supervision message group; and the node-to-node message group (CCITT Recs. Q.722 and Q.723 (Refs. 14, 16). Label capacity for the telephone user part is given in CCITT Rec. Q.725 (Ref. 17) 16,384 signaling points and up to 4096 speech circuits for each signaling point. REVIEW EXERCISES 1. What is the principal rationale for developing and implementing Signaling System No. 7? 2. Describe SS No. 7 and its relationship with OSI. Why does it truncate at OSI layer 4? 3. Give the two primary "parts" of SS No. 7. Briefly describe each part. REFERENCES 4. OSI layer 4 is subdivided into two sublayers. What are they? 5. Layer 2 carries out the functions of the signaling link. Name four of the five functions of layer 2. 6. What are the two basic categories of functions of layer 3? 7. What is a signaling relation? 8. How are signaling points defined? (Identified). 9. What does a routing label do? 10. What are the two methods of error correction in SS No. 7? What are the three types of signal units used in SS No. 7? 12. Discuss forward and backward sequence numbers. The routing label is analogous to what in our present telephone system? Name the three basic pieces of information that the routing label provides. 14. Define labeling potential. 15. What is the function of an STP? Differentiate STP with signaling point. 16. Why do we wish to limit the number of STPs in a specific relation? What are the three measures of performance of SS No. 7? 18. Regarding user parts, what is the function of the SIO? Of the network indicator? 19. What is the purpose of circuit continuity? 20. With the TUP address signals are sent digit-by-digit embedded in the last subfield of the SIF. 21. What are the two overall set of services of the SCCP? 22. What is the purpose of the SCCP? REFERENCES 1. Specifications of Signaling System No. 7 (Q.700 Series), Fascicle VI.6, CCITT Yellow Books, VIIth Plenary Assembly, Geneva, 1980. 2. W. C. Roehr, Jr., "Signaling System No. 7," in Tutorial: Integrated Services Digital Network (ISDN), W. Stallings, ed., IEE