Requirements for IP/MPLS network transmission interruption duration
draft-fan-opsawg-transmission-interruption-00

Operations and Management Area Working                            P. Fan
Group                                                              L. Li
Internet-Draft                                              China Mobile
Intended status: Standards Track                            July 9, 2012
Expires: January 10, 2013

  Requirements for IP/MPLS network transmission interruption duration
             draft-fan-opsawg-transmission-interruption-00

Abstract

   The transmission performance of IP/MPLS network affects upper layer
   services and networks, but there is no consensus in the industry on
   transmission interruption for IP/MPLS network up to now.  This memo
   studies requirements for the interruption duration criteria in
   several service scenarios.

Status of this Memo

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   This Internet-Draft will expire on January 10, 2013.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Services and Performance Criteria . . . . . . . . . . . . . . . 3
     2.1.  Softswitch  . . . . . . . . . . . . . . . . . . . . . . . . 3
     2.2.  LTE Backhaul  . . . . . . . . . . . . . . . . . . . . . . . 6
     2.3.  VPN . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     2.4.  IPTV  . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6

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1.  Introduction

   Today's IP/MPLS network is widely used as a bearer network to carry
   diversified packet switched services.  The transmission qualities of
   these services are closely related to the performance of bearer
   layers, as network failure, delay, congestion and other abnormities
   will inevitably bring about service interruption and user perception
   degradation.  However, there is no consensus in the industry on
   transmission interruption for IP/MPLS network up to now.  This memo
   studies relationships between service performance and transmission
   interruption duration in several scenarios, and is intended to reach
   a list of requirements for these interruption duration criteria.

2.  Services and Performance Criteria

   Services delivered by IP/MPLS network have different transmission
   quality requirements, thus introduce different performance criteria
   for the bearing IP/MPLS network.  In this section we will describe
   concerns on IP/MPLS network interruption duration from aspects of
   four kinds of service scenarios, namely Softswitch, LTE backhaul, VPN
   and IPTV.

2.1.  Softswitch

   From the softswitch point of view, the IP carrying nature imposes
   certain influence to the service quality.  Especially when speech is
   delivered by IP, the communication quality of voice is impaired, and
   in turn makes higher requirements for the transmission performance of
   IP.  This part will mainly focus on three communication quality
   criteria and their influence factors to give requirements for
   softswitch and IP bearer networks.

   2.1.1 Call Loss

   Call loss is used to describe the circumstance where a phone call
   fails to establish after initiated by a subscriber due to network
   faults.  In the practical network, the call loss rate is mainly
   associated by the factors as follows:

   (1) Interfaces, including Nc, Mc and interface between MSS and SG.

   (2) State machine message timer.  If a timeout takes place, the state
   machine releases signaling messages, producing a call loss.  Typical
   value of BICC timer is 10~15 seconds and value of DTAP timer about 15
   seconds.

   (3) Interface association timer.  Associations breaks off at the

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   expiration of timer.

   (4) Bearer network convergence time.

   If the configured timer duration of a state machine is shorter than
   the timer duration of interface association, then although interface
   association may not be broken off, call loss is still possible to
   occur due to message timer expiration.  If the association timer
   duration is shorter than IP routing convergence time, the association
   is considered broken off by SCTP, hence message loss at interface
   between MSS and SG as well as interface Nc results in massive call
   loss, and new calling request cannot be satisfied because of
   interface Mc breakoff.  In this case, the call loss rate can be
   calculated as

   Call Loss Rate = ( IP Convergence Time + Association Restoration Time
   ) * CAPS / BHCA.

   However, if the association timer duration is longer than IP routing
   convergence time, then the association is considered normal by SCTP,
   and data will be retransmitted.  Although this may cause buffer
   overflow leading to call loss, the call loss rate is possible to
   achieve approximately zero if buffer is big enough.

   From the analysis above and practical operation experience, the
   requirements for softswitch and IP bearer are as follows: the
   duration of SCTP interface association timer should be shorter than
   that of the state machine message timer, and this duration is further
   recommended to be no longer than 6 seconds in order to maintain
   detection sensitivity; the interruption duration of IP bearer network
   should be as short as possible to avoid call loss during the IP layer
   interruption period, and this duration is further recommended to be
   no longer than 5 seconds.

   2.1.2 Call Cut-off

   Call cut-off is referred to the abnormal release during a phone call
   due to reasons other than intentional release by any of the parties
   involved in the call.  The call cut-off rate is related with:

   (1) Interfaces, including Nc and interface between MSS and SG.

   (2) Interface association timer.

   (3) Bearer network convergence time.

   If the association timer duration is shorter than IP routing
   convergence time, established phone calls will be released once

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   interruption of interface Nc or interface connecting MSS and SG is
   detected.  In the case of association breakoff, call cut-off rate can
   be calculated as

   Call Cut-off Rate = ( CAPS * Call Duration ) * Busy Hour Association
   Breakoffs / BHCA.

   While if the association is not interrupted, the call cut-off rate
   can be approximately zero.

   In conclusion, the SCTP association should be guaranteed during IP
   layer interruption to avoid interface breakoff alert.  The
   requirements for softswitch and IP bearer are the same as those
   related to call loss.

   2.1.3 Connection Delay

   The connection delay from a call initiation by a calling party to
   PLMN should be no longer than 4 seconds.  This delay is affected by
   factors below:

   (1) RRC connection setup delay (irrelevant to whether service is
   carried by IP or not).

   (2) Core network signaling interaction delay.  The message number at
   interface Nc/Nb is 6, and is 8 (calling side) or 16 (called side, in
   case of IP-IP) at interface Mc.  Each message is with a delay of no
   longer than 50 milliseconds.  Calling message delay at interface Nc
   is no longer than 300 milliseconds.  If long distance call is made
   though CMN, the message delay is to be increased by transmission
   delay of 5 msec/km and CMN process delay.  So the message delay is
   likely to be 400 milliseconds.

   (3) IP bearer network QoS and load.

   The connection delay is influenced by the delay criterion defined in
   the IP bearer network QoS, and is raised by delay, jitter, packet
   loss caused by network overload.  In addition, if the configured
   timer duration of interface association is too long, the SCTP
   sensitivity to the retransmitted messages after packet loss will be
   decreased, which increases connection delay.

   Connection delay is generally expressed as

   Connection Delay = (IP convergence time + RRC connection setup delay
   + Signaling Interaction Delay),

   and is no longer than 4 seconds.  So the IP network in normal working

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   state should be constrained within a certain range of load to ensure
   that delay is shorter than 50 milliseconds, while in interruption
   state the IP convergence time should be no longer than 3 seconds to
   ensure that connection delay is shorter than 4 seconds.

   From the analysis of IP/MPLS performance according to the three
   criteria above, we suggest the transmission interruption duration of
   IP/MPLS network for softswitch service should be no longer than 3
   seconds.

2.2.  LTE Backhaul

   To be further analyzed.

2.3.  VPN

   To be further analyzed.

2.4.  IPTV

   To be further analyzed.

3.  Security Considerations

   TBD

4.  IANA Considerations

   This memo includes no request to IANA.

5.  Acknowledgements

   We would like to thank Kai Li for his help in the analysis of
   softswitch voice communication quality.

6.  Normative References

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Authors' Addresses

   Peng Fan
   China Mobile
   32 Xuanwumen West Street, Xicheng District
   Beijing  100053
   P.R. China

   Email: fanpeng@chinamobile.com

   Lianyuan Li
   China Mobile
   32 Xuanwumen West Street, Xicheng District
   Beijing  100053
   P.R. China

   Email: lilianyuan@chinamobile.com

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