‘Quality of Service (QoS) in VoIP’
Dr. Mashiur Rahman
ID # 072 842 056
Section # 02
Table of Content
Voice over IP - What is VoIP? 2
Reasons for Choosing Voice over IP - VoIP Advantages 2
VoIP Cons - VoIP Problems and Pitfalls 3
Quality of Service - QoS and VoIP 3
What is Quality of Service (QoS)? 4
What is Quality? 5
What is Service? 5
How to Achieve QoS? 5
Why QoS is Important 5
LAN and WAN QoS 6
Addressing QoS at the edge of the network 6
Technology standards for QoS 9
Network Diagram 9
Why Do Companies Want VoIP? 10
VoIP: Future of telephony and Bangladesh 10
Voice over IP - What is VoIP?
VoIP stands for Voice over Internet Protocol. It is also referred to as IP Telephony. It is
another way of making phone calls, though the ‘phone’ part is not always present
anymore, as we can communicate without a telephone set.
VoIP is especially popular with long-distance calls. The main reason for which people are
so massively turning to this new technology is the cost. We can use VoIP with a
telephone set or a mobile phone, but we require a paid service. This paid service is
however way cheaper than standard phone calls. This becomes thrilling when we
consider international calls. People who had their communication costs on international
calls cut down by 90 percent thanks to VoIP.
There are basically three ways of using VoIP: one is to have a PC on both communicating
sides; another is to have a Phone on one side and a PC on the other and the third is to
have two phones.
The great thing about VoIP is that it taps additional value from the already existing
infrastructure without additional costs. VoIP transmits the sounds we make over the
standard Internet infrastructure, using the IP Protocol. This is how we can communicate
without paying for more than monthly Internet bill. For instance, we can have free VoIP
calls with Skype, Gizmo, VoIPStunt and other VoIP services and applications.
Reasons for Choosing Voice over IP - VoIP Advantages
Voice over IP (VoIP) was developed in order to provide access to voice communication
in any place around the world. In most places, voice communication is quite costly.
Consider making a phone call to a person living in a country half the globe away. VoIP
solves this problem and many others.
There are of course a few drawbacks attached to the use of VoIP, as is the case with any
new technology, but the advantages largely outbalance these. Let's explore below the
benefits of VoIP and see how it can improve our home or business voice communication
• Save a lot of money-Studies have shown that, compared to using a PSTN line,
using VoIP can potentially make us save up to 40 % on local calls, and up to 90 %
on international calls.
• More than two persons-On the phone line, only two persons can speak at a time.
With VoIP, we can setup a conference with a whole team communicating in real
time. VoIP compresses data packets during transmission, and this causes more
data to be handled over the carrier. As a result, more calls can be handled on one
• Cheap user hardware and software-The only additional hardware we require
besides our computer and Internet connection for VoIP for voice communication
are a sound card, speakers and a microphone. These are quite cheap. There exist
several software packages downloadable from the Internet, which we can install
and use for the purpose. Examples of such applications are the well-known Skype
• More than voice-VoIP is based on the Internet Protocol (IP), which is in fact,
along with TCP (Transmission Control Protocol), the basic underlying protocol
for the Internet. By virtue of this, VoIP also handles media types other than voice:
we can transfer images, video and text along with the voice. For instance, we can
speak to someone while sending him/her files or even showing himself using a
• More efficient use of bandwidth-It is known that about 50 % of a voice
conversation is silence. VoIP fills the ‘empty’ silence spaces with data so that
bandwidth in data communication channels is not wasted.In other words, a user is
not given bandwidth when he is not talking, and this bandwidth is used efficiently
for other bandwidth consumers. Moreover, compression and the ability to remove
redundancy in some speech patterns add up to the efficiency.
• Flexible network layout-The underlying network for VoIP does not need to be of
a particular layout or topology. This makes it possible for an organization to make
use of the power of proven technologies like ATM, SONET, and Ethernet etc.
VoIP Cons - VoIP Problems and Pitfalls
VoIP is for sure gaining advantage over PSTN. It has seduced millions of people and
companies worldwide, especially in the US, with the numerous benefits it offers.
Whether we have already switched to VoIP or are still considering the option, we need to
be aware of the VoIP Cons - the different pitfalls it entails and the disadvantages attached
to it. Mainly, these are:
• Voice quality
• Bandwidth dependency
• Power dependency
• Emergency calls
Quality of Service - QoS and VoIP
Today’s Internet does far more than email and file transfer. Initially designed for non-
real-time (NRT) data applications, the Internet has matured far beyond these tasks. In
addition to web browsing, online imaging, and chat rooms, we now expect the Internet to
deliver such real-time (RT) media as streaming music, video, and Internet phone calls
directly to our homes and offices. Presently, Voice-over-Internet Protocol (VoIP)
technology has matured. Today, VoIP has emerged as a reliable technology that is
commercially viable, competing (and winning) against traditional phone services in
business and consumer-class markets. As a real-time application, VoIP—also known as
packet voice, packet telephony, or IP telephony—places increased demands on the
evolving Internet. VoIP users expect the Internet to deliver toll-quality voice with the
same clarity as the traditional Public Switched Telephone Network (PSTN). To meet
those expectations, the Internet connection must be more than merely reliable, it must be
time-sensitive. Each and every voice packet must be delivered without significant delay
and with consistent time intervals between packets. Advanced Quality of Service (QoS)
technology is the key to achieving voice quality that measure up to today’s high
standards. With SmartNode and Patton’s advanced QoS, toll-quality voice on every call
is the benchmark.
What is Quality of Service (QoS)?
QoS stands for Quality of Service. It is quite an elusive term since there is no finite
definition for it. Depending on where, how and why it is used, people see it in different
angles and have different appreciations of it.
The most common definition we have of QoS is the differentiation between types of
traffic and types of services so that the different types of service and traffic can be treated
differently. This way, one type can be favored over another.
QoS means the best possible VoIP service at the lowest cost. Like many technology
expressions, QoS means different things to different people. For most business
customers, the term signifies getting the best possible VoIP service quality at the lowest
possible cost. For service providers, QoS often means the exact opposite: delivering
minimally acceptable VoIP quality at the highest price the market will bear.
QoS is a traffic-management strategy that allows us to allocate network resources for
both mission-critical and normal data, based on the type of network traffic and the
priority us assign to that traffic. QoS ensures unimpeded priority traffic and provides the
capability of rate-limiting (policing) default traffic. For example, video and VoIP are
increasingly important for inter-office communication between geographically dispersed
sites, using the infrastructure of the Internet as the transport mechanism. Firewalls are
key point to network security as they control access, which includes the inspection of
VoIP protocols. QoS is the focal point to provide clear, uninterrupted voice and video
communications, while still providing a basic level of service for all other traffic that
passes through the device. For voice and video to traverse IP networks in a secure,
reliable, and toll-quality manner, QoS must be enabled at all points of the network. The
implementation of QoS allows us to:
• Simplify network operations by collapsing all data, voice, and video network
traffic onto a single backbone with the use of similar technologies.
• Enable new network applications, such as integrated call center applications and
video-based training, that can help differentiate enterprises in their respective
market spaces and increase productivity.
• Control resource use by controlling which traffic receives which resources. For
example, we can ensure that the most important, time-critical traffic receives the
network resources (available bandwidth and minimum delay) it needs, and that
other applications that use the link get their fair share of service without
interfering with mission-critical traffic.
QoS is an important tool for VoIP success. Through the years QoS mechanisms have
become more and more sophisticated.
What is Quality?
In networking, quality can mean many things. In VoIP, quality simply means being able
to listen and speak in a clear and continuous voice, without unwanted noise. Quality
depends on the following factors:
• data loss
• consistent delay characteristics (called jitter)
• latency, leading to echo
What is Service?
Service can mean many things in networking, as it carries some ambiguity in meaning. In
VoIP, it generally means what is offered to consumers in terms of communication
The very first thing we need to guarantee in order to guarantee quality for VoIP is
adequate bandwidth. And this is one of the greatest challenges in networks today: how to
achieve good voice quality with limited and often shared bandwidth. This is where QoS
comes into play.
How to Achieve QoS?
On a personal (small scale) level, QoS is set at router level. If we want to enforce QoS
policies in our network, make sure we use a router which is equipped with QoS software,
which we can use to configure the quality of service we require.
If we are an individual user, then there is a great chance that our VoIP service provider
already implements QoS on their server, though this is not always the case. This way, the
QoS configurations will be such that they favor voice over other data types. But then,
since we will be using an Internet connection from a provider of another type (our ISP),
the effect is somewhat diluted; unless we implement QoS on our router. Some IP phones
allow this as well.
Why QoS is Important
VoIP QoS is an important--perhaps the most important--consideration for businesses
moving to Internet telephony. That's because, for more than a century, businesses have
generally taken telephone QoS for granted. Enterprises that switch to a low-cost VoIP
service are often surprised to discover that they have entered a world where service
quality can be elusive and hard to achieve. The shortcomings are manifold. Latency (a
delay in packet delivery), packet loss (missing data), network jitter (data arriving out of
order) and numerous other factors can all degrade a voice connection's quality, leading to
distorted audio, disconnects and even total VoIP system collapse.
In the real world, however, enterprises and end users would be happy to settle for low-
bandwidth, yet clear, glitch-free calls. Fortunately, such a goal is attainable for the
enterprise that's willing to take QoS seriously and not cut corners.
LAN and WAN QoS
The most important factor affecting internal QoS is whether enterprise local-area
networks (LANs) and wide-area networks (WANs) use switches, routers and other
devices that support QoS standards, such as those defined in RFCs published by the
Internet Engineering Task Force (IETF) and other working groups.
Network performance monitoring tools can also help us determine the health of our
enterprise's LANs and WANs and the impact that any existing problems may have on
QoS. Finally, in networks that share data and voice services, QoS can be enhanced by
giving voice packets priority over other types of network traffic. Such special "white
glove" service will make VoIP services less susceptible to latency, jitter and other
undesirable conditions. Intelligent routing protocols, such as MPLS (multi-protocol label
switching) and OSPF (open shortest path first), are available to prioritize network traffic.
These protocols intelligently optimize the network traffic, in accordance with
Addressing QoS at the edge of the network
In VoIP systems that traverse the Internet, the bottleneck typically occurs at the access
link—the low bandwidth connection between the high-speed Internet backbone (WAN)
and the user network (LAN). Both networks typically run at 100 Mbps or above. A
typical access link may easily run about 200 times slower than the LAN residing in the
home or office (say, 512 kbps for example). Congestion, queuing delay, and queue
overflows (resulting in dropped packets) are most likely to occur on this link. Depending
on access-link bandwidth, packet size and burst size (the number of packets arriving at
once), queuing delay can be especially significant.
In a typical installation, a single access link serves both voice and data traffic, so special
measures must be employed to ensure good voice quality. Consider the case of a 256
kbps access link from the Internet’s edge router to the user’s LAN. Suppose the Internet’s
transmit queue contains five 1500-byte data packets, followed by one time-sensitive
voice packet. It will take roughly 270 ms to send those data packets over the 256 kbps
link. When the voice packet follows, it arrives with a delay longer than 120 ms (our
target), resulting in degraded voice quality.
Introducing class-of-service in the packet layer addresses queuing delay by ensuring
voice packets receive priority treatment—in much the same way that separate queues at
airport check-in counters ensure priority service to first-class customers. By creating
separate queues for Real-Time and Non-Real-Time traffic, we can assign higher priority
to the RT queue and serve that traffic with higher priority.
Upstream and downstream traffic present different problems, and are best addressed by
different Quality of Service (QoS) mechanisms. Upstream traffic flows from the (home
or office) user to the Internet, while downstream traffic flows from the Internet to the
Implementing QoS in the upstream direction is relatively easy. SmartNode ensures
that outbound voice packets get served before other packet types to prevent the Internet
access link (the bottleneck) from becoming overloaded. SmartNode also provides tuning
mechanisms for additional parameters like packet segmentation and overhead
optimization, so network administrators can further fine-tune the upstream transmission
for optimum voice quality.
Implementing QoS in the downstream direction is more complex. Typically,
customer-premise equipment (CPE) at user locations exerts no control over incoming
Figure 1: Internet Access
Figure 2: The Problem of Downstream Traffic
traffic. For traffic flowing downstream from the Internet, the access router cannot control
the volume or the sending users. In addition, local users sharing the LAN with us can
initiate file transfers or download their email at their convenience. The servers handling
these requests may be located anyplace. Since the downstream rate normally cannot be
controlled, the ISP’s edge router commonly responds to overloading by discarding VoIP
packets with the same probability as any other packet type. These factors, or a
combination of them, may degrade voice quality to a degree that users find objectionable.
Data traffic, on the other hand, can be retransmitted so the impact on the user experience
is simpler slower service response.
To resolve the problem of degraded voice quality for incoming traffic, Patton has
devised a leading- edge technology for SmartNode called DownStreamQoS. Within the
SmartNode deployed at the customer premise, DownStreamQoS dynamically creates a
virtual bottleneck against the incoming packet stream. This bottleneck can throttle back
Non-Real-Time traffic, preventing the edge router from blocking or impeding voice
traffic, to ensure voice packets are transmitted freely downstream. DownStreamQoS
employs flow-control mechanisms within the TCP standard to create the bottleneck.
Because 80% of Internet traffic is transported via TCP, DownStreamQoS is especially
effective. The dynamic bottleneck adjusts to varying traffic patterns. So whenever there
are no VoIP calls in progress, the full downstream bandwidth is available for incoming
data such as file downloads.
Figure 3: SmartNode DownStreamQoS
Technology standards for QoS
When implementing QoS to ensure good voice quality, VoIP networks may employ a
selection of mechanisms from a variety of standard communications protocols.
Such mechanisms may include:
• Tag or Label within the Packet or Frame
• Traffic (QoS) Classes
• Traffic (QoS) Conditioning or Packet treatment
Figure 4 QoS for VoIP Traffic on VPN Tunnels Configuration Example
Note: The IP addressing schemes used in this configuration are not legally routable on
the Internet. They are RFC 1918 addresses which have been used in a lab environment.
Note: Ensure that IP phones and hosts are placed in different segments (subnets). This is
recommended for a good network design.
Why Do Companies Want VoIP?
The number of companies adopting VoIP as their main communication mechanism is
increasing exponentially. It is very interesting to know why these companies are shifting
to VoIP. The benefits of VoIP are surely numerous and tempting, but the adopting VoIP
also implies a number of non-negligible risks, given the drawbacks of VoIP.
An InformationWeek Research study carried out on 280 companies currently using or
planning to use VoIP yielded some important characteristics, depicting a much-expected
trend. Note that in that survey, multiple responses were allowed.
Here are the top reasons why companies want to adopt VoIP:
Lower telecommunications costs 66%
Desire to merge voice and data networks 43%
Obtain a platform for one-stop communications in two or more areas 41%
Increase collaboration benefits in two or more areas 36%
Ease of management 31%
VoIP: Future of telephony and Bangladesh
Bangladesh is on the drive towards VoIP. In Bangladesh, the government has been trying
to encourage people to stick to its traditional telephone network. They have been cutting
down the consumer's bill by reducing the charges. However, we have seen the traditional
telephone network will never be able to allow cheaper rate compared to VoIP network.
Rather the government itself can go for VoIP businesses in different form. Moreover,
recently it is found by the GSM Association (GSMA) that the Bangladesh government
protects the BTTB's monopoly over international phone call business. Thus foreign
investors are least interested to inject fund in the country's telecom sector.
Moreover, legalization of VoIP can benefit a country in several ways. Significant cost
saving in the government, private and home telephone bills would definitely make a mark
in the economy. In the world of globalization, unlimited long distance calls are desired to
take the home businesses to the global market. VoIP is the cheapest solution to take our
voice to the world business community.
Furthermore, new software solutions will integrate real time voice system more and more
into their system. Thus VoIP infrastructure may help country like Bangladesh which is
looking for a good share from the global software outsourcing.
It is apparent that VoIP is the reality of future telephony. The cost effective VoIP solution
will continue to attract more people in the year to come. If we look closely we will realize
that voice and data network is merging. There will be one network both for data and
voice. Thus it is the same organization who will be serving both the voice and data world.
As a result their will be more users with such cost effective system. This increment in
user number will help the providers achieving revenue target.
However, if BTTB had 22 million VoIP users they could earn much higher with monthly
low fixed charge. 22 million is the current number of users in cellular world of
Bangladesh. That clearly indicates there is a high demand for telephony in Bangladesh.
With such rapid growth of users BTTB should not have any concern regarding their
income target and return on investment in VoIP infrastructure.
In April 2002, India became the first country in South Asia to officially permit VOIP of
certain types. Pakistan is awaiting a court order to remove the ban on VOIP. China
actively promotes IP telephony and Thailand, Malaysia, Singapore and South Korea
allow VOIP in some form or other.
VoIP is a relatively new technology and it has already achieved wide acceptance and use.
There is still a lot to improve and it is expected to have major technological advances in
VoIP in the future. It has so far proved to be a good candidate for replacing the POTS
(Plain Old Telephone Systems). It, of course, has drawbacks along with the numerous
advantages it brings; and its increasing use worldwide is creating new considerations
surrounding its regulations and security.
Achieving the required Quality of Service (QoS) through the proper management of
network delays, bandwidth requirements, and packet loss parameters, while maintaining
simplicity, scalability, and manageability of the network is the fundamental solution to
running an infrastructure that serves business applications end-to-end.
• Detailed Linux QoS treatise at LARTC: http://www.lartc.org/
• HTB manual and user guide:
• CBQ examples in Asterisk environments:
• Diffserv Architecture RFC 2474 & RFC 2475 (http://www.ietf.org)
• www.iifc.net/summary_voip_workshop.pdf - Similar pages