Mac Layer Bridging; Media Types; Direct-Sequence Spread Spectrum - Extreme Networks Altitude 4700 Series Product Reference Manual

MAC Layer Bridging

The Access Point provides MAC layer bridging between its interfaces. The Access Point monitors traffic
from its interfaces and, based on frame address, forwards the frames to the proper destination. The
Access Point tracks source and destination addresses to provide intelligent bridging as MUs roam or
network topologies change. The Access Point also handles broadcast and multicast messages and
responds to MU association requests.
The Access Point listens to all packets on its LAN and WAN interfaces and builds an address database
using MAC addresses. An address in the database includes the interface media that the device uses to
associate with the Access Point. The Access Point uses the database to forward packets from one
interface to another. The bridge forwards packets addressed to unknown systems to the Default Interface
(Ethernet).
The Access Point internal stack interface handles all messages directed to the Access Point. Each 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 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 802.11 specifications. 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 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.

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
Altitude 4700 Series Access Point Product Reference Guide
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