This white paper discusses the constraints imposed by WAN links in
remote office wireless systems and highlights the two basic benchmarking tests,
throughput and roaming latency, for such configurations.
Wireless LANs (WLANs) have become more popular in enterprise
applications. In a situation where a corporation does not want to install a
separate wireless solution for a branch office, a remotely installed access
point (AP) that can handle several users and use the corporate network for
other needs such as security, logging, and software upgrade, has become more
popular. The branch office network is connected to the central office network
over the WAN links. A typical scenario where a Frame Relay serial WAN link is
used, is shown in Figure 1.
Figure 1: A typical WLAN setup for a remote branch
Performance testing involves a measurement of attributes that show how
the system behaves when loaded to maximum capacity. Standard performance
measures, such as throughput, roaming delays, and scaling, are at the heart of
every performance test for wireless equipment. However, these parameters can be
impacted severely by the topology under which the equipment is deployed. This
document focuses on one such topology where bandwidth plays a more important
role that affects standard performance measures.
This white paper highlights several important constraints and
techniques used to resolve these, and tests wireless performance over WAN links
in a controller-based architecture.
This section highlights the major constraints in a remote-office
The AP uses a hello packet, also known as the heartbeat, in order to
communicate with the controller. In an event where this heartbeat is lost, the
AP rediscovers the controller. During this process, all the clients that exist
are de-authenticated. This causes disruption of the wireless services at the
branch office. Therefore, one of the goals of testing over the WAN link is not
only to keep the heartbeat alive, but also take into account the effect on the
overall performance of the system.
The default heartbeat interval is 30 seconds and it cannot be
configured manually. When a heartbeat acknowledgement from the controller is
missed, the AP resends the heartbeat up to 5 times at 1 second intervals. If an
acknowledgement is not received after 5 retries, the AP declares the controller
unreachable and searches for a new controller.
One of the techniques used in this testing is traffic prioritization.
This keeps the heartbeat alive in order to avoid any service disruption. The AP
uses two UDP ports in order to communicate with the controller. The AP uses UDP
port 12223 for all management packets and 12222 for the data packets. If the
communication via port 12223 can be kept up, the link between the controller
and the AP functions even under severe traffic load across the WAN link. This
is usually implemented on the WAN router ports that point to the WAN
class-map match-all 1
match access-group 199
ip address 22.214.171.124 255.255.255.0
clock rate 512000
frame-relay interface-dlci 101
frame-relay intf-type dce
service-policy output mypolicy
access-list 199 permit udp any any eq 12223
In a general deployment, as shown in Figure1, the authentication is performed at the central
office where all the authentication servers are hosted. A local authentication
server kept at the remote office is not advisable from a cost and maintenance
point of view. If the controller becomes inaccessible for any reason, the
traffic can be bridged locally. However, because there is no local
authentication server, only open and Wi-Fi Protected Access (WPA)
authentication types are supported locally. For most of the customers, WPA
forms the only authentication type available. This becomes a severe constraint
in the design of remote-office wireless applications.
This section analyzes the effect of these constraints on system
As mentioned earlier in this document, the throughput is severely
impacted by the bandwidth available on the WAN link, as well as the traffic
prioritization. If you assume that a fixed bandwidth on the WAN link of 512
kbps is available with a traffic prioritization channel of 64kbps, the data
bandwidth available is 448 kbps. However, when you see the throughput up to 501
kbps, you might believe that the 64 kbps is pre-emptive instead of a dedicated
Frame sizes add another twist to this. From this table, the effect of
the WAN link and the frame sizes in a topology such as this is clear. This
table also shows the comparison with the APs connected in the central office.
Also, the throughput is measured when the clients in the remote branch office
try to send data to a wired client in the central office.
Frame Sizes (in bytes)
Throughput with APs connected in the central office
Throughput with APs connected in the remote office
As you can see from this table, the throughput increases with the frame
size until the frame size becomes 1280 and then drops back to 1450 bytes. This
is due to the fragmentation that occurs for frame sizes more than 1418 bytes in
From the previous discussion, the effect on roaming delays is
understood. This table displays the actual data. It was observed that the
roaming delays were much less when the APs were connected to the switch via a
WAN Link Present?
Avg. Roaming Delay (in msec)
In a remote branch office setup, the bandwidth offered by the WAN link
plays a crucial role in the decision of the performance of the equipment. Not
only is there a need to perform traffic prioritization, but the effects on the
throughput and roaming are an issue. The WAN link determines the benchmarking
that needs to be performed. These tests differ significantly from the standard
benchmarking tests. Also, because there is no local authentication server, WPA
is the preferred security type for such applications. The WAN link capacity and
the security type are important factors to be considered when you test such