Interoperability Between Machine-to-Machine Communication System and IP Multimedia Subsystem - Interoperability and Open-Source Solutions for the Internet of Things - page 104

security credentials), required to manage M2M service capabilities in the network
domain.
Each M2M domain has its own service capability layers: Network SCL, Gate-
way SCL, and Device SCL providing functions exposed on the mIa, dIa, mId,
and mIm reference points [ 12 ].

- The mIa reference point enables an NA access to the M2M service capabilities
in the network domain. It supports an option for NA to register to the NSCL,
to subscribe for notifications for specific events, with a proper authorization to
read or write information in N/G/DSCLs, and to conduct device management
actions.
- The dIa reference point enables a DA, residing in an M2M device, the access to
different M2M service capabilities in the same M2M device or in an M2M gate-
way. Moreover, this reference point enables a GA residing in an M2M gateway
to access the different M2M service capabilities in the same M2M gateway, and
supports the ability of DA/GA to register to the GSCL or DA to register to the
DSCL. Through this reference point, DA and GA should also be able, with a
proper authorization, to read or write information in N/G/DSCLs.
- The mId reference point enables an M2M SCL residing in an M2M device or
an M2M gateway to communicate with the M2M SCL in the network domain
and vice versa. It supports the ability of G/DSCLs to register to the NSCL. It
should also give support for information exchange between N/G/DSCLs, sub-
scription to specific events, device management, and provide security related
features.
- Finally, the mIm reference point extends the reachability of services offered
over mId reference point. It is an inter-domain reference point, used for com-
munication between NSCLs of different M2M service providers, which relies
on the public core network connectivity functions.

3 IP Multimedia Subsystem

IP Multimedia Subsystem, IMS is a standardized architecture, originally designed
to enable the creation of value added services like HD voice, VoLTE, video com-
munication, HD video conferencing, PSTN transformation and multimedia com-
munication. As shown in Fig. 2 IMS has a layered architecture, which separates
functionality into three layers - an application layer, a control layer and a connec-
tivity layer [ 2 ].
The application layer consists of: applications, content servers with the pur-
pose to execute value added services to the end user. The control layer consists
of network control servers, which are in charge of call or session setup, modifi-
cation and release. The control servers may also handle functions like security,
charging and inter-working towards external networks on a control plane level.
The connectivity layer consists of routers, switches, media gateways and other
user plane elements. The routers and switches provide transport capabilities for
both control plane and user plane trac. The media gateways provide different
kinds of inter-working on a user plane level, including inter-working between