Browse Prior Art Database

Slot Based De-Jitter

IP.com Disclosure Number: IPCOM000126202D
Original Publication Date: 2005-Jul-07
Included in the Prior Art Database: 2005-Jul-07
Document File: 7 page(s) / 238K

Publishing Venue

Motorola

Related People

Mike Purcell: INVENTOR [+2]

Abstract

Slot based De-Jitter is a simple method of removing time jitter in a sequence of serial packets.

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MOTOROLA TECHNICAL DEVELOPMENTS

INFORMATION SHEET

FOR DEFENSIVE PUBLICATION

Date:                           

May 23, 2003

Title:                 Slot Based De-Jitter

Docket No.:    

IRI

05267

Author #1

Name:                          Mike Purcell

Author #2

Name:                          Leonard K. Pennock

 


Slot Based De-Jitter

Leonard K. Pennock

Mike Purcell

I.                   Introduction

Slot based De-Jitter is a simple method of removing time jitter in a sequence of serial packets.

II.                Problem Solved

This method is needed to perform jitter reduction in a Network Processor that can afford only minimal timer support (due to resource limitations of CPU cycles, or hardware capability) and stays compatible with line rate processing. This method handles reordering of early and late packets.

The traditional methods are usually based on the IETF Architecture for Differentiated Services [1]. In this model, the traffic is prioritized into flows and then each flow is managed according to the Shaper / Dropper.  In more sophisticated designs, the Shaper operates on each classification of traffic separately using a shaping per flow bases. Each flow requires a timer (or timer record) per packet that needs delay and this does not scale well to large number of flows. We feel that a method is needed to produce linear shaping (de-jitter) using a lightweight timing mechanism.

To de-jitter or flatten the profile of packets, it’s given that this will cost some delay in the overall transmission.  Against this delayed baseline, early packets will be delayed and late packets speed up.

This method can also solve jitter and interleaving problems with packets containing multiple frames of data. Packets holding several frames (such as voice over

RTP

) for the same flow are used to reduce the overhead associated with relaying the frames. This method of Slot Based De-Jitter is needed to minimize the effects of lost packets when doing bundled and/or interleaved frame formats. Jitter correction at packet level can cause all data in the packet to be discarded if slightly late, although some frames may still be usable.

  • Bundled Packets can cause large speech gaps
  • Interleaved Packets can cause many small gaps

III.             How it Works

For each flow the Shaper must manage, we have created queues that hold the maximum number of packets that can be serviced in a time period (or slot). That way, one timer in the shaper can service all packets in the flow.  For example, the Shaper will process all of the packets in the current time slot (Tnow figure 1) before moving on to the next time slot. 

A Marker feeds the Shaper’s time-oriented queues.  Packets get placed in queues that are to be sent sooner (see N-2 figure 2) if they are late, and farther out in time (see N+2 figure 2) if they are early.

A de-queuer in the Shaper will remove packets from delayed time slot (see N-4 figure 2 and Tnow figure 1) until i...