destination point. The part of the message that is used for routing is called the routing label, and it con- tains the information necessary to deliver the message to its destination point. Normally the routing label is common to all services and applications in a given signaling network, national and international. [However, if this is not the case, the particular routing label of a message is determined by means of the service indicator (SI).] The standard routing label should be used in the international signaling network and is applicable in national applications. The standard routing label is 32 bits long and is placed at the beginning of the signaling information field (SIO). Its structure is illustrated in Figure 14.6. The destination point code (DPC) indicates the destination of the message. The orig- inating point code (OPC) indicates the originating point of the message. The coding of these codes is pure binary. Within each fold, the least significant bit occupies the first position and is transmitted first. A unique numbering scheme for the coding of the fields is used for the signaling points of the international network irrespective of the user parts connected to each signaling First bit Length transmitted (bit) Routing label Label DPC Destination point code OPC Originating print code SLS Signaling link selection Figure 14.6 Routing label structure. [Based on Figure 3/Q.704, page 5, CCITT Rec. Q.704 (Ref. 8).] CCITT SIGNALING SYSTEM NO. 7 point. The signaling link selection (SLS) field is used, where appropriate, in performing load sharing. This field exists in all types of messages and always in the same position. The only exception to this rule is some message transfer part layer 3 messages (e.g., changeover order) for which the message routing function in the signaling point of origin of the message is not dependent on the field. In this particular case the field does not exist as such, but is replaced by other information (e.g., in the case of the changeover order, the identity of the faulty link