User's Manual

BER Display
Bit Error Rate (BER) Display is utilized by the operator to measure the amount bit errors present
in a link. BER Display will only work if the access point module is configured to send the BER
stream.
**IMPORTANT** If one access point module in a cluster is set to send a BER stream then
all other access point modules in that cluster must be configured in the same fashion.
The field, Measured Bit Error Rate, will display a value that corresponds to the BER at the
moment of web browser refresh. An acceptable link will have a BER that is less than 10
-4. If the
BER is greater than 10
-4 then the operator needs to evaluate the subscriber and/or access point
module installation for potential issues. To refresh the screen, click the Refresh Display button as
appropriate or configure the web page auto-refresh option.
APPENDIX
There are two basic concepts that are needed for a basic understanding of networking, IP
addresses and subnet masks. IP addresses are 32-bit binary numbers that have two
corresponding parts or sub-addresses, the first part identifying the network and the second part
identifying the hosts on the network. An imaginary boundary separates the first part from the
second. This imaginary boundary is marked by way of the subnet mask. The subnet mask is
another 32-bit binary number the acts like a filter on the IP address. When a subnet mask has a
bit set to 1, the corresponding bit in the IP address is part of the network address. A subnet is
classified as either a class A, class B, or class C network. The following table shows the common
subnet mask classes:
Class Network Portion Host Portion
A 11111111 00000000 00000000 00000000
B 11111111 11111111 00000000 00000000
C 11111111 11111111 11111111 00000000
For example, if you have an IP address of 169.254.1.1 and a subnet mask of 255.255.0.0, then the first 16-
bits of the 32-bit IP address identify the network.
10101001 11111110 00000001 00000001 IP address
11111111 11111111 00000000 00000000 Subnet Mask
There are 216 (65,536) addressable hosts in this example and 169.254 is the network. There is one last
piece of information that is needed here. Subnet masks are not shipped around in the IP
packet, the packet only contains the 32-bit IP address of the destination. So without this valuable
piece of information devices have no idea what portion of the IP address is part of the network and which is
part of the host address. How does data know where it is supposed to go? IP systems developed a unique
form of logic to make this determination. Class A network addresses always have the first bit of their IP
address set to 0. Class B network addresses always have their first bit set to 1 and their second bit set to 0.
Class C network addresses always have their first two bits set to 1 and the third bit set to 0. By examining
these first bits of the IP address a device can determine what subnet mask should be applied to the IP
address and determine where to route the data.
The following is a synopsis of an Internet Draft (http://www.ietf.org/internet-drafts/draft-ietfzeroconf-ipv4-
linklocal-05.txt) that describes how Microsoft and Apple operating systems react when a DHCP server is not
found on the network. In general, a computer needs certain configuration information to operate on a
network. Those configurations are an IP address, a subnet mask, and possibly a gateway address. A DHCP
server will automatically assign this configuration information to a computer on a network or an operator is
required to manually input these configurable items. When a computer is brought online and a DHCP server