Polycom H340 - Manual (Page 17)

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Deploying SpectraLink e340, h340 and i640 Wireless Telephones BEST PRACTICES GUIDE
October 2010
4 SpectraLink Voice Priority (SVP)
Polycom pioneered VoWLAN for the enterprise and remains the market leader today. One key success factor has
been our SpectraLink Voice Priority (SVP) mechanism for QoS. This method is proven to deliver enterprise-grade
voice quality, battery life and call capacity for SpectraLink handsets.
Quality of service (QoS) is a means of providing a level of service that will result in a network connection of
acceptable quality. Typically this results in providing different levels of service for different applications, depending on
their requirements. When data and voice are competing for bandwidth, such as in a WLAN, it is necessary to have
mechanisms to prioritize voice packets over data, preserve battery life for handhelds, and allocate appropriate AP
bandwidth for the associated device’s applications. The original 802.11 standard did not provide a QoS mechanism,
so Polycom developed SVP to allow delay-sensitive voice and asynchronous data applications to coexist on a Wi-Fi
network without compromising voice quality.
Excellent voice quality for SpectraLink handsets is ensured on a shared Wi-Fi network using SVP. Adopted by the
majority of enterprise-class WLAN vendors, SVP is well-proven and guarantees audio quality on a shared voice and
data network. SVP is compatible with 802.11 standards, but uses proprietary methods for packet prioritization,
battery management and call admission control. Access points generally use random back-off intervals that require
all types of traffic to contend for access to the wireless medium with equal rights. However, treating all traffic equally
can cause significant delays to voice traffic. Modifying the AP behavior to recognize and prioritize voice packets
increases the probability of better performance while continuing to treat asynchronous data packets normally. The
two operations that comprise SVP in the AP, minimizing random back-off and priority queuing, require a packet-
filtering mechanism. Packet filtering requires recognizing the packet’s type. SpectraLink packets are registered as IP
protocol ID 119 at layer 4. The SVP Server performs packet delivery timing through the AP to the wireless
telephones, which is critical for ensuring seamless handoffs among APs and for enhanced battery management. The
following section offers a more detailed explanation of timed delivery.
4.1.1 SVP Infrastructure
To trigger SVP in the APs from the wired side of the network, a Telephony Gateway with integrated SVP Server
and/or a standalone SVP Server is required. Telephony Gateways can provide SVP support for small installations
with four or fewer Gateways. A SVP Server is required for applications using an IP telephony server or using more
than four Telephony Gateways.
4.1.2 SVP Server Capacity
A single SVP Server supports 120 simultaneous calls when used with Telephony Gateways or 80 simultaneous calls
with an IP telephony server. Multiple SVP Servers can be used to increase capacity to support up to 850 total calls
(which can support approximately 8,000 Wireless Telephones) for IP telephony server interfaces. When used with
Telephony Gateways, the total number of users is limited to 640 (40 Telephony Gateways). For smaller IP telephony
interface deployments, 10 and 20-user SVP Servers are available. Refer to Polycom’s SpectraLink 8000 SVP Server
Administration Guide for additional information regarding the maximum number of simultaneous calls and wireless
telephones supported by multiple SVP Servers.
4.1.3 Multiple SVP Servers
For installations with multiple SVP Servers, call resources are automatically allocated between the APs and the
SpectraLink Wireless Telephones by those devices’ Media Access Control (MAC) addresses. In most instances,
because of the large number of wireless telephones and APs expected in such an application, the distribution of call
processing will be relatively even across all SVP Servers.
Some installations with multiple SVP Servers (SVP code < 17x.033) are configured to have primary (“master”) and
one or more secondary (“slave”) servers. If a secondary SVP Server fails and can no longer be detected, the packet
handling (and associated handsets) will automatically be redistributed among the remaining servers. All active calls
associated to the failed secondary SVP server will be lost during this process, however the affected wireless
telephones will check-in with available SVP servers without manual reconfiguration. In the case of a master SVP