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Internet Engineering Task Force
INTERNET DRAFT                                                      Authors
Signaling Transport Working Group                            Huai-An P. Lin
June 26, 1999                                                   Taruni Seth
Expires December 26, 1999                                   Albert Broscius
                                                          Christian Huitema
 Telcordia Technologies


      VoIP Signaling Performance Requirements and Expectations
            <draft-ietf-sigtran-performance-req-00.txt>


Status of this document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.  Internet-Drafts are working
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Abstract

This document serves as input into the IETF SIGTRAN requirements
process. It includes call setup delay requirements, derived from
relevant ISDN and SS7 standards published by ITU-T (International
Telecommunications Union--Telecommunications Standardization Sector) and
generic requirements published by Telcordia Technologies (formerly
Bellcore). To gain user acceptance of Voice-over-IP (VoIP) services and
to enable interoperability between Switched Circuit Networks (SCNs) and
VoIP systems, it is imperative that the VoIP signaling performance be
comparable to that of the current SCNs. The requirements given in this
Internet Draft are intended to be the worst-case requirements, for at
least in United States SCN calls are typically set up far faster than
the derived requirements.



1. Introduction

This document serves as input into the IETF SIGTRAN requirements
process. It includes call setup delay requirements, derived from
relevant ISDN and SS7 standards published by ITU-T (International
Telecommunications Union--Telecommunications Standardization Sector) and
generic requirements published by Telcordia Technologies (formerly
Bellcore). To gain user acceptance of Voice-over-IP (VoIP) services and


Lin, Seth, Broscius, Huitema                                    [Page 1]


to enable interoperability between Switched Circuit Networks (SCNs) and
VoIP systems, it is imperative that the VoIP signaling performance be
comparable to that of the current SCNs. The requirements given in this
Internet Draft are intended to be the worst-case requirements, since at
least in United States SCN calls are typically set up within one to two
seconds [1]--far faster than the derived requirements.

The call setup delay, also known as the post-dialing delay, in an ISDN-
SS7 environment is the period that starts when an ISDN user dials the
last digit of the called number and ends when the user receives the last
bit of the Alerting message. Call setup delays are not explicitly given
in the existing SCN performance requirements; rather, performances of
SCNs are typically expressed in terms of cross-switch (or cross-office)
transfer times. This Internet Draft uses ITU-TÆs SS7 Hypothetical
Signaling Reference Connection (HSRC) [2], cross-STP (Signaling Transfer
Point) time [Section 5.6">3], TelcordiaÆs switch response time generic requirements
[4], and a simple ISDN-SS7 call flow to derive the call setup delay
requirements. ITU-TÆs cross-switch time requirements [5] are listed as
references but not used, since the ISDN timings are missing.



2. Hypothetical Signaling Reference Connection (HSRC)

HSRC is specified in ITU-T Recommendation Q.709. A HSRC is made up by a
set of signaling points and STPs that are connected in series by
signaling data links to produce a signaling connection. Recommendation
Q.709 distinguishes the ônationalö components from the ôinternationalö
components. A HSRC for international working consists of an
international component and two national components. The size of each
country is considered; however, the definitions of ôlargeö and ôaverageö
countries was not completely precise:

ôWhen the maximum distance between an international switching center and
a subscriber who can be reached from it does not exceed 1000 km or,
exceptionally, 1500 km, and when the country has less than n Ú 10E7
subscribers, the country is considered to be of average-size. A country
with a larger distance between an international switching center and a
subscriber, or with more than n Ú 10E7 subscribers, is considered to be
of large-size. (The value of n is for further study.)ö

Recommendation Q.709 uses a probabilistic approach to specify the number
of signaling points and STPs on a signaling connection. The maximum
number of signaling points and STPs allowed in a national component and
an international component are listed in Tables 1 and 2, respectively.


  Table 1: Maximum Number of Signaling Points and STPs in a National
        Component (Source: ITU-T Recommendation Q.709, Table 3)

  Country size  Percent of      Number of       Number of
                connections     STPs            signaling points*

  Large-size       50%              3                 3
                   95%              4                 4



Lin, Seth, Broscius, Huitema                                    [Page 2]


  Average-size     50%              2                 2
                   95%              3                 3


* The terms signaling points and switches are used interchangeably in
this Internet Draft.


 Table 2: Maximum Number of Signaling Points and STPs in International
        Component (Source: ITU-T Recommendation Q.709, Table 1)

  Country size  Percent of      Number of       Number of
                connections     STPs            signaling points

  Large-size       50%              3                 3
  to
  Large-size       95%              4                 3

  Large-size       50%              4                 4
  to
  Average-size     95%              5                 4

  Average-size     50%              5                 5
  to
  Average-size     95%              7                 5



3. Switch Response Time (aka Cross-switch Transfer Time)

Most of SCN performance requirements are specified in terms of switch
response times, which are also referred to as cross-switch transport
time or cross-switch delay. This section reviews the meanings of switch
response times, several other related terms, and the generally accepted
values of switch response times published by Telcordia Technologies. The
corresponding ITU-TÆs cross-switch timing requirements are also listed
as references.

This Internet Draft reviews the switch response time requirements
intended to apply under normal loading. Normal loading is usually
associated with the notion of the Average Busy Season Busy Hour (ABSBH)
load. Simply put, it is expected that the switch response times that a
particular switch experiences at this load will be virtually load-
independent.

Switch response time is the period that starts when a stimulus occurs at
the switch and ends when the switch completes its response to the
stimulus. The occurrence of a stimulus often means the switch receives
the last bit of a message from an incoming signaling link, and
completion of a response means the switch transmits the last bit of the
message on the outgoing signaling link. If the switchÆs response to a
stimulus involves the switch sending a message on the outgoing signaling
link, then switch processing time is the sum of the switch processing
time and the link output delay:

   switch response time = switch processing time + link output delay




Lin, Seth, Broscius, Huitema                                    [Page 3]


Switch processing time is the period that starts when a stimulus occurs
at the switch and ends when the switch places the last bit of the
message in the output signaling link controller buffer. The period
between the switch placing the message in the output signaling link
controller buffer and the switch transmitting the last bit of the
message on the outgoing signaling link is defined as the link output
delay. Link output delay can be further divided into the queuing delay
and message emission time. There are separate delay requirements for
switch processing time and link output delay; however, for simplicity
only the combined delay requirements for switch response time, as given
in Table 3, will be listed in this Internet Draft.


     Table 3: Switch Response Time Assuming Typical Traffic Mix and
      Message Lengths (Source: Telcordia GR-1364-CORE, Table 5-1)

Type of Call Segment     Switch Response Time (ms)
                          Mean             95%
ISUP Message            205-218         <=337-349
Alerting                400             <=532
ISDN Access Message     220-227         <=352-359
TCAP Message            210-222         <=342-354
Announcement/Tone       300             <=432
Connection              300             <=432


Telcordia GR-1364 specifies switch response time using ôswitch call
segmentsö as a convenient way to refer to the various phases of call
processing that switches are involved in. (An alternative would be
proposing switch processing requirements for every possible type of
switch processing. Obviously, this would become burdensome and would
necessitate adding to the requirements every time an additional type of
switch processing was required.) Listed in Table 3 are:

1. ISUP message call segments that involve the switch sending an ISUP
   message as a result of a stimulus.
2. Alerting call segments that involve the switch alerting the
   originating and/or terminating lines as a result of a stimulus.
3. ISDN access message call segments that involve the switch sending an
   ISDN access message (other than an ISDN access ALERT message) as a
   result of stimulus. ISDN access message call segment processing
   occurs at originating or terminating switches where the originating
   or terminating line, respectively, is an ISDN line.
4. TCAP message call segments that involve the switch sending a TCAP
   message as a result of a stimulus.
5. Announcement/tone call segments that involve the switch playing an
   announcement, placing a tone on, or removing a tone from the
   originating or terminating line as a result of a stimulus. However,
   the announcement/tone call segments do not include dial-tone delay,
   of which the delay requirements can be found in Telcordia
   TR-TSY-000511[6].
6. Connection call segments involve the switch connecting one or more
   users as a result of a stimulus.

The ITU-TÆs cross-switch timing requirements are listed below as



Lin, Seth, Broscius, Huitema                                    [Page 4]


references. It is noted that the ITU-TÆs requirements are noticeably
stringent that those of Telcordia under the normal loading. However,
since the ITU-TÆs values are stated as ôprovisionalö and they do not
provide the timing requirements for ISDN, TelcordiaÆs values will be
used to derive the call setup delay requirements.


               Table 4: ITU-T Cross-Switch Transfer Time
             (Source: ITU-T Recommendation Q.725, Table 3)
                   Exchange call                Cross-Switch Transfer
                                                      Time (ms)*
  Message typ      attempt loading              Mean            95%

  Simple           Normal                       110             220
  (e.g. answer)    +15%                         165             330
                   +30%                         275             550

  Processing       Normal                       180             360
  intensive        +15%                         270             540
  (e.g. IAM)       +30%                         450             900

* Provisional values.



4. Cross-STP Delay

Message delay through an STP is specified as the cross-STP delay. It is
the interval that begins when the STP receives the last bit of a message
from the incoming signaling link, and ends when the STP transmits the
last bit of the message on the outgoing signaling link. As with the
switch response time discussed in the previous section, the cross-STP
can be divided into processor handling time and link output delay. This
Internet Draft adopts the cross-STP delay requirements specified in ITU-
T Q.706 Recommendation.


               Table 5: Message transfer time at an STP
             (Source: ITU-T Recommendation Q.706, Table 5)

               Message transfer Time (ms)
STP signaling traffic load              Mean            95%

  Normal                                 20              40
  +15%                                   40              80
  +30%                                  100             200



5. Maximum End-to-End Signaling Delays

Using the HSRC, switch response times, and cross-STP delays, one can
compute the maximum signaling transfer delays for ISUP messages under
normal load. As with Telcordia GR-1364, it is assumed that the
distribution of switch response time for each call segment is
approximately a normal distribution. It is further assumed that switch
response times of different switches are independent. Under these
assumptions, the end-to-end (from originating switch to terminating


Lin, Seth, Broscius, Huitema                                    [Page 5]


switch) delays for each national component and for international calls
are listed in Tables 6 and 7, respectively. The 20 ms cross-STP delay is
assumed in all cases. It should be noted that all these values must be
increased by the transmission propagation delays, which are listed in
Table 8.


Table 6: Maximum ISUP Signal Transfer Delays for Each National Component

  Country size          Percent of                   Delay (ms)
                        connections            Mean               95%

  Large-size               50%                675-714         <=904-941
                           95%                900-952         <=1164-1214

  Average-size             50%                450-476         <=637-661
                           95%                675-714         <=904-941


 Table 7: Maximum ISUP Signal Transfer Delays for International Calls

  Country size          Percent of                   Delay (ms)
                        connections            Mean               95%

  Large-size to            50%              2025-2142         <=2421-2538
  Large-size               95%              2495-2638         <=2933-3076

  Large-size to            50%              2250-2380         <=2677-2797
  Average-size             95%              2720-2876         <=3177-3333

  Average-size to          50%              2475-2618         <=2913-3056
  Average-size             95%              2965-3134         <=3441-3610


  Table 8: Calculated Terrestrial Transmission Delays for Various Call
        Distances (Source: ITU-T Recommendation Q.706, Table 1)

 Arc length             Delay terrestrial (ms)
   (km)          Wire           Fibre           Radio
    500           2.4             2.5            1.7
   1000           4.8             5.0            3.3
   2000           9.6            10.0            6.6
   5000          24.0            25.0           16.5
  10000          48.0            50.0           33.0
  15000          72.0            75.0           49.5
  17737          85.1            88.7           58.5
  20000          96.0           100.0           66.0
  25000         120.0           125.0           82.5



6. Basic Call Flow and Call Setup Delays

The following figure illustrates the simplest call flow for call setup
in an ISDN-SS7 environment. The end user terminals are assumed to be
ISDN phones and use Q.931 messages (i.e., Setup and Alerting). The
switches use ISUP messages to establish inter-switch trunks for the
subsequent voice communication.


Lin, Seth, Broscius, Huitema                                    [Page 6]


               Figure 1: Simple Call Setup Signaling Flow

     Caller          Originating                   Terminating       Called
    Terminal           Switch                        Switch        Terminal
        |                 |                             |               |
        |       Setup     |                             |               |
        |---------------->|                             |               |
        |                 |   IAM                IAM    |               |
        |                 |---------> . . . . --------->|               |
        |                 |                             |     Setup     |
        |                 |                             |-------------->|
        |                 |                             |               |
        |                 |                             |   Alerting    |
        |                 |                             |<--------------|
        |                 |   ACM                 ACM   |               |
        |                 |<--------- . . . . <---------|               |
        |       Alerting  |                             |               |
        |<----------------|                             |               |
        |                 |                             |               |
        |                 |                             |               |


Using the above call flow, the end-to-end message transfer delays in
Tables 6 and 7, and the switch response times for Q.931 messages in
Table 3, one can derive the call setup times given in the following
tables. Again, all these values must be increased by the transmission
propagation delays listed in Table 8.


        Table 9: Call Setup Delays for Each National Component

  Country size          Percent of             Call Setup Delay (ms)
                        connections            Mean              95%

  Large-size               50%               2590-2682       <=3007-3099
                           95%               3040-3158       <=3497-3615

  Average-size             50%               2140-2206       <=2513-2579
                           95%               2590-2682       <=3007-3099


          Table 10: Call Setup Delays for International Calls

  Country size          Percent of                   Delay (ms)
                        connections            Mean              95%

  Large-size to            50%               5290-5538       <=5909-6157
  Large-size               95%               6230-6530       <=6903-7203

  Large-size to            50%               5740-6014       <=6387-6661
  Average-size             95%               6680-7006       <=7378-7704

  Average-size to          50%               6190-6490       <=6863-7163
  Average-size             95%               7170-7522       <=7893-8245





Lin, Seth, Broscius, Huitema                                    [Page 7]


8. User Expectations


The requirements derived in the previous section should be interpreted
as the worst-case requirements. At least in the United States, users of
SCN typically experience far less setup delays than the derived delay
requirements. With the maturing of Common Channel Signaling (CCS)
Network, call setup time has been reduced to a mere one to two seconds
[1]. The VoIP networks are expected to achieve the same level of delay

There is no known study on expected setup delays for international
calls. As discussed, a HSRC for international working consists of an
international component and two national components, and the maximum
number of signaling points and STPs in a national component is roughly
the same as the number in an international component (Tables 1 and 2).
As a consequence, the end-to-end ISUP delays in an international call
are roughly three times of those in a national call. On the other hand,
the Q.931 signals occur only at the two ends for both national and
international calls. Based on these observations, one may expect 2.5-5
second call setup delays to be reasonable for international calls.



Acknowledgements

The authors would like to express their gratitude to Dr. Daniel Luan of
AT&T Labs for his insight into network operation and valuable
suggestions for calculating end-to-end signaling delays as well as call
setup delays.



References

[1] AT&T Webpage,
    www.att.com/technology/technologists/fellows/lawser.html.

[2] ITU-T Recommendation Q.709, Specifications of Signaling System No.
    7--Hypothetical Signaling Reference Connection, March 1993.

[3] Telcordia Technologies Generic Requirements GR-1364-CORE, Issue 1,
    LSSGR: Switch Processing Time Generic Requirements Section 5.6, June
    1995.

[4] ITU-T Recommendation Q.706, Specifications of Signaling System No.
    7ùMessage Transfer Part Signaling Performance, March 1993.

[5] ITU-T Recommendation Q.706, Specifications of Signaling System No.
    7ùSignaling performance in the Telephone Application, March 1993.

[6] Telcordia Technologies TR-TSY-000511, LSSGR: Service Standards,
    Section 11, Issue 2, July 1987.


Authors' addresses


Lin, Seth, Broscius, Huitema                                    [Page 8]


        Huai-An Lin
        Telcordia Technologies
        445 South Street, MCC-1A216R
        Morristown, NJ 07960-6438
        Phone: 973 829-2412
        Email: hlin@research.telcordia.com

        Taruni Seth
        Telcordia Technologies
        445 South Street, MCC-1G209R
        Morristown, NJ 07960-6438        Phone: 973 829-4046
        Email: taruni@research.telcordia.com

        Albert Broscius
        Telcordia Technologies
        445 South Street, MCC-1A264B
        Morristown, NJ 07960-6438
        Phone: 973 829-4781
        Email: broscius@research.telcordia.com

        Christian Huitema
        Telcordia Technologies
        445 South Street, MCC-1J244B
        Morristown, NJ 07960-6438
        Phone: 973 829-4266
        Email: huitema@research.telcordia.com
































Lin, Seth, Broscius, Huitema                                    [Page 9]


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