Recommendation E.721
                     NETWORK GRADE OF SERVICE PARAMETERS IN ISDN
         1      Introduction
               This Recommendation proposes network Grade of Service (GOS) parameters  for
         circuit-switched and packet-switched services in ISDN, based on the ISDN Grade of
         Service  concept  and  guidelines  for  selecting  GOS  parameters  provided   in
         Recommendation E.720. These parameters are defined assuming that the network  and
         the network components are fully operational.
         2      Circuit-switched services
               In the current ISDN specifications, call establishment and release for  all
         circuit-switched services provided via B-channels (voice, data, image)  will  use
         the out-of-band call control procedures defined by Recommendation Q.931 and  S.S.
         No. 7 (ISUP) signalling protocols. Thus, for  the  traffic  GOS  parameters  that
         relate to call establishment and release phases, a single set of  parameters  can
         be used for all circuit-switched services provided by the ISDN.
               The  following  four   traffic   GOS   parameters   are   recommended   for
         circuit-switched calls in ISDN:
                        1)  pre-selection delay (overlap sending),
               2) post-selection delay,
               3) call release delay, and
                              4) probability of end-to-end blocking.
               The definitions of these traffic GOS parameters are given below. The  delay
         GOS parameters are based on the message flows in Recommendation  Q.931  and  S.S.
         No. 7 (ISUP) protocols as indicated, for example, in Figure A-1/E.713.
         2.1    Pre-selection delay (overlap sending)
               pre-selection delay (overlap sending) is defined as the time interval  from
         the instant the SABME message is passed by the calling  terminal  to  the  access
         signalling system until  the  SETUP  ACK  message  is  received  by  the  calling
         terminal.
         2.2    Post-selection delay
               post-selection delay is defined as the time interval from the  instant  the
         INFO message containing the last selection digit (in the case of overlap  sending
         or the SETUP message in the case of en-bloc sending) is  passed  by  the  calling
         terminal to the access signalling system until the first message indicating  call
         disposition is received by the calling terminal.
               Note - In the ISDN the called user can choose to delay the sending  of  the
         ALERTING signal to the calling  user.  This  definition  does  not  include  such
         user-induced delays.
         2.3    Call release delay
               call release delay is defined as the time interval  from  the  instant  the
         DISCONNECT message is passed by the user terminal which terminated  the  call  to
         the access signalling system, until the RELEASE message is received by  the  same
         terminal (indicating that the terminals can initiate/receive a new call).
         2.4    Probability of end-to-end blocking
               The probability of end-to-end blocking  is  the  probability  that  a  call
         attempt will be unsuccessful due to a lack of network resources.
               Note - Resources in the access network are not part of this definition.
         3      Packet-switched services
               The ISDN user has a choice of two types of packet-switched  data  services.
         The B-channel provides 64 kbit/s packet access, while the D-channel can  also  be
         used to provide packet data access at 16 kbit/s (64 kbit/s in the case of primary
         access). For packet-switched services, the current call  control  procedures  are
         based on X.25 (inband) protocols, except  during  the  initial  B-  or  D-channel
         set-up between the DTE (Data Terminating Equipment) and the PH (Packet  Handler).
         Thus a certain number of messages will be  exchanged  between  the  TE  (Terminal
         Equipment) and the LE (Local Exchange) over  the  D-channel  during  the  initial
         establishment phase of a packet-switched call. The messages will have to  compete
         with other signalling (SAPI 0) and data (SAPI 16) traffic on the D-channel.
               Thus, the traffic GOS parameters for  the  current  specification  of  ISDN
         packet-switched services will have to be based on the Q.931 as  well  as  on  the
         X.25 call control procedures.
               The selection and definition of traffic GOS parameters for  packet-switched
         services in ISDN are for further study.
         4      Target values for GOS parameters
               The target values will be specified at the normal and  high  loads  in  the



                                                        Fascicle II.3 - Rec. E.721   PAGE1

         same sense as Recommendation E.500. The delay target values will be specified  by
         the mean and percentile levels for both normal and high loads.
               The actual target values are for further study.


































































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