| Jangho Noh (A paper written under the guidance of Prof. Raj Jain) |
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M2M communications is a technology which allows obtaining and delivering needed information easily and based on it, having access to various services. M2M devices are expected to rapidly increase in number: by 38% per annum from 0.6 million in 2015 to 3.1 billion in 2020. In addition, 34% of M2M 3.1 billion devices in 2020 are expected to be connected through LTE or LTE-A networks. They are expected to be used more extensively than Low-Power, and Wide-Area (LPWA) which have the second largest market share with 28% [cisco16] [cisco16a].
As M2M devices using LTE-A networks and the traffics these devices are rapidly increasing, network operators need to constantly increase their access and backhaul capacities, or introduce the technologies to reduce their network loads. D2D communications is the technology to reduce network loads. It allows data offloading by providing a direct communication.
The remainder of this paper is organized as follows. Section 2 examines 3GPP Architectures of LTE-A, M2M and D2D; and Section 3 looks at their data flow procedures and service requirements.
This chapter examines LTE-A, D2D and M2M reference model defined in 3GPP and describes the essential entities that constitute the network and the function of each of the entities. LTE-A referred to 3GPP TS 23.401; D2D to 3GPP TR 23.703; and M2M to 3GPP TS 23.888 respectively. The overlapping functions in each of the models were dealt with only in the part of LTE-A architecture.
LTE-A is divided largely into two parts. The first part is related to the radio access network (E-UTRAN). The related entities are the User Equipment (UE) and the evolved Node B (eNB). The second part is the Evolved Packet Core (EPC) which constitutes the core network. Major EPC entities include MME, PGW, SGW, HSS, and PCRF. Figure 1 represents an LTE-A network reference model [3gpp401].
Figure 1: LTE-A Reference Model
M2M Network is composed of Radio Access Network, EPC, Machine-Type Communications (MTC) device, MTC server and MTC user. The entities newly added to in the LTE-A Network are the MTC server and MTC-IWF.Figure 2 shows an M2M network reference model [3gpp888] [faye15].
Figure 2: M2M Reference Model
There are three architectural models which differ depending on the ways that the MTC user is connected to the MTC device. The first model is a direct model which communicates with the LTE-A network without going through the MTC server. The second model is an indirect model which is connected to the LTE-A network through the MTC server. The last model is a hybrid model which uses both the direct and indirect model at the same time. In the hybrid model, the user plane is transmitted through the direct model and the control plane is processed by using the indirect model.
D2D communication allows a direct communication between devices without going through the core network, and thus improves the end-to-end latency as it decreases loads on the core network and reduces the number of the entities through which data should pass. The D2D reference model, which is work in progress, can be modified in the future. Figure 3 shows a D2D network reference model [3gpp703].
Figure 3: D2D Reference Model
This chapter briefly examines which traffic path the network use when data are sent from one UE to another UE. In addition, it describes necessary service requirements for proposed data flow when deploying M2M and D2D based on the LTE-A network.
As M2M assumes a connection through the interface of the existing LTE-A network, there is no difference in the data flow between the LTE-A and M2M network. Therefore, when a packet sent from one UE to another UE in the LTE-A and M2M network, it has to go through eNB, SGW, and PGW. Figure 4 shows this [3gpp401] [3gpp888].
Figure 4: LTE-A Network Traffic Flow
However, M2M has several service requirements differentiated from the existing LTE-A network. Firstly, M2M requires different address system from the existing ones to accept a numerous number of devices. Secondly, it needs to control the overload arising when a countless number of devices are simultaneously connected to the network. Thirdly, the M2M applications cause mainly the uplink data traffic in comparison with the LTE applications which cause the downlink data traffic of video and others. Table 1 describes the characteristics of M2M network [3gpp368] [krjung10].
| Characteristics | Description |
| Low Mobility | Do not move, move infrequently, Move within a certain region |
| Time Controlled | Tolerate to send or receive data only during defined time interval |
| Time Tolerant | Delay data transfer |
| Packet Switched (PS) Only | Only require packet switched services |
| Small Data Transmissions | Send or receive small amounts of data |
| Mobile Originated Only | Only utilize mobile originated communications |
| MTC Monitoring | Monitoring MTC device related events |
| Priority Alarm Message (PAM) | Issue a priority alarm in the event of theft or other immediate attention |
| Secure Connection | Require a secure connection between the MTC server and services |
| Location Specific Trigger | Trigger MTC devices which are known by the MTC application in certain area |
| Network Provided Destination for Uplink Data | All data from an MTC device to be directed to a network provided destination |
| Infrequent Transmission | Send or receive data infrequently |
| Group Based MTC Features | Group based collection of MTS features |
| Group Based Policing | Group based management of MTS policies |
| Group Based Addressing | Group based addressing of MTS identification |
Table 1: the characteristics of M2M network (source ref: krjung10)
The data paths of D2D are classified into two: direct mode and locally-routed path. Direct path means a direct communication between UEs without using eNB while locally-routed data path represents communication between UEs using eNB. Both paths don't use EPC in the data plane but only in the control plane. Figure 5 shows this [3gpp703] [balji13].
Figure 5: D2D Data Flow
UE discovers other UEs which are located around it. It measures the channel, shares the results with other UEs or the eNB through synchronizing with peer UEs, and generates the link with other UEs based on the results. eNB should control the D2D links to prevent D2D communication from interfering the link between eNB and other UEs. Table 2 describes the service requirements and proposed solutions [3gpp703].
| Service Requirements | Proposed Solutions |
| Discovery |
- Direct discovery - EPC-level ProSe discovery - Targeted discovery - IMS based discovery - Network based discovery |
| Communication |
- Group Owner mode - Ad-Hoc mode - Hybrid mode for one-to-many communication - Network independent LTE direct communication (one-to-one) - Network-authorized LTE direct communication (one-to-one) - LTE direct communication (one-to-many) |
| WLAN Direct communications |
- EPC support for WLAN Direct communication - ProSe assisted WLAN Direct communication - Network-assisted WLAN Direct communication |
| Relay |
- UE-to-UE Relay using IP Routing and Forwarding - UE-to-UE Relay operating at the application layer - UE-to-Network Relay using Layer 3 Routing based on EPS Bearer - Relay as v4/v6 IP router with application level gateway functions |
| Management |
- Identity - Authorization - Charging - Capability Handling - Service Continuity |
Table 2: D2D Required Functions and Proposed Solutions
We have examined the reference models and data flow of LTE-A network-based M2M and D2D network so far. The use of the M2M network is rapidly spreading as it can be used for wide range of applications such as e-Health, smart city, and public safety. The LTE-A, already extensively used, is playing an important role in deploying the M2M network. The advantageous points provided by the data offloading are also definite. M2M is expected to be used along with the D2D network as D2D network will reduce local end-to-end delay and decrease the local M2M traffic.
| 3GPP | 3rd Generation Partnership Project |
| DPF | Direct Provisioning Function |
| eNB | Evolved Node B |
| EPC | Evolved Packet Core |
| EPS | Evolved Packet System |
| E-UTRAN | Evolved Universal Terrestrial Radio Access Network |
| GTP | GPRS Tunneling Protocol |
| HSS | Home Subscriber Server |
| IMS | IP Multimedia Subsystem |
| IWF | Interworking Function |
| LPWA | Low-Power, Wide-Area |
| MME | Mobility Management Entity |
| MTC | Machine-Type Communications |
| MTC-IWF | Machine-Type Communication-InterWorking Function |
| PCRF | Policy and Charging Rules Function |
| PDN | Public Data Network |
| PGW | PDN Gateway |
| ProSe | Proximity-based Services |
| PSAP | Public-safety answering point |
| SGSN | Serving GPRS Support Node |
| SGW | Serving Gateway |
| UE | User Equipment |