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 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.