1. Assignment
Measurement Based VoIP Quality
Management
Sub: IP Telephony
ETE: 605
By:
Nuruddin Ahmed
ID: 063 429 056
Sec – 1
Submitted to:
Dr. Mashiur Rahman
3. Contents
1 General Information ..................................................... 4
1.1 Purpose............................................................ 4
1.2 Terms and Concepts ........................................ 4
2 Function ........................................................................ 6
2.1 Function Summary ........................................... 6
2.2 Quality Monitoring on Network Path Groups .... 7
2.2.1 Network Path Groups ....................................... 7
2.2.2 Quality Measurements and Categories ............ 9
2.2.2.1 Packet Loss........................................................ 9
2.2.2.2 Jitter ............................................................... 11
2.2.2.3 Number of Monitored Channels Per Network
Path Group ..................................................... 12
2.2.3 Performance Measurement............................ 13
2.2.4 Quality of Service Alarm................................. 13
2.3 Route Selection Based on Measured QoS..... 13
2.3.1 Routing Restriction Based on Measured QoS 14
2.3.2 Determination of Restriction Level for Each
Network Path Group....................................... 15
2.3.3 Transit and Diversion Node Considerations... 16
3 Operational Conditions ............................................. 18
3.2 Operation and Maintenance ........................... 18
3.2.1 System Management ..................................... 18
3.2.1.1 Configuration Operations ............................... 18
3.2.1.2 Performance Information................................ 18
3.2.1.3 Alarms ............................................................ 18
3.2.1.4 Performance Measurement............................ 19
3.3 Characteristics ............................................... 19
4 Compliance with Standards ...................................... 20
5 Acronyms and Abbreviations ................................... 21
6 References.................................................................. 22
4. 1 General Information
1.1 Purpose
Measurement Based VoIP Quality Management will be a node local function that provides
the ability to select the best path through the network for voice channels, based on quality
measurements on active Real-Time Transport Protocol (RTP) voice channels.
Two levels of service will be provided:
• Monitoring quality of service on paths
• Route selection based on monitored quality of service on paths.
1.2 Terms and Concepts
Channel An RTP channel between source and destination nodes is identified by source
and destination IP addresses and UDP port numbers. The Synchronization
Source (SSRC) uniquely identifies the source within this channel and can be
used to detect changes in the connection, for example, diversion.
Codec Codec is a shortening of coder-decoder. The codec is responsible for the
coding and decoding of the voice samples. The codec can provide
compression of the data and special handling for silence suppression and
comfort noise generation.
Delayed backward setup
Delayed backward setup is a channel establishment process where the media
gateway waits for the remote media gateway bearer interworking function
address and backbone network connection identifier before seizing an
interface.
Ephemeral termination
An ephemeral termination is a temporary termination. It is created for a
connection and it only exists while the connection exists.
Fast forward setup
Fast forward setup is a channel establishment process where the media
gateway seizes an interface without knowing the backbone network
connection identifier.
5. Hardware element
A hardware element is either a Voice Processing Board or a Packet
Termination Board. The Packet Termination Board is used for G.711 without
silence suppression or packet loss concealment. All other codec types use the
Voice Processing Board.
Jitter The variation in packet arrival time, as determined by comparing the received
RTP timestamp and the local clock. The jitter is calculated in accordance with
the RTCP definition within RFC 1889 (Ref. [1]).
Masking process
Masking of IP addresses is used in the definition of Network Path Groups as
defined in Section 2.2.1. The masking process allows source and destination
voice application IP address prefixes to be used to identify the path of the
RTP voice channels through the network.
Media loss
Media loss is a specific type of packet loss. Media loss occurs when no
packets are received for a specific time.
Network Path Group
An operator-configurable group determined by the destination and source
voice application IP address prefixes. This can be managed to be the first and
last router along the path of the RTP voice channels.
Packet loss Packet loss is when packets are lost or discarded in the network and therefore
do not arrive at the receiving Media Gateway.
Packet loss is determined by comparing the number of received packets
against the number of expected packets. The number of expected packets is
found by comparison of sequence numbers in the RTP header. This is
performed as defined in RFC 1889 (Ref. [1]).
It should be noted that G.711 without silence suppression does not support
packet reordering on reception and therefore discards packets that are out of
order.
Restriction Provides the ability to reject a percentage of calls that are attempting to use a
Network Path Group. Rejected calls will fail or take alternative paths.
6. 2 Function
2.1 Function Summary
Measurement Based VoIP Quality Management provides the ability to select the best path
through the network for new RTP voice channels based on quality measurements on active
RTP voice channels.
Quality measurement is provided by a passive quality monitoring function which takes
measurements on established RTP voice channels (calls) in order to determine the current
quality of service being experienced. The quality of service is determined by the percentage
packet loss and the jitter. Measurements are grouped according to operator-defined Network
Path Groups. These groups are defined by the source and destination IP prefixes and can be
configured to correspond to the first and last router along the paths of RTP voice channels.
Selection of best path is provided by a route selection function. This function takes the
information provided by the monitoring function and actively uses it to influence the IP and
MPLS routing path selection. This is achieved by applying a restriction level to each
Network Path Group.
If the Network Path Group is showing good quality, the level of restriction will be reduced.
If the Network Path Group is showing poor quality, the level of restriction will be increased.
At allocation of a channel, the restriction level will be considered and may result in the
chosen path being rejected. Alternative routings will be taken, if available under these
conditions. Semipermanent connections and data calls will always be established, regardless
of the current restriction level.
Measurement Based VoIP Quality Management has the following levels of operation:
• Off
- Function not active.
• Monitoring
- Quality Monitoring is active.
• On
- Quality Monitoring and Route Selection is active.
7. 2.2 Quality Monitoring on Network Path Groups
Monitoring of Quality of Service (QoS) takes measurements of jitter and percentage packet
loss on a sample of active channels. This information is grouped according to the path that
the packets have taken for this channel. This grouping is termed the Network Path Group.
Grouped information is presented via MEASUREMENT BASED VOIP QUALITY
MANAGEMENT SYSTEM and is available for performance monitoring.
2.2.1 Network Path Groups
This function monitors the quality of the network. Within this function, the network is
considered to be a collection of paths determined by the local and remote IP addresses. These
paths are grouped by screening the IP addresses by operator defined masks. Optimal
grouping of these paths occurs when the masks correspond to the first and last routers along
the paths of the voice channels. Any channel that has common local and remote voice
interface IP addresses after the masking process will be measured in the same Network Path
Group.
In order to simplify the mask definition process a default mask is provided that is applied if
no specific masks are defined. The default mask can be changed from the preset value by the
operator. There is no restriction on the definition of the default and specific masks, for
example, IP prefix lengths of 20, 24, or 32 bits can be defined.
Network Path Group identities only exist whilst the Network Path Group is in regular use. If
a Network Path Group is inactive for more than one day, it will be removed from the system,
and all associated data are lost.
Setting the Administrative state of Measurement Based VoIP Quality Management to Off
makes all Network Path Groups undefined.
The example in Figure 1 illustrates the network path grouping function.
8. Figure 1
Example network with IP Address
If the default IP mask selected was 255.255.255.0 then this would result in the following
Network Path Group allocation in Media Gateway 1.
Table 1
Possible Network Path Group configuration for MG-1 for the network in Figure 1
9. This is only one possible configuration for the Network Path Groups defined for Media
Gateway 1. It is important that multiple paths are not combined in the same group. This
would happen if the prefixes and masks are set so that data from more than one router is
combined in the same Network Path Group. One way this could occur in the above
example is if the default mask were set to 255.255.204.0. This would result in the
measurements for routers 2, 4, and 5 being combined in MG-1. The information collected
from such a group will be impossible to interpret correctly, as one path or router may be
experiencing different degradation levels from the others. The result would be a
measurement that would be an average of the information for routers 2, 4, and 5 and may
not be correct for any of them.
Paths directly connected to Media Gateways are locally monitored by the Media Gateway
interface and therefore do not need to be supervised by this function. Thus the optimal
configuration for this function is to define the IP masks, such that the Network Path Groups
correspond to the first and last router along the path of the voice channels in the network.
2.2.2 Quality Measurements and Categories
When monitoring of QoS is active, individual RTP channels are monitored for packet
loss and jitter. Packet loss and jitter are derived from information in the RTP protocol.
Channels are monitored from channel establishment until channel release. The number of
channels that can be monitored by a hardware element is limited. In order to ensure that
the monitoring capacity is distributed between the Network Path Groups, a limit is
applied. This limit allows monitoring of up to 20 channels per Network Path Group per
hardware element at any one time. If no resources are available for monitoring at channel
establishment or the monitoring limit has been reached for the hardware element, that
channel will not be monitored for the duration of the channel allocation.
Each monitored channel continuously collects data related to the packet loss and the jitter
being experienced on that channel at a fixed period of 5 seconds.
Data collected against individual RTP channels are allocated to the Network Path Group
related to the local and remote voice application IP address for the channel.
2.2.2.1 Packet Loss
Packet loss is recognized by gaps in the received packet stream, using the sequence
numbers in the received packets. Gaps are determined by counting the received valid
packets and calculating how many packets should have been received. The amount of
expected packets is found by subtracting the last received packet sequence number from
the first received packet sequence number since the last measurement.
Packet loss is expressed of a percentage of the expected packets with a resolution of 0.1
percent.
Media loss occurs when no packets are received for a defined period of time. This
function provides the option of detecting media loss. Media loss is not detected in the
same way as packet loss, as there is no way to determine the expected number of packets.
Media loss is detected when there has been no change in the last received RTP sequence
10. number within the last 5–10 seconds, that is, no new packets have been received. Media
loss detection should only be applied in networks where media loss indicates a network
failure, for example, media loss detection should not be activated in multimedia networks,
as multimedia networks may not send packets when there is no speech activity.
If media loss detection is enabled and media loss is detected, the channel is considered to
have a packet loss category of Faulty and 100% packet loss.
Percentage packet loss is presented as both a weighted average and as a category.
Weighted Average Percentage Packet Loss
This value is a weighted average percentage packet loss for all monitored channels using
a Network Path Group within a Media Gateway.
The value is presented on MEASUREMENT BASED VOIP QUALITY
MANAGEMENT SYSTEM pages and is available for performance monitoring. The
weighted average is recalculated every 5 seconds:
WAPPL: = WAPPL + (Current Percentage Packet Loss – WAPPL) / 6
where
WAPPL = Weighted Average Percentage Packet Loss
and
Current Percentage Packet Loss =
= Average value of percentage packet loss for all monitored channels over the last 5
seconds
Percentage Packet Loss Category
The percentage packet loss category is a representation of the current quality being
experienced by the Network Path Group. It is defined by six categories with thresholds
that can be set by the operator.
11. Figure 2
Percentage packet loss categories
The threshold levels can be configured by the operator. Consideration should be given to the
codecs and use of packet loss concealment when configuring threshold values, as some
codecs are more affected by packet loss than others. The default values have been assigned to
provide high quality voice characteristics for G.711 without silence suppression or packet
loss concealment.
2.2.2.2 Jitter
Jitter is caused by variations in the relative transmission times of packets. This is
determined by comparing the RTP timestamp with the local clock at successive packet
arrivals. The jitter calculation is compliant with the RTCP definition of jitter in Ref. [1].
Jitter on an individual channel is expressed in terms of the clock frequency, that is,
multiples of 125 μs.
Jitter is presented both as a weighted average and as a category.
Weighted Average Jitter
This value is a weighted average jitter for all monitored channels using a Network Path
Group within a Media Gateway. The jitter is expressed in microseconds and may have a
value other than a multiple of the 125 μs period, due to the averaging activity.
12. The value is presented on MEASUREMENT BASED VOIP QUALITY
MANAGEMENT SYSTEM pages and is available for performance monitoring. The
weighted average is recalculated every 5 seconds:
WAJ := WAJ + ( Current Jitter – WAJ ) / 6
where
WAJ = Weighted Average Jitter
and
Current Jitter =
= Average value of jitter for all monitored channels over the last 5 seconds
Jitter category
The jitter category is a representation of the current quality being experienced by the
Network Path Group. It is defined by six categories with operator definable thresholds.
Figure 3
Jitter categories
2.2.2.3 Number of Monitored Channels Per Network Path Group
When percentage packet loss and jitter are presented in the MEASUREMENT BASED
VOIP QUALITY MANAGEMENT SYSTEM, additional information regarding the
number of monitored channels per Network Path Group is also available. This
information is expressed as a weighted average in the same way as percentage packet loss
and jitter so that the data can be related. The value is recalculated every 5 seconds:
13. WAC: = WAC + (Current Number of Channels - WAC) / 6
where
WAC = Weighted Average Number of Channels
and
Current Number of Channels =
= Average number of channels monitored over the last 5 seconds
2.2.3 Performance Measurement
The Measurement Based VoIP Quality Management system provides functionality for
regular measurements on counters and gauges. In this context, Measurement Based
VoIP Quality Management provides the following performance measurement types on a
per Network Path Group basis:
• Average percentage lost packets
• Average jitter
• Average number of channels
• Restriction.
The counters are reset when read for performance monitoring purposes.
Only one performance measurement period can be applied to a Network Path Group at a
time.
2.2.4 Quality of Service Alarm
A quality of service category alarm alerts the operator when a Network Path Group is
experiencing quality degradation. The alarm is issued if either the percentage packet loss
category or jitter category reaches a level that can be specified by the operator. Hysteresis
is provided by a second category at which the alarm will be cleared.
An alarm administrative state is provided which allows the operator to turn the alarm
handling on and off. Any active alarms will be terminated if the Administrative state is set
to Off.
2.3 Route Selection Based on Measured QoS
When the required bandwidth across a Network Path Group exceeds the available
bandwidth, a reduction in quality is detected by an increase in the percentage packet loss
and/or jitter measurements. In order to return the Network Path Group to an acceptable
quality level the system will reduce the bandwidth being passed over this path.
Reduction in bandwidth is achieved by restricting the rate at which new channels are
allowed to be established over this path. The restriction is applied on a percentage basis,
that is, if the Network Path Group has a restriction level of 30% then 3 out of 10 channel
establishments attempting to use this Network Path Group will be rejected. Any rejected
channel establishments can use alternative paths, if available.
14. 2.3.1 Routing Restriction Based on Measured QoS
When a path (IP interface or LSP pair) is selected to carry a voice channel, the
Measurement Based VoIP Quality Management function checks if the available local and
remote IP addresses match any of the existing Network Path Groups.
If the local and remote IP addresses provide a Network Path Group, this will be checked to
determine if the call should be allowed to proceed using this path. A percentage restriction is
applied to each Network Path Group. If the current restriction level denotes that this call
should not be allowed to proceed over this Network Path Group then the current path will be
rejected and an alternative path selected. If all alternative paths have been attempted, the call
will be rejected, as there is no path with sufficient quality from this node to the required
destination.
Figure 4
Example network path group allocation with restriction levels
For example, in the network in Figure 4, a channel may be being allocated from Media
Gateway 1 to Media Gateway 2. Media Gateway 1 will attempt to select interface A due to
this being the preferred routing selection to reach Media Gateway 2. This would result in the
call using Network Path Group 1. This Network Path Group is currently showing a restriction
level of 100%. This means that this path cannot be used for the call. An alternative routing
selection is then made via interface B. This results in Network Path Group 3 being used,
which shows no restriction and the call is therefore allowed to proceed using this path.
In the case of delayed backward setup, the restriction is applied at modification of the
ephemeral termination in the originating Media Gateway. No restriction will be applied
in the terminating Media Gateway for delayed backward setup.
In the case of fast forward setup, the restriction will be applied at reservation of the
ephemeral termination in the terminating Media Gateway. No restriction will be applied
in the originating Media Gateway for fast forward setup.
15. Network Path Groups are activated by the first channel that uses the Network Path Group.
Subsequent channels using the Network Path Group can be subject to monitoring and routing
restriction.
2.3.2 Determination of Restriction Level for Each Network Path
Group
Each Network Path Group classifies the percentage packet loss and jitter into
corresponding categories every 5 seconds.
If a Network Path Group has moved to a lower quality category, this is reported
immediately.
Improvements in quality category must be received over two consecutive 5 second
periods within each hardware element before the change is reported. Any reported change
is acted on immediately.
This asymmetric treatment of quality improvement and deterioration prevents oscillation
in the system when a Network Path Group is on a category boundary.
The jitter category and percentage packet loss category are compared, and the worst
category, that is, lowest quality, will be used to modify the existing restriction level for
the Network Path Group. For example, if a Network Path Group determines that the
Percentage Packet Loss is within the ‘Good’ category but the jitter is within the ‘Poor’
category, the ‘Poor’ category will be used to modify the percentage restriction value.
Each category modifies the percentage restriction on a Network Path Group as described
in Table 2.
Table 2
Category – percentage restriction relationship
The effects of the percentage packet loss category and jitter category on the restriction of
a particular Network Path Group are illustrated in Figure 5.
16. Figure 5
Example of effects of percentage packet loss and jitter on percentage restriction
2.3.3 Transit and Diversion Node Considerations
Measurements are taken on percentage packet loss and jitter for each Network Path
Group. The Network Path Group is determined by the local and remote IP addresses. If
individual channels within a Network Path Group are diverted, deflected or passed
through a transit Media Gateway, measurements on these channels may cause
degradation in the accuracy of Network Path Group information.
17. Channels passing through diverting and transit nodes will affect the collated information
on a Network Path Group.
Figure 6
Example network for Media Gateway diversion or transit situation
The proportion of channels that are diverted and transited determines if the interference
to measurements is significant.
When viewed from the MG-1 (Figure 6) the Network Path Group being measured is
between routers R1 and R2. Channels that are diverted or transited onto the R3-R4 leg
will result in the quality of the R3-R4 leg being measured along with the R1-R2 leg, as
the RTP session is relayed by MG-2.
The effect of this is that any channels diverted or transited onto R3-R4 will be counted
with the R1-R2 Network Path Group. If the proportion of channels being diverted or
transited is low, then the effect of the R3-R4 leg will be diluted by the measurements on
the R1-R2 leg, for example, if 10% of channels are diverted or transited, then the effect
on the Network Path Group R1-R2 will not be significant.
It is not recommended to use Measurement Based VoIP Quality Management when the
proportion of diverted or transit calls is greater than 10 percent.
18. 3 Operational Conditions
3.2 Operation and Maintenance
3.2.1 System Management
Application functions are provided for management of all Measurement Based VoIP
Quality Management MIB objects.
3.2.1.1 Configuration Operations
The following items are configurable via the management interface:
• Functional operation level
• Support of media loss detection
• Jitter category thresholds
• Percentage packet loss category thresholds
• Restriction characteristics per category
• IP prefixes and masks for Network Path Groups.
3.2.1.2 Performance Information
The following performance information is available, on a per Network Path Group basis,
via the management interface. Both current and weighted average information is updated
every 5 seconds. The current information is a reflection of categories and restrictions for
the last 5 seconds only. The weighted average information is based on historical
measurements but the information is weighted towards the last received information.
• Current percentage packet loss category
• Current jitter category
• Current percentage restriction level
• Weighted average percentage packet loss
• Weighted average jitter
• Weighted average number of channels.
Detailed definitions of the category and weighted average information are included in
Section 2.2.2.
3.2.1.3 Alarms
An alarm is provided that is initiated if measurements on a Network Path Group show
that the quality level is below an operator-defined threshold. Alarm handling is
configured with the following parameters:
• Alarm administrative state
19. • Jitter and percentage packet loss category for alarm initiation
• Jitter and percentage packet loss category for alarm cancellation Hysteresis is
provided by separate initiation and cancellation categories.
3.2.1.4 Performance Measurement
Measurement Based VoIP Quality Management provides the following performance
measurement types on a per Network Path Group basis:
• Average percentage lost packets
• Average jitter
• Average number of channels
• Current percentage restriction level
Parameters are reset when accessed for performance monitoring purposes.
Only one performance measurement period can be applied to a Network Path Group at a
time.
3.3 Characteristics
Please configure this function with care, see below.
This function will detect degradation on received channels. This information will be used
to influence routing for outgoing channels. This works well for MPLS and small or
hierarchical, pure IP networks. The function must be applied with care in large or meshed
pure IP networks, as there is only a limited relationship between the forward and
backward paths through the network. For example, the measured channel may exhibit bad
quality but the channel in the other direction takes a different path through the network
which is working perfectly.
For delayed backward setup, the restriction will be applied at modification of the
ephemeral termination in the originating Media Gateway. No restriction is applied in the
terminating Media Gateway for delayed backward setup.
For fast forward setup, the restriction will be applied at reservation of the ephemeral
termination in the terminating Media Gateway. No restriction is applied in the originating
Media Gateway for fast forward setup.
Activation of this function has an impact on call processing capacity. The effect is
minimum when the function is monitoring only. The effect is increased when restriction
is being performed.
Packet forwarding capabilities are not affected by this function.
20. 4 Compliance with Standards
Jitter and packet loss are derived as defined for the corresponding RTCP fields in RFC
1889 Ref. [1]. This usage of RTCP fields does not result in general support of RTCP.
RTCP is only supported for test calls.
21. 5 Acronyms and Abbreviations
IETF Internet Engineering Task Force
IP Internet Protocol
LSP Label Switched Path
MG Media Gateway
MPLS Multi-Protocol Label Switching
QoS Quality of Service
RTCP Real-Time Transport Control Protocol
RTP Real-Time Transport Protocol
SSRC Synchronization Source
UDP User Datagram Protocol
WAC Weighted Average Number of Channels
WAJ Weighted Average Jitter
WAPPL Weighted Average Percentage Packet Loss
22. 6 References
1. IETF RFC 1889, RTP: A Transport Protocol for Real-Time Applications
