Internet Standards/CD2.mdf/ccitt/1992/e/e525.asc

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         All drawings appearing in this Recommendation have been done in Autocad.
         Recommendation E.525
                              SERVICE PROTECTION METHODS
         1      Introduction
               The objective of service protection methods is  to  control  the  grade  of
         service for certain streams of traffic  by  restricting  the  access  to  circuit
         groups. Several methods are available, with the  common  feature  that  they  may
         reject certain call attempts when the considered circuit group has  little  spare
         capacity. Service protection is generally used in alternative routing networks to
         restrict overflow traffic, but can also be used to give priority service  to  one
         class of traffic over another.
               Failure or overload conditions may require  temporary  changes  to  service
         protection parameters. This is considered to be network management  action  which
         is described in the E.400 Series Recommendations.
               Applications of service protection methods are described in S  2,  and  the
         available methods are described in S 3.
               The use  of  service  protection  generally  increases  the  complexity  of
         dimensioning algorithms. Appropriate dimensioning algorithms are presented  in  S
         4.
               The choice between available methods will generally depend  on  performance
         characteristics and ease of implementation. These are discussed in SS 5 and 6.
         2      Applications
         2.1    Traffic routing
         2.1.1  Overflow routing strategies - General principles
               Routing strategies that involve overflow  often  have  direct  first-choice
         (high-usage) routes, and indirect alternative routes. In  conditions  of  traffic
         overload the proportion of alternatively-routed traffic increases  rapidly,  with
         the risk of severe degradation of network performance. Service protection methods
         should be used to prevent calls overflowing from a direct route to an alternative
         route when circuit groups on the alternative route are heavily loaded. In  Figure
         1/E.525, which shows a hierarchical case only for  the  sake  of  simplification,
         calls from A to B have a direct first-choice route and an alternative  route  via
         D. Exchange A should apply service protection on the circuit group AD. When AD is
         occupied over a certain limit, overflow calls (e.g. from  AB)  are  rejected  and
         priority is given to first-choice traffic (e.g. from A to C).
                                        Figure 1/E.525 - T0200640-87

               In Figure 1/E.525 traffic from A to B has access  to  two  possible  routes
         but traffic from A to C has only one. In this situation traffic from A  to  B  is
         likely to experience a much better end-to-end grade  of  service  unless  service
         protection is used to restrict its access to AD. This control of grade of service
         allows optimal (minimum cost) dimensioning for planned traffic loads in  addition
         to giving protection against heavy overloads.
         2.1.2  Fixed hierarchical alternative routing
               An example of fixed hierarchical  alternative  routing  is  illustrated  in
         Figure 1/E.525. Here exchange D is a tandem exchange  at  a  higher  hierarchical
         level than A, B and C. Direct routes at the lower level (e.g.  AB)  overflow  via
         the hierarchical route  (ADB).  This  hierarchical  route  is  always  the  final
         alternative routing. In such networks it is highly recommended to  apply  service
         protection to restrict traffic overflowing to final choice routes.
         2.1.3  Fixed non-hierarchical alternative routing
               This term describes routing strategies which are based on  fixed  sequences
         of alternative routes (as in hierarchical alternative routing) but which  do  not
         have a hierarchical final-choice route for all overflow traffic.  Figure  2/E.525
         may be used to illustrate some simple but common cases. Traffic from A to B has a
         first-choice route AB and an alternative ACB which  is  final  to  this  traffic,
         while traffic from A to C may use AC as a first choice and then overflow to  ADC.
         Traffic from D to B is either first offered to the route DAB and then overflowing
         to DCB or vice versa. The latter routing principle is commonly  known  as  mutual
         overflow.
                                        Figure 2/E.525 - T0200650-87




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               In both routing schemes a certain hierarchy is  distinguishable.  They  are
         however non-hierarchical, in the sense that no hierarchical trunk group  that  is
         final to all its carried  traffic  can  be  found.  The  application  of  service
         protection methods may be less simple than  for  hierarchical  routing,  but  the
         general principles presented in S 2.1.1 still apply.
         2.1.4  Dynamic routing
               Many  different  forms  of  preplanned  or  adaptive  dynamic  routing  are
         possible, with either centralized  or  distributed  control  (see  Recommendation
         E.170). One feature that is  common  to  most  dynamic  routing  schemes  is  the
         availability of a large number of potential  alternative  routes  for  any  given
         connection. With this type of routing scheme, service protection  is  of  crucial
         importance and has several special features:
               -   Protection should be stronger than with other overflow routing schemes
                  (i.e. larger reservation parameters should be used).
               -   If possible, service protection should be applied on all circuit groups
                  in  an  alternative  route.  This  requires   a   certain   amount   of
                  information-passing between exchanges or to a central processor.
               -   In connection with adaptive routing, the service protection concept can
                  be used not only to block overflow calls but also in the selection of a
                  good alternative route (generally this will be a  route  on  which  all
                  circuit groups have at least a requested number of free circuits).
         2.2    Priority service
               Service protection methods can also be used to  give  priority  service  to
         one type of traffic, for example in a multiservice network, e.g. ISDN.
         2.3    Stability
               In order to provide stability in  networks  with  non-hierarchical  routing
         schemes, service protection should be used to restrict overflow traffic  if  that
         traffic overflows to an alternative  route  which  is  shared  with  first-choice
         traffic.
         3      Available methods
         3.1    Split circuit group
               A  straightforward  technique  is  to  divide  a  circuit  group  into  two
         components. Priority traffic is allowed access to the whole circuit group,  while
         non-priority (usually overflow) traffic is only allowed access to one  component.
         Normally the priority traffic is offered first to the reserved component  -  this
         is then equivalent to a separate high usage group.
         3.2    Trunk reservation
               This method is also known  as  priority  reservation  system.  Non-priority
         calls are accepted on the considered circuit group only when the momentary number
         of free circuits in that group observed at the arrival  of  a  non-priority  call
         exceeds a specified lower limit (irrespective of which  particular  circuits  are
         free). Priority calls are always accepted if any circuits are free.
               Trunk  reservation  may  also  be  applied  selectively,  for  example,  to
         restrict call attempts to hard-to-reach destinations. This  method  is  known  as
         selective trunk reservation.
         4      Evaluation and dimensioning
         4.1    Cluster engineering concept
               In hierarchical automatic alternative routing (AAR) a cluster  consists  of
         a final-choice circuit group together with those high  usage  groups  from  which
         traffic overflows to the final group. This cluster  should  be  engineered  as  a
         whole. This implies  firstly  that  grade-of-service  (GOS)  criteria  should  be
         applied to the whole cluster rather than separately to  final  groups.  Secondly,
         the question of high-load dimensioning must be considered for the whole  cluster.
         In order to meet normal and high load cluster GOS criteria in the most  efficient
         way,  the  parameters  of  service  protection   methods   must   be   determined
         appropriately as a part of the dimensioning process.
         4.2    Split circuit group
               With hierarchical AAR, the split final circuit  group  creates  a  separate
         high-usage group for first-choice traffic. This should be dimensioned  so  as  to
         achieve the cluster GOS criteria. Standard evaluation methods that  can  be  used
         include the Wilkinson Equivalent Random Traffic theory [1].  Interrupted  Poisson
         Process methods can be used to give more  precise  evaluation  [2],  [3]  and  to
         evaluate network performance [4].
               Split circuit groups may be  useful  to  control  GOS  in  non-hierarchical




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         routing. The precise  dimensioning  and  evaluation  depends  on  the  individual
         situation and it is generally more practical to use 1-moment methods of  analysis
         [5], [6].
         4.3    Trunk reservation
               With hierarchical AAR, a trunk reservation parameter should be  applied  to
         the final group so as to achieve the  cluster  GOS  criteria  optimally  for  all
         traffic offered to the  cluster.  In  most  situations  a  small  value  of  this
         parameter is appropriate. For evaluation of Poisson streams a recursion method is
         available  which  may  be  extended,  using  equivalent  random   traffic   (ERT)
         techniques, into overflow situations [7]. Interrupted Poisson Process [3] methods
         can be used to give a more precise evaluation and to evaluate network performance
         [8].





















































                                                        Fascicle II.3 - Rec. E.525   PAGE1

               For non-hierarchical strategies,  1-moment  evaluation  methods  are  again
         recommended. Simple recursion formulas are available for a  circuit  group  using
         trunk reservation and offered Poisson traffics. 1-moment [7] methods can also  be
         extended to give better accuracy by taking account  of  downstream  blocking  and
         traffic correlations [6] and [8].
         5      Performance characteristics
         5.1    Efficiency
               Efficiency can be measured by traffic capacity at normal  load  subject  to
         GOS criteria. In this respect there is little to choose between trunk reservation
         and split circuit group methods, provided each is correctly dimensioned.
         5.2    Overload protection
               The two service protection methods, trunk reservation and split final  with
         a reserved high usage group, provide considerably better overload protection  for
         first-choice-final traffic in cases of general and overflow  overload  than  does
         the less usual method, split final with reserved final group.
         5.3    Robustness
               A significant advantage of trunk reservation is that it provides  a  robust
         performance profile with respect to  traffic  load  variations  (decreasing  high
         priority traffic  in  combination  with  increasing  low  priority  traffic)  and
         reservation parameter settings. Independent of  the  trunk  group  size,  traffic
         variations (which have not been forecasted) are relatively well carried.
               With trunk reservation the same parameter value is  likely  to  be  optimal
         for a wide range of configurations at both normal load and overloads.
               In contrast, the reserved section  of  a  split  circuit  group  should  be
         redimensioned for different configurations and (when dimensioned according to the
         normal traffic load pattern) will not give optimal values at overload.
         5.4    Peakedness
               Changes in the peakedness of overflowing traffic have  a  slightly  greater
         impact on the blocking probabilities within split circuit group  arrangements  in
         comparison with trunk reservation.
         6      Implementation consequences
         6.1    Dimensioning methods
               Methods  for  the  calculation  of  a  split  circuit  group  or  a   trunk
         reservation parameter are available [7], [9], [10].
         6.2    Traffic measurements
               Both  service   protection   methods   require   the   estimates   of   the
         first-choice-final traffic which is to be protected and the  overflowing  traffic
         from the high-usage trunk  group(s)  (i.e.  measurements  on  a  per  destination
         basis).
               With the split circuit group method, first routed  traffic  can  be  easily
         measured on a trunk group basis. With the trunk reservation method,  measurements
         other than traditional are required to identify the first offered traffic.
         6.3    Operational aspects
               Since trunk reservation is a software controlled technique, protection  for
         different traffic streams can easily be changed by  changing  parameters  in  the
         software. This allows temporary changes  to  be  made  under  network  management
         control. Precautions should  be  taken  in  such  situations  to  restore  design
         parameter values.
         6.4    Technology requirements
               Split circuit group methods can be installed in both electromechanical  and
         processor controlled exchanges.
               Trunk reservation may, in practice, only  be  realized  in  software  as  a
         conditional  overflow  facility  and  consequently  only  be  installed  in   SPC
         exchanges.
               Both methods require that the exchange  have  the  ability  to  distinguish
         between priority and non-priority traffic.
               References
         [1]    WILKINSON (R. I).: Theories for toll traffic engineering in the USA.  Bell
               System Technical Journal, Vol. 35, March 1956.
         [2]     MATSUMOTO  (J.),  WATANABE   (Y.):   Analysis   of   individual   traffic
               characteristics for queueing systems with multiple  Poisson  and  overflow
               inputs. Proc. 10th ITC, paper 5.3.1, Montreal, 1983.
         [3]    KUCZURA (A.): The interrupted Poisson Process as  an  overflow  processor.
               Bell System Technical Journal, Vol. 52, No. 3, 1973.
         [4]    MANSFIELD (D. R.), DOWNS  (T.):  A  moment  method  for  the  analysis  of




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         telephone traffic networks by decomposition. Proc. 9th ITC,  paper  2.4.4,
               Torremolinos, 1979.
         [5]    MANSFIELD (D. R.), DOWNS (T.): On the  one-moment  analysis  of  telephone
               traffic networks. IEEE Trans. Comm. 27, pp. 1169-1174, 1979.
         [6]     LE  GALL  (F.),   BERNUSSOU   (J.):   An   analytical   formulation   for
               grade-of-service determination in telephone networks.  IEE  Trans.  Comm.,
               31, pp. 420-424, 1983.
         [7]    COOPER (R. B.): Introduction to queueing theory. North Holland, 1977.
         [8]    SONGHURST (D. J.): Protection against  traffic  overload  in  hierarchical
               networks employing alternative routing.  Proc.  Telecommunication  Network
               Planning Symposium, pp. 214-220, Paris, 1980.
         [9]    LEBOURGES (M.), PASSERON (A.): Contribution to a network sizing  procedure
               using probability distributions of  traffic  data,  Networks  '86,  Tarpon
               Springs, 1986.
         [10]   LINDBERG (K.): Simple approximations of  overflow  system  quantities  for
               additional demands in the optimization. Proc. 10th ITC, Montreal, 1983.
               Bibliography
         LEBOURGES (M.), BECQUE (C. R.), SONGHURST, (D. J.): Analysis and dimensioning  on
         non- hierarchical telephone networks. Proc. 11th ITC, paper 2.28-4, Kyoto, 1985.














































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