Nachum Shacham

Packet recovery in high-speed networks using coding and buffer management

A technique for fiber-optic networks based on forward-error correction (FEC) that allows the destination to reconstruct missing data packets by using redundant parity packets that the source adds to each block of data packets is presented. Methods for generating several types of parity packets are described, along with decoding techniques and their implementations. Algorithms are presented for packet interleaving and selective rejection of packets from node buffers, both of which disperse missing packets among many blocks, thereby reducing the required coding complexity. Performance evaluation, by both analytic and simulation models, shows that this technique can result in a reduction of up to three orders of magnitude in the packet loss rate.<<ETX>>

Multipoint communication by hierarchically encoded data

A novel multipoint communication paradigm, in which each destination receives a subset of the sources signal that corresponds to that destinations terminal and access bandwidth constraints, is presented. The approach to realizing this paradigm is based on integration of layered coding of the sources signal, routing based on bandwidth demand, optimization of signal parameters, and layered error control. The author gives an overview of several hierarchical signal coding techniques; presents methods for finding maximum bandwidth available to destination, establishing maximum-bandwidth routes; and optimally assigns bandwidth to the signal layers to maximize overall reception quality. Error control procedures whereby the network, source, and destinations cooperate to maintain layered-based data integrity, using erasure recovery coding and prioritized packet detection are also presented.<<ETX>>

Multicast security and its extension to a mobile environment

Multicast is rapidly becoming an important mode of communication and a good platform for building group-oriented services. To be used for trusted communication, however, current multicast schemes must be supplemented by mechanisms for protecting traffic, controlling participation, and restricting access of unauthorized users to data exchanged by the participants. In this paper, we consider fundamental security issues in building a trusted multicast facility. We discuss techniques for group-based data encryption, authentication of participants, and preventing unauthorized transmissions and receptions. We also describe the application of these principles and techniques in designing an architecture for secure multicast in a mobile environment.

Distributed algorithms for computing shortest pairs of disjoint paths

Distributed algorithms for finding two disjoint paths of minimum total length from each node to a destination are presented. The algorithms have both node-disjoint and link-disjoint versions and provide each node with information sufficient to forward data on the disjoint paths. It is shown that this problem can be reduced to the problem of finding a minimal shortest path from each node to the destination in a modified network, and a distributed algorithm on the original network that simulates a shortest-paths algorithm running on the modified network is presented. The algorithm has a smaller space complexity than any previous distributed algorithm for the same problem, and a method for forwarding packets is presented that does not require any additional space complexity. A synchronous implementation of the algorithm is also presented and studied. >

Architectures and Performance of Multichannel Multihop Packet Radio Networks

Packet radio networks that employ several parallel multiple-access channels are considered. An architecture for such a network dictates the selection of channels for packet transmission. We propose and analyze two multichannel architectures. In the first, each node employs a single radio and is assigned a channel on which it listens when it does not transmit. To transmit a packet, the node tunes its radio to the channel of the intended receiver for that transmission only. The second architecture requires each radio to use a single channel for both transmission and reception, but provides some of the nodes with more than one radio each, allowing them to serve as bridges between channels. Within these architectures, one can further select the amount of routing information held by each node and the channel-access protocol, both of which greatly affect the network performance. To ascertain the effects of the various parameters, we calculate the throughput in both architectures. The channel-access protocols we consider are slotted ALOHA and CSMA with and without capture. We also evaluate the effect of increasing the amount of routing information held by the nodes.

A selective-repeat-ARQ protocol for parallel channels and its resequencing analysis

A communications system in which multiple parallel channels are available to carry traffic from a transmitter to a receiver is considered, and an extension of the selective-repeat automatic repeat request (SR-ARQ) protocol that dynamically assigns packets to channels for each (re)transmission is presented. Because of selective retransmission, packets arrive at the receiver out of order and must be stored in a resequencing buffer. A queuing model for the resequencing buffer is constructed. The generating function of the buffer occupancy and the packet-delay distribution are derived, and procedures for simplifying the computation are presented. The dynamic assignment scheme is compared with, and shown to have performance superior to, a static assignment scheme. >

A Packet Radio Network for Library Automation

The progress of libraries towards establishing online databases and automated search and retrieval tools allows them to share their resources and provide access to remote users. Unfortunately, the steady increase in the cost of leased lines makes them very undesirable as the media for access networks for budget-constrained libraries. An alternative approach-a packet switching radio network-is therefore presented here. Low-cost, commercially available radio transceivers and standard personal computers are the hardware building blocks of the system. The resulting network employs a suite of protocols, including channel access, routing, congestion control, and the higher-level TCP/IP. This network, which is planned to cover a large part of northern California, will provide remote access to University of Californias automated library online catalog system. It will also support communication between any two end-points, provide for alternative routes in cases of link or nodal failure, and monitor its own performance. This paper, then, presents the networks topology, its architecture, its basic elements, and the functions it performs.

Packet recovery and error correction in high-speed wide-area networks

The author presents a novel technique for reducing packet loss rate in high-speed wide-area networks in which the BER (bit-error rate) is low. Grouping packets into blocks and adding a packet that computes parity over bits of all packets in a block allow a data recipient to reconstruct any single packet in a block, using the other packets and the block parity packet. The missing packet is identified by observing a sequence-number gap in the stream of incoming packets. Adding another packet containing parity information over the diagonals of a series of blocks allows the decoder to correct a single bit error and reconstruct a missing packet, both occurring in the same block. The performance of the scheme was evaluated using a model of a single-server, discrete-time, finite-capacity queue. It was found that, if the input rate of packets to the queue is such that the packet rejection probability is 10/sup -3/ and below, it is possible to find a proper value of block size for which the decoding yields a substantial reduction in packet loss rate. Further reductions are possible if the server discards not necessarily newly arrived packets but takes into consideration their block affiliations and attempts to distribute the rejected packets among the blocks to maximize the decoding capability.<<ETX>>

A distributed algorithm for finding shortest pairs of disjoint paths

A distributed asynchronous algorithm is presented for finding two disjoint paths of minimum total length from each possible source node to a destination. The algorithm has both node-disjoint and link-disjoint versions, and provides each node with information sufficient to make incremental routing decisions for forwarding packets over the disjoint paths. For a network in which all links have unit length, it is shown that a synchronous implementation of the algorithm has communication and time complexities O( mod E mod +D mod N mod ) and O(D/sub 2/), respectively, where D is the networks diameter and D/sub 2/ is the maximum, over all nodes i, of the total number of links in the shortest pair of disjoint paths from i to the destination.<<ETX>>

Self-organizing networks

Abstract With the advent of inexpensive, distributed, powerful computers has come the need for network telecommunications that can transfer data between machines and between users and machines. Fortunately, during the past two decades, major advances have been achieved in telecommunications, ranging from basic improvements in hardware and software design to the creation of such fundamental architectural concepts as the Open System Interconnect (OSI) model developed by the International Standards Organization (ISO). However, although significant progress has been made, one area in telecommunications still requires further development. Until now, we have generally designed protocols for complex telecommunication networks, founded on the assumption that these networkds are static. This assumption facilitates the design of routing algorithms, algorithms for network management, and network topology control. This assumption, however, imposes constraints on the end-user of such networks. Specifically, a user with a high-performance workstation should be able to easily move his/her machine from network to network. However, this movement cannot occur in most networks without first notifying system administrators, in order that appropriate changes in host tables are established and distributed—a process that can take days or weeks to complete. Similarly, many networks today are constrained as to how they can modify their operating parameters (topology, channeltransmission rates, data-flow rates, and alternate routing) to provide service under adverse conditions, such as network-node failures. Note, though, that these constraints are not fundamental to network designs. They have come about primarily because it has been easier to implement ‘first-generation’ networks under these ‘static’ assumptions. However, current telecommunications technology has matured to the point where we can now strive toward developing ‘second-generation’ networks, which we shall call in this paper ‘self-organizing networks’. Our approach to discussing these networks will be to begin with a description of an open system architecture and an example of a network that meets most of the attributes of self-organization. Using these introductory examples, presented in Section 2, we will proceed to discuss the generic issue, ‘binding’, fundamental to self-organizing network designs. We show that if a network is to be self organizing, dynamic binding must be supported. We will then describe research we are pursuing in developing architectures, algorithms, and protocols that permit networks to self-organize. The results of this research will be networks that permit flexible access and that are inherently robust.