Dolors Sala

A protocol for efficient transfer of data over hybrid fiber/coax systems

The cable TV plant is a broad bandwidth channel into the home and community that has been used primarily for the distribution of television. As the plant is upgraded, there is an opportunity to use it for the transport of high-speed digital signals. While the downstream channel is relatively easy to exploit, the topology and physical characteristics of the upstream channel present new challenges at both the physical and the media access layers. We present a media access protocol that efficiently transfers data on this channel. A primary goal of the design is to keep that portion of the protocol resident in the station as simple as possible. Thus, we use centralized control located in the cable head-end. A station wishing to transmit sends a request to the head-end using a contention channel. The head-end acknowledges the request and then schedules the request, informing the station, by means of a grant message, exactly when to transmit. The protocol performs efficiently under a wide range of conditions, and is robust in the presence of channel errors.

A protocol for efficient transfer of data over fiber/cable systems

A revolution is occurring in the scope and range of information, communication and education services that will be made available to schools, libraries, town-halls, clinics and, most importantly, residences. These services will be provided initially, primarily over hybrid fiber-cable systems, either by telephone companies or cable companies. The old cable plant is being upgraded and used in totally new ways. The topology and physical characteristics of the upstream channel present new challenges for efficient channel access. We present a media access protocol that efficiently transfers data on this channel. A primary goal in the design was to keep the portion of the protocol resident in the station as simple as possible. Thus we use centralized control located in the cable head-end and minimize intelligence in the station. We refer to this protocol as centralized priority reservation or CPR. A station wishing to transmit sends a request to the head-end using a contention channel. The head-end acknowledges the request and then schedules the request, informing the station by means of a grant message when to transmit. The protocol performs well under heavy load. Performance is affected little by the number of stations, the speed of the system and the physical length of the system.

An access protocol to support multimedia traffic over hybrid fiber/coax systems

Hybrid fiber/coax plant is being widely deployed to deliver broadband interactive services to the home. Many of these services require a two-way high-speed channel, for example, extending the office LAN into the home. This is a challenging problem over HFC because most of the plant has only a small fraction of the bandwidth allocated to the upstream path, it is a noisy path and it is not possible to sense the medium prior to transmitting. For these reasons a protocol tuned to the medium is necessary. The centralized priority reservation (CPR) protocol is specifically designed for HFC plant with a primary goal of keeping the protocol within the station as simple as possible, consistent with efficient use of the channel. The basic CPR protocol has been described previously. In this paper we describe the application of CPR to the transmission of multimedia type traffic and the performance that is obtained. It supports multiple levels of priority in the transmission of asynchronous data with fine control over the level of priority. It also supports synchronous streams of information. Channel capacity not required by the streams is available to the asynchronous data.

Comparison of contention resolution algorithms for a cable modem MAC protocol

A reservation-based MAC protocol has been adopted by standards bodies as the basic protocol for data communication within the upstream channel of a cable plant. The available capacity is divided in two separate channels. One is a random access channel, or contention channel, used to send requests to the head-end, with the possibility of collisions occurring therein. When collisions happen, they are resolved by using a contention resolution algorithm (CRA). The second channel is a data channel to send user information in a collision-free manner. Adaptive algorithms are used to divide the system capacity dynamically, between the data and contention channels depending on the system requirements. In this paper the performance and implementation issues of p-persistence and tree-based CRAs under this adaptive environment are compared. While the complexity of the p-persistent algorithm remains as simple as in a traditional Aloha channel, traditional implementations of the tree-based algorithms becomes more complex. Simple and efficient implementations of the tree-based algorithms are proposed. It is shown that the performance difference between the studied algorithms is very small.

Maximizing packet loss monitoring accuracy for reliable trace collections

Network traces are a valuable source of information for modeling and analysis of network behavior and for the evaluation of network protocols. These crucial activities should be supported with reliable traffic traces. Reliable packet capturing facilities should be devoted to avoid losing packets and at the very last, report when packets have been lost with the highest accuracy possible. This paper describes a methodological approach to maximize the accuracy in the packet loss monitoring. The approach lies on the monitoring of appropriate statistical indicators directly from Ethernet hardware. These indicators become the enablers for the collection of metadata traces in parallel with the standard trace collections. The resulting meta-traces are proposed as a mechanism to monitor packet loss accuracy at the packet level as they provide the means to identify the exact location and amount of the packet loss in the collected traces files. We describe an evaluation use case of our approach with commodity hard- and software. The conclusion drawn from our experimental set up configuration is that it is possible to maximize the accuracy of the packet loss monitoring for reliable trace collections at affordable costs.

Accurate real-time monitoring of bottlenecks and performance of packet trace collection

Collection of packet traces for future analysis is a very meticulous work that must guarantee accurate traces in order for these traces to be valuable for analysis. Current platforms do not provide a means to measure this accuracy. This paper describes a real-time monitoring method to measure the quality of a collected trace. The method takes a system architecture approach monitoring different points of the system to account for all potential drops of the packet journey. A set of metadata is stored in metatraces to be analyzed together with the trace after the capturing. The primary information is taken from standard Ethernet counters which are available in all commodity hardware and therefore performs very well without expensive specific hardware. The paper presents the evaluation of the real-time monitoring method concluding that the processing overhead does not produce significant performance degradation and that it improves packet loss detection up to orders of magnitude depending on different scenarios.

Interactive Web service via satellite to the home

The satellite distribution of digital broadcast signals to the home has become widespread in the last seven years, delivering hundreds of television channels. We we have also seen other services appear, such as Direct PC/sup TM/ where data is broadcast. This service may be used to deliver information like stock quotes to a large number of users, or information such as Web pages may be requested by a single user. In the latter case, however, a return path is needed from the end user to the service provider in order to provide the specific information requested by the user. In services offered today this information is usually provided by a terrestrial return link, most often a telephone line. While a telephone line with a modem is adequate in most cases, it has the drawback of tying up an existing telephone line for a considerable amount of time for very little data transmission, or requiring an additional line. Furthermore, in many parts of the world a telephone line may not be readily available. In these cases a satellite return link may be a viable option using VSATs. We describe an investigation and simulation of a return path VSAT link designed specifically for delivery of Web traffic requests to a base station and thence via a terrestrial link, using the Internet, to a Web server. The focus in this work is on the MAC protocol to support such a satellite channel and on modeling the whole service from traffic generation through transport, network, and MAC layers to the physical channel.

Scheduling disciplines for HFC systems: what can we learn from ATM scheduling?

The support of multimedia traffic with different quality of service (QoS) requirements on an integrated network has proven to be difficult. It requires simple design of conceptually complex mechanisms. We study the application of proposed ATM scheduling disciplines to a hybrid fiber/coaxial (HFC) system. Switching networks are based on point-to-point communication links whereas HFC is a shared medium. If we can show that the structural differences between the environments do not affect the functionality of the scheduler a great amount of knowledge on supporting QoS can be imported to HFC systems from switching studies. As an example self clock fair queueing (SCFQ) is embedded in an HFC system. Simulation results show that the scheduler maintains its capabilities in guaranteeing the QoS in the HFC system.

RSTP-SP: Shortest path extensions to RSTP

The spanning tree protocol is the component of the Ethernet architecture that establishes the network connectivity. Its plug-and-play property and ease of configuration have been some of the pillars of Ethernets success. However, the new provider applications require improving the protocol capabilities such as response time, path optimality and path control. Optimal paths can be achieved if we deploy one tree rooted at each node. Nevertheless, this introduces the challenge of maintaining the path symmetry requirement of Ethernet networks. In this paper we propose RSTP-SP as an extension to RSTP that meets the performance objectives and keeps the bridging requirements. We evaluate RSTP-SP by means of a simulation analysis and we compare it to Shortest Path Bridging (SPB). Simulation results show that RSTP-SP outperforms SPB in terms of recovery time and outage experienced. In contrast, the message overhead introduced by RSTP-SP is higher than in the SPB case.

Building Ethernet Connectivity Services for Provider Networks

Ethernet has become a dominant technology and it is now a universal communication interface more than just an access technology. Its low cost, high data rates, low complexity and simple maintenance offer Network Providers a good opportunity for using Ethernet data networks at very large scale replacing the existing ATM/SONET or IP networking [1].