Mac Layer Bridging - Symbol AP-51 Series Product Reference Manual

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Introduction
access points with the same ESSID defines a coverage area. A valid ESSID is an alphanumeric, case-
sensitive identifier up to 32 characters. An MU searches for an access point with a matching ESSID
and synchronizes (associates) to establish communications. This device association allows MUs
within the coverage area to move about or roam. As the MU roams from cell to cell, it associates with
a different access point. The roam occurs when the MU analyzes the reception quality at a location
and determines a different access point provides better signal strength and lower MU load
distribution.
If the MU does not find an access point with a workable signal, it can perform a scan to find any AP.
As MUs switch APs, the AP updates its association statistics.
The user can configure the ESSID to correspond to up to 16 WLANs on each 802.11a or 802.11b/g
radio. A Wireless Local Area Network (WLAN) is a data-communications system that flexibly extends
the functionalities of a wired LAN. A WLAN does not require lining up devices for line-of-sight
transmission, and are thus, desirable. Within the WLAN, roaming users can be handed off from one
access point to another like a cellular phone system. WLANs can therefore be configured around the
needs of specific groups of users, even when they are not in physical proximity.

1.3.2 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
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