Internet Engineering Task Force INTERNET DRAFT Authors Signaling Transport Working Group Huai-An P. Lin June 26, 1999 Taruni Seth Expires December 26, 1999 Albert Broscius Christian Huitema Telcordia Technologies VoIP Signaling Performance Requirements and Expectations Status of this document This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document serves as input into the IETF SIGTRAN requirements process. It includes call setup delay requirements, derived from relevant ISDN and SS7 standards published by ITU-T (International Telecommunications Union--Telecommunications Standardization Sector) and generic requirements published by Telcordia Technologies (formerly Bellcore). To gain user acceptance of Voice-over-IP (VoIP) services and to enable interoperability between Switched Circuit Networks (SCNs) and VoIP systems, it is imperative that the VoIP signaling performance be comparable to that of the current SCNs. The requirements given in this Internet Draft are intended to be the worst-case requirements, for at least in United States SCN calls are typically set up far faster than the derived requirements. 1. Introduction This document serves as input into the IETF SIGTRAN requirements process. It includes call setup delay requirements, derived from relevant ISDN and SS7 standards published by ITU-T (International Telecommunications Union--Telecommunications Standardization Sector) and generic requirements published by Telcordia Technologies (formerly Bellcore). To gain user acceptance of Voice-over-IP (VoIP) services and Lin, Seth, Broscius, Huitema [Page 1] to enable interoperability between Switched Circuit Networks (SCNs) and VoIP systems, it is imperative that the VoIP signaling performance be comparable to that of the current SCNs. The requirements given in this Internet Draft are intended to be the worst-case requirements, since at least in United States SCN calls are typically set up within one to two seconds [1]--far faster than the derived requirements. The call setup delay, also known as the post-dialing delay, in an ISDN- SS7 environment is the period that starts when an ISDN user dials the last digit of the called number and ends when the user receives the last bit of the Alerting message. Call setup delays are not explicitly given in the existing SCN performance requirements; rather, performances of SCNs are typically expressed in terms of cross-switch (or cross-office) transfer times. This Internet Draft uses ITU-TÆs SS7 Hypothetical Signaling Reference Connection (HSRC) [2], cross-STP (Signaling Transfer Point) time [3], TelcordiaÆs switch response time generic requirements [4], and a simple ISDN-SS7 call flow to derive the call setup delay requirements. ITU-TÆs cross-switch time requirements [5] are listed as references but not used, since the ISDN timings are missing. 2. Hypothetical Signaling Reference Connection (HSRC) HSRC is specified in ITU-T Recommendation Q.709. A HSRC is made up by a set of signaling points and STPs that are connected in series by signaling data links to produce a signaling connection. Recommendation Q.709 distinguishes the ônationalö components from the ôinternationalö components. A HSRC for international working consists of an international component and two national components. The size of each country is considered; however, the definitions of ôlargeö and ôaverageö countries was not completely precise: ôWhen the maximum distance between an international switching center and a subscriber who can be reached from it does not exceed 1000 km or, exceptionally, 1500 km, and when the country has less than n Ú 10E7 subscribers, the country is considered to be of average-size. A country with a larger distance between an international switching center and a subscriber, or with more than n Ú 10E7 subscribers, is considered to be of large-size. (The value of n is for further study.)ö Recommendation Q.709 uses a probabilistic approach to specify the number of signaling points and STPs on a signaling connection. The maximum number of signaling points and STPs allowed in a national component and an international component are listed in Tables 1 and 2, respectively. Table 1: Maximum Number of Signaling Points and STPs in a National Component (Source: ITU-T Recommendation Q.709, Table 3) Country size Percent of Number of Number of connections STPs signaling points* Large-size 50% 3 3 95% 4 4 Lin, Seth, Broscius, Huitema [Page 2] Average-size 50% 2 2 95% 3 3 * The terms signaling points and switches are used interchangeably in this Internet Draft. Table 2: Maximum Number of Signaling Points and STPs in International Component (Source: ITU-T Recommendation Q.709, Table 1) Country size Percent of Number of Number of connections STPs signaling points Large-size 50% 3 3 to Large-size 95% 4 3 Large-size 50% 4 4 to Average-size 95% 5 4 Average-size 50% 5 5 to Average-size 95% 7 5 3. Switch Response Time (aka Cross-switch Transfer Time) Most of SCN performance requirements are specified in terms of switch response times, which are also referred to as cross-switch transport time or cross-switch delay. This section reviews the meanings of switch response times, several other related terms, and the generally accepted values of switch response times published by Telcordia Technologies. The corresponding ITU-TÆs cross-switch timing requirements are also listed as references. This Internet Draft reviews the switch response time requirements intended to apply under normal loading. Normal loading is usually associated with the notion of the Average Busy Season Busy Hour (ABSBH) load. Simply put, it is expected that the switch response times that a particular switch experiences at this load will be virtually load- independent. Switch response time is the period that starts when a stimulus occurs at the switch and ends when the switch completes its response to the stimulus. The occurrence of a stimulus often means the switch receives the last bit of a message from an incoming signaling link, and completion of a response means the switch transmits the last bit of the message on the outgoing signaling link. If the switchÆs response to a stimulus involves the switch sending a message on the outgoing signaling link, then switch processing time is the sum of the switch processing time and the link output delay: switch response time = switch processing time + link output delay Lin, Seth, Broscius, Huitema [Page 3] Switch processing time is the period that starts when a stimulus occurs at the switch and ends when the switch places the last bit of the message in the output signaling link controller buffer. The period between the switch placing the message in the output signaling link controller buffer and the switch transmitting the last bit of the message on the outgoing signaling link is defined as the link output delay. Link output delay can be further divided into the queuing delay and message emission time. There are separate delay requirements for switch processing time and link output delay; however, for simplicity only the combined delay requirements for switch response time, as given in Table 3, will be listed in this Internet Draft. Table 3: Switch Response Time Assuming Typical Traffic Mix and Message Lengths (Source: Telcordia GR-1364-CORE, Table 5-1) Type of Call Segment Switch Response Time (ms) Mean 95% ISUP Message 205-218 <=337-349 Alerting 400 <=532 ISDN Access Message 220-227 <=352-359 TCAP Message 210-222 <=342-354 Announcement/Tone 300 <=432 Connection 300 <=432 Telcordia GR-1364 specifies switch response time using ôswitch call segmentsö as a convenient way to refer to the various phases of call processing that switches are involved in. (An alternative would be proposing switch processing requirements for every possible type of switch processing. Obviously, this would become burdensome and would necessitate adding to the requirements every time an additional type of switch processing was required.) Listed in Table 3 are: 1. ISUP message call segments that involve the switch sending an ISUP message as a result of a stimulus. 2. Alerting call segments that involve the switch alerting the originating and/or terminating lines as a result of a stimulus. 3. ISDN access message call segments that involve the switch sending an ISDN access message (other than an ISDN access ALERT message) as a result of stimulus. ISDN access message call segment processing occurs at originating or terminating switches where the originating or terminating line, respectively, is an ISDN line. 4. TCAP message call segments that involve the switch sending a TCAP message as a result of a stimulus. 5. Announcement/tone call segments that involve the switch playing an announcement, placing a tone on, or removing a tone from the originating or terminating line as a result of a stimulus. However, the announcement/tone call segments do not include dial-tone delay, of which the delay requirements can be found in Telcordia TR-TSY-000511[6]. 6. Connection call segments involve the switch connecting one or more users as a result of a stimulus. The ITU-TÆs cross-switch timing requirements are listed below as Lin, Seth, Broscius, Huitema [Page 4] references. It is noted that the ITU-TÆs requirements are noticeably stringent that those of Telcordia under the normal loading. However, since the ITU-TÆs values are stated as ôprovisionalö and they do not provide the timing requirements for ISDN, TelcordiaÆs values will be used to derive the call setup delay requirements. Table 4: ITU-T Cross-Switch Transfer Time (Source: ITU-T Recommendation Q.725, Table 3) Exchange call Cross-Switch Transfer Time (ms)* Message typ attempt loading Mean 95% Simple Normal 110 220 (e.g. answer) +15% 165 330 +30% 275 550 Processing Normal 180 360 intensive +15% 270 540 (e.g. IAM) +30% 450 900 * Provisional values. 4. Cross-STP Delay Message delay through an STP is specified as the cross-STP delay. It is the interval that begins when the STP receives the last bit of a message from the incoming signaling link, and ends when the STP transmits the last bit of the message on the outgoing signaling link. As with the switch response time discussed in the previous section, the cross-STP can be divided into processor handling time and link output delay. This Internet Draft adopts the cross-STP delay requirements specified in ITU- T Q.706 Recommendation. Table 5: Message transfer time at an STP (Source: ITU-T Recommendation Q.706, Table 5) Message transfer Time (ms) STP signaling traffic load Mean 95% Normal 20 40 +15% 40 80 +30% 100 200 5. Maximum End-to-End Signaling Delays Using the HSRC, switch response times, and cross-STP delays, one can compute the maximum signaling transfer delays for ISUP messages under normal load. As with Telcordia GR-1364, it is assumed that the distribution of switch response time for each call segment is approximately a normal distribution. It is further assumed that switch response times of different switches are independent. Under these assumptions, the end-to-end (from originating switch to terminating Lin, Seth, Broscius, Huitema [Page 5] switch) delays for each national component and for international calls are listed in Tables 6 and 7, respectively. The 20 ms cross-STP delay is assumed in all cases. It should be noted that all these values must be increased by the transmission propagation delays, which are listed in Table 8. Table 6: Maximum ISUP Signal Transfer Delays for Each National Component Country size Percent of Delay (ms) connections Mean 95% Large-size 50% 675-714 <=904-941 95% 900-952 <=1164-1214 Average-size 50% 450-476 <=637-661 95% 675-714 <=904-941 Table 7: Maximum ISUP Signal Transfer Delays for International Calls Country size Percent of Delay (ms) connections Mean 95% Large-size to 50% 2025-2142 <=2421-2538 Large-size 95% 2495-2638 <=2933-3076 Large-size to 50% 2250-2380 <=2677-2797 Average-size 95% 2720-2876 <=3177-3333 Average-size to 50% 2475-2618 <=2913-3056 Average-size 95% 2965-3134 <=3441-3610 Table 8: Calculated Terrestrial Transmission Delays for Various Call Distances (Source: ITU-T Recommendation Q.706, Table 1) Arc length Delay terrestrial (ms) (km) Wire Fibre Radio 500 2.4 2.5 1.7 1000 4.8 5.0 3.3 2000 9.6 10.0 6.6 5000 24.0 25.0 16.5 10000 48.0 50.0 33.0 15000 72.0 75.0 49.5 17737 85.1 88.7 58.5 20000 96.0 100.0 66.0 25000 120.0 125.0 82.5 6. Basic Call Flow and Call Setup Delays The following figure illustrates the simplest call flow for call setup in an ISDN-SS7 environment. The end user terminals are assumed to be ISDN phones and use Q.931 messages (i.e., Setup and Alerting). The switches use ISUP messages to establish inter-switch trunks for the subsequent voice communication. Lin, Seth, Broscius, Huitema [Page 6] Figure 1: Simple Call Setup Signaling Flow Caller Originating Terminating Called Terminal Switch Switch Terminal | | | | | Setup | | | |---------------->| | | | | IAM IAM | | | |---------> . . . . --------->| | | | | Setup | | | |-------------->| | | | | | | | Alerting | | | |<--------------| | | ACM ACM | | | |<--------- . . . . <---------| | | Alerting | | | |<----------------| | | | | | | | | | | Using the above call flow, the end-to-end message transfer delays in Tables 6 and 7, and the switch response times for Q.931 messages in Table 3, one can derive the call setup times given in the following tables. Again, all these values must be increased by the transmission propagation delays listed in Table 8. Table 9: Call Setup Delays for Each National Component Country size Percent of Call Setup Delay (ms) connections Mean 95% Large-size 50% 2590-2682 <=3007-3099 95% 3040-3158 <=3497-3615 Average-size 50% 2140-2206 <=2513-2579 95% 2590-2682 <=3007-3099 Table 10: Call Setup Delays for International Calls Country size Percent of Delay (ms) connections Mean 95% Large-size to 50% 5290-5538 <=5909-6157 Large-size 95% 6230-6530 <=6903-7203 Large-size to 50% 5740-6014 <=6387-6661 Average-size 95% 6680-7006 <=7378-7704 Average-size to 50% 6190-6490 <=6863-7163 Average-size 95% 7170-7522 <=7893-8245 Lin, Seth, Broscius, Huitema [Page 7] 8. User Expectations The requirements derived in the previous section should be interpreted as the worst-case requirements. At least in the United States, users of SCN typically experience far less setup delays than the derived delay requirements. With the maturing of Common Channel Signaling (CCS) Network, call setup time has been reduced to a mere one to two seconds [1]. The VoIP networks are expected to achieve the same level of delay There is no known study on expected setup delays for international calls. As discussed, a HSRC for international working consists of an international component and two national components, and the maximum number of signaling points and STPs in a national component is roughly the same as the number in an international component (Tables 1 and 2). As a consequence, the end-to-end ISUP delays in an international call are roughly three times of those in a national call. On the other hand, the Q.931 signals occur only at the two ends for both national and international calls. Based on these observations, one may expect 2.5-5 second call setup delays to be reasonable for international calls. Acknowledgements The authors would like to express their gratitude to Dr. Daniel Luan of AT&T Labs for his insight into network operation and valuable suggestions for calculating end-to-end signaling delays as well as call setup delays. References [1] AT&T Webpage, www.att.com/technology/technologists/fellows/lawser.html. [2] ITU-T Recommendation Q.709, Specifications of Signaling System No. 7--Hypothetical Signaling Reference Connection, March 1993. [3] Telcordia Technologies Generic Requirements GR-1364-CORE, Issue 1, LSSGR: Switch Processing Time Generic Requirements Section 5.6, June 1995. [4] ITU-T Recommendation Q.706, Specifications of Signaling System No. 7ùMessage Transfer Part Signaling Performance, March 1993. [5] ITU-T Recommendation Q.706, Specifications of Signaling System No. 7ùSignaling performance in the Telephone Application, March 1993. [6] Telcordia Technologies TR-TSY-000511, LSSGR: Service Standards, Section 11, Issue 2, July 1987. Authors' addresses Lin, Seth, Broscius, Huitema [Page 8] Huai-An Lin Telcordia Technologies 445 South Street, MCC-1A216R Morristown, NJ 07960-6438 Phone: 973 829-2412 Email: hlin@research.telcordia.com Taruni Seth Telcordia Technologies 445 South Street, MCC-1G209R Morristown, NJ 07960-6438 Phone: 973 829-4046 Email: taruni@research.telcordia.com Albert Broscius Telcordia Technologies 445 South Street, MCC-1A264B Morristown, NJ 07960-6438 Phone: 973 829-4781 Email: broscius@research.telcordia.com Christian Huitema Telcordia Technologies 445 South Street, MCC-1J244B Morristown, NJ 07960-6438 Phone: 973 829-4266 Email: huitema@research.telcordia.com Lin, Seth, Broscius, Huitema [Page 9]