BSS, SSID, ESS, BSSID Alphabet Soup

In the tech world, there is no shortage of acronyms; and Wi-Fi is no different. If you are someone who uses Wi-Fi regularly, chances are you’ve heard of SSID, but there are other related abbreviations like BSS, ESS, and BSSID that Wi-Fi operators need to know. If you find it confusing, I don’t blame you. I once used these terms interchangeably as if they were the same. This makes it difficult to troubleshoot when we are referring to a different thing.

So in this article, I’ll explain what each term is. I’ll point out the differences, and close with a few advices and best practices on how to improve your WLAN.

First, let’s define a BSS. BSS or Basic Service Set, is the foundation that makes up a wireless LAN (WLAN). It comprises an AP radio and at least one client station.

There can be many BSSs in the air, so we use SSID or Service Set Identification to label our wireless networks. These are names that show up on your devices when you search for Wi-Fi. You can name your SSID whatever you want, and it doesn’t have to be unique (there are a few rules like SSID must be less than 32 chars and exclude some special chars).

A company typically have many APs within its network, which all configured using the same SSID. We refer to a collection of these BSSs as an ESS or Extended Service Set.

ESS is typically managed by wireless LAN controllers. The main purpose of ESS is to expand the coverage of WLAN while facilitating client roaming. When a client device successfully connects to a WLAN, the client stores SSID in its profile, and constantly broadcasts Probe Request frames with that SSID. When a target BSS hears the frame, it will send back a Probe Response to get that client associated. For properly designed WLANs, roaming should be seamless, but very often we found it hit-and-miss.

Ok, I hope you are still with me, because here comes the most confusing part; let me introduce BSSID. It stands for Basic Service Set Identification; and no, BSSID is not the same as SSID.

BSSID is a layer 2 MAC address of an AP’s radio that makes up a BSS. Like any MAC address, BSSID is unique and provided by the AP manufacturer. So if you have a dual-band AP setup with the same SSID, each radio will have its own BSSID.

In another word, each BSS has its unique BSSID, and each BSSID can have the same SSID.

It’s worth mentioning that most Wi-Fi vendors support converged network and allow multiple BSSs per radio; in this case, each BSS will have a unique BSSID as well.

A word of caution: be mindful of how many BSSs you have on your network. Each BSS (including hidden ones) adds management overhead that eats up airtime; therefore, having too many BSSs particularly in 2.4GHz can slow down your Wi-Fi.

Also, keep in mind that AP can’t force its clients to connect to a specific BSSID. That said, if both 2.4GHz and 5GHz bands use the same SSID, a common issue is clients get associated to the slower and highly congested 2.4GHz BSSID instead of 5GHz. Yes, there are workarounds, but it may involve using proprietary feature like band steering, which can be cumbersome to implement especially in a large network. So my advice is if you don’t have a good reason to use single SSID for 2.4GHz and 5Ghz, choose a different name for each band. Your users will thank you for this.

Ed Chuchaisri

Wi-Fi Consultant @ siamwireless.com | We help Thailand’s businesses build fast, secure, and reliable WLANs

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