: INFOCOM'96, March 1996). In this paper, we extend the work of Yin and Jagannath (End-to-end traffic management in IP/ATM internetworks, ATM Forum Contribution 96-1406, October 1996) on TCP over available bit rate (ABR) and unspecified bit rate (UBR) services in an IP/ATM internetworking environment, and provide a more comprehensive study on this important issue. We compare TCP performance in terms of goodput and fairness and discuss the performance tradeoff among different types of services. We also investigate the ATM edge congestion with different traffic scenarios in various networks configurations and system setups. In particular, a simple and effective scheme to prevent ATM edge buffer overflow is proposed in this paper. Simulation results are provided to show that our scheme can reduce the ATM edge congestion, and therefore provide significant improvement of TCP goodput.

Performance of TCP over UBR in ATM with EPD and virtual queuing techniques

While both available bit rate (ABR) and unspecified bit rate (UBR) services can be used to support data traffic in asynchronous transfer mode (ATM), many ATM switch vendors consider UBR to be more attractive because of its low implementation cost in comparison with ABR. However, since there is no congestion control at the cell level in UBR and cells are simply discarded when buffer overflow occurs, the effective throughput of transmission control protocol (TCP) can degrade significantly over UBR service in a congested network. Early Packet Discard (EPD) techniques have been proposed by Romanow and Floyd (Dynamics of TCP traffic over ATM networks, IEEE Journal on Selected Areas in Communications 13 (4) (1995) 633–641) and shown to improve the throughput of TCP over ATM. In our earlier work (A simulation study of TCP performance in ATM networks with ABR and UBR services, in: Proceedings of the International Phoenix Conference on Computers and Communications, March 1996), we have shown that TCP with EPD can suffer significant degradation in fairness of throughputs among competing VCs in a congested ATM LAN environment, but the degree of fairness can be improved by using per-VC accounting/queuing techniques. In this paper, we continue our studies in designing better and low cost methods for the support of TCP over ATM. We apply the virtual queuing technique proposed in Chiussi et al. (Virtual queuing techniques for ABR service: improving ABR/VBR interaction, Infocom'97) to emulate on a FIFO queue the service provided by per-VC queuing. This technique requires low implementation cost and delivers performance of TCP over UBR comparable to the expensive per-VC queuing technique. Simulation results are presented to demonstrate that our technique combined with EPD can drastically improve the performance of TCP over UBR service.

Performance and buffering requirements of Internet protocols over ATM ABR and UBR services

The Asynchronous Transfer Mode (ATM) networks are quickly being adopted as backbones over various parts of the Internet. This paper analyzes the performance of TCP/IP protocols over ATM network's Available Bit Rate (ABR) and Unspecified Bit Rate (UBR) services. It is shown that ABR pushes congestion to the edges of the ATM network while UBR leaves it inside the ATM portion.

On TCP performance in ATM networks with per-VC early packet discard mechanisms

Both available bit rate (ABR) service and unspecified bit rate (UBR) service with early packet discard (EPD) schemes have been considered for supporting data applications in ATM networks. Since the transmission control protocol (TCP) is perhaps the most widely used transport layer protocol in existing data networks, the performance of TCP over ATM using ABR and UBR services with EPD schemes is of great interest to ATM equipment vendors and service providers. With regard to this interesting issue, we demonstrate in earlier work [1] some simulation results, which show that (1) with UBR service and EPD schemes, TCP suffers significant performance degradation in terms of fairness and requires a relatively large switch buffer even with a small number of active virtual connections over a LAN configuration, and (2) for the same set of network configurations and with ABR service, TCP achieves good performance in terms of fairness and link utilization, and requires relatively small switch buffer. In this paper, we extend our work [1] by studying the performance of TCP over UBR service where EPD schemes are improved by using per-VC accounting and per-VC queueing techniques. In particular, our simulation results show that in a LAN environment, per-VC based EPD schemes can significantly improve the performance of TCP over UBR service in terms of fairness.