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Browse Prior Art Database

SMART JITA DELAY ADJUSTMENT

IP.com Disclosure Number: IPCOM000007833D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2002-Apr-29
Document File: 3 page(s) / 123K

Publishing Venue

Motorola

Related People

Michael Bychowsky: AUTHOR [+2]

Abstract

Within the iDEN system, a technique called bypass is used to prevent audio degradation when two mobiles units communicate with each other dur- ing telephone interconnect mode. This is accom- plished by simply routing the HDLC packets through the PSTN as they were originally encoded from the transmitting subscriber. This avoids the XCDR Vocoder from performing the VSELP/linear to linear/VSELP conversions (See Figure 1).

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Page 1 of 3

M-LA Technical Developments

SMART JITA DELAY ADJUSTMENT

by Michael Bychowsky and Michael Kloos

BACKGROUND

THE SOLUTION

  Within the iDEN system, a technique called bypass is used to prevent audio degradation when two mobiles units communicate with each other dur- ing telephone interconnect mode. This is accom- plished by simply routing the HDLC packets through the PSTN as they were originally encoded from the transmitting subscriber. This avoids the XCDR Vocoder from performing the VSELP/linear to linear/VSELP conversions (See Figure 1).

  HDLC is a protocol that transmits information of variable length that is bounded by a start and start flag. A stream ofones (fill frame) are transmit- ted when packets are not being sent. During packet transmission, no more than six ones can be trans- mitted consecutively. Zero stuffing is used to pre- vent this from happening within a packet.

  In order to ensure data packets will arrive at the BRC exactly as they are needed, a delay message is sent to the BSC to synchronize the data transfer. This JITA (Just In Time Audio) message is used by the BSC to offset a delay buffer pointer to match the required delay needed in the downward path.

  Realizing that no more than six consecutive ones can be sent within a valid HDLC packet, a determi- nation can be made as to when a frame begins or ends. Analysis of the data stream can be made to determine the number ofconsecutive ones that have passed through the system. If more than six have been sent, the output pointer must be within a till frame. As soon as this condition changes (a zero is found) the data must have found the beginning of the start flag (0111110). Similarly, ifthe pointer enters a state where it just starts a fill frame, the system must be at the end of an HDLC packet. This simple determination can be used to establish the bounds of the packet without performing an entire HDLC decode routine.

  Since the problem stated that JITA updates could be processed regardless where the output pointer was within the HDLC packet, a method of limiting those movements can now be employed. By limit- ing movements only to-negativ...