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All drawings appearing in this Fascicle have been done in Autocad.
Recommendation Q.706
MESSAGE TRANSFER PART SIGNALLING PERFORMANCE
The message transfer part of Signalling System No. 7 is designed as a
joint transport system for the messages of different users. The requirements of
the different users have to be met by the Message Transfer Part. These
requirements are not necessarily the same and may differ in importance and
stringency.
In order to satisfy the individual requirements of each user the Message
Transfer Part of Signalling System No. 7 is designed in such a way that it meets
the most stringent User Part requirements envisaged at the time of specification.
To this end, the requirements of the telephone service, the data transmission
service and the signalling network management, in particular, were investigated.
It is assumed that a signalling performance which satisfies the requirements
mentioned above will also meet those of future users.
In the light of the above, signalling system performance is understood to
be the capability of the Message Transfer Part to transfer messages of variable
length for different users in a defined manner. In order to achieve a proper
signalling performance, three groups of parameters have to be taken into account:
- The first group covers the objectives derived from the requirements of
the different users. The aims are limitation of message delay,
protection against all kinds of failures and guarantee of availability.
- The second group covers the features of the signalling traffic, such as
the loading potential and the structure of the signalling traffic.
- The third group covers the given environmental influences, such as the
characteristics (e.g. error rate and proneness to burst) of the
transmission media.
The three groups of parameters are considered in the specification of the
procedures to enable the Message Transfer Part to transfer the messages in such a
way that the signalling requirements of all users are met and that a uniform and
satisfactory overall signalling system performance is achieved.
1 Basic parameters related to Message Transfer Part signalling
performance
Signalling performance is defined by a great number of different
parameters. In order to ensure a proper signalling performance for all users to
be served by the common Message Transfer Part, the following design objectives
are established for the Message Transfer Part.
1.1 Unavailability of a signalling route set
The unavailability of a signalling route set is determined by the
unavailability of the individual components of the signalling network (signalling
links and the signalling points) and by the structure of a signalling network.
The unavailability of a signalling route set should not exceed a total of
10 minutes per year.
The unavailability of a signalling route set within a signalling network
may be improved by replication of signalling links, signalling paths and
signalling routes.
1.2 Unavoidable message transfer part malfunction
The Message Transfer Part of Signalling System No. 7 is designed to
transport messages in a correct sequence. In addition, the messages are protected
against transmission errors. However, a protection against transmission errors
cannot be absolute. Furthermore, mis-sequencing and loss of messages in the
Message Transfer Part cannot be excluded in extreme cases.
For all User Parts, the following conditions are guaranteed by the Message
Transfer Part:
a) Undetected errors
On a signalling link employing a signalling data link which
has the error rate characteristic as described in
Recommendation Q.702 not more than one in 1010 of all signa
unit errors will be undetected by the message Transfer
Part.
Fascicle VI.7 - Rec. Q.706 PAGE1
b) Loss of messages
Not more than one in 107 messages will be lost due to
failure in the message transfer part.
c) Messages out-of-sequence
Not mo e than one in 1010 messages will be delivered
out-of-sequence to the User Parts due to failure in the
message transfer part. This value also includes duplication
of messages.
1.3 Message transfer times
This parameter includes:
- handling times at the signalling points (see S 4.3);
- queueing delays including retransmission delays (see S 4.2);
- signalling data link propagation times.
1.4 Signalling traffic throughput capability
Needs further study (see S 2.2).
2 Signalling traffic characteristics
2.1 Labelling potential
The design of Signalling System No. 7 provides the potential for labels to
identify 16 384 signalling points. For each of the 16 different User Parts a
number of user transactions may be identified, e.g. in the case of the telephone
service up to 4096 speech circuits.
2.2 Loading potential
Considering that the load per signalling channel will vary according to
the traffic characteristics of the service, to the user transactions served and
to the number of signals in use, it is not practicable to specify a general
maximum limit of user transactions that a signalling channel can handle. The
maximum number of user transactions to be served must be determined for each
situation, taking into account the traffic characteristics applied so that the
total signalling load is held to a level which is acceptable from different
points of view.
When determining the normal load of the signalling channel, account must
be taken of the need to ensure a sufficient margin for peak traffic loads.
The loading of a signalling channel is restricted by several factors which
are itemized below.
2.2.1 Queueing delay
The queueing delay in absence of disturbances is considerably influenced
by the distribution of the message length and the signalling traffic load (see S
4.2).
2.2.2 Security requirements
The most important security arrangement is redundancy in conjunction with
changeover. As load sharing is applied in normal operation, the load on the
individual signalling channels has to be restricted so that, in the case of
changeover, the queueing delays do not exceed a reasonable limit. This
requirement has to be met not only in the case of changeover to one predetermined
link but also in the case of load distribution to the remaining links.
2.2.3 Capacity of sequence numbering
The use of 7 bits for sequence numbering finally limits the number of
signal units sent but not yet acknowledged to the value of 127.
In practice this will not impose a limitation on the loading potential.
PAGE16 Fascicle VI.7 - Rec. Q.706
2.2.4 Signalling channels using lower bit rates
A loading value for a signalling channel using bit rates of less than 64
kbit/s will result in greater queueing delays than the same loading value for a
64-kbit/s signalling channel.
2.3 Structure of signalling traffic
The Message Transfer Part of Signalling System No. 7 serves different User
Parts as a joint transport system for messages. As a result, the structure of the
signalling traffic largely depends on the types of User Parts served. It can be
assumed that at least in the near future the telephone service will represent the
main part of the signalling traffic also in integrated networks.
It cannot be foreseen yet how the signalling traffic is influenced by the
integration of existing and future services. The traffic models given in S 4.2.4
have been introduced in order to consider as far as possible the characteristics
and features of different services within an integrated network. If new or more
stringent requirements are imposed on signalling (e.g. shorter delays) as a
consequence of future services, they should be met by appropriate dimensioning of
the load or by improving the structure of the signalling network.
3 Parameters related to transmission characteristics
No special transmission requirements are envisaged for the signalling
links of Signalling System No. 7. Therefore, System No. 7 provides appropriate
means in order to cope with the given transmission characteristics of ordinary
links. The following items indicate the actual characteristics to be expected -
as determined by the responsible Study Groups - and their consequences on the
specifications of the Signalling System No. 7 Message Transfer Part.
3.1 Application of Signalling System No. 7 to 64-kbit/s links
The Message Transfer Part is designed to operate satisfactorily with the
following transmission characteristics:
a) a long-term bit error rate of the signalling data link of less than
10-6 [1];
b) a medium-term bit error rate of less than 10-4;
c) random errors and error bursts including long bursts which might occur
in the digital link due to, for instance, loss of frame alignment or
octet slips in the digital link. The maximum tolerable interruption
period is specified for the signal unit error rate monitor (see
Recommendation Q.703, S 10.2).
3.2 Application of Signalling System No. 7 to links using lower bit rates
(Needs further study.)
4 Parameters of influence on signalling performance
4.1 Signalling network
Signalling System No. 7 is designed for both associated and nonassociated
applications. The reference section in such applications is the signalling route
set, irrespective of whether it is served in the associated or quasi-associated
mode of operation.
For every signalling route set in a signalling network, the unavailability
limit indicated in S 1.1 has to be observed irrespective of the number of
signalling links in tandem of which it is composed.
Fascicle VI.7 - Rec. Q.706 PAGE1
4.1.1 International signalling network
(Needs further study.)
4.1.2 National signalling network
(Needs further study.)
4.2 Queueing delays
The Message Transfer Part handles messages from different User Parts on a
time-shared basis. With time-sharing, signalling delay occurs when it is
necessary to process more than one message in a given interval of time. When this
occurs, a queue is built up from which messages are transmitted in order of their
times of arrival.
There are two different types of queueing delays: queueing delay in the
absence of disturbances and total queueing delay.
4.2.1 Assumptions for derivation of the formulas
The queueing delay formulas are basically derived from the M/G/1 queue
with priority assignment. The assumptions for the derivation of the formulas in
the absence of disturbances are as follows:
a) the interarrival time distribution is exponential (M);
b) the service time distribution is general (G);
c) the number of server is one (1);
d) the service priority refers to the transmission priority within level 2
(see Recommendation Q.703, S 11.2); however, the link status signal
unit and the independent flag are not considered;
e) the signalling link loop propagation time is constant including the
process time in signalling terminals; and
f) the forced retransmission case of the preventive cyclic retransmission
method is not considered.
In addition, for the formulas in the presence of disturbances, the
assumptions are as follows:
g) the transmission error of the message signal unit is random;
h) the errors are statistically independent of each other;
i) the additional delay caused by the retransmission of the erroneous
signal unit is considered as a part of the waiting time of the
concerned signal unit; and
j) in case of the preventive cyclic retransmission method, after the error
occurs, the retransmitted signal units of second priority are accepted
at the receiving end until the sequence number of the last sent new
signal unit is caught up by that of the last retransmitted signal unit.
Furthermore, the formula of the proportion of messages delayed more than a
given time is derived from the assumption that the probability density function
of the queueing delay distribution may be exponentially decreasing where the
delay time is relatively large.
4.2.2 Factors and parameters
a) The notations and factors required for calculation of the queueing
delays are as follows:
Qa mean queueing delay in the absence of disturbances
eq s\s(a,2) variance of queueing delay in the absence of
disturbances
Qt mean total queueing delay
eq s\s(2,t) variance of total queueing delay
P(T) proportion of messages delayed more than T
a traffic loading by message signal units (MSU) (excluding
retransmission)
PAGE16 Fascicle VI.7 - Rec. Q.706
Tm mean emission time of message signal units
Tf emission time of fill-in signal units
TL signalling loop propagation time including processing time in
signalling terminal
Pu error probability of message signal units
k1 = eq \f(2nd moment of message signal units emission time,T\o(m,2))
k2 = eq \f( 3rd moment of message signal units emission time,T\o(m,3))
k3 = eq \f( 4th moment of message signal units emission time,T\o(m,4))
Note - As a consequence of zero insertion at level 2 (see
Recommendation Q.703, S 3.2), the length of the emitted signal unit
will be increased by approximately 1.6 percent on average. However,
this increase has negligible effect on the calculation.
b) The parameters used in the formulas are as follows:
tf = Tf/Tm
tL = TL/Tm
for the basic method,
E1 = 1 + PutL
E2 = k1 + PutL(tL + 2)
E3 = k2 + PutL(tL2 + 3tL + 3k1)
for the preventive cyclic retransmission (PCR) method,
a3 = exp(-atL): traffic loading caused by fill-in signal units.
az = 1 - a - a3
H1 = atL
H2 = atL(k1 + atL)
H3 = atL(k2 + 3atLk1 + a2tL2)
F1 = atL/2
F2 = atL(k1/2 + atL/3)
F3 = atL(k2/2 + atLk1 + a2tL2/4)
qa = eq \f( k1 (a + az) + a3tf,2(1 - a))
sa = eq \f( ak1,1 - a) qa + eq \f( k2 (a + az) + a3tf2,3(1 - a))
ta = eq \f( 3ak1sa + 2ak2qa,2(1 - a)) + eq \f( (a + az)k3 + a3tf3,4(1 - a))
Fascicle VI.7 - Rec. Q.706 PAGE1
Z1 = 2 + Pu(1 + H1)
Z2 = 4K1 + Pu(5k1 + 3H1 + H2)
Z3 = 8k2 + Pu(19k2 + 27k1H1 + 9H2 + H3)
Y2 = sa + 4k1 + F2 + 2{qa(2 + F1) + 2F1}
Y3 = ta + 8k2 + F3 + 3{sa(2 + F1) + qa(4k1 + F2) + 2F + 2 + 4k1F1} + 12qaF1
a = eq \f( 1 - a{2 + Pu(1 + atL)},2 + qa + atL/2)
qd = eq \f( aZ2 + aY2,2(1 - aZ1))
sd = eq \f( aZ2,1 - aZ1) qd + eq \f( aZ3 + aY3,3(1 - aZ1))
qb = eq \f( qa + 1 + F1,1 - a)
sb = eq \f( sa + k1 + F2,(1 - a)3) + eq \f( 2{qa(1 + F1) + F1},(1 - a)2)
qc = eq \f( qd + 1 + Pu(1 + H1),1 - a)
sc = eq \f( sd + k1 + Pu(3k1 + H2),(1 - a)3) + 2 eq \f( qd + Pu{qd(1 +
+ 2H1},(1 - a)2)
PV = Pua eq \f( qa + 2 + atL/2,1 - 2a) eq \b\bc\( ( 1 + Pu\f( a + a2tL
- 2a))
4.2.3 Formulas
The formulas of the mean and the variance of the queueing delays are
described in Table 1/Q.706. The proportion of messages delayed more than a given
time Tx is:
P (Tx) @ expeq \b\bc\( ( -\f( Tx - Qx + sx,sx))
where Qx and sx denote the mean and the standard deviation of queueing delay,
respectively. This approximation is better suited in absence of disturbances. In
the presence of disturbances the actual distribution may be deviated further.
Relation between P(Tx) and Tx is shown in Figure 1/Q.706.
4.2.4 Examples
Assuming the traffic models given in Table 2/Q.706, examples of queueing
delays are calculated as listed in Table 3/Q.706.
Note - The values in the table were determined based on TUP messages. With
the increase of the effective message length, using ISUP and TC, these values may
be expected to be increased during the course of further study.
Figure 1/Q.706 - CCITT 35211
TABLE 1/Q.706
Queueing delay formula
Error
correcti Disturba Mean Q Variance s2
on nce
method
eq \f(s\s(2,a),T\s(2,m)) = \f(
Absence eq \f( Qa,Tm) = \f( tf,2) + \f( t\s(2,f),12) + \f(a[4k2 - (4k2 -
ak1,2(1 - a)) 3k\s(2,1))a], 12 (1 - a)2)
Basic eq \f (s\s(2,t),T\s(2,m)) = \f(
eq \f( Qt,Tm) = \f( tf,2) + \f( t\s(2,f),12) + \f( a[4E3 - (4E1E3
Presence aE2,2(1 - aE1)) + E1 - 1 - 3E\s(2,2))a],12(1 - aE1)2)
eq \d\fo40() + Pu(1 -
Pu)t\s(2,L)
Absence
Preventi
ve
cyclic
PAGE16 Fascicle VI.7 - Rec. Q.706
eq \f( Qa,Tm) = qa eq \f( s\s(2,t),T\s(2,m)) =sa -
q\s(2,a)
retrans- eq \f( s\s(2,t),T\s(2,m)) = (1 -
mission eq \f( Qt,Tm) = (1 - Pu - Pv) qa Pu - Pv) sa
Presence + Puqb + Pvqc eq \d\fo40()+ Pusb + Pvsc - \f(
Qs(2,t),T\s(2,m))
TABLE 2/Q.706
Traffic model
Model A B
Message length (bits) 120 104 304
Percent 100 92 8
Mean message length 120 120
(bits)
k1 1.0 1.2
k2 1.0 1.9
k3 1.0 3.8
TABLE 3/Q.706
List of examples
Figure
Fascicle VI.7 - Rec. Q.706 PAGE1
Error Queueing delay Disturbance Model
control
2/Q.706 Basic/PCR Mean Absence A and B
3/Q.706 Basic/PCR Standard Absence A and B
deviation
4/Q.706 Basic Mean Presence A
5/Q.706 Basic Standard Presence A
deviation
6/Q.706 PCR Mean Presence A
7/Q.706 PCR Standard Presence
deviation
PAGE16 Fascicle VI.7 - Rec. Q.706
A
Fascicle VI.7 - Rec. Q.706 PAGE1
Figure 2/Q.706 - CCITT 35220
Figure 3/Q.706 - CCITT 41040
Figure 4/Q.706 - CCITT 41211
Figure 5/Q.706 - CCITT 41200
Figure 6/Q.706 - CCITT 41222
Figure 7/Q.706 - CCITT 41226
4.3 Message transfer times
Within a signalling relation, the Message Transfer Part transports
messages from the originating User Part to the User Part of destination, using
several signalling paths. The overall message transfer time needed depends on the
message transfer time components (a) to (e) involved in each signalling path.
4.3.1 Message transfer time components and functional reference points
A signalling path may include the following functional signalling network
components and transfer time components.
a) Message Transfer Part sending function at the point of origin (see
Figure 8/Q.706).
b) Signalling transfer point function (see Figure 9/Q.706).
c) Message Transfer Part receiving function at the point of destination
(see Figure 10/Q.706).
d) Signalling data link propagation time (see Figure 11/Q.706).
e) Queueing delay.
An additional increase of the overall message transfer times is caused
by the queueing delays. These are described in S 4.2.
Figure 8/Q.706 - CCITT 35270
Figure 9/Q.706 - CCITT 35280
Figure 10/Q.706 - CCITT 35290
Figure 11/Q.706 - CCITT 35300
4.3.2 Definitions
4.3.2.1 message transfer part sending time Tms
F: temps d'émission du Sous-système Transport de Messages Tms
S: tiempo de emisión de la parte de transferencia de mensajes Tms
Tms is the period which starts when the last bit of the message has left
the User Part and ends when the last bit of the signal unit enters the signalling
data link for the first time. It includes the queueing delay in the absence of
disturbances, the transfer time from level 4 to level 3, the handling time at
level 3, the transfer time from level 3 to level 2, and the handling time in
level 2.
4.3.2.2 message transfer time at signalling transfer points Tcs
F: temps de transfert des messages aux points de transfert sémaphore Tcs
S: tiempo de transferencia de mensajes en los puntos de transferencia de
la señalización Tcs
Tcs is the period, which starts when the last bit of the signal unit
leaves the incoming signalling data link and ends when the last bit of the signal
unit enters the outgoing signalling data link for the first time. It also
includes the queueing delay in the absence of disturbances but not the additional
queueing delay caused by retransmission.
4.3.2.3 message transfer part receiving time Tmr
F: temps de réception du Sous-système Transport de Messages Tmr
S: tiempo de recepción de la parte de transferencia de mensajes Tmr
Tmr is the period which starts when the last bit of the signal unit leaves
the signalling data link and ends when the last bit of the message has entered
the User Part. It includes the handling time in level 2, the transfer time from
level 2 to level 3, the handling time in level 3 and the transfer time from level
3 to level 4.
4.3.2.4 data channel propagation time Tp
PAGE16 Fascicle VI.7 - Rec. Q.706
F: temps de propagation sur la voie de données Tp
S: tiempo de propagación del canal de datos Tp
Tp is the period which starts when the last bit of the signal unit has
entered the data channel at the sending side and ends when the last bit of the
signal unit leaves the data channel at the receiving end irrespective of whether
the signal unit is disturbed or not.
4.3.3 Overall message transfer times
The overall message transfer time To is referred to the signalling
relation. To starts when the message has left the user part (level 4) at the
point of origin and ends when the message has entered the user part (level 4) at
the point of destination.
The definition of the overall message transfer time and the definitions of
the individual message transfer time components give rise to the following
relationships:
a) In the absence of disturbances
eq Toa = Tms + \i\su(i=1,n+1, )\d\fo10()Tpi +
\i\su(i=1,n, )\d\fo10()Tcsi + Tmr
b) In the presence of disturbances
To = Toa + (Qt - Qa)
Here
Toa overall message transfer time in the absence of disturbances
Tms Message Transfer Part sending time
Tmr Message Transfer Part receiving time
Tcs Message transfer time at signalling transfer points
n number of STPs involved
Tp data channel propagation time
To overall message transfer time in the presence of disturbances
Qt total queueing delay (see S 4.2)
Qa queueing delay in the absence of disturbances (see S 4.2)
Note - For S(Qt - Qa), all signalling points in the signalling relation
must be taken into account.
Fascicle VI.7 - Rec. Q.706 PAGE1
4.3.4 Estimates for message transfer times
(Needs further study.)
The estimates must take account of:
- the length of the signal unit,
- the signalling traffic load,
- the signalling bit rate.
The estimates for Tmr, Tms and Tcs will be presented in the form of:
- mean values,
- 95% level values.
The estimates for Tcs for a signalling transfer point are given in Table
4/Q.706.
TABLE 4/Q.706
STP signalling Message transfer
traffic load time at an STP
(Tcs) in ms
Mean 95 %
Normal 20 40
+15 % 40 80
+30 % 100 200
Note - the values in the table were determined based on TUP messages. With
the increase of the effective message length, using ISUP and TC, these values may
be expected to be increased during the course of further study.
These figures are related to 64-kbit/s signalling bit rate. The normal
signalling traffic load is that load for which the signalling transfer point is
engineered. A mean value of 0.2 Erlang per signalling link is assumed. The
message length distribution is as given in Table 2/Q.706.
4.4 Error control
During transmission, the signal units are subject to disturbances which
lead to a falsification of the signalling information. The error control reduces
the effects of these disturbances to an acceptable value.
Error control is based on error detection by redundant coding and on error
correction by retransmission. Redundant coding is performed by generation of 16
check bits per signal unit based on the polynomial described in Recommendation
Q.703, S 4.2. Moreover, the error control does not introduce loss, duplication or
mis-sequencing of messages on an individual signalling link.
However, abnormal situations may occur in a signalling relation, which are
caused by failures, so that the error control for the signalling link involved
cannot ensure the correct message sequence.
4.5 Security arrangements
The security arrangements have an essential influence on the observance of
the availability requirements listed in S 1.1 for a signalling relation.
In the case of Signalling System No. 7, the security arrangements are
mainly formed by redundancy in conjunction with changeover.
PAGE16 Fascicle VI.7 - Rec. Q.706
4.5.1 Types of security arrangements
In general, a distinction has to be made between security arrangements for
the individual components of the signalling network and security arrangements for
the signalling relation. Within a signalling network, any security arrangement
may be used, but it must be ensured that the availability requirements are met.
4.5.1.1 Security arrangements for the components of the signalling network
Network components, which form a signalling path when being
interconnected, either have constructional security arrangements which exist from
the very beginning (e.g. replication of the controls at the exchanges and
signalling transfer points) or can be replicated, if need be (e.g. signalling
data links). For security reasons, however, replication of signalling data links
is effected only if the replicated links are independent of one another (e.g.
multipath routing). In the case of availability calculations for a signalling
path set, special care has to be taken that the individual signalling links are
independent of one another.
4.5.1.2 Security arrangements for signalling relations
In quasi-associated signalling networks where several signalling links in
tandem serve one signalling relation, the security arrangements for the network
components, as a rule, do not ensure sufficient availability of the signalling
relation. Appropriate security arrangements must therefore be made for the
signalling relations by the provision of redundant signalling path sets, which
have likewise to be independent of one another.
4.5.2 Security requirements
In the case of 64-kbit/s signalling links, a signalling network has to be
provided with sufficient redundancy so that the quality of the signalling traffic
handled is still satisfactory. (Application of the above to signalling links
using lower bit rates needs further study.)
4.5.3 Time to initiate changeover
If individual signalling data links fail, due to excessive error rates,
changeover is initiated by signal unit error monitoring (see Recommendation
Q.703, S 8). With signal unit error monitoring, the time between the occurrence
of the failure and the initiation of changeover is dependent on the message error
rate (a complete interruption will result in an error rate equal to 1).
Changeover leads to substantial additional queueing delays. To keep the
latter as short as possible, the signalling traffic affected by an outage is
reduced to a minimum by the use of load sharing on all existing signalling links.
4.5.4 Changeover performance times
There are two performance times associated with link changeover. Both
times are maximum time values (not normal values). They are defined to be the
point at which 95% of the events occur within the recommended performance time at
a signalling point traffic load that is 30% above normal.
The performance times are measured from outside the signalling point.
4.5.4.1 Failure response time
This time describes the time taken by a signalling point to recognize that
a changeover is needed for a signalling link. This time begins when the
signalling link is unavailable, and ends when the signalling point sends a
changeover (or emergency changeover) order to the remote signalling point. A link
is unavailable when a signalling unit with status indication out of service
(SIOS) or processor outage (SIPO) is sent or received on the link.
Failure response time (maximum permissible): 500 ms.
4.5.4.2 Answer time to changeover order
This time describes the time taken by a signalling point to answer a
changeover (or emergency changeover) order. This time begins when the signalling
point receives a changeover (or emergency changeover) order message, and ends
when the signalling point sends a changeover (or emergency changeover)
acknowledgement message.
Answer time to changeover order (maximum permissible): 300 ms.
4.6 Failures
4.6.1 Link failures
During transmission, the messages may be subject to disturbances. A
measure of the quality of the signalling data link is its signal unit error rate.
Signal unit error monitoring initiates the changeover at a signal unit
error rate of about 4 . 10-3.
The error rate, which Signalling System No. 7 has to cope with, represents
a parameter of decisive influence on its efficiency.
Fascicle VI.7 - Rec. Q.706 PAGE1
As a result of error correction by retransmission, a high error rate
causes frequent retransmission of the message signal units and thus long queueing
delays.
4.6.2 Failures in signalling points
(Needs further study.)
4.7 Priorities
Priorities resulting from the meaning of the individual signals are not
envisaged. Basically, the principle "first-in - first-out" applies.
Although the service indicator offers the possibility of determining
different priorities on a user basis, such user priorities are not yet foreseen.
Transmission priorities are determined by Message Transfer Part functions.
They are solely dependent on the present state of the Message Transfer Part and
completely independent of the meaning of the signals (see Recommendation Q.703, S
11).
5 Performance under adverse conditions
5.1 Adverse conditions
(Needs further study.)
5.2 Influence of adverse conditions
(Needs further study.)
Reference
[1] CCITT Recommendation Error performance on an international digital
connection forming part of an integrated services digital network, Vol.
III, Rec. G.821.
PAGE16 Fascicle VI.7 - Rec. Q.706