M3UA explained

M3UA is a communication protocol of the SIGTRAN family, used in telephone networks to carry signaling over Internet Protocol (IP). M3UA enables the SS7 protocol's User Parts (e.g. ISUP, SCCP and TUP) to run over virtually any network technology breaking its limitation to telephony equipment like T-carrier, E-carrier or Asynchronous Transfer Mode (ATM), which highly improves scalability of the signaling networks.

M3UA stands for MTP Level 3 (MTP3) User Adaptation Layer as defined by the IETF SIGTRAN working group in (which replaces and supersedes). Like other adaptation protocols, M3UA uses SCTP to transmit messages between its network elements.

Implementation scheme

Image:M3UA-scheme.svg|404x404px|thumb|M3UA: Transmitting MTP-3 messages: on the left is the TDM-network for SS7. On the right is the IP network with SIGTRAN as a transport.|centerrect 1 1 100 72 Point code

rect 1 90 593 123 ISUPrect 1 123 593 160 SCCP

rect 1 160 100 232 MTP-3rect 1 232 100 445 MTP-2rect 1 445 100 515 MTP-1

rect 248 1 347 72 Point code

rect 199 160 296 232 MTP-3rect 199 232 296 445 MTP-2rect 199 445 296 515 MTP-1

rect 298 160 397 232 M3UArect 298 232 397 302 SCTPrect 298 302 397 372 IPrect 298 372 397 445 L2rect 298 445 397 515 L1

rect 493 1 593 72 Point code

rect 493 160 593 232 M3UArect 493 232 593 302 SCTPrect 493 302 593 372 IPrect 493 372 593 445 L2rect 493 445 593 515 L1

rect 15 522 85 565 ISDNrect 248 522 347 565 Signaling gatewayrect 493 522 580 565 Media Gateway Controller

Typical scheme

________                   _________                __________ 
|      |                   |       |                |     MGC|
|  SP  |<----------------->|  SGW  |<---------------|-->(AS) |
|______|   SS7 network     |_______|   IP network   |________|
  MTP3
point-code                         common point-code
   PC1                                    PC2

Use SGW as STP

Several AS owns PC itself and uses SGW as STP (transit pointcode).

________                 _____________               ___________ 
|      |                 |    SGW    |               |      MGC|
|      |                 |           | /-------------|-->(AS)  | point-code PC3
|  SP  |<----------------|-->(STP)<--|-              |         |
|      |                 |           | \-------------|-->(AS)  | point-code PC4
|______|  SS7 network    |___________|   IP network  |_________|
  MTP3                    point-code
point-code                   PC2                         
   PC1

Protocol

M3UA uses a complex state machine to manage and indicate states it's running. Several M3UA messages are mandatory to make an M3UA association or peering fully functional (ASP UP, ASP UP Acknowledge, ASP Active, ASP Active Acknowledge), some others are recommended (Notify, Destination Audits (DAUD)).

Additional info

An open implementation of the M3UA standard can be found at OpenSS7's web site.^[1]

Wireshark is shipped with a dissector for M3UA, sample packets can be found in Wireshark's wiki page, which shows ISUP Packet samples (including M3UA^[2]).

Notes and References

http://www.openss7.org/m3ua.html OpenSS7 web site
http://wiki.wireshark.org/SampleCaptures?action=AttachFile&do=view&target=isup.cap Wireshark wiki page