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An Enhanced Selective ARQ Protocol to Increase System Throughput: Polling for Acknowledgement with the Transmission of the Last Negatively Acknowledged Packet in the Transmit Window

IP.com Disclosure Number: IPCOM000005069D
Original Publication Date: 2001-Aug-14
Included in the Prior Art Database: 2001-Aug-14
Document File: 5 page(s) / 124K

Publishing Venue

Motorola

Related People

Walter Featherstone: AUTHOR [+2]

Abstract

In systems employing automatic repeat request (ARQ) the receiver (Rx-end) is periodically polled for an Ack/Nack (acknowledgement / negative acknowledgement) message. The Ack/Nack message generated by the Rx-end contains the state of the previously received packets. In addition to the periodic polling, it is also customary to poll with the transmission of the last new packet, i.e. the last previously un-transmitted packet. However, currently there is no provision for polling the Rx-end with the last transmitted Nacked packet. This paper proposes to poll for acknowledgement with the last negatively acknowledged block in addition to polling with the last new block. The advantages of the approach are demonstrated through simulation results.

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An Enhanced Selective ARQ Protocol to Increase System Throughput: "Polling for Acknowledgement with the Transmission of the Last Negatively Acknowledged Packet in the Transmit Window"

by Walter Featherstone and Davood Molkdar

Abstract

In systems employing automatic repeat request (ARQ) the receiver (Rx-end) is periodically polled for an Ack/Nack (acknowledgement negative acknowledgement) message. The Ack/Nack message generated by the Rx-end contains the state of the previously received packets. In addition to the periodic polling, it is also customary to poll with the transmission of the last new packet, i.e. the last previously un-transmitted packet. However, currently there is no provision for polling the Rx-end with the last transmitted Nacked packet. This paper proposes to poll for acknowledgement with the last negatively acknowledged block in addition to polling with the last new block. The advantages of the approach are demonstrated through simulation results.

Introduction

In packet data systems employing an ARQ mechanism, the transmit-end (Tx-end) periodically polls the Rx-end for an Ack/Nack message. Upon the reception of a poll message the Rx-end transmits an Ack/Nack message back to the Tx-end. The Ack/Nack message contains the Ack/Nack state of the previous packets sent to the Rx-end by the Tx-end. It is usual in most selective ARQ based systems; for example in GPRS (General Packet Radio System) and EGPRS (Enhanced GPRS), to include the poll message (a simple flag) with the last new packet sent to the Rx-end to ensure timely completion of the data transfer. A new packet is considered to be a packet that has not been previously transmitted. In summary, in a system employing ARQ the Rx-end is periodically polled; and in addition the Rx-end is polled with the last new packet.

This paper proposes to poll for acknowledgement with the transmission of the last negatively acknowledged block in addition to polling with the last new block. The problems with the standard polling procedure are highlighted in the next section, before describing the advantages of the proposed technique.

Problem(s) To Be Solved

For an error free channel the polling scheme described in the introduction works well. However, if the channel is erroneous this scheme can perform sub-optimally. The reason being that the transmission may not be complete since it is highly likely there will be Nacked packets in the Ack/Nack message received. In such a situation, the Tx-end will resend the Nacked packets repeatedly until the next polling period. The impact is wasteful retransmissions; hence reduction in system throughput. This paper proposes a solution to this problem of needlessly transmitting the same packet(s) repeatedly. To illustrate the problem and solution, EGPRS/GPRS RLC protocol is used; however the technique is equally applicable to other ARQ systems employing similar type of protocol.

Proposed Solution to the Problem(s)

This paper proposes to include the poll message with eve...