Cloud server products that charge based on traffic volume, when billed, refer to "traffic" in most cases specifically to "outbound traffic" generated by the server, i.e., the amount of data corresponding to "download" or "downlink bandwidth." The "peak bandwidth" metric, however, represents the capacity limit of the shared bidirectional (upload and download) bandwidth.

To ensure complete understanding, we need to separate the concepts of "traffic-based billing" and "bandwidth capacity."

When cloud service providers bill network usage for lightweight application servers based on traffic volume, they typically bill for "public network outbound traffic." This refers to data flowing from your cloud server to the internet. Corresponding to your everyday understanding of "downloading": when a user accesses a website on your server through a browser, the server needs to "send" the webpage's HTML, images, CSS/JS files, etc., to the user. This process, from your server's perspective, is "outbound" traffic and is the subject of billing.

Similarly, if your server provides data as an API interface or sends status packets to players as a game server, this data outflow will consume billed traffic.

Why primarily charge for "outbound" traffic? Considering business models and costs, data centers need to pay for the bandwidth connecting to their upstream ISPs (Internet Service Providers). Outbound traffic from servers directly consumes the expensive "uplink" bandwidth resources purchased by cloud service providers. Inbound traffic (user requests for data), however, often has a different cost structure for data center networks. On the other hand, this encourages optimization, prompting developers and operations personnel to optimize websites and applications, reducing unnecessary data transmission (such as compressing images and enabling CDN), thereby lowering overall network costs.

This is the common billing model in the international cloud computing industry, with the significant exception that inbound traffic is usually free. In the default policies of almost all major cloud service providers, public network inbound traffic (i.e., "uploading" to the server or users "downloading" from the server) is free. Uploading files from your local machine to the server, or a user sending a request to your server (such as submitting a form), this data volume is usually not charged.

Unlike traffic-based billing, when you purchase or configure a lightweight server, the selected "peak bandwidth" (such as 1Mbps, 5Mbps, 100Mbps) is a speed cap that simultaneously limits upload and download speeds, and this total capacity is shared.

How to understand "sharing" and "bidirectional"? Let's assume your server's peak bandwidth is 5Mbps. This means your server's maximum outbound data transmission rate is 5Mbps. Simultaneously, the maximum inbound data transmission rate is also 5Mbps. Crucially, this 5Mbps is bidirectionally shared, not exclusively allocated. If a download task fully utilizes the 5Mbps outbound bandwidth, the inbound bandwidth will become extremely congested, potentially reaching zero. New requests may fail to load, leading to service instability.

You can think of it as a two-way single-lane bridge. The bridge's width (5Mbps) is fixed. When too many vehicles (outbound traffic) are flowing out of the server, clogging the bridge, vehicles (inbound requests) wanting to enter from the user end must queue.

Core Concept Summary and Analogy

How to confirm and optimize?

Check official documentation: This is the most accurate way. Please be sure to review your cloud service provider's product documentation and billing instructions, which will clearly state whether "traffic billing" only applies to "public network outbound traffic." 

Check billing and console monitoring: In the cloud service provider's console, you can view traffic monitoring charts, which usually clearly distinguish between "inbound bandwidth" and "outbound bandwidth." Billing details will also list the consumed "public network outbound traffic."

Optimization recommendations include reducing outbound traffic by enabling GZIP compression, optimizing images and videos (using formats like WebP), and hosting static resources in object storage (OSS/COS) and binding them to a Content Delivery Network (CDN). A CDN caches your content closer to users, allowing them to retrieve data directly from the CDN, significantly reducing outbound traffic consumption from your origin server. Also, consider choosing the appropriate peak bandwidth based on your business type. If users primarily download large files, higher outbound bandwidth is needed; if users primarily upload data (e.g., to cloud storage), focus on ensuring sufficient inbound bandwidth. Although it's not billed, insufficient peak bandwidth will lead to slow upload speeds.

In short, the next time you see network configurations for a lightweight server, remember: "Traffic packages" are your "outbound data volume quotas," while "peak bandwidth" is your "two-way overall network speed limiter."