ching system, i.e. SS7 over the IP network; the transmission over the IP network will be done using the User Datagram Proto- col (UDP) from the TCP/IP stack; both signalling transmission and media transmis- sion will use the protocol; data, fax and compressed speech will be packetized to IP packets and transmitted to the other switch using the Real-Time Transport Protocol (RTP) on the UDP. Other key interfaces in the evolution to 3G include: A-Interface MSC to GSM BSS will continue as needed for applications like Radio Resource Management (RRM), Mobility Management (MM), and Link Manage- ment (LM); MAP performing signalling between the MSC and other NSS elements and per- forming critical operations between switching and database elements to support roaming; CCS7 - Common Channel Signalling system (7) links the MSC to a PSTN or to an ISDN using a single channel to carry the signalling of multiple speech circuits; the digital Channel Associated Signalling (CAS) used between exchanges will also continue as needed; in the short term, the File Transfer Access and Management (X.25 FTAM) inter- face will continue to communicate with billing systems as IP links to new billing centres develop; standard V.24 interfaces connecting O&M terminals to the MSC will probably con- tinue, while more sophisticated WWW type interfaces will be implemented with evolving MSC operating systems. The UMTS Development Platform In conclusion, we can say that the two critical interfaces that UMTS introduces to the CN are primarily the Iu and IP. These interfaces add new dimension to the existing GSM infrastructures besides enriching the type of links a CN may have. In the following we cover the essential transition steps in terms of the 3G architecture requirements. 3.2.2 Key Release 99 Architectural Requirements The general working assumptions for Release 99 (R99), which cover the phase 1 UMTS/Release '99 GSM standards and reflecting in part the elements illustrated in Figure 3.3, can be summarized from [3] as follows: a Core Network based o