3月 252020
 

The purpose of this FAQ is to address some of the most common questions and misconceptions that keep coming up in roaming related discussions.

WLAN Roaming FAQ

What is roaming?

Simply put, roaming is the act or process by which a client device transitions from one cell (BSS) to another.

Is roaming a feature of the infrastructure (APs, controller, etc.)?

No. In the 802.11 standard (a.k.a Wi-Fi), roaming decisions are left to the client device and does not specify any criteria for determining when and where to roam to. All the wireless infrastructure can do is leverage standards-based and/or proprietary mechanisms to try to influence client device roaming behavior and maybe provide for reduced roaming times. But that doesn’t always work well.

How does a client device decides when and where to roam?

Different vendors use different criteria differently. Most of them take into account received signal strength, data rates (PHY rates), frame retry rates, and other metrics in their roaming algorithms. More often than not, roaming criteria isn’t properly documented if at all. In the consumer space, iOS devices are the best documented ones. Continue reading »

3月 252020
 

要想理解 WLAN 漫游技术,首先要区分 SSID、BSSID 与 ESSID。

  • SSID(Specific Service Set Identifier):一个逻辑上的无线网络热点的名称,例如 CMCCChinaNet ,会显示在客户端的 WLAN 列表中,用来表示不同的无线网络。
  • BSSID(Basic Service Set Identifier):基本服务集,是无线网络的基本单元。无线接入点(Access Point)的硬件接口 MAC(Medium Access Control)地址。当多个 AP 组成集群(Cluster)时,一个 SSID 即对应了多个 BSSID。对于 Android 设备,在开发者选项中开启“开启 WLAN 详细日志记录”,即可查看某 SSID 所对应的 BSSID 地址,以及 RSSI(Received Signal Strength Indicator,接收信号强度)。
  • ESSID(Extended Service Set Identifier):扩展服务集,由多个 BSSID 组成的一个使用相同 SSID 的网络,不同接入点比邻安置,通过以太网、Mesh(网状网)、WDS(Wireless Distributed System,无线分布式系统)等方式互联1

在开始讲解 WLAN 漫游技术前,先引用 Cisco WLAN 漫游白皮书中的一句话2 Continue reading »