IEC_61158-4-7-2007_cor1-2014.pdf

January 2014 IEC 61158-4-7(First edition 2007)Industrial communication networks Fieldbus specifications Part 4-7:Data-link layer protocol specification Type 7 elements C O R R I G E N D U M 1 Annex C Topology of multi-segment DL-subnetwork Replace the existing text of the annex by the following:Annex C(informative)Topology of multi-segment DL-subnetwork C.1 Introduction This annex describes how to specify the topology of a multi-segment DL-subnetwork.The aim is to propose a data structure,which could be minimal while allowing correct operation of the bridge retransmission function.The topology of a DL-subnetwork can first be specified globally,in order to verify a certain number of properties(topological connectivity,non-meshing,etc.);then on the basis of this specification the local data base specific to each bridge must be calculated in order to ensure it operates correctly.Although this appendix proposes a method to achieve this goal,only the specifications of the data structures,global or local to each bridge,which define the DL-subnetwork topology,as well as the properties which it should fulfil,must be taken into account in the standard.The suggested method shows how to obtain a solution to the problem by taking into account certain optimization problems.C.2 Global specification The topology of a multi-segment DL-subnetwork can be defined by the following elements:the set S of its segments:nisSi,1=the set B of its bridges:mkbBk,1=and for each bridge of B,the data of a matrix Bk of dimension n n.whose coefficients kijb are defined by:January 2014 0=kijb if i=j;=kijb if the bridge bk does not allow transfer of messages from segment si to segment sj;=kijb with R+*,if the bridge bk allows the transfer of messages from segment si towards segment sj,with as load coefficient which allows taking into account of a different efficiency rate according to the transfers.A load coefficient kijb can represent the load,as a rate of occupation of the medium,of the retransmission segment sj.In reality,either the destination is directly sj,or there are several paths possible,passing through intermediate segments,to reach sj and in this case the choice shall be to pass by the least loaded path.It is of course possible to take as coefficients the same value(1 for example).If a bridge allows two-way retransmission with the same load coefficient for the two directions,its matrix is symmetrical.The matrix Bk of a bridge also allows knowing all the segments to which it is connected:either in reception,Srk=segments whose corresponding line in the matrix includes at least one non-null finite coefficient;note nrk=card(Srk),or in transmission,Sek=segments whose corresponding column in the matrix includes at least one non-null finite coefficient;note nek=card(Sek).C.3 Local specification The information which a bridge must have locally allows it to answer the following question:when I receive a message on a segment sri Srk destined for another segment sj,must I do nothing or must I retransmit on segment seh Sek).To fulfil this purpose,it is enough to allocate to each bridge bk a transfer matrix Tk with dimensions nrk n,whose elements kijr are defined by:the line index i 1,nrk references segments sri connected in reception(Srk),the column index j 1,n references the segments sj of the DL-subnetwork(S),0=kijr if on reception of a message on segment sri Srk addressed to segment sj,the bridge shall not do anything,either because sj cannot be reached via this bridge,or because sri=sj(a bridge shall not retransmit a message received from a segment towards this same segment),hkijser=,with seh Sek,if on reception of a message on segment sri Srk addressed to segment sj,the bridge must retransmit to segment seh.NOTE Indexes i and h correspond to channel numbers whereas sri is the segment connected in reception to channel i and seh is the segment connected in transmission to channel h.January 2014 C.4 Properties The properties which should satisfy the DL-subnetwork are topological connectivity and non-meshing.Topological connectivity consists in ensuring that there is always a path from any given segment of S to any other segment of S.Non-meshing consists in ensuring that the transmission of a message from a transmitter located on segment se and addressed to a receiver located on segment sr can be routed by only one path(thus preventing the message from being received more than once).In fact,it is the definition of the local specification of each bridge and the calculation of its transfer matrix which ensure this property:by definition,on reception of a message on segment sri addressed to segment sj,either the bridge does not retransmit it,in particular if segment sri is equal to segment sj,or the bridge retransmits it on a single segment seh,whereas by calculation of the matrix,it is necessary to make sure that one and only one bridge,connected in reception to segment sri,retransmits the message.C.5 Methods The method consists in calculating