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[From Beginner to Expert - WLAN Fundamentals] Section 6 Basic Concepts in WLAN Highlighted

Latest reply: Apr 11, 2018 10:14:36 5902 2 2 0 1

There's a joke on the Internet that goes like this: A man sets his Wi-Fi network name to "who will ** me" and password to "nobody". The next day, his Internet is slow all the people in the building had connected to his Wi-Fi network.

Though maybe not such a good joke, it does touch upon a basic WLAN concept.

A network name, in the joke "who will ** me", is called a service set identifier (SSID) in WLAN. SSID is the most prominent WLAN concept in everyday life, as it's the one we interact with the most as casual users. In Starbucks shops, when we use our phones to search for Wi-Fi networks, the network name CMCC-STARBUCKS is an SSID. An SSID identifies a service set, which is what most people think of as a network (though they're only partly right). A service set is a set of wireless devices that can communicate with each other. For example, the AP in the Starbucks shop, together with the wireless STAs associated with it, form a service set. Wireless STAs are mobile phones, tablets, or notebook computers with wireless network interface cards (NICs). In a service set, STAs can communicate with each other or visit external networks through the AP after STAs have been associated with the AP.
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The Starbucks in this example is relatively small, and one AP on its own is enough to serve it. Such a service set is called a basic service set (BSS). BSSs provide the basic building-blocks of an 802.11 WLAN. All STAs associate with one AP, which is connected to other wired devices and manages data transmission in the BSS.
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Usually, the maximum range of a BSS is 100 meters. If a WLAN serves a larger coffee shop, or an entire shopping mall, multiple APs are required to build an extended service set (ESS).

In simple words, an ESS is a set of two or more BSSs that share the same SSID and meet the following two conditions:
1. The BSSs are adjacent.
2. The BSSs are interconnected through wired or wireless distribution systems, usually Ethernet networks.

When two Starbucks shops are far away from each other, their networks are not in the same ESS although they have the same network name. 

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Since the SSID is the same, users will not notice that they are connecting to multiple BSSs. From the perspective of users, they connect to one AP. Communications between STAs in an ESS is similar to that in a BSS. However, if STA A wants to communicate with STA B in another BSS, data is transmitted through AP1 and AP2. 

A process called roaming is needed when a STA moves out of the original BSS and associates with another BSS in the ESS. For example, STA A in BSS1 roams to BSS2 ('A' in the previous figure). STA A can still communicate with STA B, but its serving AP changes.

An ESS has a unique SSID, called extended service set identifier (ESSID), which has a maximum length of 32 characters. In an ESS, while users don't typically notice that there are multiple BSSs, STAs have to know each BSS. Therefore, a 48-bit binary basic service set identifier (BSSID), usually the MAC address of an AP, is used to identify a BSS. 

In a coffee shop, we may obtain two similar SSIDs when we search for Wi-Fi networks. One of the SSID identifies the network provided for guests. This network has a public password, and its quality is not high. Another SSID identifies the network provided for employees, and the password is not open to the public. This aims to provide employees with better network performance. The previous paragraphs mentioned that each service set has one AP. So, do we have to deploy two APs in this situation?

No. We can configure the virtual access point (VAP) function on Huawei APs to provide differentiated WLAN services. On a physical AP, up to 16 VAPs can be configured. Each VAP provides the same functions as a physical AP. The VAP function brings the following advantages:
1. Multiple VAPs work on one physical hardware platform, improving hardware utilization.
2. Different SSIDs, security settings, and QoS policies can be applied to different VAPs. The network is more flexible.
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The previous paragraphs described how to distinguish BSSs in an ESS. But how can we distinguish BSSs on an AP? Actually, VAPs are distinguished by BSSIDs which are the MAC addresses of VAPs instead of physical APs. MAC addresses of VAPs on a physical AP are mapped to the MAC address of the physical AP.
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Usually, the MAC address of the first VAP is the MAC address of the physical AP. For a subsequent VAP, the last digit of its MAC address is the result of the last digit of the physical AP's MAC address plus sequence number of the VAP minus one. A BSSID is a string of random numbers. It is easier to remember an ESSID. Therefore, an SSID is assigned to a VAP.

The following table summarizes the basic WLAN concepts mentioned in this section.

 Concept  Full Name  Description
 BSS  Basic Service Set  A BSS provides a basic building-block of a WLAN and consists of one AP and all STAs associated with the AP.
 ESS  Extended Service Set  An ESS is a set of two or more BSSs that share the same SSID. This extends the coverage range of a BSS.
 SSID  Service Set Identifier  An SSID identifies a wireless network.
 ESSID  Extended Service Set Identifier  An ESSID identifies an extended service set.
 BSSID  Basic Service Set Identifier  BSSIDs identify VAPs on the same physical AP on the link layer. They are also used to identify BSSs in an ESS.
 VAP  Virtual Access Point  A VAP is a functional entity on an AP. Different VAPs can be created on an AP to provide wireless access service for different user groups.

I hope that this brief explanation of these key concepts will help you to better understand WLANs.

The post is synchronized to: From Beginner to Expert-WLAN Fundamentals

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Created Jun 25, 2016 02:25:40

Thank you fror sharing.

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MVE Created Apr 11, 2018 10:14:36

useful document, thanks
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