Introduction and RRC of mobile network

1. Introduction to Mobile networks

• 1G: Represents the first generation of mobile communication technology, based on analog technology, for cellular telephone systems, such as the now obsolete analog mobile network. 1G Wireless systems are designed to transmit only voice traffic and are limited by network capacity.
• 2G: The second generation of mobile communication technology specifications, with digital voice transmission technology as the core. It is generally defined as the inability to directly transmit information such as e-mail, software, etc., only with calls and some mobile communication specifications such as the date and time of transmission. But SMS can be executed in some of its specifications. In the second generation of mobile communication technology, GSM is the most widely used.
• gprs: In order to support from the traditional voice service to the emerging data service, GPRS overlay the network which supports the high-speed packet data based on the original GSM network, To provide users with WAP browsing (browse the Internet page), e-mail and other functions, to promote the first rapid development of mobile data Services, the realization of mobile communication technology and data communication technology (especially Internet technology) perfect combination. GPRS is a technology between 2G and 3G, also known as 2.5G . It also has a younger brother Edge, known as 2.75G. They lay the groundwork for a smooth transition from GSM to 3G.
• 3G: The third generation of mobile communication technology, refers to the support of high-speed data transmission of cellular mobile communication technology. The 3G service is capable of transmitting both sound and data at a rate of more than hundreds of Kbps. 3g refers to the wireless communication and the Internet and other multimedia communications, a new generation of mobile communication systems, the current 3G there are 3 kinds of standards: CDMA2000, WCDMA, TD-SCDMA.
• 4g: Fourth generation mobile phone communication standard, refers to the fourth generation of mobile communication technology, foreign language abbreviation: 4G. The technology includes two formats for Td-lte and Fdd-lte (strictly speaking, LTE is only 3.9G and, despite being advertised as a 4G wireless standard, it has not been recognized by 3GPP as the next generation of wireless communications standard imt-advanced described by the International Telecommunication Union, so it has not reached 4G standards in the strictest sense.

Comparison of network transmission rates under different standards

hspa+, LTE, lte-advanced contrast

2. Radio Resource Controller

In 3G and 4G networks, there is a unique module-Radio resource Controller (RRC). RRC distributes the wireless resources and sends the related signaling, the main part of the control signaling between UE and Utran is the RRC message, RRC message hosts all the parameters necessary to establish, modify and release the media access control layer and Physical layer protocol entity, and also carries some signaling of the NAS (non-access layer), such as MM, CM, SM and so on.

Because the power of the mobile device is a key factor affecting the use of the device, and not all the time the device is transmitting data, if the device is always in a high-power connection state will undoubtedly consume a large amount of electricity, which requires a mechanism to regulate the state of the device to conserve power. When the data is transmitted in a high power state, no data transmission in the idle state, so that the device can be very good to save power. This mechanism is RRC state machine, in different network standards in the implementation of the state machine is different, the following is the specific description:

• LTE-RRC State Machine

LTE-RRC state machine: The 3GPP standard defines a complete state machine that describes the power state of each device connected to the network.

• RRC Idle: The radio module of the device is in a low power state (<15MW) and only listens for control signals from the network. The client in the carrier network does not have radio resources.
• RRC Connection: The device's Radio module is in a high-resolution state (1000-3500MW), either transmitting data or waiting for data. The carrier network specifies the method of data hosting and also allocates a dedicated radio resource.

Simply put, the device is either idle or in a connected state. When the device is idle, the device cannot send or receive any data. To receive data, the device needs to listen to the network to synchronize itself with the network before sending a request to RRC to switch it to the connected state. When switched to connected state, the network environment between the wireless signal tower and the LTE device is ready to transmit data at any time. However, if the side of the communication ends the data transfer, how does the RRC know when to switch the device to a low power state? The RRC state machine relies on a set of timers to trigger RRC state switching.

Because of the high power required for the connection state, W has more than one sub-state to complete the operation more efficiently:

• Continuous receive: The highest power, network environment is ready, has allocated network resources.
• Short discontinuous receive (short DRX): Network environment is ready, network resources are not allocated.
• Long discontinuous receive (long DRX): Network environment is ready, network resources are not allocated.

The only difference between long Drx and short drx is that the sleep time before waking is different and the rest is the same.

When a device or network needs to transmit data in a short drx or long drx state, the device and RRC need to exchange control information to negotiate when to start the transmission and when to listen for radio broadcasts, which has a delay.

When the device or network is in an idle state to transfer data, the device needs to listen to the network in order to synchronize with the network, and then exchange control information with the RRC, negotiate the relevant information, switch to the connection status, the network environment is ready, waiting for data transmission, which also has a delay.

RRC delay

Delay	Lte	LTE Advanced
Idle to Connection	<100ms	<50ms
Drx to Connection	<50ms	<10ms
User face one-way	<5ms	<5ms

• HSPA and hspa+ (UMTS) RRC state machine

The 3GPP network had similar state machines before LTE, but the early state was more complicated and the delay was longer. LTE has better performance and less latency, in part because it simplifies the state machine architecture and improves the performance of state switching.

• Idle: Similar to LTE, Radio, module in low power state, only monitor the network control signal, the client does not occupy radio resources (such as channel resources, etc.).
• Cell DCH: A continuous receive is similar to a connected state of LTE. The radio module is in high power state and allocates network resources for uplink and downlink data transmission. 、
• Cell Fach: The device Radio module is in a medium power state and consumes much less power than the DCH state. The device department occupies a dedicated network resource, but can transmit a small amount of data through a shared low-speed channel (<20KBIT/S).

The idle and DCH states are basically the same as idle and connected states in LTE. However, Fach is unique to the UMTS network, where a small amount of data can be transmitted over a common channel-slow, steady, and consume only half of the DCH state. This state is used to handle non-interactive communication, such as polling and stateful detection in the background.

DCH and Fach state switching is also done through timers, each device caches some data to be sent, as long as the data does not exceed the network configuration threshold (typically 100-1000b), the device will remain in the middle state. After the device has been fach for a certain amount of time and there is still no data to transfer, another timer switches the device to an idle state. Earlier 3G networks had a state switching delay higher than LTE, as shown in table 1. Hspa+ has made a big improvement on it, which is already similar to LTE.

Note: The UMTS device has only one intermediate state. In theory, LTE is capable of better control of power, but the radio module itself consumes power. Higher throughput also requires more power consumption, and in fact, LTE devices consume more energy than 3G devices.

• Ev-do (CDMA) RRC state machine

Although CDMA is not the latest technology, but its user volume can not be underestimated. All mobile devices have their share of limitations--limited battery capacity. Therefore, the CDMA network also has the RRC state machine, is used to control the Radio module state of the device, in order to save power.

• Idle: Similar to 3GPP and LTE, the device Radio module is in a low power state, listening only to control signals from the network, the client does not occupy the radio resources.
• Connection: The device Radio module is in a high power state, allocating network resources for data transmission up and down.

This state machine is relatively simple, either in a high power state or in an idle state. As long as you want to transfer data, you have to switch to the connection state, the corresponding delay is similar to HSPA. It has no intermediate state, and the transition to an idle state is also controlled by the operator's configured timeout period.

Low-efficiency periodic transmission

A radio state switch controlled by a time-out period can have serious consequences and must wait long enough for the radio module to switch to a low-power state and then trigger the RRC state switch over network access. Each transmission, regardless of the amount of data, needs to switch to a high-power state. After the transfer is complete, you must wait until the timer expires to switch to a low-power state, and the size of the data does not affect the timer's timeout length. The state switch has a delay time, then the transmission begins, the final radio module is idle, the power is wasted, until all timers expire, the device switches to a low-powered state.

Introduction and RRC of mobile network