industrial wireless book special edition - Networking ...

Industrial Wireless
Controllers to centralise the
management of big WLANs
The use of Wireless LAN has gained increasing acceptance in automation. Hardware manufacturers have
accordingly designed products for use in line with industrial requirements. This radio technology mostly finds
employment within application islands; additional requirements have to be fulfilled if it is to be used over a
wider area. Achieving coordinated wireless operation requires a degree of centralised management. A new
design take on the WLAN controller enables the creation of extended networks comprising hundreds or
thousands of access points says Hirschmann’s Olaf Schilperoort
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IN WHAT SEEMS like an age ago back in 2002,
a few entrepreneurs linked to American universities
set out to turn the WLAN world on its
head. Before then, radio networks consisted of
one or more access points connected to a wired
network which enabled mobile clients such as
laptops and PDAs to gain network access. These
access points, which were used predominantly
in the office sector, had just the basic functions
necessary for a Layer 2 infrastructure. Only few
of them included any sort of routing or firewall
mechanisms.
The start-up companies initially considered
adding extensive Layer 3 functions. They
wanted to take WLAN beyond local applications
in order to construct and centrally administer
campus and company-wide networks. These
networks could extend to several thousand
access points. There was also a question of the
software needed to manage this number of
devices. But the promise was to administer
large networks with a server-like device.
The original idea
This device was initially known as a WLAN
switch, because it was meant to control all the
access point functions centrally. The access
points were given the commercially attractive
name Thin AP. Their task was limited to that of
providing an interface between cable and radio
link. Every packet sent and received – including
all control and management information – was
initially sent to the central WLAN switch, before
the latter relayed the user data to the local
area network (LAN). In order to make this
possible, the switch and access points were
inter-connected by a Layer 2 tunnel.
This central approach was actually advantageous
for large installations since WLAN clients
that move around an extended site had to
remain within the same IP sub-network at all
times. The reason for this was to avoid new IP
addresses having to be assigned when
switching to other sub-networks. The inevitable
interruption of the network connection would
otherwise last several seconds, causing VoIP
for example to suffer service interruptions.
WLAN switches were therefore regarded as a
solution to cut down roaming handover time.
Roaming in this context is the term given to
the change by a client from one access point
or sub-network to another.
However WLAN switches turned out to be a
bottleneck as each data packet first had to be
transported from the receiving access point
across the entire network to the central device
which only then dispatched it to the assigned
destination in the LAN (Fig. 1). This led to the
switch’s bandwidth quickly becoming
exhausted. This problem was further exacerbated
with the advent of WLAN Standard IEEE
802.11n, which hiked the data rate possible
with each access point. With bandwidth
continuing to be an important sales argument
in the office sector, the development of WLAN
switches looked as though it was going to
reach an impasse.
Decentralised improvement
For such reasons of bandwidth management, it
became necessary to work around some aspects
of the centralised approach. WLAN switches
became WLAN controllers, which now only
exchange control data with the access points.
Many functions, which had previously been
centralised, migrated back to the access points.
In addition, the CAPWAP Standard (Control and
Provisioning of Wireless Access Points) had
since been defined, which describes how the
Layer 2 tunnel between controller and access
industrial ethernet book
point should be structured; this standard was
later also included in IEEE 802.11w. This subsequently
led to a symbiosis of the centralised
and decentralised approaches. In other words,
it combined the flexibility of the local, standalone
access point with broadband network
connection and the benefits of a high-performance
controller solution.
A WLAN controller locates all available access
points when the network is first configured,
provided a cable connection exists to the
Fig. 1. WLAN switches were regarded as a solution to cut
down roaming handover time. However WLAN switches
turned out to be a bottleneck as each data packet first
had to be transported from the receiving access point
across the entire network to the central device which only
then dispatched it to the assigned destination in the LAN
sponsored by Advantech
PHOTO: BELDEN