IP Telephony Project

IP Telephony Project

By:

Liane Lewin

Shahar Eytan

Guided By:

Ran Cohen - IBM

Vitali Sokhin - Technion

Network Lab - Technion 2

Agenda

• What is IP Telephony

• The Jitter Buffer

• The Jitter Buffer Algorithm

• Implementation

• The Code

• Simulations

• Conclusions


Agenda

• What is IP Telephony• The Jitter Buffer


• Implementation

• The Code

• Simulations

• Conclusions


What is IP Telephony?

• IP stands for Internet Protocol

• Basically, it enables passing of audio data over the Internet.

• The data is passed by UDP packets.


Why was that needed?• Imagine being able to call anywhere in the

world for the cost of a local call…

• Imagine being able to link your telephone system into your web page…

• Imagine being able to give your customers a world wide toll-free number…

• IP Telephony enables all that, and more...


• We’re dealing with real time data, delays are critical– Remember the last time you talked to South

America...

• You would like a continuous conversation– You will not like to hear half words or

sentences ( “ I ………….. you “ )

Sounds easy? Well, it’s not that easy!


• Audio data is sent by UDP.

• The network is not that reliable.– There are Routers on the way– Datagrams are delayed– Some times they change their order– Some times they just vanish– Some times they all come at once

• Some one has to put an end to this mess...


Agenda


• The Jitter Buffer• The Jitter Buffer Algorithm

• Implementation

• The Code

• Simulations

• Conclusions


Jitter Buffer (JB)

• Jitter - the speed variation between slowly and quickly traveling packets.

• The JB stores packets, allowing most of the slower packets to catch up.

• The less control in routing, the more jitter that results.


The Jitter Buffer Catch:

More jitter means longer JB

Longer JB introduces more latency

Shorter JB loses too many packets

Lose of packets cause voice to tumble


The Goal: How to control the Jitter Buffer?

• Need to find a good algorithm that will control the JB.

• A convenient Test Bench should be designed in order to simulate the algorithm.

• The Test Bench will not be a full VoIP system (with gateways, routing, etc.)


Our Solution:

The Packet Source

The Network Module

JB

Destination Module


Agenda



• The Jitter Buffer Algorithm• Implementation

• The Code

• Simulations

• Conclusions


The JB Algorithm:

The frames number in the JB adjusts according to the High Water Mark (HWM)

• Whenever the JB is empty, wait till packets will fill it to the HWM.

• Whenever the jitter changes, check if the change is less then the last jitters.


• If the jitter is consistently shorter then reduce the HWM.

• If the number of packets exceeds HWM X Const then throw the remains.

• The HWM will increase whenever the Jitter exceeds a certain limit.

• Remember, the goal is to have the shorter latency and the fewer breaks!

What is the Const ?

What is the limit?

By how much? Define consistent


The Algorithm Parameters:

• Prefetch - Initialized at the beginning of the JB operation. It is constantly updated according to the network behavior.

• Stable History - Initialized at the beginning of the JB operation. It defines the time limit (in JB operations) needed to decide that the network is relatively stable.


The Algorithm Parameters (cont):

• Prefetch reduction:Only when all the previous jitters during the last

Stable History cycle where below the HWM.

The Prefetch is reduced according to the difference between the current HWM and the maximal jitter.

• Prefetch increment:Each time the jitter is higher then the Prefetch, it

will increase to the jitter value.


The Algorithm Parameters (cont):

• Packets are thrown whenever their number is 50% higher then the HWM.

• The algorithm parameters are updated at every mode switch.

Note:

The parameters values are subject to change according to the packet size.

The only well defined parameter is the Stable History.


Agenda




• Implementation• The Code

• Simulations

• Conclusions


Implementation Notes

• The project consist of 2 main blocks– The Packet Source and the Network module

• It’s all part of the Test Bench

– The Destination module• This is the main module. Improvement of this

module is the goal of the project.


2 ways to implement the Network module:

1. Simulation:– The path between source and destination is

divided by Routers.– Number of Routers could be modeled by one

queue as well.– For this queue we will add working parameters

that characterize a network• Frequency of the breaks in its operation

• The duration of a break


2. Using the ‘Real’ Net– We used a Mirror Application

• It runs on a far computer

• All it does is replying the sender by sending his packets back immediately

– While traveling through the net, packets:• where delayed

• changed order

• vanished away


The Destination:

• Receives the voice packets

• Uses the Jitter Buffer Algorithm

• Tries to provide the user with smooth and short delayed voice


Agenda




• Implementation

• The Code• Simulations

• Conclusions


The Code:

• It contains 2 basic applications– The Network Simulator– The Destination

• Each contains number of threads

• Both NT and UNIX compatible

• Few basic classes are being used to simplify the code (Sockets, Threads and Synchronization)


Implementing the Jitter Buffer

• It’s basically a cyclic array

• Allocated in memory at the beginning of the operation

• Its size is a multiple of the Frame SizeGet Put

Wrap around


• 3 Threads controls the JB I/O:1. JB Input Thread:

• Poles the network for incoming frames and inserts them to the JB according to their sequence number.

2. JB Output Thread:• Reads from the JB in constant frequency (equal to

the sender’s), and plays the data to the user.

3. User Interaction Thread:• Gets the user commands

The JB Control


The Network Simulator

• The Network is modeled as queue (Cyclic Queue)

• 3 threads control its operation (Input, Output and User Interaction)

• The input/output of the frames is done in order - FIFO


• The simulated network works according to the following parameters:– Breaks - Caused by heavy traffic in the net– Break frequency

Those parameters could be entered by the user, or selected randomly

Remember, if we want real data we still got the Mirror app.

The Network Simulator (cont.)


Agenda




• Implementation

• The Code

• Simulations• Conclusions


The Simulations Goal

• Validate the code

• Fix the constants values

• Find the JB advantages over Simple algorithms


In each simulation we checked:

• At any time:– Number of packets in the buffer (Buffer)– The number of input tries vs. output tries (Level)– The jitter (Jitter)– The HWM (Prefetch)– The approximate packet delay (Delay)

• For comparison we managed also a simple buffer


Simulation Area:

• Simulations ran on the “real net” using the Mirror application. It ran on computers in USA, Japan and closer.

• To exercise the algorithm on different scenarios we used the network simulator.

• For better view, we processed the data using Matlab.


Simulation Output Example


On the “Real” net with the Mirror


Agenda




• Implementation

• The Code

• Simulations

• Conclusions


Conclusions:

• When coming to improve network connected applications, dynamic algorithms are needed.

• Applying a JB to the destination improves the latency and breaks.

• Its advantage is mainly when the net stabilizes. Not significant when it is static.


Date post:	09-Jan-2016
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IP Telephony Project

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