Mission Critical Push To Talk and EPS bearers

Mission critical push to talk architecture defines several reference points and uses multiple
protocols over different EPS bearers.

23.179 specifies that UE connets to MCPTT specific APN in order to use MCPTT service. It is defined that SIP signaling uses QCI 69 bearer. Additionally HTTP is used for signaling. 23.179 defines also that QCI 8 or better is used for HTTP messaging.
Voice media is transmitted over secure RTP either using unicast GBR QCI65 bearer or optionally in multicasting downlink media over MBMS GBR QCI65 bearer. Same bearer (unicast or multicast) is used also floor control.

Following figure depicts how these MCPTT reference points can be mapped to unicast and multicast bearers. Although the 23.179 states that SIP and HTTP can be mapped to different unicast bearers that have different QoS, it can be also interpreted that QCI69 is better than QCI8 and therefore it should be possible to carry both SIP and HTTP signalling over the default QCI69 bearer.

After LTE attach, UE performs authentication and authorization (33.179):

  • A: MCPTT user authentication (CSC-1)
  • B: SIP Registration and Authentication
  • C: MCPTT Service Authorization.

A and B can be performed in any order. A i.e MCPTT user authentication is performed over secure connection TLS, which can be established e.g. based on certificates.
B is based on IMS AKA as specific in 33.203. Confidentiality and integrity is provided according to Gm interface i.e. using IPsec.
MCPTT client uses credentials received in A for MCPTT service authorization in C.

MCPTT reference points:

  • CSC-1 is used for MCPTT user authentication between identity management client and server over secure HTTP/TLS. 
  • CSC-4 provides configuration information for MCPTT service. HTTP is used for non-subscription/notification related signaling and SIP is used for subscription/notification related signaling.
  • CSC-2 is used for configuration of group management data between server and client. HTTP is used for non-subscription/notification related signaling and SIP is used for subscription/notification related signaling. 
  • CSC-8 provides key material over HTTP/TLS from key management server for e2e communication security (keys for SRTP and SRTCP).
  • MCPTT-1 is used for MCPTT session establishment. MCPTT may also provides also location information with respect to multicast service availability. MCPTT-1 shall use SIP and it may use also HTTP. MCPTT-1 includes GC1 as described in 23.468.
  • MCPTT-4 is reference point for floor control over unicast bearer. Secure RTCP (SRTCP) is used for floor control.
  • MCPTT-5 reference point is used for policy control (QoS).
  • MCPTT-6 is used for requesting multicast resources. It uses MB2-C as defined in 29.468.
  • MCPTT-7 is for media distribution over unicast bearer using SGi interface. Secure RTP (SRTP) is used for media.
  • MCPTT-8 sends multicast media to MCPTT clients using MB2-U as defined in 23.468. Secure RTP (SRTP) is used for media.
  • MCPTT-9 provides floor control signaling over multicast bearer using MB2-U as defined in 23.468. Secure RTCP (SRTCP) is used for floor control .


  1. Hi Mika,

    Just read 3gpp 23.179 v13.2.0 (ch and it is possible to use the well-known "ims" APN for MCPTT with some conditions:

    NOTE: The APN value of "IMS" is a well-known APN, whose PDN connection characteristics are defined in GSMA PRD IR.92 [18] and GSMA PRD IR.88 [19], and which is used in some deployments for operator IMS based services e.g. Voice over LTE. This well-known APN can be used for the MCPTT service APN if the SIP core belongs to the PLMN operator and both the PLMN operator and MCPTT service provider have agreed which QoS aspects to utilise i.e. either the QoS aspects defined in subclause or the QoS aspects defined in GSMA PRD IR.92 [18] and GSMA PRD IR.88 [19].

    Best Regards
    Montreal, 2017-03-10

    1. That's correct. 3GPP concept is just getting a bit strange, if VoLTE and MCPTT can optionally share at least partially same QoS. Why did 3GPP define own QCIs for MCPTT? MC services are getting quite complex making market growth unnecessarily difficult and slow.


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