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

1-21
Introduction
The access point internal stack interface handles all messages directed to the access point. Each
access point stores information on destinations and their interfaces to facilitate forwarding. When a
user sends an ARP (Address Resolution Protocol) request packet, the access point forwards it over all
enabled interfaces except over the interface the ARP request packet was received.
On receiving the ARP response packet, the access point database keeps a record of the destination
address along with the receiving interface. With this information, the access point forwards any
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.