"Heavyweight" uncover the performance of mobile networks (in)--specific explanations for mobile network components

Simple Introduction

Over the past few years we have made significant progress in mobile cellular network performance. But because of the expansion of network latency, many applications have not been improved.

Latency has long been a constraint on mobile networks. While some progress has been made in recent years, the reduction in network latency has not been keeping pace with the delayed growth rate. It is because of such an incorrect delay, rather than throughput, that it becomes the biggest factor that affects network performance.

This article logically consists of two chapters. The first section discusses the details that lead to mobile network latency. The second part introduces the software technology to improve the delay of network performance reduction.

What are you waiting for?

Latency indicates the time required for a packet to pass through one or a series of networks.

Because there are so many factors. Mobile networks can have a higher latency that already exists in most network traffic, including network types (such as hspa+ and LTE), carriers (at and/or Verizon), or environmental factors such as standing, driving, geography, and the difference between a day and a point.

So. It is very difficult to accurately predict the latency of the mobile network. But we can see that its latency ranges from dozens of to hundreds of milliseconds.

Round Trip Time (RTT) is a way to test latency by measuring the round trip time from the source server to the destination by testing the packet. And the size of the RTT has a significant impact on most network performance. Such reasons can be explained by the table tennis movement.

In the usual table tennis, the time it takes to move between the players in the table tennis game is almost not noticeable. However, suppose that the farther the athletes stand. The longer they wait for the ball, the more they can't do other things in the process of waiting.

The athlete is under normal distance. 5-minute table tennis match, assuming a distance of 1000 feet. Then it will take a few hours to finish the skill. Even though it sounds ridiculous.

But suppose that the source server and the destination server are considered to be the two athletes in table tennis, the round-trip time represents the time required to travel between two athletes. Then you will start to clarify the problem.

Part of the general operation of most network protocols is like playing table tennis. These "out-of-the-ball", assuming you want to, is to establish and maintain a network link session (sessions) required for two-way exchange of information (such as TCP) or to run a service request (such as HTTP). These messages are very little or no data transferred during the exchange process and the network bandwidth is largely unused.

Can be seen Latency to a large extent does not take full advantage of bandwidth. This also results in a delay of at least one RTT for each information exchange, which constantly accumulates the attention that leads to the performance implications.

Just imagine. An HTTP request to download 10K of something will have 4 information exchange.

Assume that each RTT is 100 milliseconds (it is reasonable for a mobile network). 4 swaps all taken into account. Then the throughput is 10k/400ms or 25k/s.

Note that the sample above is completely not related to bandwidth-no matter how fast the network results are the same, it is 25k/s. But the performance of the above operation. or any similar operation, can be improved by a simple, clear strategy: Avoid the network client and the server to exchange information.

Mobile cellular networks

The next step is to introduce a simple introduction to the components and conventions that affect latency in mobile cellular networks.

Mobile cellular networks are representatives of a number of interconnected components with highly specialized functions. Each of these components has an impact on latency, but the individual components have varying degrees of impact.

And in the mobile network alone, such as the management of radio resources, has become a mobile cellular network delay among the factors.

Figure 1: Mobile cellular network components

Baseband processor

In most mobile devices are actually two very complex computers. The application processor is responsible for managing the operating system and applications, which are similar to computers and laptops, and the baseband processor is responsible for all wireless network functions, equivalent to a computer modem. Just this modem is using radio waves instead of telephone lines.

The baseband processor is a consistent, but often delayed, resource. The Fast wireless network is an incredibly complex thing, and the complex signal processing that it requires leads to a fixed, guaranteed delay, in microseconds to milliseconds for most network traffic.

Cellular Base Station

The cellular base station, which is synonymous with the transceiver Base station or the transmitter tower, acts as an access point for the mobile network. The responsibility of cellular base stations is to provide an area of network coverage, also known as Honeycomb.

Cellular base stations handle complex, fast wireless networks just like mobile devices. The same is mostly a trivial delay; but a cellular base station must serve hundreds of thousands of mobile devices at the same time, and the difference in system load will lead to different throughput and latency.

At the same time, slow, unreliable network performance tends to limit the processing delivery rate of cellular base stations in crowded public activities.

The latest generation of mobile networks has expanded the role of cellular base stations. Contains the mobile device that manages it. Very many once the functions of the Infinite Network controller have been given to the cellular base station processing, such as: network registration and transmission scheduling. The reason for this is explained later in this chapter, where the role conversion has greatly improved the latency of the new generation of mobile cellular networks.

Backhaul Network

The Backhaul network is a dedicated wide area network (WAN) connection between the cellular base station, the controller of the base station, and the core network. Backhaul networks have been and continue to be notorious for delays.

A backhaul network is a transition from a classic circuit or from a frame-based old mobile network (such as a gsm,ev-do) transmission protocol. Such a protocol will have a delay, mainly due to its synchronization characteristics. A logical connection represents a channel that only accepts or sends data in a short, pre-allocated period of time. In contrast, the latest generation of mobile networks uses IP packet-switched backhaul networks and supports the asynchronous transfer of data. Such a switchover greatly reduces the return delay.

The bandwidth constraints of physical facilities have always been a bottleneck.

Many backhaul are not designed to handle peak traffic loads, so modern fast mobile networks can exhibit significant latency and throughput differences in the event of network congestion. While operators are trying to upgrade these networks as quickly as possible, the component remains a weakness in many network infrastructures.

Wireless network Controller

In general, wireless network controllers manage neighboring cellular base stations and the mobile devices they serve.

The wireless network controller coordinates the mobile device directly through a message-based management scheme called signaling. And because of the topology of the mobile network. All messages for mobile devices and controllers must be sent over a high-latency backhaul network. This itself is not ideal, but because a lot of network operations, such as network registration and transmission scheduling, so the need to back and forth several times to exchange information operations to make the delay worse. Typically, an infinite network controller is also known for this reason as an important factor in latency.

As mentioned earlier, the controller in the latest generation of mobile networks out of the responsibility of equipment management; So many tasks are now handled by the cellular base station itself. This design also determines that the backhaul network latency in most networks is eliminated.

Core network

The core is the gateway between the operator's internal network and the public network, and it is here that operators use inline network devices to implement network service quality policies or bandwidth metering. As a rule, no matter what the interception of network traffic will produce a delay.

And in reality such delays are prevalent. But this also needs to be noted.

Power protection

One of the most important sources of mobile network latency is directly related to the usage limits of cell phone batteries.

The power consumption of the fast mobile device's network station is 3 watts. This power dissipation is so great that iphone5 's battery can only support one hours, and that's why mobile devices seize every opportunity to remove or reduce the radio path.

This approach not only prolongs battery life but also delays the activation of the radio route, providing power to the wireless circuitry once again, only when receiving or transmitting data.

All mobile phone networks use standard formalized wireless resource management (RRM) programs to conserve energy. Most RRM conventions define 3 states--active, spare, unlinked--each state represents a tradeoff between certain startup latencies and power consumption.

Figure 2: Wireless resource management state transitions

Active (Active)

The activation state represents the data perhaps with the smallest delay in telling the transmission or receiving.

This state of the electricity consumption is very large even in the case of spare. In this state, the short time network spare, usually less than 1 seconds, will start to transition to the spare state.

This impact on performance is noteworthy: prolonged pauses in network processing can cause the device to switch back and forth between the activation and spare states, which can also cause delays in adding.

Spare (Idle)

The spare state is a tradeoff between low power usage and proper delay loading.

In this state, the mobile device's network is still connected, unable to send or receive data, but able to receive network requests to meet the requirements of the transition to the activation state (such as input data). After a reasonable period of time if the network is still inactive. At the same time for one minute or less, the mobile device switches to the disconnected state.

Spare status can cause delays in both respects. First, it takes some time to provide energy and synchronous analog circuitry for an infinite communication device again, and secondly, to save a lot of other power. Wireless communication Equipment However, intermittent monitoring of network notifications, and the response to the notification will be slightly delayed.

Not Connected (disconnected)

The non-connected state consumes the least power but has the highest startup latency.

In this state, the device's network is not connected and the wireless device is inactive. The wireless communication device is activated and listens to the arrival of the network request through a special broadcast channel , but such activation is not very frequent .

The delay of the non-connected delay and the spare state is the same, but the difference is that the disconnected state also counts the delay in connecting to the network again. Connecting to a mobile network is a complex process that includes multiple back-and-forth information exchanges (that is, signaling).

Restoring a network connection takes at least hundreds of milliseconds, and the link time is often a matter of seconds.

(To be continued, the next chapter is more exciting ...) ）

1. This paper is translated by the program Ape architecture

2. This document is translated from the performance of Open Source software | Secrets of Mobile Network performance

3. Reprint Please be sure to indicate this article from : Program Ape Architecture (No.:archleaner )

4. Many other articles please scan the code:

Copyright notice: This article Bo Master original articles, blogs, without consent may not be reproduced.

Heavyweight reveals the performance of mobile networks (in)--specific explanations for mobile network components