Table 4 List of the reference points and network elements of the 3GPP reference model for MTC communications (taken from [45, 51–53]). The acronyms of network elements are given in Table 1

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