Wireless voice can be a great asset to any wireless network. Mobile users such as doctors, nurses, warehouse associates, sales floor staff, and call center personnel are no longer tied to a desk where they can miss important, perhaps life saving, phone calls. The Cisco Unified Wireless Networking (CUWN) solution provides a robust wireless infrastructure for voice applications.
Cisco currently offers two wireless phone models: the 7921 and 7925. The 7920 model was the first Cisco wireless phone and is still found in the field, but it is End-of-Life and reached End-of-Support in June 2009. The 7920 is an 802.11b-only device and does not have the security protocol support of the newer models. This topic does not cover the 7920.
The controllers also support any wireless voice device of a vendor as long as it adheres to the wireless standards. Some examples of third-party wireless devices are Vocera badges and SpectraLink wireless phones.
Although you might be able to get away with simply placing access points (AP) in the network based on what looks good on a floor map for a data-only deployment, voice is a radio frequency (RF)-sensitive, throughput demanding, and latency-unforgiving application. If a data client has to retransmit a packet, odds are the end user will never notice. That is not so for voice. Lost packets, delayed packets, coverage gaps, and incorrect AP power and channel assignments will wreck a wireless voice implementation. The end user on a voice call will definitely notice if part of the conversation is lost.
Note It is highly recommended that you take advantage of the 802.11a capability of the 7921 and 7925 phones and conduct the site survey accordingly. In most cases, the 802.11b/g network is already saturated, which is detrimental to a voice deployment. This is especially true if you are planning to use the coexistence feature of the 7925.
The one prerequisite you must have completed is a site survey for voice. You must use a Cisco certified partner that has advanced wireless LAN (WLAN) specialization. Walking around with a laptop using a Cisco CB21 card with the site survey tool is not going to be sufficient. In fact, you are not even supposed to be able to purchase a 792x phone without a professional site survey. You must use nonoverlapping channels and allow at least 20 percent overlap with adjacent channels when deploying phones in the environment. Figure 11-1 shows a 20 percent cell overlap.
Figure 11-1 Twenty Percent Cell Overlap
For acceptable voice quality, the phones should always have a signal of -67 dBm or higher when using 2.4 or 5 GHz. You should also ensure that the packet error rate (PER) is no higher than 1 percent. You also want to maintain a minimum signal-to-noise ratio (SNR) of 25 dB.
You should design the voice deployment for a data rate of 24 Mbps. You can enable the higher data rates if you desire.
Cisco recommends that you set the minimum data rate on the 802.11 networks to 11 Mbps or 12 Mbps for 2.4 GHz as long as the other wireless clients can support it. For the 802.11a network, the minimum data rate should be 12 Mbps, and only 12 Mbps should be set to mandatory.
Other prerequisites you want to keep in mind are having the correct CallManager and controller code to support the phones. All the wireless controller and AP models support the 792x phones. Table 11-1 shows the supported versions of CallManager and controller for the 792x phones.
Table 11-1 792x CallManager and Controller Versions
|
Phone Model |
CallManager |
CallManager Express |
Wireless Controller |
|
7921 |
4.1 and later* |
4.1 and later (minimum of 12.4[15]T7) |
4.0.217.0, 5.1 or higher-recommended |
|
7925 |
4.1 and later* |
4.1 and later (minimum of 12.4[15]T7) |
4.0.217.0, 5.1 or higher-recommended |
You also want to make sure that your wired network quality of service (QoS) is properly configured for a voice deployment. QoS is covered later in this topic in the "QoS" section.
Tip It is usually a best practice to make sure that the phones are running the latest firmware. Unlike some devices, the latest firmware for the phones is usually the best and can clear up many intermittent issues.
Remember to separate voice, data, and management VLANs. Doing so isolates the different types of traffic and allows you greater control over that traffic. You would never want to have your data and voice clients sharing a service set identifier (SSID)/VLAN.
