Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
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Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 9 VoWLAN Design Recommendations
Call Capacity
Call Capacity
The number of calls on a Wi-Fi channel is limited by a number of factors. First, the media used by the
AP and VoWLAN clients is the RF spectrum, which cannot be shielded from electromagnetic
interference like shielded twisted-pair CAT 5 cable. The closest Wi-Fi comes to segmentation is channel
separation. This open shared media of 802.11 creates the possibility for high packet loss. Most of this
packet loss is addressed through retransmission of 802.11 frames, which in turn causes jitter.
AP and VoWLAN clients is the RF spectrum, which cannot be shielded from electromagnetic
interference like shielded twisted-pair CAT 5 cable. The closest Wi-Fi comes to segmentation is channel
separation. This open shared media of 802.11 creates the possibility for high packet loss. Most of this
packet loss is addressed through retransmission of 802.11 frames, which in turn causes jitter.
illustrates the packet loss relationship as a mean opinion score (MOS).
Figure 9-6
Effective Packet Loss Graphic
In 802.11a as well as 802.11g, the highest coverage range is achieved by the lowest data rate, which is
6 Mbps. The lowest packet error rate is also at 6 Mbps, for the same given power level.
6 Mbps. The lowest packet error rate is also at 6 Mbps, for the same given power level.
An acceptable coverage area for voice is an area that maintains a packet error rate of 5 percent or less.
The MOS scores are ranked as follows:
The MOS scores are ranked as follows:
•
4.4—Top G.711 MOS score
•
4.3–4.0—“Very satisfied” to “satisfied”
•
4.0–3.6—“Some users satisfied”
shows that a packet error rate of 5 percent reduces the MOS to a level of “some users
satisfied” quality of speech.
The coverage area edge for a phone is where the coverage area drops the MOS to the “very satisfied”
category. This coverage area edge is referred to as a cell edge in this chapter. A cell edge with a 1 percent
packet error rate is needed for voice because of the likelihood of multiple phone clients, data clients,
co-channel interference, and other un-accounted for interferers. Cell edge and coverage design are
defined in detail in other sections of this chapter.
category. This coverage area edge is referred to as a cell edge in this chapter. A cell edge with a 1 percent
packet error rate is needed for voice because of the likelihood of multiple phone clients, data clients,
co-channel interference, and other un-accounted for interferers. Cell edge and coverage design are
defined in detail in other sections of this chapter.
If 802.11 and 802.11b are not required to support legacy 2.4 GHz Wi-Fi clients, Cisco recommends
disabling the rates of 1, 2, 5.5, and 11. If those rates are disabled, one or more 802.11g data rates must
be set to “required”. The data rate of 6 is generally the recommended data rate to be set to “required”,
but this depends on the cell size design requirements, which may require using a higher bit rate. If
possible, an 802.11g-only network is recommended rather than a 802.11b/g network. Most data clients
and phone clients recognize the data rates advertised by the AP in its beacons and probe response.
disabling the rates of 1, 2, 5.5, and 11. If those rates are disabled, one or more 802.11g data rates must
be set to “required”. The data rate of 6 is generally the recommended data rate to be set to “required”,
but this depends on the cell size design requirements, which may require using a higher bit rate. If
possible, an 802.11g-only network is recommended rather than a 802.11b/g network. Most data clients
and phone clients recognize the data rates advertised by the AP in its beacons and probe response.
220342
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.0
1.5
0.5
0
0
1
2
3
4
5
% Loss - 20 ms packets
MOS