4-4
Direct-sequence systems communicate by continuously transmitting a redundant
pattern of bits called a chipping sequence. Each bit of transmitted data is mapped
into chips and rearranged into a pseudorandom spreading code to form the
chipping sequence. The chipping sequence is combined with a transmitted data
stream to produce the output signal.
Wireless mobile clients receiving a direct-sequence transmission use the spreading
code to map the chips within the chipping sequence back into bits to recreate the
original data transmitted by the wireless device. Intercepting and decoding a direct-
sequence transmission requires a predened algorithm to associate the spreading
code used by the transmitting wireless device to the receiving wireless mobile
client.
This algorithm is established by IEEE 802.11b specications. The bit redundancy
within the chipping sequence enables the receiving wireless mobile client to
recreate the original data pattern, even if bits in the chipping sequence are
corrupted by interference. The ratio of chips per bit is called the spreading ratio.
A high spreading ratio increases the resistance of the signal to interference. A
low spreading ratio increases the bandwidth available to the user. The wireless
device uses a constant chip rate of 11Mchips/s for all data rates, but uses different
modulation schemes to encode more bits per chip at the higher data rates. The
wireless device is capable of an 11 Mbps data transmission rate, but the coverage
area is less than a 1 or 2 Mbps wireless device since coverage area decreases as
bandwidth increases.
Encryption
This provides wireless data transmissions with a level of security. This option
allows you to specify a 64-bit or a 128-bit WEP key. A 64-bit encryption contains
10 hexadecimal digits or 5 ASCII characters. A 128-bit encryption contains 26
hexadecimal digits or 13 ASCII characters.
64-bit and 40-bit WEP keys use the same encryption method and can interoperate
on wireless networks. This lower level of WEP encryption uses a 40-bit (10
hexadecimal digits assigned by the user) secret key and a 24-bit Initialization
Vector assigned by the device. 104-bit and 128-bit WEP keys use the same
encryption method.
All wireless clients in a network must have identical WEP keys with the access
point to establish connection. Keep a record of the WEP encryption keys.
Extended Service Set (ESS)
A set of one or more interconnected basic service set (BSSs) and integrated local
area networks (LANs) can be congured as an Extended Service Set.
ESSID (Extended Service Set Identier)
You must have the same ESSID entered into the gateway and each of its wireless
clients. The ESSID is a unique identier for your wireless network.
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