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Figure 35

Example WPS Process: PIN Method

5.10.8.3

How WPS Works

When two WPS-enabled devices connect, each device must assume a specific role. One device acts

as the registrar (the device that supplies network and security settings) and the other device acts

as the enrollee (the device that receives network and security settings. The registrar creates a

secure EAP (Extensible Authentication Protocol) tunnel and sends the network name (SSID) and the

WPA-PSK or WPA2-PSK pre-shared key to the enrollee. Whether WPA-PSK or WPA2-PSK is used

depends on the standards supported by the devices. If the registrar is already part of a network, it

sends the existing information. If not, it generates the SSID and WPA(2)-PSK randomly.

The following figure shows a WPS-enabled client (installed in a notebook computer) connecting to a

WPS-enabled access point.

ENROLLEE

SECURE EAP TUNNEL

SSID

WPA(2)-PSK

WITHIN 2 MINUTES

COMMUNICATION

This device’s

WPS

Enter WPS PIN

WPS

from other device:

WPS PIN:

123456

WPS

START

WPS

START

REGISTRAR

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Figure 36

How WPS works

The roles of registrar and enrollee last only as long as the WPS setup process is active (two

minutes). The next time you use WPS, a different device can be the registrar if necessary.

The WPS connection process is like a handshake; only two devices participate in each WPS

transaction. If you want to add more devices you should repeat the process with one of the existing

networked devices and the new device.

Note that the access point (AP) is not always the registrar, and the wireless client is not always the

enrollee. All WPS-certified APs can be a registrar, and so can some WPS-enabled wireless clients.

By default, a WPS devices is “unconfigured”. This means that it is not part of an existing network

and can act as either enrollee or registrar (if it supports both functions). If the registrar is

unconfigured, the security settings it transmits to the enrollee are randomly-generated. Once a

WPS-enabled device has connected to another device using WPS, it becomes “configured”. A

configured wireless client can still act as enrollee or registrar in subsequent WPS connections, but a

configured access point can no longer act as enrollee. It will be the registrar in all subsequent WPS

connections in which it is involved. If you want a configured AP to act as an enrollee, you must reset

it to its factory defaults.

5.10.8.4

Example WPS Network Setup

This section shows how security settings are distributed in an example WPS setup.

The following figure shows an example network. In step

1

, both

AP1

and

Client 1

are

unconfigured. When WPS is activated on both, they perform the handshake. In this example,

AP1

is the registrar, and

Client 1

is the enrollee. The registrar randomly generates the security

information to set up the network, since it is unconfigured and has no existing information.

SECURE TUNNEL

SECURITY INFO

WITHIN 2 MINUTES

COMMUNICATION

ACTIVATE

WPS

ACTIVATE

WPS

WPS HANDSHAKE

REGISTRAR

ENROLLEE

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Figure 37

WPS: Example Network Step 1

In step

2

, you add another wireless client to the network. You know that

Client 1

supports registrar

mode, but it is better to use

AP1

for the WPS handshake with the new client since you must

connect to the access point anyway in order to use the network. In this case,

AP1

must be the

registrar, since it is configured (it already has security information for the network).

AP1

supplies

the existing security information to

Client 2

.

Figure 38

WPS: Example Network Step 2

In step 3, you add another access point (

AP2

) to your network.

AP2

is out of range of

AP1

, so you

cannot use

AP1

for the WPS handshake with the new access point. However, you know that

Client

2

supports the registrar function, so you use it to perform the WPS handshake instead.

REGISTRAR

ENROLLEE

SECURITY INFO

CLIENT 1

AP1

REGISTRAR

CLIENT 1

AP1

ENROLLEE

CLIENT 2

EXISTING CONNECTION

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Figure 39

WPS: Example Network Step 3

5.10.8.5

Limitations of WPS

WPS has some limitations of which you should be aware.

•

WPS works in Infrastructure networks only (where an AP and a wireless client communicate). It

does not work in Ad-Hoc networks (where there is no AP).

•

When you use WPS, it works between two devices only. You cannot enroll multiple devices

simultaneously, you must enroll one after the other.

For instance, if you have two enrollees and one registrar you must set up the first enrollee (by

pressing the WPS button on the registrar and the first enrollee, for example), then check that it

successfully enrolled, then set up the second device in the same way.

•

WPS works only with other WPS-enabled devices. However, you can still add non-WPS devices to

a network you already set up using WPS.

WPS works by automatically issuing a randomly-generated WPA-PSK or WPA2-PSK pre-shared

key from the registrar device to the enrollee devices. Whether the network uses WPA-PSK or

WPA2-PSK depends on the device. You can check the configuration interface of the registrar

device to discover the key the network is using (if the device supports this feature). Then, you

can enter the key into the non-WPS device and join the network as normal (the non-WPS device

must also support WPA-PSK or WPA2-PSK).

•

When you use the PBC method, there is a short period (from the moment you press the button

on one device to the moment you press the button on the other device) when any WPS-enabled

device could join the network. This is because the registrar has no way of identifying the

“correct” enrollee, and cannot differentiate between your enrollee and a rogue device. This is a

possible way for a hacker to gain access to a network.

You can easily check to see if this has happened. WPS works between only two devices

simultaneously, so if another device has enrolled your device will be unable to enroll, and will not

have access to the network. If this happens, open the access point’s configuration interface and

look at the list of associated clients (usually displayed by MAC address). It does not matter if the

CLIENT 1

AP1

REGISTRAR

CLIENT 2

EXISTING CONNECTION

ENROLLEE

AP2

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access point is the WPS registrar, the enrollee, or was not involved in the WPS handshake; a

rogue device must still associate with the access point to gain access to the network. Check the

MAC addresses of your wireless clients (usually printed on a label on the bottom of the device). If

there is an unknown MAC address you can remove it or reset the AP.