FAQs - Access Point

What steps can I take to enhance security?

First, change the AP's SSID from the default that is set at the factory. Second, make the AP's SSID invisble (turn off the SSID broadcast). Implement a password for access to the AP and CPE configuration software. Enable MAC address filtering in the AP. Enable 128 bit WEP with open key authentication. Deny unencrypted data in WEP. Change the WEP keys at least weekly. Disallow administrative access over wireless. Use secure protocols such as VPN connections or SSH (secure shell) or SSL (secure sockets layer). MAC filtering won't actually deny association, it just won't pass any traffic from a non-authorized MAC address. A good measure to implement at the next hop past the wireless nextwork is IP/MAC pair filtering. A LINUX box with iptables can do this by creating a chain which gets all traffic from X.X.X.X/X.X.X.X processed by a chain which only allows traffic from MAC x:x:x:x:x:x.

Is Shared Key Authentication better than Open Key in WEP?

Shared Key Authentication is not considered secure, because a hacker who detects both the clear-text challenge and the same challenge encrypted with a WEP key can decipher the WEP key. With Open Key Authentication, even if a client can complete authentication and associate with an AP, the use of WEP prevents the client from sending data to and receiving data from the AP, unless the client has the correct WEP key.

What is the maximum distance to an 802.11b Access Point or another CPE?

The maximum distance supported is approximately 12 miles (19 km). Timing issues in 802.11b transmissions limit the maximum distance.

Should I use a lower RTS Threshold in my AP?

For WISP applications, an AP should NEVER ask clients for permission to speak. Set your RTS threshold high on the AP, between 1500 and 2347.

In my Access Point wireless configuration, what are the differences between "Basic Rates", "Supported Rates" and "Transmission Rate"?

The "Basic Rates" are used only for management frames, broadcast and multicast frames, and NOT for data frames. The highest rate specified is the rate the AP uses to transmit such frames. The "Supported Rates" allows an 802.11 network to specify the data rate it supports. When a station attempts to join the network, it checks the data rate used on the network. The "Transmission Rate" is used to control the current transmit rate of a data packet. This setting basically provides a means to set a Fixed or Auto rate value when the AP and a station are enabled to communicate at the current rate. Starting from the highest rate of the association, the fallback algorithm is triggered by the transmit failure of a frame (i.e. all retries have been sent without acknowledgement). The fallback rate will last for an internally fixed time or until a number of successful transmissions controls upgrade back to the highest rate.

What is the DTIM Interval?

The DTIM Interval determines the number of AP beacons between each Delivery Traffic Indication Message (DTIM). This informs clients of the next window for listening to broadcast and multicast messages. When the AP has buffered broadcast or multicast messages for associated clients, it sends the next DTIM with a DTIM Interval value. Clients for that AP hear beacons and awaken to receive the broadcast and multicast messages.

What are the surge ratings of the AP and CPE?

Here are the electrical specifications of the surge protection:

For the 2 power lines (PoE):
Peak Pulse Power (tp=8/20us) = 500 watts
Maximum Clamping Voltage (@Ip=1A) = 43 volts
Maximum Clamping Voltage (@8/20us) = 49 volts @ 12A
IEC 61000-4-2 (ESD): Air = 15 kV, Contact = 8 kV
IEC 61000-4-4 (EFT): 40A - 5/50ns
IEC 61000-4-5 (Surge): 24A, 8/20us - Level 2
(Line-Ground) & Level 3 (Line-Line)

For the data lines:
Peak Pulse Power (tp=8/20us) = 500 watts
Maximum Clamping Voltage (@Ip=1A) = 13.4 volts
Maximum Clamping Voltage (@8/20us) = 16.9 volts @ 34A
IEC 61000-4-2 (ESD): Air = 15 kV, Contact = 8 kV
IEC 61000-4-4 (EFT): 40A - 5/50ns
IEC 61000-4-5 (Surge): 12A, 8/20us - Level 1
(Line-Ground) & Level 2 (Line-Line)

How can I prevent inter-client file sharing in my WISP?

We recommend that all wireless clients use a NAT router between the CPE and their computers. This will block all NETBIOS traffic and also supply the customer with limited firewall protection against hackers and broadcast based viruses.

How can I tell if my Access Point antenna is working correctly?

There are 2 main properties that you can check if you have the proper equipment. The first property is the antenna's VSWR (voltage standing wave ratio). The second main property is the radiation pattern. For an omnidirectional antenna, the received signal strength at a client should be similar for all angles at a fixed radius from the AP. Since the client antenna may have directional properties and terrain & obstacles may affect the AP coverage area, the received signal may vary as much as 6 to 10 dB over different paths at a fixed distance from an AP. For a directional or sector antenna, the recieved signal strength at a client should be at least 15 dB stronger off the front side of the antenna than off the back side at a fixed distance. If it isn't, then the antenna may be defective or damaged. Another way to check to see if an antenna is working is to unplug the coaxial cable from the antenna. If the received signal off the front side of the antenna doesn't change significantly, then the antenna may be defective or damaged. This may also indicate a problem with the cable or connectors too.

Why is the range so poor for my Access Point?

There can be many reasons why you are seeing reduced range from an AP. First, make sure that you are getting at least 30 dBm EIRP from your AP and antenna combination. The maximum allowed by the FCC is 36 dBm. Use a short length of low-loss cable between the AP and antenna, and if you are just using one antenna, make sure that it is connected to the TX/RX port. If you have to use an omnidirectional antenna, make sure that it has at least 7 dBi gain. 10 dBi is better. On the client end, outdoor antennas with at least 10 dBi gain should be used if possible. To obtain distances over 6 miles, the client EIRP should be at least 36 dBm. If the clients use a laptop with a built-in antenna, they should use systems with at least 100 mW transmit power. Laptops with built-in antennas and 30 mW or less should not expect more than 300-500 feet range from an AP.

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Technical FAQ Categories

Basics, Access Point, Antenna Properties, Canadian RSS-210, Client Equipment, FCC Part 15, Point to Multi-Point, Point to Point, Pre-installation and Site Preparation, Radio Propagation, Reference Material, Troubleshooting.