Voice over Wireless LAN Solution Guide v1.0 December 2005
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Figure 7: Single telephony VLAN implementation
Consolidating VoWLAN handsets into one VLAN/subnet has a few advantages. First, it allows the
WTM 2245 design to be greatly simplified. Instead of purchasing and deploying at least one WTM
2245 per voice subnet, you can now install one WTM 2245 for the single voice subnet. For larger
VoWLAN deployments, more WTM 2245s may be required in that single subnet to support the
number of calls, but overall fewer WTM 2245s are needed than in an equivalent multisubnet
deployment. Deciding on the number of WTM 2245s needed becomes strictly a call engineering
exercise (as it should be).
A second advantage is that external security measures are easier and less costly to implement. It
is common practice to put a telephony WLAN behind a firewall for security reasons. This is
because security features on handsets, particularly authentication capabilities, tend to lag behind
the industry. So to mitigate risks, a firewall can be used to block all but the ports needed for IP
Telephony. This practice gets complex and costly when multiplied by a number of subnets. A
more cost-effective alternative to implementing a firewall is to assign private addresses to the
handsets and let the WTM 2245 Network Address Translation (NAT) capabilities serve as a form
of secure firewall to the telephony LAN (T-LAN). Of course this is not as secure as using a
traditional firewall to secure the T-LAN.
The downside of putting all telephony devices into the same subnet is that broadcasts are
increased. Also, while security is simplified, the importance of implementing adequate security
measures increases because more devices will be impacted in the event of a security breach.
2.2.2.6 Roaming
The concept of roaming in the WSS 2300 solution is a simple one. Given the previous discussion
about Layer 3 implementation and VLANs being determined by policy, roaming is a very simple
extension. In fact, the section could be renamed “roaming upon startup” because a device being
tunneled to a remote VLAN upon connection to the network is like starting out in a roamed state
from a WSS perspective. When moving from WSS to WSS, the tunnel back to the remote VLAN
is simply moved with the device.
In the previous VoWLAN solution for the WLAN 2200 series, one of the design limits was that
Push-to-Talk (PTT) required all WSS 2270s to be physically located in the same subnet due to
roaming problems. With the WLAN 2300 series this restriction is now lifted due to symmetric
tunneling, and PTT works across WSSs in different subnets. Symmetric tunneling describes the
traffic flow in the event that a device is not local to the subnet to which it is assigned. Both