Last time we talked about the features of the new NB-IoT standard from the point of view of the radio access network architecture. Today we will discuss what has changed in the core of the network (Core Network) with NB-IoT. So let's go.
Significant changes have taken place in the core of the network. To begin with, a new element has appeared, as well as a number of mechanisms that are defined by the standard as “CIoT EPS Optimization” or core network optimization for the cellular Internet of things.
As you know, there are two main communication channels in mobile networks, which are called Control Plane (CP) and User Plane (UP). Control Plane is intended for the exchange of service messages between various network elements and serves to provide mobility (Mobility management) of devices (UE) and establish / maintain a data transfer session (Session Management). User Plane is, in fact, a channel for transmitting user traffic. In classic LTE, the distribution of CP and UP across interfaces is as follows:
CP and UP optimization mechanisms for NB-IoT are implemented on MME, SGW and PGW nodes, which are conditionally combined into a single element called C-SGN (Cellular IoT Serving Gateway Node). The standard also assumes the emergence of a new network element - SCEF (Service Capability Exposure Function). The interface between MME and SCEF is called T6a and is based on the DIAMETER protocol. Despite the fact that DIAMETER is a signaling protocol, in NB-IoT it is adapted to transfer small amounts of non-IP data.
As the name suggests, SCEF is a service capability exposure node. In other words, SCEF hides the complexity of the operator's network, and also removes the need for application developers to identify and authenticate mobile devices (UE), allowing application servers (Application Server, hereinafter AS) to receive data and manage devices through a single API interface.
The UE identifier is not a telephone number (MSISDN) or an IP address, as it was in the classic 2G / 3G / LTE network, but the so-called “external ID”, which is defined by the standard in the format familiar to application developers “ @ ". This is a separate large topic that deserves a separate material, so we will not talk about it in detail now.
Now let's look at the most significant innovations. "CIoT EPS Optimization" is the optimization of traffic transmission mechanisms and subscriber session management. Here are the main ones:
- DoNAS
- NIDD
- Power Saving Mechanisms PSM and eDRX
- HLCOM
DoNAS (Data over NAS):
This is a mechanism designed to optimize the transmission of small amounts of data.
In classical LTE, a subscriber device establishes a PDN connection (hereinafter referred to as PDN) via eNodeB to MME-SGW-PGW when registering with the network. The UE-eNodeB-MME connection is the so-called “Signaling Radio Bearer” (SRB). If it is necessary to send / receive data, the UE establishes another connection with the eNodeB - “Data Radio Bearer” (DRB), to transfer user traffic to the SGW and then to the PGW (interfaces S1-U and S5, respectively). At the end of the exchange and in the absence of traffic for some time (usually 5-20 seconds), these connections are broken and the device goes into standby mode or “Idle Mode”. If it is necessary to exchange a new portion of data, SRB and DRB are reset.
In NB-IoT, user traffic can be transmitted via a signaling channel (SRB), in NAS protocol messages (). Establishing DRB is no longer required. This significantly reduces the signal load, saves network radio resources and, most importantly, extends the battery life of the device.
In the eNodeB - MME section, user data begins to be transmitted over the S1-MME interface, which was not the case in the classical LTE technology, and the NAS protocol is used for this, in which the “User data container” appears.
In order to carry out the “User Plane” transfer from MME to SGW, a new S11-U interface appears, which is designed to transfer small amounts of user data. The S11-U protocol is based on GTP-U v1, which is used to transmit the User Plane and on other network interfaces of the 3GPP architecture.
NIDD (non-IP data delivery):
As part of the further optimization of mechanisms for transferring small amounts of data, in addition to the already existing PDN types, such as IPv4, IPv6 and IPv4v6, another type has appeared - non-IP. In this case, the UE is not assigned an IP address, and data is transmitted without using the IP protocol. There are several reasons for this:
- IoT devices such as sensors can transfer very small amounts of data, 20 bytes or even less. Given that the minimum IP header size is 20 bytes, IP encapsulation can sometimes be quite expensive;
- There is no need to implement an IP stack in a chip, which leads to their reduction in price (a question for discussion in the comments).
By and large, an IP address is required by IoT devices in order to transmit data over the Internet. In the NB-IoT concept, SCEF acts as a single AS connection point, and data exchange between devices and application servers occurs through the API. In the absence of SCEF, non-IP data can be transmitted to the AS through the Point-to-Point (PtP) tunnel from the PGW and IP encapsulation will be performed already on it.
All this fits into the NB-IoT paradigm - the maximum simplification and reduction in the cost of devices.
Power Saving Mechanisms PSM and eDRX:
One of the key benefits of LPWAN networks is energy efficiency. A period of up to 10 years of autonomous operation of the device on a single battery is declared. Let's see how these values are achieved.
When does a device use the least amount of power? That's right when it's off. And if it is impossible to completely de-energize the device, let's de-energize the radio module, for the time when it is not needed. You just need to agree with the network first.
PSM (Power saving mode):
The PSM power saving mode allows the device to turn off the radio module for a long time, while remaining registered on the network, and not to reset the PDN every time it needs to transfer data.
In order for the network to know that the device is still available, it periodically initiates an update procedure - Tracking Area Update (TAU). The frequency of this procedure is set by the network using timer T3412, the value of which is transmitted to the device during the Attach procedure or the next TAU. In classic LTE, the default value of this timer is 54 minutes, and the maximum is 186 minutes. However, to ensure high energy efficiency, the need to go on the air every 186 minutes is too expensive. To solve this problem, the PSM mechanism was developed.
The device activates the PSM mode by sending the values of two timers T3324 and T3412-Extended in the Attach Request or Tracking Area Request messages. The first determines the time that the device will be available after switching to "Idle Mode". The second is the time after which the TAU must be generated, only now its value can reach 35712000 seconds or 413 days. Depending on the settings, the MME may accept the timer values received from the device, or change them by passing new values in the Attach Accept or Tracking Area Update Accept messages. Now the device may not turn on the radio module for 413 days and still remain registered in the network. As a result, we get tremendous savings in network resources and energy efficiency of devices!
However, in this mode, the device is not available for incoming communications only. If it is necessary to send something to the application server, the device can exit PSM at any time and send data, remaining then active during the T3324 timer to receive information messages from the AS (if any).
eDRX (extended discontinuous reception):
eDRX, Enhanced Discontinuous Reception. To transfer data to a device that is in "Idle mode", the network performs a notification procedure - "Paging". Upon receiving a page, the device initiates the establishment of an SRB for further communication with the network. But in order not to miss the Paging message addressed to it, the device must constantly monitor the radio air, which is also quite energy-consuming.
eDRX is a mode in which the device receives messages from the network not constantly, but periodically. During the Attach or TAU procedures, the device coordinates with the network the time intervals during which it will “listen” to the air. Accordingly, the Paging procedure will be performed at the same intervals. In eDRX mode, the operation of the device is divided into cycles (eDRX cycle). At the beginning of each cycle, there is a so-called “paging window” (Paging Time Window, hereinafter referred to as PTW) - this is the time that the device listens to the radio channel. At the end of the PTW, the device turns off the radio module until the end of the cycle.
HLCOM (high latency communication):
If it is necessary to transfer data to Uplink, the device can exit any of these two power saving modes without waiting for the end of the PSM or eDRX cycle. But it is possible to transfer data to the device only when it is active.
The HLCOM functionality or high latency communication is the buffering of Downlink packets on the SGW while the device is in power saving mode and not available for communication. Buffered packets will be delivered as soon as the device exits PSM by making TAU or sending Uplink traffic, or when PTW occurs.
This, of course, requires awareness on the part of developers of IoT products, since communication with the device is not obtained in real time and requires a certain approach to designing the business logic of applications.
In conclusion, let's say: the introduction of a new one is always exciting, and now we are dealing with a standard that has not been fully tested even among the world's "bisons" like Vodafone and Telefonica - therefore it is doubly exciting. Our presentation of the material does not claim to be absolute, but we hope it provides a sufficient understanding of the technology. We will be grateful for feedback.
Author: Expert of Convergent Solutions and Multimedia Services Department Alexey Lapshin
Source: habr.com