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MO7VROLA
OPTIMIZING PACKET DATA RE-TRANSMISSIONS
IN AN INTEGRATED VOICE AND DATA SYSTEM
by Jim Eastwood, Andy Stankoskey, Mike Tanner, Karen Upp, Ray Van Tassle
ABSTRACT
Motorola has been developing communication systems to comply with the APCO Project 25 speci- fications. The first phase of this project was ASTRO release 1.7. This release introduced APCO data on a conventional RF digital voice communications system. This invention provides controls to prevent excess dam packet re-transmissions in an integrated voice and data system.
MOTIVATION
In a data only system the maximum length of data packets is known. This allows prediction of worst case acknowledgement (ACK) response times.
In an integrated data and voice system, voice calls are random in length and can be long with respect to data packets. Due to the extended length and randomness of voice, the ACK response times become difficult to predict.
The ability to predict worst case ACK response times is important in a dam system. If an ACK to a data packet isn't received within the predicted time (retry window) the data packet is re-transmitted. If the retry window is too short the message will be
retried too often creating excess traffic. If the retry window is too long, a message that does need to be re-transmitted will be delayed.
voice call the ACK will be delayed until the voice call ends. If the voice call is long the retry window will close. The data packet will be re-transmitted on the transmit channel, and a new retry window will be started. The retry sequence will repeat until a maximum number of retries has occurred, or the receive channel becomes available and an ACK is received.
If the retry window is extended to take the voice activity into account, data packets that do need to be retried will delayed.
A method is needed to dynamically change the length of the retry window based on voice activity.
SOLUTION
In a data system the retry window is based on the worst case time required to send a message and then receive an ACK.
The retry window is the time difference between when a message is sent to and when an ACK'is received. The retry window can be broken down into two parts:
The first part, called the message window, includes the time to send the message, the delay through the tranmission media, and the time for t...
