Polycom H340 - Manual (Page 27)

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Deploying SpectraLink e340, h340 and i640 Wireless Telephones BEST PRACTICES GUIDE
October 2010
configuration, regardless of what subnet the phone is operating in. This scenario requires fewer SVP Servers to be
installed, but requires higher performance from the router (see performance requirements in Section 6.3)
The ability to cross a subnet boundary exists in either scenario, but the SpectraLink handsets will need to be power
cycled to obtain a new IP address within the new subnet. In addition, other configuration considerations must be
addressed. Because users will not want to re-administer the wireless telephones to a separate subnet, Extended
Service Set Identifier (ESSIDs) should be the same or the handsets should be set to the “Learn Always” mode, the
security mode and associated key should be the same or turned off, and DHCP should be used.
6.3 Network Performance Requirements
Ethernet packets containing voice as their payload have short, useful lifetimes, making the timely delivery of voice
packets essential. Routers can introduce latency and delay between the SVP Server and the APs, resulting in poor
voice quality.
Ethernet connectivity from the call server or other voice endpoint to the SVP Server should never exceed 100
milliseconds of network delay (one way), 30 milliseconds of network jitter, and 2 percent packet loss end-to-end,
regardless of the physical properties of the link. The link from the SVP Server to the APs should be under 100
milliseconds of network delay, one millisecond of jitter and less than two percent packet loss. In both cases, the jitter
requirements are for wired network jitter and do not include the RF link.
One function of the SVP Server is to control the timing of packets through the AP. The SVP server delivers audio
packets to the wireless telephone every 30 millisecond. The delay between the SVP Server and the AP needs to be
controlled and consistent. Wired QoS (DSCP) is one aspect of ensuring voice packets have highest priority. The jitter
requirement between the call server and the SVP Server is a function of how the audio is packetized for
encapsulation in the SpectraLink Radio Protocol (SRP) and the packet queuing in the SVP Server.
Jitter between the SVP Server and the AP should be measured at the wired Ethernet connection to the AP. If the AP
is a lightweight AP attached to a wireless controller and Polycom has VIEW Certified the system, jitter can be
measured at the entry to the wireless controller. However it is better to measure jitter at the AP’s Ethernet interface if
the AP does not connect directly to the wireless controller. For this measurement, the SVP Server is delivering
packets at 30 millisecond intervals with no jitter. The time is measured from the arrival of one packet from the SVP
Server directed to a single wireless telephone to the next packet from the SVP Server to the same wireless
telephone. The jitter measurement is the time difference from the ideal 30 millisecond arrival of packets at the AP.
See Figure 9.