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Media Gateway
Benoit Bégué
2006 Study for EE department. EE526 with Professor Dan Keun Sung
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Table of Content
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Introduction: what is a Media Gateway ?
Operation principles
Related standard
Performance issues
Problems in performance measurements
Experiment environment
Tests and results
Others: quick market analysis
Conclusion
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What is a media Gateway ?
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Media Gateway :translation unit between disparate
telecommunications networks such as
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PSTN
Next Generation Networks
2G, 2.5G and 3G radio access networks
PBX
Media Gateways enable multimedia communications
across Next Generation Networks over multiple
transport protocols such as ATM and IP
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Operation principles
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A couple translating and linking different
protocols and technologies:
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Gatekeeper or Media Gateway Controller (MGC)
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deals with the signaling
Media Gateway (MG)
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deals with the data/voice
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Related standards : signaling protocols
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Signaling protocol between an MGC and gateways :
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ITU-T Rec. H.323. This is employed where all network
elements (NEs) have software intelligence.
SIP (Session Initiation Protocol, Ref. 2) is used when the
end devices have software intelligence and the network
itself is without such intelligence.
MGCP (media gateway control protocol) is another
gateway control protocol.
MEGACO (ITU-T Rec. H.248, Ref. 13) is a gateway control
protocol applicable when end devices are without software
intelligence and the network has software intelligence.
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Related standards : voice codecs
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The terminals may have different capabilities.
That’s why we need a negotiation about
terminal compatibility.
PCM, ADPCM, LPC, AMR, A-CECP,G.7xx
H.245 is used to negotiate service
capabilities between terminals and gateway
controllers
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Related standards : terminations on a GW
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Terminations are logical entities on a GW:
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Analog telephone line
Digital telephone (64 Kbps)
T1 trunk line (1.544 Mbps)
E1 (2.048 Mbps)
RTP (VoIP)
Example of Cisco T1 card interface (RJ48 port)
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Performance analysis
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Reference Paper
A Study of IP Network Impairments Impact on
Media Gateway Performance
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Sherry Wang, Luis Nieto, Edward Zielinski
Next Generation Networks, Bell-Labs, Lucent Technologies
Ask one question :
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Can voice quality be guaranteed in a connectionless and
best effort data network ?
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Performance issues
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A GW involves many complex processes:
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packetization / depacketization of voice frames
jitter smoothing
delay
error treatment (lost packets)
If Quality of Service (QoS) engineered IP
network is available, the GW is a bottleneck
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Problems in performance measurements
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Voice quality is defined as the qualitative and quantitative
measurement of the sound and conversation quality.
 Subjective !
Mean Opinion Score (MOS)
 Gives a subjective opinion score
 expensive in terms of time and human efforts
Perceptual
Speech Quality Measurement (PSQM)
 defined by ITU-T Recommendation P.861
 Repeatable
 Objective
 Reasonably inexpensive
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More about PSQM
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PSQM compares an original voice signal to a
received copy of this signal
PSQM measures the dissimilarity of the
received signal to the original signal from
human auditory perceptions.
A lower PSQM score implies higher similarity
between two signals (good !). The score of
zero corresponds to equality of two signals to
the human ear (excellent !).
Big PSQM score -> bad !
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Experiment environment
Performance
measurement
tools (acts as
2 terminals)
IP
Network
(can simulate
Impairments)
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Experiment environment
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G.711 vocoder was used through out this study
The choice of the number of voice frames per IP
packet is a trade off between a fast delivery and
network bandwidth efficiency. In this study, 10ms
voice frame per RTP packet was used as
recommended by the manufacture.
An adaptive jitter buffer was used to accommodate
jitter smoothing and reduce delay at the same time.
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Tests
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1st: Analog communication: only switch
2nd: Analog and IP communication (using
switches and gateways) in ideal IP network
condition
3rd: 2 with impaired IP network
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Delay
Jitter
Packet Loss
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Tests 1 & 2
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Theory for IP Network Delay Impact (on
Voice Quality)
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The end-to-end delay or the speech latency
can be summarized as a function of
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gateway performance g
IP network hop behavior h
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Dpkt is packet-processing delay
Th is transmission delay
Qh is queuing delay
Ph is propagation delay
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Theory 2
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The hop behavior is replaced by a fixed delay D(i)
generated by the IP network emulator.
D(i) is a constant in ith test, i = 1, 2...
• N is the number of frames per packet –constant in this study• f is the frame size
C was measured at approximately 26.5ms
that is about 6ms in switch and about 10ms in each gateway
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Results for IP Network Delay Impact (on
Voice Quality)
D(i) was selected from 10ms to 70ms with 10ms increments.
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Theory for IP Network Delay Variation
Impact (on Voice Quality)
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Quality is affected by not only the end-to-end delay,
but also the delay variation = jitter
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gateway jitter: J(di, ji)
where di is a probabilitydistribution
 ji is an average jitter value in ms for the ith test.
A uniform distribution and an Internet distribution
were used.
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Theory 2
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An adaptive jitter buffer is used in a gateway
to effectively smooth out jitter without
introducing too much delay
Speech latency is affected if
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Jitter exceeds buffer capacity
Jitter triggers buffer adaptations frequently
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Results of impact of IP Network Jitter
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Theory of IP Network Packet Loss
Impact
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Voice quality, in terms of intelligibility, is
directly affected by packet loss. Two types of
packet loss can occur:
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Single-packet loss: occasional bit error on the
transmission facility
Consecutive packet losses: due to traffic
congestion
We will see which one is worst
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Results for IP Network Packet Loss
Impact
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Others: Market analysis
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Cisco Systems is the leader for general VoIP
network equipment
Huawei is the current market leader for media
gateway port shipments for 2005
With a projected increase of over 300% in the
next 5 years and a total world market of over
$8.7 Billion, demand for Media Gateways
signal the accelerating transition from PSTN
to VoIP
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Conclusion
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The network delay affects only the conversation quality, not the
voice quality
 This is achieved through an optimal selection of a jitter buffer size
and a packet drop rate in a gateway
The burst packet loss is more detrimental to voice quality than
the single-packet loss
What is missing in this presentation:
 Codec comparison
 and their echo cancellation capability
From a network-service provider’s perspective, simple system
configurations and consistent parameter settings are desirable
features of a gateway and of other network components for a
speedy VoIP network deployment.
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