Implementing Multiple SSIDs

A service set identifier (SSID)
is a unique label that distinguishes one WLAN from another. Wireless devices
use the SSID to establish and maintain connectivity. As part of the association
process, a wireless network interface card (NIC) must have the same SSID as
the access point. An SSID contains up to 32 alphanumeric characters, which are
case sensitive. Traditional access points are only capable of supporting a singe
SSID.

Now some companies, such as Cisco
and Symbol, are offering enterprise-class access
points that support multiple SSIDs. Some examples of products that support multiple
SSIDs are the Cisco 1100 Series access point, which can support up to 16 separate
SSIDs, and the Symbol Mobius Axon Wireless Switch, which can support up to 32
separate SSIDs. This logically divides the access point into several virtual
access points all within a single hardware platform. Many companies want to
take advantage of this technology because using access points to support more
than one application, such as public Internet
access
and inventory control, increases flexibility and keeps costs down.

Options for Multiple SSIDs

Multiple SSIDs allow users to access different networks through a single access
point. Network managers can assign different policies and functions for each
SSID, increasing the flexibility and efficiency of the network infrastructure.

Here are some possible settings you could assign to each SSID:

  • Virtual Local Area Network (VLAN) . If your network uses VLANs, you can assign an SSID to VLAN1, and
    the access point groups client devices using that SSID into VLAN1. This
    enables the separation of wireless applications based on security and performance
    requirements. For example, you could enable encryption and authentication
    on one SSID to protect private applications and no security on another SSID
    to maximize open connectivity for public usage.
  • SSID broadcasting. In some cases, such as public
    Internet access applications, you can broadcast the SSID to enable user radio
    cards to automatically find available access points. For private applications,
    it’s generally best to not broadcast the SSID for security reasons — it invites
    intruders. Multiple SSIDs means you can mix and match the broadcasting of
    SSIDs.
  • Maximum number of client associations. You can
    set the number of users that can associate via a particular SSID, which makes
    it possible to control usage of particular applications. This can help provide
    a somewhat limited form of bandwidth control for particular applications.

Shared WLANs Come Alive

The use of multiple SSIDs means more flexibility when deploying a shared WLAN
infrastructure. Instead of supporting only one type of application, possibly
one that requires significant authentication and encryption, the WLAN can also
maintain other applications that don’t require such stringent controls. For
example, the access point could support both public and operational users from
a single access point.

The benefits of a shared infrastructure are certainly cost savings and enabling
of mobile applications. Rather than having two separate WLANs (which probably
isn’t feasible), a company can deploy one WLAN and satisfy all requirements.
The combination of multiple applications enables the ones having lower return
on investment to be part of the WLAN. Sometimes a company needs to have several
applications supported together to make the costs of deploying a WLAN feasible.

Jim Geier provides independent
consulting services
to companies developing and deploying
wireless network solutions.
He is the author of the book, Wireless LANs and offers workshops on deploying
WLANs.

Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

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