Specifications

Altitude 3500 Series Access Point Product Reference Guide
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packet to the correct destination. Transmitted ARP request packets echo back to other MUs. The access
point removes from its database the destination or interface information that is not used for a specified
time. The AP refreshes its database when it transmits or receives data from these destinations and
interfaces.
Media Types
The access point radio interface conforms to IEEE 802.11a/b/g specifications. The interface operates at a
maximum 54Mbps (802.11a radio) using direct-sequence radio technology. The access point supports
multiple-cell operations with fast roaming between cells. Within a direct-sequence system, each cell can
operate independently. Adding cells to the network provides an increased coverage area and total
system capacity.
The RS-232 serial port provides a Command Line Interface (CLI) connection. The serial link supports a
direct serial connection (assuming a DB9 connector is used). The access point is a Data Terminal
Equipment (DTE) device with male pin connectors for the RS-232 port. Connecting the access point to a
PC requires a null modem serial cable.
Direct-Sequence Spread Spectrum
Spread spectrum (broadband) uses a narrowband signal to spread the transmission over a segment of
the radio frequency band or spectrum. Direct-sequence is a spread spectrum technique where the
transmitted signal is spread over a particular frequency range. The access point uses Direct-Sequence
Spread Spectrum (DSSS) for radio communication.
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 by the access point 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.
MUs 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 access point.
Intercepting and decoding a direct-sequence transmission requires a predefined algorithm to associate
the spreading code used by the transmitting access point to the receiving MU. This algorithm is
established by IEEE 802.11 specifications. The bit redundancy within the chipping sequence enables the
receiving MU 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
access point uses different modulation schemes to encode more bits per chip at higher data rates. The
access point is capable of a maximum 54Mbps data transmission rate (802.11a radio), but the coverage
area is less than that of an access point operating at lower data rates since coverage area decreases as
bandwidth increases.
MU Association Process
An access point recognizes MUs as they begin the association process. An access point keeps a list of
the MUs it services. MUs associate with an access point based on the following conditions:
● Signal strength between the access point and MU
● Number of MUs currently associated with the access point