Brocade Mobility 7131N-FGR Access Point Product Reference Guide (Supporting software release 4.0.0.0-35GRN and later) - Manual (Page 31)

Brocade Mobility 7131N-FGR Product Reference Guide 19
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Theory of operations
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Access points with the same ESSID define 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 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/n or
802.11b/g/n 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 phone system. WLANs can therefore be configured
around the needs of specific groups of users, even when they are not in physical proximity.
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 operates independently. Adding cells to the network provides an increased
coverage area and total system capacity.