Introduction
1-5
1.2.2 Media Access Control (MAC) Layer Bridging
Like other Ethernet devices, the CB3000 has a hardware factory encoded address called a MAC address. The
address consists of a 48-bit number written as six hexadecimal bytes separated by colons.
The CB3000 maintains a list of up to 16 Ethernet clients. Using the CB3000 client list, the administrator can
determine which device is receiving or sending data at any given time.
1.2.3 DHCP Support
The CB3000 can use Dynamic Host Configuration Protocol (DHCP) to obtain a leased IP address and
configuration information from a remote server. DHCP is based on the BOOTP protocol and can co-exist or
interoperate with BOOTP.
Configure the CB3000 to send out a DHCP request searching for a server to acquire the required IP address
information. If DHCP server support is unavailable, an IP address can be assigned to the CB3000 manually
(static).
If CB3000 is configured to use DHCP, but there is no DHCP server to service the request, the CB3000 by default
takes the address 10.10.1.1. If it is not (as per the Ethernet settings) then one needs to set the address
manually.
1.2.4 Modulation
Modulation is the process of modulating an electromagnetic carrier wave. (e.g., radio wave), by imposing
digital information on it, enabling the user to transmit the information in analogue form. The 802.11b standard
uses Direct Sequence Spread System (DSSS), while 802.11a/g use Orthogonal Frequency Division
Multiplexing (OFDM) to accommodate higher data rates.
1.2.4.1 Direct Sequence Spread Spectrum (DSSS)
The 802.11b standard supported by the CB3000 uses DSSS for radio communication. Spread Spectrum
(broadband) uses a narrowband signal to spread the transmission over a segment of the radio frequency band.
Direct-sequence is a spread spectrum technique where the transmitted signal is spread over a particular
frequency range.
DSSS communicates by continuously transmitting a redundant pattern of bits called a chipping sequence. Each
bit of transmitted data is mapped into chips by the CB3000 and rearranged into a pseudo-random spreading
code to form the chipping sequence. The chipping sequence is combined with a transmitted data stream to
produce the output signal used by the CB3000 and its associated access point or peer device (if in Ad Hoc
mode).
Devices receiving a direct sequence signal use the spreading code to map the chips within the chipping
sequence back into bits to recreate the original transmitted data. Intercepting and decoding a direct sequence
transmission requires a predefined algorithm to associate the spreading code used by the transmitting access
point and CB3000 to the receiving device. This algorithm is defined within the IEEE 802.11b specification. The
bit redundancy within the chipping sequence enables the receiving device 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. 802.11b supported
devices are capable of an 11 Mbps data transmission rate, but the coverage area is less than 1 or 2 Mbps
(Frequency-Hopping devices) since coverage area decreases as bandwidth increases.
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