Media Types; Direct-Sequence Spread Spectrum - Symbol AP-51 Series Product Reference Manual

1-22 AP-51xx Access Point Product Reference Guide
directed 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.

1.3.3 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 operates independently. Adding cells to the network provides 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. 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.

1.3.4 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 Symbol 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 AP -5131's 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 802.11b 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.