School of Specs IMS — the call as an internet sessionOverview

What IMS is, and where it came from · chapter 3 of 7 · 7 minutes

3 Rebuilt for every new network: circuits, LTE, the browser, 5G

The pattern that makes this subject unlike a young one — every new way of carrying IP sends the same system back to the drawing board, from circuit networks in Release 6 to the 5G core in Releases 15 to 17.

3.1 The promise that keeps the work open

What IMS is, in the words of its own documents ends on one definition out of TS 22.228: subscribers should reach their IP Multimedia services over any access network able to carry IP.

That sentence is cheap to write and expensive to keep. Every time a new way of carrying IP arrives — or an old way that has to be reached — the same system goes back to the drawing board, and a fresh ring of work plan rows opens around it.

This chapter walks that pattern from Release 6 to Release 17. It is the shortest answer to why a subject opened in Release 5 still has documents filed against it today.

3.2 Circuits, the network that was already there

The first thing IMS had to reach was not a new network but the old one. The Release 6 row is IMS-CCR-IWCS, and its reasoning is blunt. IMS-CCR-IWCS

Its objective is the same sentence turned into a job: address interworking between the IM CN subsystem and circuit-switched networks, in order to support basic voice calls. Not video, not messaging — basic voice calls. [8]

Release 6 carries 17 work plan rows in this subject [9], and this one row has 207 documents against it in the local record. [10]

3.3 Emergency calls, and carrying on when something moves

The rows marked Rel-9 are where the emergency call, service continuity and recovery from failure arrive. Rel-8 carries 64 rows and Rel-9 carries 45 [11] [12], and several of the Rel-9 rows open by naming what Release 8 had already specified.

Emergency calling came first. IMS_EMER_GPRS_EPS set out to update the requirements for IMS emergency calls over GERAN, UTRAN and E-UTRAN access — three kinds of access named in one sentence. [13] IMS_EMER_GPRS_EPS

That is a regulator's argument rather than an engineer's, and emergency calling turns up again in the 5G work at the end of this chapter.

IMS_SCC follows, and its justification is a plain statement of what Release 8 had already built. IMS_SCC

Those two ideas have a document each: TS 23.292 for centralized services and TS 23.237 for service continuity. TS 23.292 TS 23.237

The third Rel-9 row is about what happens when a piece of the system falls over. eIMS_RP set out to finish the restoration procedures studied in TR 23.820 and written into TS 23.380. [14] TS 23.380

One part of them — the P-CSCF procedures — had not been finished in the Release 8 timeframe. [15]

3.4 The browser

Release 12 is where the browser arrives. IMS_WebRTC is a requirements row, and its justification names who was doing the work elsewhere. IMS_WebRTC

The IETF and the W3C are standards bodies outside 3GPP, and the justification names them, not 3GPP, as the ones specifying the browser client.

3GPP's own row is a requirements row — specify service requirements for WebRTC clients to reach IMS, reusing IMS client security credentials and public identities where that made sense. [16]

The pattern of one subject arriving at several groups shows plainly here. The requirements row IMS_WebRTC has 139 documents against it, and the protocol row IMS_WebRTC-CT has 101. IMS_WebRTC-CT

The document they produced is TS 24.371, which 160 documents in this record change. TS 24.371

3.5 Telepresence, and the media groups

Release 12 also opened telepresence, and the word is worth having from the document rather than from a brochure.

The objective is careful about what it is not: use cases and requirements for an IMS-based telepresence conference as an extension of the existing IMS conference services, for mobile and fixed devices alike. [17] IMS_TELEP

The media side took its own row a release later. IMS_TELEP_S4 is marked Rel-13 and belongs to SA4, and the document behind it is TS 26.223, the media handling for IMS-based telepresence. IMS_TELEP_S4 TS 26.223

Release 12 carries 60 work plan rows in this subject [18], more than Rel-9's 45 and under Rel-8's 64.

3.6 The 5G core

Release 15 is where the pattern repeats against the 5G core. 5GS_Ph1-IMSo5G set out to specify the core-network and terminal protocol aspects needed to carry IMS services, emergency services among them, over 5G. 5GS_Ph1-IMSo5G

The same objective says the technical issues of the 5G core itself are not covered by this work item. [19]

Read that twice. 5G did not bring its own way of making a call. The call stayed in IMS, and the new core became one more way of carrying IP to it — the access independence of What IMS is, in the words of its own documents, made good once again. That row has 476 documents against it. 5GS_Ph1-IMSo5G

Release 16 then reopened the question that first answer had left. FS_eIMS5G is a study, and a study says why it was needed.

The study's own aim keeps the same restraint: investigate what should change, with the existing IMS architecture as the baseline, and no change expected to the split between the parts or to the SIP signalling between them. [20]

The report is TR 23.794, and 190 documents in this record name it. TR 23.794

What the study concluded, eIMS5G_SBA then wrote: the service-based architecture conclusions of TR 23.794, including new services for the IMS service logic held in the combined UDM and HSS. [21]

It carries 543 documents, more than any other acronym in this register. [22]

Release 17 carried the leftovers. FS_eIMS5G2 was opened because key issues 1, 2, 3 and 8 of the first study were never finalised and no solution was agreed for them; its report for the CT1 side is TR 23.700-10. [23] TR 23.700-10

3.7 Where this goes next

Every row named here landed on a group, and the same handful of groups keeps appearing: the architecture group, the protocol groups, the media group, the test group. Who does the work: fourteen groups, and a core-network shape is that map, with the counts that go with it.

Where the numbers in this chapter come from

  1. why IMS-CCR-IWCS was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/2047/NP-030292/NP-030292.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with NP-030292, the document the work plan points at
  2. why IMS_EMER_GPRS_EPS was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/380064/SP-080844/SP-080844.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-080844, the document the work plan points at
  3. why IMS_SCC was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/410032/SP-080633 SP-080557rev_New-IMSSSC-Nortel/SP-080633 SP-080557rev_New-IMSSSC-Nortel.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-080633, the document the work plan points at
  4. why IMS_WebRTC was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/580062/SP-130707_clean/SP-130707_clean.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-130707, the document the work plan points at
  5. why IMS_TELEP was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/530042/SP-110588/SP-110588.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-110588, the document the work plan points at
  6. why 5GS_Ph1-IMSo5G was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/760029/CP-180094/CP-180094.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because it states the unique identifier of this work plan row
  7. why FS_eIMS5G was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/780029/SP-180736_S2-188366_revised SID 5GC IMS Enhancements/SP-180736_S2-188366_revised SID 5GC IMS Enhancements.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because it states the unique identifier of this work plan row
  8. what IMS-CCR-IWCS set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/2047/NP-030292/NP-030292.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with NP-030292, the document the work plan points at
  9. 17 IMS work plan rows are marked Rel-6 rows of table `item` in /var/www/whatthespec.net/data/database/api/wp.sqlite taken in by the acronym rule whose release field is exactly 'Rel-6', read 2026-08-05
  10. 207 documents carry the work item IMS-CCR-IWCS rows of `tdoc` whose work item column names IMS-CCR-IWCS as a whole word, asked of /var/www/whatthespec.net/data/database/api/api.sqlite on 2026-08-05; the first and last meeting are those of its earliest and latest upload stamp
  11. 64 IMS work plan rows are marked Rel-8 rows of table `item` in /var/www/whatthespec.net/data/database/api/wp.sqlite taken in by the acronym rule whose release field is exactly 'Rel-8', read 2026-08-05
  12. 45 IMS work plan rows are marked Rel-9 rows of table `item` in /var/www/whatthespec.net/data/database/api/wp.sqlite taken in by the acronym rule whose release field is exactly 'Rel-9', read 2026-08-05
  13. what IMS_EMER_GPRS_EPS set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/380064/SP-080844/SP-080844.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-080844, the document the work plan points at
  14. what eIMS_RP set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/440017/CP-091050-WID-IMS-Restoration/CP-091050-WID-IMS-Restoration.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with CP-091050, the document the work plan points at
  15. why eIMS_RP was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/440017/CP-091050-WID-IMS-Restoration/CP-091050-WID-IMS-Restoration.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with CP-091050, the document the work plan points at
  16. what IMS_WebRTC set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/580062/SP-130707_clean/SP-130707_clean.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-130707, the document the work plan points at
  17. what IMS_TELEP set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/530042/SP-110588/SP-110588.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-110588, the document the work plan points at
  18. 60 IMS work plan rows are marked Rel-12 rows of table `item` in /var/www/whatthespec.net/data/database/api/wp.sqlite taken in by the acronym rule whose release field is exactly 'Rel-12', read 2026-08-05
  19. what 5GS_Ph1-IMSo5G set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/760029/CP-180094/CP-180094.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because it states the unique identifier of this work plan row
  20. what FS_eIMS5G set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/780029/SP-180736_S2-188366_revised SID 5GC IMS Enhancements/SP-180736_S2-188366_revised SID 5GC IMS Enhancements.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because it states the unique identifier of this work plan row
  21. what eIMS5G_SBA set out to do the opening sentences of the objective of the work item description /var/www/whatthespec.net/data/data/wis/840062/SP-190181_S2-1902837_eIMS5G_SBA WID/SP-190181_S2-1902837_eIMS5G_SBA WID.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with SP-190181, the document the work plan points at
  22. 543 documents carry the work item eIMS5G_SBA rows of `tdoc` whose work item column names eIMS5G_SBA as a whole word, asked of /var/www/whatthespec.net/data/database/api/api.sqlite on 2026-08-05; the first and last meeting are those of its earliest and latest upload stamp
  23. why FS_eIMS5G2 was proposed the opening sentences of the justification of the work item description /var/www/whatthespec.net/data/data/wis/911032/CP-201358/CP-201358.md, copied word for word, read 2026-08-05. That file was taken as this work item's own because its name begins with CP-201358, the document the work plan points at
  24. 13 IMS work plan rows are marked Rel-16 rows of table `item` in /var/www/whatthespec.net/data/database/api/wp.sqlite taken in by the acronym rule whose release field is exactly 'Rel-16', read 2026-08-05

Every source this course is built on

Check yourself

Answers appear when you pick one, with where they come from.

Q3.1 Why did Release 6 open interworking between IMS and circuit-switched networks?

Q3.2 What does the Release 15 work item record SA2 as having agreed about voice on 5G?

Q3.3 Why did the first 5G core work leave the existing IMS largely alone?

Q3.4 How many work plan rows of this subject are marked Rel-12?

Q3.5 What did the WebRTC work item set out to specify?

This chapter was built from a source register generated 2026-08-05. A fresher build of the register may hold different numbers.