This document provides an overview of VoIP techniques and challenges. It begins with an introduction to VoIP and what it is. It then discusses quality of service (QoS) and the importance of providing prioritized delivery services for applications like VoIP. The main challenges for VoIP are then outlined as system capacity/available bandwidth, packet loss, delay/network latency, jitter, echo, and security. Specific causes and issues are described for each challenge. The conclusion is that while VoIP provides a cheaper alternative to PSTN, it has lower quality of service due to these challenges, and the internet is not perfectly designed to carry voice.

1. VoIP Techniques and Challenges
By: Karama Said
Mohamed

2. Presentation outline
 Introduction
 What is VoIP
 QoS
 VoIP Challenges
 Conclusion
 References
 Q &A Session

3. Introduction
 VoIP stands for Voice Over Internet Protocol
 Voice is carried through internet architecture
 Alternative for traditional PSTN
 Poor QoS due to factors affecting
transmission

4. What is VoIP?
 Voice carried by IP (Internet Protocol)
 Voice sent in same manner as e-mails and other
internet data
 RTP header added to IP packet to make a VoIP
packet
 At transmitter data (voice) is compressed, converted
to digital and packets carrying distinct serial
numbers
 At receiver data is decompressed, converted back to
analogue and packets rearranged

5. VoIP Packet

6. QoS
 Quality of Service
 Overall set of network standards and mechanisms
that ensure that the services offered are of high
performance
 Provide prioritized delivery services to the
applications that need it more
 Main models of QoS defined by the Internet
Engineering Task Force (IETF) :(Intserv) and
(Diffserv).

7. Functions of QoS
Source:

8. VoIP Challenges
 System capacity/ Available bandwidth
 Packet loss
 Delay/Network Latency
 Jitter
 Echo
 Security

9. System capacity
 Provision of sufficient bandwidth is the first
crucial step towards achieving a desirable
QoS
 The challenge is that the available bandwidth
is a limited resource
 A bandwidth imbalance may exist in the
uplink and the downlink due to symmetrical
bandwidth

10. Packet loss
 Voice treated as normal data in network
 Packets may be dropped when network is congested
 Re-transmission of packets is not a solution for voice
data as they contain 40 to 80 ms of speech
information
 Packet Loss Concealment (PLC) or Packet Loss
Recovery (PLR ) algorithms compensate lost data
 Identification and classification of VoIP packets can
reduce packet loss

11. Delay/Network Latency
 When voice packets take a longer time than
expected to arrive at their destinations
 Packets arrive late or never at all
 There are fixed delays and variable
delays(jitter)

12. Causes of delay
 Codec
 Queuing
 Wait for packet being transmitted
 Serialization
 Jitter buffer

13. Jitter
 A variable delay caused mainly by queuing,
contention and serialization along the
network
 Occurs in slow and heavy congested links
 queuing based on class, reservation of
bandwidth and faster links can reduce jitter

14. Types of jitter
 Type A: this type is classified as a constant
jitter.
 Type B: this type is termed as the transient
jitter..
 Type C: this is the jitter composed of short
term delay variations.

15. Causes of Jitter
 Sharing of load between many access links
or IP service providers
 Sharing of load within IP service
 Inter-router load sharing
 Routing table updates
 Route flapping
 Timing drift

16. Echo
 Annoying repetition of voice
 Echo time interval varies with the causes of
the echo
 PSTN have less echo due to less delays
compared to VoIP
 Two types of echoes: talkers echo and
listeners echo
 Echo cancellers are used to remove echoes

17. Security
 Many consumer VoIP solutions do not
support encryption yet
 This makes it easy to eavesdrop
 Compression used to prevent eavesdropping
 This doesn’t completely prevent
eavesdropping
 Encryption and cryptography are the best
solutions against eavesdropping

18. Conclusion
 VoIP is a cheaper alternative of PSTN
 Uses internet as the backbone network
 Although cheap, VoIP has a very low QoS
 Performance of VoIP is hindered by factors like
delay and packet loss
 Internet is not perfectly designed for carrying voice
 VoIP can only be used in conjunction with the PSTN
and not to replace it yet

19. Reference
 ‘Voice over IP’ available at:
http://en.wikipedia.org/w/index.php?title=Voice_over_IP&redirect=no
 Leading Edge-VoIP over HSPA: running in the fast lane, By Li Xuanbo available
at: http://www.huawei.com/publications/view.do?id=2938&cid=5331&pid=61
 Quality of Service for Voice over IP (QoS for VoIP) Presented by: Dr. Peter J.
Welcher. Available at www.netcraftsmen.net/welcher/seminars/qos-voip.pdf
 In depth: jitter. Available at
http://www.voiptroubleshooter.com/indepth/jittersources.html
 Jonathan Davidson, James Peters Contributor: Brian Gracely, Voice over IP
Fundamentals, Cisco press, 2000.
 Olivier Hersent, Jean-Pierre Petit, David Gurle, Beyond VoIP Protocols:
Understanding Voice Technology and Networking techniques for IP telephony,
John Wiley and Sons, 2005.

20. Q&A Session
Any Questions?