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Method to detect audio source change for same site RF takeover, in a packet-switched network Disclosure Number: IPCOM000004683D
Original Publication Date: 2001-Apr-05
Included in the Prior Art Database: 2001-Apr-05
Document File: 2 page(s) / 27K

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Vinayak Naik: AUTHOR [+3]

Related Documents

1889: OTHER


Method to detect audio source change for same site RF takeover, in a packet-switched network

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This is the abbreviated version, containing approximately 72% of the total text.

Method to detect audio source change for same site RF takeover, in a packet-switched network

By Vinayak Naik, Thomas Senese and Calvin Jestice

In the event of a radio interruption (i.e. RF takeover) at the same RF Site in the X-Zone IP-based network, the transcoder cannot reliably detect audio source changes. If the call happens to be encrypted, and if the source change is not cleanly detected, the transcoder shall continue to decrypt the audio with the old (existing) Crypto sync. This causes loss of intelligible audio for periods greater than one second. By using the Timestamp field in the RTP header, in sync with the Sequence number field, we are able to reliably detect the audio source change and take appropriate corrective action to prevent playout of unintelligible audio.

The transcoding device recognizes source change based on one of the following factors:

1. Control Plane (new source at different site)

Controller indicates a new source is keying

Voice stream from the new source matches identifiers provided by the Controller, namely the URID, which is embedded in the Audio packets and is assigned by the controller upon a source change. The transcoder filters out all audio packets whose embedded URID does not match the most recent URID that has been assigned by the controller.

The Universal Resource ID (URID) is a unique identifier of infrastructure equipment on the X-Zone network that is independent of any protocol layer. During call set-up, the controller notifies all destination infrastructure devices of the URID for the source infrastructure device.

2. User Plane (new source at same site)

A break in the IP Sequencing indicates a potential source change

The URID can not be used as a filter of an old audio source, since it does not change for same-site radio interruptions.

The problem, is that in a packet-switched network, a break in sequence numbers is not always tantamount to a source change. By its very nature, a packet-switched network does not guarantee all the packets sent from the source to reach the destination.

In order to overcome this problem, we propose using two pieces of information in the audio packet header: 1) a sequence number, 2) a value that synchronizes the packet stream to a specific audio source. For this purpose, we chose to use the Sequence Number and Timestamp fields in the RTP header.

All transmissions start with a deterministic sequence number value. Also, the timestamp delta, which is the time interval between successive packets in the same stream, shall remain constant.

Assuming N to be the next sequence number and P to be the previous sequence number, and TSx to be the time stamp value for packet X, the following rule shall be able to detect a source change with optimum usage of resources:

N P 1; indicates that N is the expected packet in the stream. Confirm using (TSn TSp) delta 1;

N P 1; either N is for the same stream or for a different stream (after a Source change). Use the timestamp value to determine source change. Us...