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Table 4 List of the reference points and network elements of the 3GPP reference model for MTC communications (taken from [45, 5153]). The acronyms of network elements are given in Table 1

From: Mobile M2M communication architectures, upcoming challenges, applications, and future directions

  Descriptions
Reference points  
Tsms It enables the communication between an entity lying outside the 3GPP network and
  MTC using SMS
Tsp It connects MTC-IWF to SCSs to provide several functionalities
  such as device triggering
T4 It performs SME tasks, e.g., connects MTC-IWF to the HPLMN domain.
  T4 also supports device triggering functionlities
T5a It enables communication between MTC-IWF and serving SGSN
T5b It establishes the communication path between MTC-IWF and serving MME
T5c It creates the communication link between MTC-IWF and serving MSC
S6m It performs subscription and MTC-related information tasks by establishing
  the communication path between MTC-IWF to HSS/HLR
S6n It also performs subscription and MTC-related information tasks by establishing
  communication link between MTC-AAA and HSS/HLR and supports
  interrogation of HSS/HLR
Network elements  
MTC-IWF One of the instances of MTC-IWF exists in HPLMN to support indirect and hybrid models
  Receives trigger request of an MTC device from SCS
  Reporting and updating SCS about the acceptance or rejection of the device trigger request
HSS/HLR It determines whether an SCS is allowed to send a trigger to a particular MTC
  It also performs mapping tasks between IMSI and external identifier(s)
GGSN/P-GW It enquires an MTC AAA server for regenerating external identifier(s)
SGSN/MME/MSC Accepting an MTC device trigger from MTC-IWF
  It confines the device trigger information in NAS messages sent to MTC
  It accepts a device trigger acknowledgement from the triggering MTC
  It reports to the MTC-IWF about the device trigger delivery success/failure status
  It also delivers SGSN/MME congestion/load information to MTC-IWF
SMS-SC It delivers the routing information from MTC-IWF to SMS-GMSC
MTC AAA It uses standard procedures such as RADIUS to identify the destination external AAA server
SCEF It performs several tasks such as device authentication and profile management
  It makes external entities capable to discover various service capabilities