How to guarantee QoS for IP telephony and unified communications:

How to guarantee QoS for IP telephony and unified
communications: a technical white paper examining the
functional requirements of implementing and maintaining
quality of service
Published: 1 July 2007
© 2007 SAS Global Communications Limited. Reproduction of this publication in any form without express
permission is forbidden. The information contained herein has been obtained from sources believed to be reliable.
SAS does not accept liability for errors, omissions or inadequacies in the information or its interpretation. The
opinions expressed in this document are based on judgements at the date of publication and are subject to change
without notice. All Rights Reserved.
Table of contents
Introduction ............................................................................................................................... 3
Glossary ................................................................................................................................... 4
What is quality of service (QoS)? ............................................................................................. 5
Class of service- considerations ........................................................................................... 5
Defining class of service ....................................................................................................... 6
Implementing class of service on the network....................................................................... 7
Why is QoS important to IP telephony and unified communications? ....................................... 8
Setting class of service parameters .................................................................................... 10
How to achieve QoS throughout your network ....................................................................... 11
Switches.............................................................................................................................. 11
Routers ............................................................................................................................... 11
Testing QoS and setting benchmarks for performance .......................................................... 13
The MOS test ...................................................................................................................... 13
Maintaining QoS and policy enforcement. .............................................................................. 15
Conclusion .............................................................................................................................. 16
About SAS .............................................................................................................................. 16
List of Figures
Figure 1-1 Class of service categories ............................................................................................................... 5
Figure 1-2 Class of service applications ............................................................................................................ 6
Figure 1-3 How CoS works at an IP packet level ............................................................................................... 7
Figure 1-4 QoS-enabled infrastructure .............................................................................................................. 8
Figure 1-5 Standard data ................................................................................................................................... 8
Figure 1-6 Assured data .................................................................................................................................... 9
Figure 1-7 Expedited data ................................................................................................................................. 9
Figure 1-8 Dynamic demand allocation ........................................................................................................... 10
Figure 1-9 Infrastructure components requiring QoS enablement ................................................................... 11
Figure 2-1 QoS on the WAN ............................................................................................................................ 12
Figure 2-2 MOS testing ................................................................................................................................... 13
Figure 2-3 Monitoring QoS .............................................................................................................................. 15
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© SAS Global Communications Limited
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Introduction
When voice, data and video applications had their own dedicated networks, quality of service
(QoS) played a fairly small role because each type of traffic behaved in a similar way;
networks could be finely tuned to support that behaviour.
In the converged network, QoS is critical to the success or failure of the network because the
performance of applications like voice and video can become unacceptable if subjected to
many of the common features of a best-effort IP data network, such as packet loss and
delays.
This technical white paper provides a detailed explanation of where QoS is required within a
network infrastructure, to support IP telephony and unified communications.
It also provides guidance on how QoS may be achieved; offering guidelines on establishing
benchmarks for testing and measuring QoS on the network, as well as providing
recommendations for maintaining and proactively monitoring its delivery.
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Glossary
AF - Assured forwarding (video / applications)
CoS - Class of service (AF/EF/standard)
CODEC - Coder/decoder
CODEC - Compression/decompression
EF - Expedited forwarding (voice)
IPC - IP communications – integrating applications into IP telephony
IPT - IP telephony – voice traffic is 100% IP
QoS - Quality of service
UC - Unified communications – integrating all multimedia communications
VoIP - Voice over IP – analogue traffic converted to IP at the router
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What is quality of service (QoS)?
Quality of service is best defined as the capability of the network to guarantee priority and
performance for different data types, ensuring smooth delivery of applications across the
network.
QoS is primarily achieved by applying class of service (CoS) functionality to the network
infrastructure.
Figure 1-1 Class of service categories
EF
AF
Standard
Figure 1-1 shows an example of six categories of CoS being used across a single network.
The diagram incorporates three important categorisations:
expedited forwarding (EF), used for voice-specific traffic.
assured forwarding (AF), for primary applications
standard, used for generic data.
Class of service- considerations
Currently, in most network environments, class of service is not used. The implications of this
are that each network treats all traffic as generic data, according no priorities and, therefore,
offering no time guarantee. The absence of such parameters results in the data being broken
down into packets and potentially arriving in a non-sequential manner.
This process takes time and causes delays. Occasionally, data is lost and has to be retransmitted.
The timescale of the delay is, of course, miniscule, and of no consequence to many users.
Depending on the application in use at the time, they may not even notice the delay.
With voice and other primary applications, however, such delay can be a serious performance
issue.
Whilst a one-second delay in the delivery of an email may cause no problems, since the
despatch of the email is never so precisely time-critical, a similar delay in voice traffic is, by
any yardstick, unacceptable. It leads to callers talking across one another and creates a
barrier to conversation clarity in a way that was common on transatlantic calls ten to fifteen
years ago.
Fluctuations in network activity can also cause problems when class of service is not present.
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Email and web browsing applications that may have been categorised as standard data, can
generate sudden bursts of traffic capable of flooding the network for a short period of time.
This can result in the delay of primary applications when routing through the network at the
same time.
Defining class of service
Figure 1-2 Class of service applications
EF
AF
.
Standard
Figure 1-2 shows an example of service categorisations working in a live environment, as
follows:
Expedited forwarding- used for voice traffic.
Assured forwarding- used for primary applications; shown for illustrative purposes
are a CRM application, a database and an ERP application plus multimedia in the
form of video conferencing.
Standard- used to describe traffic other than voice and primary applications; in
this example, web-browsing and email.
Class of service facilitates efficient and timely delivery of all data traffic through the network
by assigning each application to a specified CoS category, thereby ensuring that delay, jitter
and packet loss is kept to a minimum.
The alternative simply does not offer such high network delivery standards.
Traditional networks are able to differentiate between standard and primary applications
through the use of packet shapers. Such products are not dynamic and typically divide the
network into different segments and permanently allocate these segments to specific
applications, whether or not they are used.
Also in common usage are local area network and wide area network accelerators, often
referred to as expand boxes; of limited functionality however, able to recognise different
systems but only useable within the specified network. They become less effective when the
traffic is handed off to a carrier’s wide area network, where there is no in-transit control
mechanism.
Expedited forwarding class of service (EF CoS) provides the highest level of priority, so is
particularly suitable for voice traffic. There is negligible packet loss and data delivery is
optimal.
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Assured forwarding class of service (AF CoS) has a high level of network performance but
can experience some low level, and basic packet, interruption.
Standard data does not need any priority; it fits into the background around high priority class
of service traffic.
Implementing class of service on the network.
Figure 1-3 How CoS works at an IP packet level
Layer 2 – 802.1Q/p
Pream.
SFD
DA
SA
Type
PRI
CFI
TAG
(4 bytes)
PT
DATA
FCS
VLAN ID
Three bits used for CoS
Layer 3 – IPv4
Version
Length
ToS
(1 byte)
Len
ID
Offset
TTL
Proto
FCS
7
6
5
4
3
2
1
0
IP-SA
IP-DA
DATA
Unused bits;
Flow control for DSCP
IP Precedence
DSCP
Figure 1-3 represents the composition of a data packet at both Layer 2 and Layer 3 levels.
At any one time, millions of data packets are moving around every network.
The application itself automatically provides the relevant packet information, whilst
configuration of the network infrastructure delivers the class of service as well as the quality of
service mechanism.
In the diagram, header information is carried at the front of the packet, for Layer 2 packets.
The actual size of the data within the packet is relatively small compared to the overall size of
the information associated with it. Of particular interest here, with regards to class of service,
is the tag. (The tag is shown in blue and expanded in the middle of the diagram).
The tag provides a simple identifier for class of service, recognised by routing devices to route
the data to the correct location. This is made up from basic information, including the VLAN
address identifying the data destination.
At Layer 3 there is more segmentation of information on the packet and the proportion of the
packet represented by data is even smaller. The header, or service identifier, used in Layer 3
provides more detailed IP information on how the data should be managed. The type of
service (ToS) header ensures that each application has a unique identifier which, in turn,
facilitates a far more granular level of management and routing capability.
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Why is QoS important to IP telephony and unified communications?
A short sequence of diagrams serves to answer this question in a relatively self-explanatory
fashion.
Figure 1-4 QoS-enabled infrastructure
Figure 1-4 shows a London/ New York office set-up with identical product sets in use at each
location: voice runs over an IP platform and assured data is deployed for video conferencing;
there is also a CRM application, a database application and an ERP application. Standard
data is represented by web browsing and email applications.
With a wide area network, when London users are in communication with each other, nothing
passes across the network connection to New York. This means that two levels of quality of
service are required, one on the local area network and the other on the 2MB wide area
network which connects the two sites.
Figure 1-5 Standard data
If nothing other than standard data were being routed across the network, the traffic would
simply utilise the available bandwidth; in such a situation, a large email being routed between
the two offices, with no control mechanism, would use as much bandwidth as were available.
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Figure 1-6 Assured data
As other applications are brought on line, class of service mechanisms are introduced to
prioritise each of the application types; shown here are four assured data services, voice
conferencing, CRM, database and an ERP application.
The implication for standard data is that it is then condensed down to the remaining
bandwidth. In this scenario, video conferencing is using 512K, and each of the other three
applications (CRM, database and ERP) is using 256K. Each has an independently allocated
amount of bandwidth. All assured data receives priority over standard data.
Figure 1-7 Expedited data
As the voice application is brought on line, it receives the highest level of priority, allocated by
the expedited forwarding parameters for class of service. The voice in this scenario might be
using 512K. This produces an aggregated total through the 2MB pipe of 512K for voice, 512K
for video and 256K for each of the remaining three services. Voice and video together fill half
the pipe. Web-browsing and email applications are throttled to a bandwidth allowance of just
256K, preventing any large email distribution from consuming all of the network bandwidth.
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Figure 1-8 Dynamic demand allocation
Both the management of class of service over the network, and delivery of quality of service,
are dynamic.
As voice traffic drops, the other applications automatically take up the available bandwidth
according to their class of service categorisation.
Setting class of service parameters
There are a number of areas where, in traditional networking, the requirements for class of
service do not necessarily match a pure IP environment.
Video conferencing demonstrates this point, where a significant difference exists between the
old system and the new.
Video conferencing over ISDN, using a system of 35 frames a second, would traditionally
require four ISDN2 circuits, which is 8 channels or 512K bandwidth.
The same performance using the H.323 video standard, the requirement for transmission
over an IP network, would need 768K bandwidth.
There is a big difference in the way in which the bandwidth is allocated between ISDN and IP.
Because the class of service and the way data is handled across the network is dynamic, IP
is much more flexible than ISDN, which locks out all channels for the duration of the video
conference, If, in a video conferencing scenario, all participants were sitting fairly still with not
much conversation occurring, then no more than 128K would be needed for the majority of
the session. As people become more animated and there is more conversation, then the
network will flex and push the bandwidth up to 768K as required.
It is important to note that standard data can flood the network and stop other applications
from working. This observation does not apply to email and web usage alone.
On a wide area network with no quality of service, other applications, such as back-up
systems running over the wide area network or servers sending file data to other servers, can
also flood the network. This potential danger also arises through continual anti-virus updating,
causing considerable problems that impede the other applications for which quality of service
needs to be available.
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How to achieve QoS throughout your network
Figure 1-9 Infrastructure components requiring QoS enablement
WAN service
Carrier router
Firewall
PSTN / BRI / PRI
VPN
Voice gateway router
WAN
router
Default gateway
Application server
SAN
Application server
Email server
Switch
Printer
Switch gateway
WiFi switch
WIP phone
UM server
Call control server
Switch gateway
IP phone
Laptop
Figure 1-9 shows the typical components of a network infrastructure. At the most basic layer
is the switch infrastructure. This is where the packet information is processed; the detail and
break-down between the Layer 3 packet and the Layer 2 packet.
Switches
The ideal switch infrastructure is a Layer 3 switch.
This ensures a faster path from the device through to the ultimate destination.
Reliance on a Layer 2 switch configuration, if the IP subnet, or even the device, is not
present, on that switch, means that the data could end up at a router and require further
multiple hops before reaching its final destination; the whole process being noticeably slower
than directly switching out through the Layer 3 device.
Attention should also be paid to WiFi switches, particularly with regard to voice. With the
increasing use of mobile voice WiFi 802.11 standard WiFi technology, a WiFi access point is
also a switch.
Not all wireless access points support the voice standard required for WiFi voice. Radial
distance, or bandwidth, required for voice is shorter than that required for standard data. If
access points were in place to provide coverage for the whole office, and voice traffic was
routed over that area, additional access points may be required; or at least tuning of those in
place to provide better coverage.
Routers
Figure 1-9 shows three routers indicated by the Voice, default and WAN gateways. The
routers manage any information which cannot be handled by the switches; in the case of a
Layer 3 switch, the gateway will pass the information to the intended destination.
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The two most likely routes for that information are either via a voice gateway router to the
outside world, or via the wide area network router to other company locations. In the former
situation the information would be passed to the PSTN dial network through a primary rate or
basic rate ISDN. At this point any control over quality of service is handed off to the carrier
network.
In the latter situation, where the router passes information to the wide area network router, it
is necessary for users to put their own control mechanisms in place. This practice ensures
that the router devices support the auto class of service recognition. It also ensures that, as
they pass the information out across the wide area network, the relevant wide area network
carrier also supports the same class of service parameters.
This means that, as the traffic flows from the local area network, across the WAN, to the final
destination, class of service is maintained and equipment at the intended destination can
receive the information, maintaining quality of service throughout transmission.
Figure 2-1 QoS on the WAN
Cisco voice-enabled router
DSL or Fractional T1
Catalyst switch with
Catalyst switch with
Secure LAN features
Secure LAN
Gigabit
or
10/100
Ethernet
features
PSTN
Catalyst switch
with inline
power and
Secure LAN
features
IP WAN
Internet
PSTN
Desktops/laptops with
third-party anti-virus
software
Cisco distributed
CallManager
platform
Cisco voice
application servers
Main business location
Cisco IP Phone
XML apps or
IPCC Express
agent
POTS phones
Corporate servers
Cisco voice-enabled router
with firewall and VPN
Dedicated connection
Teleworker/remote access
Branch Office
DSL or Cable
Broadband
access modem Laptops/desktops w/Cisco
VPN Client, Cisco Secure
Agent and PC-based
SoftPhone
Fax
Catalyst switch
with inline
power and
Secure LAN
features
Cisco IP
Desktops/laptops with
Phone
third-party anti-virus
XML apps or
software
IPCC Express
agent
Figure 2-1 uses three scenarios to illustrate how quality of service might be achieved within a
business environment.
The main business location is where most of the local area network infrastructure would be
located; where call control servers are installed for an IP telephony environment or where the
main switch infrastructure is located and where the main outside routers would be connected.
For the shortest path, the best possible network configuration, the switches will, ideally, be
running Layer 3, the routers running auto-CoS and the shortest path is available from end
device to the outside world and to the ultimate destination.
A branch office can be connected in a number of ways. The optimum configuration would be
either through an IP WAN, with class of service invoked, and across the carrier PSTN
network, where quality of service can be guaranteed. With teleworkers or mobile workers,
class of service is not possible over the Internet; quality of calls over the network can
therefore not be guaranteed for those users.
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Testing QoS and setting benchmarks for performance
Even if Layer 2 switches are replaced with Layer 3 switches, and devices are configured to
offer the optimal routing path, ensuring that the quality of service is being achieved on the
network is an ongoing challenge.
The first step to overcoming this challenge is to establish a baseline for monitoring the
performance and delivery of QoS on the network.
The most effective way to establish a baseline is using a testing system known as mean
opinion score (the MOS test) which covers all aspects of the network and is not limited to
specific devices or components.
The MOS test
Figure 2-2 MOS testing
Figure 2-2 illustrates the results from the MOS test report for a company with six offices, each
of which has two IP addresses, shown on the left, which represent the wide area network
router and the wide area network switch.
From the subnet at the beginning, it can be seen that the address starts at 10.1.1, then 241
for the wide area network router and 253 for the switch.
The rankings range from 5 for ‘excellent’ to 1 for ‘bad’; these are used to judge voice quality.
The very top result is the overall score for the MOS test on that particular wide area network
and network infrastructure.
The performance is seen to be improving over time.
A score of 4.34 is judged to be ‘carrier quality’ and users are always very satisfied with this
level of service.
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3.6 is judged to be ‘business quality’ but some users might be dissatisfied with this score.
3.1 is where users are really dissatisfied.
The example in Figure 2-2 shows an overall ‘fail’ ranking due to the 10.8.1. location, where
both devices failed on separate tests.
The first device, 1.240, failed on jitter, (refer to the third column, in green), and the second
device, 253, failed due to loss of service. Looking above that at the 240, it will be noticed that
there were delays caused by jitter and loss.
On further analysis, it becomes evident that these failures were caused by the switch running
at half duplex which, in turn, was linked with other devices running at full duplex; a mismatch
in their configuration.
Due to that simple error on one particular switch, the entire network failed the test and quality
of service thus failed as well.
When looking at quality of service across the infrastructure, every node throughout the
infrastructure needs to be optimally configured and enabled for quality of service routing. This
means that adjustments to the infrastructure may be required to reduce data paths, and
network equipment may need to be upgraded to ensure that QoS can be delivered.
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Maintaining QoS and policy enforcement.
Maintaining quality of service, and ensuring policy enforcement, are two prime responsibilities
in ensuring an efficient and effective system.
One of the most successful techniques is that of conducting long-term trending of quality of
service across the network, so that MOS testing is deployed on a daily basis to maintain
quality of service running across the network.
Figure 2-3 Monitoring QoS
Figure 2-3 shows the overall testing results for a network over a four-day period.
At the top, in blue, is the overall MOS test result for the network. In green are shown the
results for delay, in yellow the results for jitter and in aqua, the results for packet loss.
The test was conducted over a four-day period (from 27th April), at the commencement of
which the system failed, dropping below the acceptable threshold for quality of service
delivery.
Configuration changes were made through the first day, with demonstrable improvements;
packet loss, jitter and delay were all reduced. By the end of the day a suitable level of service
had been established.
Nonetheless there were still peaks, outside the threshold parameters. On the afternoon of the
28th there was a spike in the delay which went above and beyond the threshold.
By monitoring such occurrences longer term, user’s expectations can be managed; interim
adjustments can be made to correct problem issues before the overall quality of service is lost
on the network.
Ongoing monitoring also permits remedial investigations if users complain of voice quality
problems after the event.
Long-term trending also serves as a useful tool for capacity planning for the future. If the
inclusion of additional users on the network is being considered, or if a new CoS-dependant
application is being rolled out, trending will enable realistic impact analysis.
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Conclusion
There are nine foolproof reasons why quality of service simply cannot be ignored.
Poor QoS configuration is the major cause of IP telephony and unified communications
project failure.
Under-optimised QoS causes end-user dissatisfaction. It has frequently proved to be the
case, with organisations undertaking an IP telephony pilot or a small project, that the
system does not performing to anticipated levels. Upon investigation it invariably emerges
that the infrastructure is not supporting quality of service to deliver the voice quality
required.
Critical applications need a guaranteed packet delivery time. As well as voice, this can
also apply to other critical applications such as the CRM, ERP, database and video
conferencing systems.
Quality of service may well require the replacement of the core infrastructure and/or at
least upgrades to components and certainly the configuration of the network.
QoS is not possible over the Internet. In the future, with IP6, this will be possible,
however, the estimated delivery date for IP6 is currently the year 2015.
QoS acceptance testing must be done prior to deployment. Acceptance testing is
essential to ensure that the quality of service is there; simply having an up-to-date and
correctly configured infrastructure is no guarantee that it will work.
Change control is a pre-requisite for maintaining QoS. All networks evolve and change
and the organisation must be fully and operationally cognisant of the fact.
IT staff must fully understand QoS and its significance to performance. If separate
personnel maintain the server infrastructure, with no responsibility overlap in the area of
IP telephony, they may not realise the impact of plugging a particular device into a
particular port on a switch. Quality of service will definitely be affected everywhere else.
The duplex mismatch mentioned earlier in this white paper is a classic example of such
an event.
Supporting and maintaining QoS requires specialist tools. This was demonstrated with the
MOS test results and also needs some comprehensive research and planning on how to
support it within the infrastructure.
About SAS
SAS Global Communications is a major provider of managed and professional network
services. The company provides consultancy and implementation services to design, build
and manage converged IP networks for enterprises of all sizes. Since its inception in 1989,
the company has successfully delivered more than 2000 highly commended solutions, to a
broad spectrum of clients including many renowned brands, such as Coca Cola Enterprises,
The Body Shop International and Millennium and Copthorne Hotels.
For more information contact:
SAS Global Communications Limited
SAS House
Blackhouse Road
Colgate, Horsham
West Sussex RH13 6HS
Tel: +44 (0) 1293 851951
Fax: +44 (0) 1293 852200
Email: [email protected]
www.sas.co.uk
Published: 1 July 2007
© SAS Global Communications Limited
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