IP Telephony Project
By:
Liane Lewin
Shahar Eytan
Guided By:
Ran Cohen - IBM
Vitali Sokhin - Technion
Agenda
•
•
•
•
•
•
•
What is IP Telephony
The Jitter Buffer
The Jitter Buffer Algorithm
Implementation
The Code
Simulations
Conclusions
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Agenda
• What is IP Telephony
•
•
•
•
•
•
The Jitter Buffer
The Jitter Buffer Algorithm
Implementation
The Code
Simulations
Conclusions
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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.
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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...
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Sounds easy?
Well, it’s not that easy!
• 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 “ )
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• 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...
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Agenda
• What is IP Telephony
• The Jitter Buffer
•
•
•
•
•
The Jitter Buffer Algorithm
Implementation
The Code
Simulations
Conclusions
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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.
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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
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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.)
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Our Solution:
The Packet
Source
The Network
Module
JB
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Destination
Module
12
Agenda
• What is IP Telephony
• The Jitter Buffer
• The Jitter Buffer Algorithm
•
•
•
•
Implementation
The Code
Simulations
Conclusions
Network Lab - Technion
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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.
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• 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!
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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.
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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.
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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.
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Agenda
• What is IP Telephony
• The Jitter Buffer
• The Jitter Buffer Algorithm
• Implementation
• The Code
• Simulations
• Conclusions
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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.
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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
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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
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The Destination:
• Receives the voice packets
• Uses the Jitter Buffer Algorithm
• Tries to provide the user with smooth and
short delayed voice
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Agenda
•
•
•
•
What is IP Telephony
The Jitter Buffer
The Jitter Buffer Algorithm
Implementation
• The Code
• Simulations
• Conclusions
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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)
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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 Size
Get
Put
Wrap around
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The JB Control
• 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
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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
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The Network Simulator (cont.)
• 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.
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Agenda
•
•
•
•
•
What is IP Telephony
The Jitter Buffer
The Jitter Buffer Algorithm
Implementation
The Code
• Simulations
• Conclusions
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The Simulations Goal
• Validate the code
• Fix the constants values
• Find the JB advantages over Simple
algorithms
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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
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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.
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Simulation Output Example
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On the “Real” net with the Mirror
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Agenda
•
•
•
•
•
•
What is IP Telephony
The Jitter Buffer
The Jitter Buffer Algorithm
Implementation
The Code
Simulations
• Conclusions
Network Lab - Technion
36
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.
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