WPCL 2BJ|x H   X  6p&6p&   H   c4 P  Fascicle VI.7 - Rec. Q.706 PAGE1  c4 P    HH  c4 P PAGE16  c4 P  Fascicle VI.7 - Rec. Q.706  HH Hp P X`h!(# X  Ё c4 P All drawings appearing in this Fascicle have been done in Autocad. Recommendation Q.706 HP X`h!(#8< c4 P MESSAGE TRANSFER PART SIGNALLING PERFORMANCE  H Hp P X`h!(#Ё c4 P   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.  H  In order to satisfy the individual requirements of each user the Message Transfer Part of Signalling SystemNo. 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.  H  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:  H  - 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.  H  - The second group covers the features of the signalling traffic, such as the loading potential and the structure of the signalling traffic.  H  - The third group covers the given environmental influences, such as the characteristics (e.g. error rate and proneness to burst) of the transmission media.  H  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. HP X`h!(#X X  H 1  c4 P Basic parameters related to Message Transfer Part signalling performance Hp P X`h!(#Ё c4 P   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. HP X`h!(#X X  H1.1   c4 P Unavailability of a signalling route set Hp P X`h!(# c4 P  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.  H  The unavailability of a signalling route set should not exceed a total of 10 minutes per year.  H  The unavailability of a signalling route set within a signalling network may be improved by replication of signalling links, signalling paths and signalling routes. 1.2h  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.  H  For all User Parts, the following conditions are guaranteed by the Message Transfer Part: HP X`h!(#X h  h a) c4 P Undetected errors Hp P X`h!(#  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 10 c4 P 10 c4 P  of all signal unit errors will be undetected by the message Transfer Part.  b)pLoss of messages  Not more than one in 10 c4 P 7 c4 P  messages will be lost due to failure in the message transfer part. HP X`h!(#X h  h c)Messages out-of-sequence Hp P X`h!(# Not more than one in 10 c4 P 10 c4 P  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.  HH  c4 P 1.3h  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. 1.4h  Signalling traffic throughput capability  Needs further study (see S 2.2). HP X`h!(#X X  H 2  c4 P Signalling traffic characteristics Hp P X`h!(#Ё c4 P  2.1h  Labelling potential  H  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  H a number of user transactions may be identified, e.g. in the case of the telephone service up to 4096 speech circuits. 2.2h  Loading potential  H  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.  H  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.  H  The loading of a signalling channel is restricted by several factors which are itemized below.  H2.2.1 Queueing delay  H  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).  H2.2.2 Security requirements  H  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.  H2.2.3 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.  H  In practice this will not impose a limitation on the loading potential.  2.2.4 Signalling channels using lower bit rates  H  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. 2.3h  Structure of signalling traffic  H  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.  H  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. 3X 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. 3.1h  Application of Signalling System No. 7 to 64-kbit/s links  H  The Message Transfer Part is designed to operate satisfactorily with the following transmission characteristics:  H   a)pa long-term bit error rate of the signalling data link of less than 10 c4 P -6 c4 P  [1];   b)pa medium-term bit error rate of less than 10 c4 P -4 c4 P ;  H   c)prandom 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).  H 3.2h  Application of Signalling System No. 7 to links using lower bit rates  (Needs further study.) 4X Parameters of influence on signalling performance 4.1h  Signalling network  H  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.  H  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.  4.1.1 International signalling network  HH  (Needs further study.)  H4.1.2 National signalling network  (Needs further study.) 4.2h  Queueing delays  H  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.  H  There are two different types of queueing delays: queueing delay in the absence of disturbances and total queueing delay.  H4.2.1 Assumptions for derivation of the formulas  H  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)pthe interarrival time distribution is exponential (M);   b)pthe service time distribution is general (G);   c)pthe number of server is one (1);  H   d)pthe service priority refers to the transmission priority within level 2 (see Recommendation Q.703, S11.2); however, the link status signal unit and the independent flag are not considered;  H   e)pthe signalling link loop propagation time is constant including the process time in signalling terminals; and  H   f)pthe forced retransmission case of the preventive cyclic retransmission method is not considered.  H  In addition, for the formulas in the presence of disturbances, the assumptions are as follows:  Hh   g)pthe transmission error of the message signal unit is random;   h)pthe errors are statistically independent of each other;  H   i)pthe 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  H   j)pin 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.  H  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.  H4.2.2 Factors and parameters  H   a)pThe notations and factors required for calculation of the queueing delays are as follows:  H  hpQa  mean queueing delay in the absence of disturbances  H  hpeq s\s(a,2) X%variance of queueing delay in the absence of disturbances  hpQt  mean total queueing delay  hpeq s\s(2,t) X%variance of total queueing delay  hpP(T)  proportion of messages delayed more than T  Hh  ap traffic loading by message signal units (MSU) (excluding retransmission) hpTm  mean emission time of message signal units  hpTf  emission time of fill-in signal units  H  hpTL  signalling loop propagation time including processing time in signalling terminal  hpPu  error probability of message signal units  H  hpk c4 P 1 c4 P  = eq \f(2nd moment of message signal units emission time,T\o( c4 P m,2) c4 P )  H  hpk c4 P 2 c4 P  = eq \f( 3rd moment of message signal units emission time,T\o( c4 P m,3) c4 P )  H  hpk c4 P 3 c4 P  = eq \f( 4th moment of message signal units emission time,T\o( c4 P m,4 c4 P ))  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)pThe parameters used in the formulas are as follows:  HH Hp8"(# Ђ p8%tf = Tf/Tm  p8%tL = TL/Tm Hp P X`h!(# hpfor the basic method, Hp8"(# Ђ p8#E c4 P 1 c4 P  = 1 + P c4 P u c4 P t c4 P L c4 P   p8 E c4 P 2 c4 P  = k c4 P 1 c4 P  + P c4 P u c4 P t c4 P L c4 P (t c4 P L c4 P  + 2)  p8E c4 P 3 c4 P  = k c4 P 2 c4 P  + P c4 P u c4 P t c4 P L c4 P (t c4 P L2 c4 P  + 3t c4 P L c4 P  + 3k c4 P 1 c4 P ) Hp P X`h!(#Ё for the preventive cyclic retransmission (PCR) method,  Hh  hpa3 = exp(-atL): traffic loading caused by fill-in signal units.  hpa c4 P z c4 P  = 1 - a - a c4 P 3 c4 P   hpH c4 P 1 c4 P  = at c4 P L c4 P   hpH c4 P 2 c4 P  = at c4 P L c4 P (k c4 P 1 c4 P  + at c4 P L c4 P )  hpH c4 P 3 c4 P  = at c4 P L c4 P (k c4 P 2 c4 P  + 3at c4 P L c4 P k c4 P 1 c4 P  + a c4 P 2 c4 P t c4 P L2 c4 P )  hpF c4 P 1 c4 P  = at c4 P L c4 P /2  hpF c4 P 2 c4 P  = at c4 P L c4 P (k c4 P 1 c4 P /2 + at c4 P L c4 P /3)  hpF c4 P 3 c4 P  = at c4 P L c4 P (k c4 P 2 c4 P /2 + at c4 P L c4 P k c4 P 1 c4 P  + a c4 P 2 c4 P t c4 P L2 c4 P /4)  hpq c4 P a c4 P  = eq \f( k c4 P 1 c4 P  (a + a c4 P z c4 P ) + a c4 P 3 c4 P t c4 P f c4 P ,2(1 - a))  H  hps c4 P a c4 P  = eq \f( ak c4 P 1 c4 P ,1 - a) q c4 P a c4 P  + eq \f( k c4 P 2 c4 P  (a + a c4 P z c4 P ) + a c4 P 3 c4 P t c4 P f2 c4 P ,3(1 - a))  HX  hpt c4 P a c4 P  = eq \f( 3ak c4 P 1 c4 P s c4 P a c4 P  + 2ak c4 P 2 c4 P q c4 P a c4 P ,2(1 - a)) + eq \f( (a + a c4 P z c4 P )k c4 P 3 c4 P  + a c4 P 3 c4 P t c4 P f3 c4 P ,4(1 - a)) hpZ c4 P 1 c4 P  = 2 + P c4 P u c4 P (1 + H c4 P 1 c4 P )  hpZ c4 P 2 c4 P  = 4K c4 P 1 c4 P  + P c4 P u c4 P (5k c4 P 1 c4 P  + 3H c4 P 1 c4 P  + H c4 P 2 c4 P )  hpZ c4 P 3 c4 P  = 8k c4 P 2 c4 P  + P c4 P u c4 P (19k c4 P 2 c4 P  + 27k c4 P 1 c4 P H c4 P 1 c4 P  + 9H c4 P 2 c4 P  + H c4 P 3 c4 P )  hpY c4 P 2 c4 P  = s c4 P a c4 P  + 4k c4 P 1 c4 P  + F c4 P 2 c4 P  + 2{q c4 P a c4 P (2 + F c4 P 1 c4 P ) + 2F c4 P 1 c4 P }  HX  hpY c4 P 3 c4 P  = t c4 P a c4 P  + 8k c4 P 2 c4 P  + F c4 P 3 c4 P  + 3{s c4 P a c4 P (2 + F c4 P 1 c4 P ) + q c4 P a c4 P (4k c4 P 1 c4 P  + F c4 P 2 c4 P ) + 2F + 2 + 4k c4 P 1 c4 P F c4 P 1 c4 P } + 12q c4 P a c4 P F c4 P 1 c4 P   hpa = eq \f( 1 - a{2 + P c4 P u c4 P (1 + at c4 P L c4 P )},2 + q c4 P a c4 P  + at c4 P L c4 P /2)  hpq c4 P d c4 P  = eq \f( aZ c4 P 2 c4 P  + aY c4 P 2 c4 P ,2(1 - aZ c4 P 1 c4 P ))  hps c4 P d c4 P  = eq \f( aZ c4 P 2 c4 P ,1 - aZ c4 P 1 c4 P ) q c4 P d c4 P  + eq \f( aZ c4 P 3 c4 P  + aY c4 P 3 c4 P ,3(1 - aZ c4 P 1 c4 P ))  hpq c4 P b c4 P  = eq \f( q c4 P a c4 P  + 1 + F c4 P 1 c4 P ,1 - a)  HX  hps c4 P b c4 P  = eq \f( s c4 P a c4 P  + k c4 P 1 c4 P  + F c4 P 2 c4 P ,(1 - a) c4 P 3 c4 P ) + eq \f( 2{q c4 P a c4 P (1 + F c4 P 1 c4 P ) + F c4 P 1 c4 P },(1 - a) c4 P 2 c4 P )  hpq c4 P c c4 P  = eq \f( q c4 P d c4 P  + 1 + P c4 P u c4 P (1 + H c4 P 1 c4 P ),1 - a)  H  hps c4 P c c4 P  = eq \f( s c4 P d c4 P  + k c4 P 1 c4 P  + P c4 P u c4 P (3k c4 P 1 c4 P  + H c4 P 2 c4 P ),(1 - a) c4 P 3 c4 P ) + 2 eq \f( q c4 P d c4 P  + P c4 P u c4 P {q c4 P d c4 P (1 + H c4 P 1 c4 P ) + 2H c4 P 1 c4 P },(1 - a) c4 P 2 c4 P )  H  hpP c4 P V c4 P  = P c4 P u c4 P a eq \f( q c4 P a c4 P  + 2 + at c4 P L c4 P /2,1 - 2a) eq \b\bc\( ( 1 + P c4 P u c4 P \f( a + a c4 P 2 c4 P t c4 P L c4 P ,1 - 2a))  HH  H4.2.3 Formulas  H  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: Hp8"(# Ђ p8P (T c4 P x c4 P ) @ expeq \b\bc\( ( -\f( T c4 P x c4 P  - Q c4 P x c4 P  + s c4 P x c4 P ,s c4 P x c4 P ))  H Hp P X`h!(#Ёwhere 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.  H4.2.4 Examples  H  Assuming the traffic models given in Table 2/Q.706, examples of queueing delays are calculated as listed in Table 3/Q.706.  H  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. K c4 P Figure 1/Q.706 CCITT 35211  c4 P  S c4 P TABLE 1/Q.706 N Queueing delay formula  c4 P H 8"҇Hp N c4 P Error correction method H` 0  UDisturba Xnce H X( VMean Q HP8" TVariance s c4 P 2 c4 P   8 P8" c4 P ш 8  c4 P H 8"҇p  c4 P  ` 0  0 vAbsence  X(  x eq \f( Q c4 P a c4 P ,T c4 P m c4 P ) = \f( t c4 P f c4 P ,2) + \f( ak c4 P 1 c4 P ,2(1 - a)) P8"eq \f(s\s(2,a),T\s(2,m)) = \f( t\s(2,f),12) + \f(a[4k c4 P 2 c4 P  - (4k c4 P 2 c4 P  - 3k\s(2,1))a], 12 (1 - a) c4 P 2 c4 P )  8  c4 P ш 8  c4 P H 8"҇p 0 w c4 P Basic ` 0  0 uPresence  X(  x eq \f( Q c4 P t c4 P ,T c4 P m c4 P ) = \f( t c4 P f c4 P ,2) + \f( aE c4 P 2 c4 P ,2(1 - aE c4 P 1 c4 P )) +  E c4 P 1 c4 P  - 1 P8"eq \f (s\s(2,t),T\s(2,m)) = \f( t\s(2,f),12) + \f( a[4E c4 P 3 c4 P  - (4E c4 P 1 c4 P E c4 P 3 c4 P  - 3E\s(2,2))a],12(1 - aE c4 P 1 c4 P ) c4 P 2 c4 P ) 0qeq \d\fo40() + P c4 P u c4 P (1 - P c4 P u c4 P )t\s(2,L)  8  c4 P ш 8  c4 P H 8"҇p  c4 P  0 uPreventi 0 tve cyclic ` 0  0 vAbsence  X( 0 peq \f( Q c4 P a c4 P ,T c4 P m c4 P ) = q c4 P a  c4 P   x P8"eq \f( s\s(2,t),T\s(2,m)) =sa - q\s(2,a)  8  c4 P ш 8  c4 P H 8"҇p 0 r c4 P retransmission ` 0  0 uPresence  X(   eq \f( Q c4 P t c4 P ,T c4 P m c4 P ) = (1 - Pu - Pv) qa + Puqb + Pvqc P8"eq \f( s\s(2,t),T\s(2,m)) = (1 - Pu - Pv) sa eq \d\fo40()+ Pusb + Pvsc - \f( Qs(2,t),T\s(2,m))  8  c4 P ш HH Hp P X`h!(#Ё 8L c4 P TABLE 2/Q.706 8L Traffic model  c4 P H x(҇Hp P  c4 P Model Hh88RA 8RB  xH xh8 c4 P ш xH  c4 P H xXH(҇ x p P x c4 P Message length (bits) xh8!W120 !W104 !W304  HH hH8 c4 P ш HH  c4 P H xXH(҇ Hh p P H c4 P Percent hH8'(U100 '(U92 '(V8  HH  c4 P ш HH  c4 P H x(҇ HP p P H c4 P Mean message length (bits) hH8'b120 'b120  xH xh8 c4 P ш xH  c4 P H x(҇p P x c4 P k1 xh8!K1.0 !J 1.2  xH  c4 P ш xH  c4 P H x(҇p P x c4 P k2 xh8!K1.0 !J 1.9  xH  c4 P ш xH  c4 P H x(҇p P x c4 P k3 xh8!K1.0 !J 3.8  xH  c4 P ш HH Hp P X`h!(# c4 P  8LTABLE 3/Q.706 8J List of examples  c4 P H XHp҇8O c4 P Figure 8LError control 8KQueueing delay 8MDisturbance 8PModel  p p P Xp`h!(# c4 P ш p  c4 P H XHp҇ c4 P 2/Q.706 Basic/PCR Mean Absence A and B  p  c4 P ш p  c4 P H XHp҇ c4 P 3/Q.706 Basic/PCR Standard deviation Absence A and B  p  c4 P ш p  c4 P H XHp҇ c4 P 4/Q.706 Basic Mean Presence A  p  c4 P ш p  c4 P H XHp҇ c4 P 5/Q.706 Basic Standard deviation Presence A  p  c4 P ш p  c4 P H XHp҇ c4 P 6/Q.706 PCR Mean Presence A  p  c4 P ш p  c4 P H XHp҇ c4 P 7/Q.706 PCR Standard deviation Presence A  p  c4 P ш HH Hp P X`h!(# c4 P  8DFigure 2/Q.706 CCITT 35220  c4 P  8D c4 P Figure 3/Q.706 CCITT 41040  c4 P  8D c4 P Figure 4/Q.706 CCITT 41211  c4 P  8D c4 P Figure 5/Q.706 CCITT 41200  c4 P  8D c4 P Figure 6/Q.706 CCITT 41222  c4 P  8D c4 P Figure 7/Q.706 CCITT 41226  c4 P  4.3h  Message transfer times  Hx  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.  H4.3.1 Message transfer time components and functional reference points  H  A signalling path may include the following functional signalling network components and transfer time components.  H   a)pMessage Transfer Part sending function at the point of origin (see Figure 8/Q.706).   b)pSignalling transfer point function (see Figure 9/Q.706).  H   c)pMessage Transfer Part receiving function at the point of destination (see Figure 10/Q.706).  Hh   d)pSignalling data link propagation time (see Figure 11/Q.706).   e)pQueueing delay.  H  An additional increase of the overall message transfer times is caused by the queueing delays. These are described in S 4.2.  HH Ђ8D c4 P Figure 8/Q.706 CCITT 35270  c4 P  8D c4 P Figure 9/Q.706 CCITT 35280  c4 P  8D c4 P Figure 10/Q.706 CCITT 35290  c4 P  8D c4 P Figure 11/Q.706 CCITT 35300  c4 P   H4.3.2 Definitions  H4.3.2.1p message transfer part sending time Tms   F: temps d')mission du Sous-syst/me Transport de Messages Tms  Hh   S: tiempo de emisi;n de la parte de transferencia de mensajes Tms  H  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.  H4.3.2.2p message transfer time at signalling transfer points Tcs  H   F: temps de transfert des messages aux points de transfert s)maphore Tcs  H   S: tiempo de transferencia de mensajes en los puntos de transferencia de la se9alizaci;n Tcs  H  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.  H4.3.2.3p message transfer part receiving time Tmr  HX   F: temps de r)ception du Sous-syst/me Transport de Messages Tmr  Hx   S: tiempo de recepci;n de la parte de transferencia de mensajes Tmr  H  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.  H4.3.2.4p data channel propagation time Tp   F: temps de propagation sur la voie de donn)es Tp   S: tiempo de propagaci;n del canal de datos Tp  H  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. HP X`h!(#X X  H4.3.3  c4 P Overall message transfer times Hp P X`h!(# c4 P  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.  H  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)pIn the absence of disturbances  H Hp8"(# Ђ p8'eq H*"*T c4 P oa c4 P  = T c4 P ms c4 P  + \i\su(i=1,n+1, )\d\fo10()T c4 P pi c4 P  + \i\su(i=1,n, )\d\fo10()T c4 P csi c4 P  + T c4 P mr c4 P  Hp P X`h!(#  b)pIn the presence of disturbances  HH Hp8"(# Ђ p8!T c4 P o c4 P  = T c4 P oa c4 P  + (Q c4 P t c4 P  - Q c4 P a c4 P ) Hp P X`h!(#Ё Here  H   Toap overall message transfer time in the absence of disturbances   Tmsp Message Transfer Part sending time   Tmrp Message Transfer Part receiving time   Tcsp Message transfer time at signalling transfer points   np number of STPs involved   Tpp data channel propagation time  H   Top overall message transfer time in the presence of disturbances   Qtp total queueing delay (see S 4.2)  Hh   Qap queueing delay in the absence of disturbances (see S 4.2)  H  Note - For S(Qt - Qa), all signalling points in the signalling relation must be taken into account.  4.3.4  c4 P Estimates for message transfer times  HH  c4 P  (Needs further study.)  The estimates must take account of:  - the length of the signal unit,  - the signalling traffic load,  - the signalling bit rate.  H  The estimates for Tmr, Tms and Tcs will be presented in the form of:  - mean values,  - 95% level values.  H  The estimates for Tcs for a signalling transfer point are given in Table 4/Q.706. S c4 P TABLE 4/Q.706  c4 P H X҇L c4 P STP signalling traffic load H XXMessage transfer time at an STP (Tcs) in ms    XX c4 P ш   c4 P H X҇ c4 P  :Mean :95 %    XX c4 P ш   c4 P H X҇A c4 P Normal C20 C40    c4 P ш   c4 P H X҇B c4 P +15 % C40 C80    c4 P ш   c4 P H X҇B c4 P +30 % C100 C200    c4 P ш HH Hp P X`h!(#Ё  H  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.  H  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. 4.4h  Error control  H  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.  H  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.  H  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. HP X`h!(#X X  H4.5   c4 P Security arrangements  H Hp P X`h!(# c4 P  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.  4.5.1 Types of security arrangements  H  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.  H  H4.5.1.1pSecurity 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  H (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.  H4.5.1.2pSecurity arrangements for signalling relations  H  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.  H4.5.2 Security requirements  H  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.) HP X`h!(#X X  H4.5.3  c4 P Time to initiate changeover  H Hp P X`h!(# c4 P  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).  H  Changeover leads to substantial additional queueing delays. To keep the latter as short as possible, the signalling traffic affected by an outage  H is reduced to a minimum by the use of load sharing on all existing signalling links. HP X`h!(#X X  H4.5.4  c4 P Changeover performance times Hp P X`h!(# c4 P  There are two performance times associated with link changeover. Both times are maximum time values (not normal values). They are defined to be  H 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. HP X`h!(#X X  H4.5.4.1 c4 P Failure response time  H Hp P X`h!(# c4 P  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  H 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. HP X`h!(#X X  H4.5.4.2 c4 P Answer time to changeover order Hp P X`h!(# c4 P  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. 4.6h  Failures  H4.6.1 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.  H  Signal unit error monitoring initiates the changeover at a signal unit error rate of about 4 . 10 c4 P -3 c4 P .  H  The error rate, which Signalling System No. 7 has to cope with, represents a parameter of decisive influence on its efficiency.  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.  H4.6.2 Failures in signalling points  (Needs further study.) 4.7h  Priorities  H  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.  H  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). 5X Performance under adverse conditions 5.1h  Adverse conditions  (Needs further study.) 5.2h  Influence of adverse conditions  (Needs further study.)  Reference [1]h  CCITT Recommendation Error performance on an international digital connection forming part of an integrated services digital network, Vol. III, Rec. G.821.