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1991-12-30
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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.
PAGE2 Fascicle II.3 - Rec. E.721