Transcript
SIP Pee
ring KPI’s
- How
to Measure Answer
Seize Ratio [ASR] and
other Key Performance
Indicators in a VoIP
World
Teraquant Corporation
2400 Central Ave, Suite P-2
Boulder
CO 80301
Phone: +1 719 488 1003
Fax: +1 303 547 3248
www.teraquant.com
White Paper
SIP Peering KPI’s
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IXC’s always measure KPI’s such as ASR [Answer Seize Ratio]
or Post Dial Delay at SS7 Interconnects with other operators.
This allows them to optimize Billable MOU’s versus cost of
termination.
Service providers have for many decades measured key performance indicators
for their SS7 interconnects with long-distance or international operators or
peering partners. Such measurements are defined in ITU-T Recommendation
E.411 "International Network Management
–
Operational Guidance" and E.422
"Quality of Service for Outgoing International Calls" and include Answer Seize
Ratio [ASR], Post Dial Delay [PDD] and Network Efficiency Ratio [NER].
Name
Description
Counted Number of calls
ASR Answered calls (percent)
SSB Subscriber busy (percent)
CGC Circuit Group Congestion (percent)
SEC Switching Equipment Congestion (percent)
CFL Call Failure (percent)
RSC Reset Circuit Signal (percent)
UNN Unallocated Number (percent)
ADI Address Incomplete (percent)
CLF Clear Forward (percent)
LOS Line Out of Service (percent)
rt Response time (average)
wt Wait time with answer (average)
wna Wait time with no answer (average)
ct Call time (average)
ht Hold time (average)
Minutes Total Call time in minutes
Table 1
SIP Peering KPI’s
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Answer Seize Ratio [ASR] is the common measure for call
completion but it discounts calls that are not answered by
the Callee. This is not necessarily a network fault but it does
depress the success rate.
Answer Seize Ratio [ASR] is used as a measure of network quality although the
measurement also includes user behavior. In other words, if the call was not
answered, the network could not be faulted, although the ASR measurement
would be reduced by the uncompleted call, indicating lower quality. However,
because for a given hour within a day, unanswered calls would always represent
the same percentage, from day-to-day, this offset would be normalized out and
carriers are able to monitor the trend of ASR and treat it as a relative
measurement
Network Efficiency Ratio is a measurement that more
accurately shows calls succeeding and calls failing due to
network errors.
Network Efficiency Ratio
was
designed to eliminate the impact of user
behavior on the measurement and better represent pure network performance.
SIP is a more flexible protocol and has wider uses than simple call control.
Therefore, use cases differ widely and systems such as voicemail and call
forwarding skew expected behavior for answered calls
Furthermore, SIP is a more flexible protocol and has wider range of response
messages which can be used to specifically indicate certain types of failures from
either servers, network devices or the movement or absence or other behavior of
users. Accordingly, IETF has defined KPI's equivalent to Answer Seize Ratio and
Network Efficiency Ratio. These are described under SIP End-to-End Performance
Metrics
draft ietf-pmol-sip-perf-metrics-04
. For example, the equivalent of ASR is
Session Establishment Ratio (SER)
Network Efficiency Ratio [NER] is defined as:
User Answers or Normal call clearing - Cause code: 16
+ User Busy - Cause code: 17
+ Ring No Answer - Cause code: 18 & 19
+ Terminal Rejects) - Cause code: 21
NER
= -----------------------------------------------------------------------------------
x 100
(Total # of Call Attempts i.e. IAM’s)
SIP Peering KPI’s
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Session Establishment Ratio (SER) is defined as follows, to quote the IETF
Here is the message flow which defines the SER.
UA1 UA2
| |
|INVITE |
+-------------------->|------------------>|
| | 180|
| |<------------------|
Session Established | |
| | |
| | 200|
+-------------------->|<------------------|
| |
A more realistic representation of a SIP call setup is shown below using IPTEGO's
Palladion message flow presentation
IPTEGO Palladion’s Messag
e Flow
The SER is calculated using the following formula:
# of INVITE Requests w/ associated 200 OK
SER =
--------------------------------------------
x 100
(Total # of INVITE Requests)
-
(# of INVITE Requests w/ 3XX Response)
SIP Peering KPI’s
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In SS7, the fate of the call is determined by the RELEASE message which ends
the call. The fate of a SIP call is determined by the Response to each Request for
each transaction within the SIP call. For example as above, session establishment
is determined by a successful outcome from the call session establishment phase
Similar to NER in SS7 ISUP, Session Establishment
Effectiveness Ratio (SEER) is SIP INVITES which successfully
setup a voice call plus busy scenarios or VoIP Phones has
been moved i.e. the network has done its job
Similar to NER in SS7 ISUP, SIP End-to-End Performance Metrics also defines
Session Establishment Effectiveness Ratio (SEER)
This metric is complimentary to SER, but is intended to exclude the potential
effects of the terminating UAS from the metric and therefore more closely reflect
the performance of the network. SEER is defined as the number of INVITE
requests resulting in a 200 OK response and INVITE requests resulting in a 480,
486, or 600; to the total number of attempted INVITE requests less INVITE
requests resulting in a 3XX, 401, 402, and 407 response.
In order to simplify the formula, the following variable ‘a’ is us
ed to summarize
multiple SIP responses
All these KPIs for both SS7 ISUP and for SIP are easily selected for graphing with
in IPTEGO's advanced service assurance monitoring platform, Palladion. As
shown below, such KPI's can be selected from a GUI, graphed and alerts can be
raised [SNMP, PCAP/Wireshark Trace, or e-mail sent to or communication to
a = 3XX, 401, 402, and 407
The SEER is calculated using the following formula:
# of INVITE Requests
w/ associated 200 OK, 480, 486, or 600
SEER
= -------------------------------------------------------------------
x 100
(Total # of INVITE Requests) -
(# of INVITE Requests w/ 'a' Response)
SIP Peering KPI’s
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other network devices over API] in response to KPI thresholds being exceeded.
In addition, these Palladion KPI metrics can be selected for network elements,
realms or trunks specified by you in your network. So this makes it very easy for
you to monitor the
performance provided to you
by each peering partner, SIP
trunk or route/LCR or the
quality you are delivering to
each of your realms or
enterprise customers.
IPTEGO Palladion supports hundreds of KPI’s measured
for each network device or Trunk
SIP Response Codes
o
2xx
—
Successful Responses
o
200 OK
o
4xx
—
Client Failure Responses
o
401 Unauthorized (Used only by
registrars or
user agents
.
Proxies
should use proxy authorization 407)
o
402 Payment Required (Reserved for
future use)
o
480 Temporarily Unavailable
o
486 Busy Here
o
6xx
—
Global Failure Responses
o
600 Busy Everywhere
SIP Peering KPI’s
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Another very useful measurement defined by the IETF here is Session Defects
Ratio (SDR), also graphed by Palladion by checking a box. ‘503’ SIP Response
messages commonly indicate a route to one of your peering partners is failing
due to congestion of the Gateway or SoftSwitch.
Session Defects Ratio (SDR) is the percentage of call attempts receiving the
following responses, in relation to total call attempts or INVITES:
o
500 Server Internal Error
o
503 Service Unavailable
o
504 Server Timeout
Palladion can be set up to monitor, measure, graph and alert
on any specific SIP response code. The diagram below shows
‘404’s’ and ‘503's
The SDR is calculated using the following formula
:
# of INVITE Requests w/ associated 500, 503, or 504
SDR
=--------------------------------------------------------------
x 100
Total # of INVITE Requests
SIP Peering KPI’s
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IPTEGO Palladion set to measure ‘503’s
for each network device or Trunk
In addition to this, some carriers like to plot Post Dial Delay, [PDD] and to alert
when it exceeds a certain time threshold. PDD is defined as the time interval
between transmission of the INVITE and reception of the ‘180 Ringing’ response
message. This is an earlier indication of congestion soon to occur on a given
route and at this time, Palladion can be configured to provide an SNMP trap all
other notification to the Network Operations Center [NOC].
A trend of increasing Post Dial Delay, [PDD] is used to
indicate the on-site or imminence of congestion at a peering
point. Early alerts can allow rerouting of LCR’s to avoid
dropped calls
SIP Peering KPI’s
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The IETF defines many more KPI’s. the table given below maps these to the
equivalent SS7 or circuit switched performance measurement, wherever a
comparison makes sense or is valid.
SIP End-to-End
Performance
Metrics
draft-ietf-pmol-
sip-perf-metrics-
04
SIP Definition ISUP
Equivalent
ISUP
Definition
Successful Request
Attempt [SRD]
INVITE to 100
Response Time IAM to ACM
Session Request
Delay [SRD]
INVITE to non-100
Trying eg 180
Post-Dial Delay
(PDD)
IAM to ANM
Failed Session Setup
[SRD] and
INVITE to 4XX (excluding 401,
402, and 407 non-
failure challenge response
codes), 5XX, or possible 6XX
message.
N/A IAM to REL
Successful session
completion [SDD]
BYE, to 2XX Ack Time to Clear
Call or CIC
REL to RLC
Failed session
completion SDD
BYE, to Timer F Expires Release with
no release
complete
Missing RLC
Session Duration
Time [SDT]
Average Call
Hold Time
(ACHT)
Successful
Session
Duration
Time [SSDT]
200 OK response to an INVITE
to BYE
Failed
session
duration SDT
200 OK response to an INVITE,
and the resulting Timer F
expiration.
Average Call
Hold Time
(ACHT)
Session
Establishment [SER ]
Ratio
(# of INVITE Requests w/ associated 200 OK)
x 100/-----------------------
(Total # of INVITE Requests)
-
(# of INVITE Requests w/ 3XX Response)
ASR
Session
Establishment
Effectiveness Ratio
(SEER)
(# of INVITE Requests
w/ associated 200 OK, 480, 486, or 600)
x 100------------------------------
(Total # of INVITE Requests)
-
(# of INVITE Requests w/ 'a' Response)
N/A
Session Defects
Ratio (SDR)
(# of INVITE Requests
w/ associated 500, 503, or 504)
x100---------------------------
Total # of INVITE Requests
N/A
Ineffective Session
Ineffective
SIP Peering KPI’s
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Attempts (ISA)
Invites resulting in:-
o 408 Request
Timeout
o 500 Server
Internal Error
o 503 Service
Unavailable
o 504 Server
Timeout
# of ISA x 100
-----------------------------
Total # of Session Requests
Machine
Attempts (IMA)
in telephony
applications of
SIP, and was
adopted from
Telcordia GR-
512-CORE
[GR-512]
.
Session Disconnect
Failures [SDF]
Session disconnect
failures occur when an
active session is
terminated due to a
failure condition that
can be identified by a
REASON header
[RFC3326]
in a BYE or
CANCEL request
.
# of SDF's x 100
-----------------------------
Total # of Session Requests
Cutoff Calls Telcordia GR-
512-CORE
[GR-512]
.
Session Completion
Ratio (SCR)
# of Successfully Completed
Sessions x 100
------------------------------------------
Total # of Session Requests
Call Completion
Ratio (CCR)
Successful Session
Completion
A session completes successfully
when it begins with a setup
request
and ends with a session
completion message ie SIP ‘200’
Failed Session
Completion
Session completion fails when an INVITE is sent from a UAC, but
there is no indication the INVITE reached the intended UAS. This
can also occur if the intended UAS does not respond to the UAC or
the response never reaches the UAC associated with the session.
Eg only responses are from proxies eg 407 or 408
Session Success
Ratio [SSR]
SSR = 100% - (ISA% + SDF%) Call Success
Ratio (CSR)
References
-
SIP End-to-End Performance Metrics draft-ietf-pmol-sip-perf-
metrics-04
-
ITU-T Recommendation E.411 : International network management -
Operational guidance
-
ITU-T Recommendation E.422 : Observations on international outgoing
telephone calls for quality of service
-
Telcordia GR-512-CORE - LSSGR: Reliability, Section 12
SIP Peering KPI’s
Page 11/12
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For more information on
SIP KPI's
, visit our website at
www.teraquant.com
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Palladion
, please click the "mail to" or email
[email protected]
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