User Manual

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2009 Industrial Wireless Guidebook
Understanding Industrial WLAN – IEEE 802.11
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Wireless VLAN
A Virtual LAN (VLAN), as defined in IEEE, is a group of hosts grouped together as if they were attached to the
Broadcast domains in a Layer 2 network. Traditional networks use routers to define broadcast domain, but it
is now possible to set the broadcast domain boundaries with Layer 2 switches. That is to say, a VLAN can add
two or more hosts that were in different subnets to be grouped into the same LAN segment disregard of their
geographical locations. VLAN provides the leeway to the network administrators to address network security,
management and scalability issues.
How to Set Up VLAN with IEEE 802.11
Every time a packet is sent from one switch to another over a VLAN, VLAN tagging is required. VLAN
tagging is the practice of inserting a VLAN ID in the packet header so the packet can be identified and
forwarded to the right port or interface. The IEEE 802.1q standard is the most commonly seen VLAN tagging
protocol created by IEEE group. The tagging protocol supports a maximum of 4096 VLANs.
By borrowing the same concepts, it is now also possible to apply VLAN to an IEEE802.11 wireless
network. Many wireless access points (AP) are now equipped with VLAN capability. A single AP can now
be configured to assign a different service set identifier (SSID) to different VLANs. Also the authentication
settings like MAC, EAP, and VLAN ID are required to configure a wireless VLAN.
Wireless VLAN Limitations
Wireless VLANs bring many benefits to WLAN applications, but there are some potential limitations when a
wireless VLAN reaches a certain scale. The first limitation arises from its 12 bit VLAN identifier (VID). The size
of the VID limits the number of wireless VLAN to 4094. The number might look big enough to accommodate
most WLAN applications. However, as wireless applications grow at a tremendous pace, it will soon be not
enough for some large scale WLAN applications.
Large scale Wireless VLAN also causes the second and third limitations. That is, when the Wireless VLAN
grows too large, traffic flowing through the routers also increases. This large volume of traffic makes routers
another potential bottleneck for the Wireless VLAN.
The third constraint is the potential security loopholes. As the VLAN grows, there is a possibility that the
wireless VLAN will stretch over large geographical areas that require the VLAN to pass through a third party
network. This creates security loopholes as there is almost nothing to stop the virus from spreading inside
the VLAN.