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Browse Prior Art Database

PREDICTIVE KEY LOADING DURING LATE ENTRY RECEPTIONS

IP.com Disclosure Number: IPCOM000007893D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-May-02
Document File: 2 page(s) / 90K

Publishing Venue

Motorola

Related People

Curtis D. Lehman: AUTHOR [+2]

Abstract

Since the keyload takes longer than that the EMC will not be ready for decryption, with the result another LDU will be discarded before the call is decrypted. This would result in a total truncation of 540ms of voice.

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M-ROLA Technical Developments

PREDICTIVE KEY LOADING DURING LATE ENTRY RECEPTIONS

by Curtis D. Lehman and Mark Gonsalves

Since the keyload takes longer than that the EMC will not be ready for decryption, with the result another LDU will be discarded before the call is decrypted. This would result in a total truncation of 540ms of voice.

BACKGROUND:

  An APCO-25 CA1 voice message structure con- sists ofa header called the Header Code Word @Icw) and a number for superframes contsining voice and imbedded data. Each superframe is divided into two Logical Data Units (LDU) that are 180ms in dura- tion each. LDUl has Link Control (LC) informa- tion and Low Speed Data imbedded with in it, while LDU2 has Encryption Syncronization (ESYNC) information along with LSD imbedded within it.

  In a secure conventional talkgroup environment, the receiving radio is able to determine information regarding the key the call is being encrypted with from the HCW or the ESYNC.

THE SOLUTION:

THE PROBLEM:

  During late entry calls, when the receiving radio misses the HCW the receiving radio has to derive the LC from LDUl and ESYNC from LDU2. While the LDUs are being decoded to derive LC or ESYNC the DSP does not buffer the frames.

  If the first complete LDU that the radio receives is LDUl, then it will take 360ms for the radio to determine which key to use. Ifthe desired key is not already loaded, a keyload will have to be performed. This can take up to 135ms (including overheads) for radios with hardware encryption using an Encryption Module Controller (EMC) with DVP-XL encryption.

The DSP will buffer only 25 ms of a LDU before it can start decrypting, otherwise it will be discarded.

  During late entry reception ofcalls in a multikey conventional talkgroup environment, if the f...