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Resource Management in IP Telephony Networks
Matthew Caesar, Dipak Ghosal, Randy H. Katz
{mccaesar, randy}@[email protected]
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Motivation What is IP Telephony?
Packetized voice over IP PSTN access through Internet Telephony Gateway (ITG)
Benefits: Improved network utilization Next generation services (POTS PANS)
Growth: Revenues $1.7 billion in 2001, 6% of international traffic
was over IP, growing [Frost 2002] [Telegeography 2002] Standardized, deployed protocols (TRIP, SIP, H.323)
Requires scalable architecture to limit congestion.
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Goals High quality, economically efficient
telephony over the Internet. Low blocking probability Provide preferential treatment, high QoS
Questions: How to perform call admission control? How best to route calls through converged
network?
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Approach Mechanisms
Congestion sensitive call admission control
ITG selection Techniques
Awareness of ITG congestion
Path quality between important points in network
Dis
tance
ITG Utilization
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**
*
* *
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Overview IP Telephony Networks Pricing-based Admission Control Redirection Techniques Experimental Design Results Future Work
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System Architecture
ITG
LS
Example Call SetupExample AdvertisementGateway (ITG)
IP TerminalLocation Server (LS)
InternetAdmin. Domain (AD)
Example Call Session
ITGITG
ITG
ITG
ITG
ITG
LS
LS
LS
LS
LS
LS
1 2
3
4
5
6
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Scope of Study 1. All calls are net-to-phone2. ADs cooperate to provide service.3. Use IETF’s TRIP architecture to
support interoperability.4. Disregard degradation in access
network.5. Prices determined at start of call.6. ITGs offer equal PSTN reachability.
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Pricing PSTN
distance pricing time of day pricing
IP Telephony richer user interface allows for more dynamic pricing
schemes Baseline: Flat-rate Admission
Control (FAC)
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Congestion Sensitive Call Admission Control (CAC) Goal: prevent system overload and
generate revenue Price of call
function of number of voice ports in use
rises when highly utilized More dynamic than PSTN
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Price-Congestion Function Used M/M/m/m (m-
server loss system) responsive server loss system discouraged arrivals
Found price-congestion function that maximized revenue with respect to
0
1
2
m-1
m
...
m-1
m
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Congestion Pricing Analysis Exponential function generates most
revenue Stepwise linear function almost as good
Maximum system price charged early Approximation to function minimizes price
fluctuationsPrice-congestion Function Used in this Study
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60Utilization [voice ports]
No
rmal
ized
Pri
ce C
har
ged
Revenue-maximizing Price-congestion Function
00.10.20.30.40.50.60.70.80.9
1
0 10 20 30 40 50 60Utilization [voice ports]
No
rma
lize
d P
ric
e C
ha
rge
d
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Redirection Problem: finding the “best” ITG Approach: tradeoffs between quality and load Method: LS maintains
Average measured path quality Number voice ports in use
Algorithms: Random Redirection (RR) (baseline) QoS Sensitive Redirection (QR) Congestion Sensitive Redirection (CR) Hybrid Scheme (CQR)
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Redirection Schemes QoS Sensitive Redirection (QR)
Different paths provide different service Technique:
Use RTCP RRs to monitor path congestion Route over best paths
Congestion Sensitive Redirection (CR) Unbalanced load causes call blocks Technique:
Use TRIP advertisements to estimate ITG utilization
Route to least utilized ITG
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Hybrid Redirection (CQR) Choosing nearby ITG improves call quality, but
can unbalance load. Algorithm:
Compute Rdm = *Mi+(1-)*Qi Mi is utilization, Qi is loss rate
Select randomly from k ITGs with lowest Rdm Tradeoffs:
Use to trade off call quality and load balance Use k to vary flash crowd protection
Price Sensitive CQR (PCQR) Decrease for higher bids
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Overview IP Telephony Networks Pricing-based Admission Control Redirection Techniques Experimental Design Results Future Work
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Experimental Method Modified ns-2 Ran for 1.5 simulated hours
Eliminated first half-hour User Model
Bid uniformly distributed Voice traffic on-off Markov process
Self-similar cross-traffic Data points stable across several time
scales
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Evaluation: Metrics Blocking Probability Average call QoS
Used Mean Opinion Score (MOS) based on RTP loss rate
Economic efficiency Ratio of service tier to QoS achieved
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Admission Control: Blocking Probability
Flat pricing unnecessarily blocks many callers
Congestion pricing changes system price dynamically with load
Call Blocking Probability
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.2 0.4 0.6 0.8 1Offered Load
Blo
ckin
g P
rob
ab
ilit
y
QR+FAC
QR+CAC
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Call Blocking Probability
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Beta
Blo
ckin
g P
ro
bab
ilit
y
CQR+NAC k=1CQR+NAC k=3CQR+NAC k=6RR+NAC
Redirection: Blocking Probability
Congestion sensitivity decreases blocking probability Small k few blocked calls Congestion Sensitive Redirection (CR) improves balance over
Random Redirection (RR)
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Redirection: Background Traffic Effects
Effects of Background Traffic
0
1
2
3
4
5
6
0 1 2 3 4 5Background Traffic Multiplier
Qo
S [
MO
S]
CQR+NAC Beta=0
CQR+NAC Beta=0.9
CQR+NAC Beta=1
RR+NAC
QoS sensitivity minimizes effects of cross traffic Small amount of sensitivity vastly
improves call quality
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Summary Admission Control Schemes:
Congestion sensitive pricing decreases unnecessary call blocking, increases revenue, and improves economic efficiency
Derived exponential price-congestion function that maximizes revenue
Redirection Schemes: Hybrid scheme achieves “best of both worlds” Price sensitivity improves economic efficiency
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Future Work Realistic workload Improve user model
Develop price-congestion function for real users
Study flash-crowd effects ITG Placement Competitive Network
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Resource Management in IP Telephony Networks
Matthew Caesar, Dipak Ghosal, Randy H. Katz
{mccaesar, randy}@[email protected]