2013 Latest Cisco 350-001 Exam Section 5: VoWLAN (5 Questions)
QUESTION NO 1:
TestKing is utilizing VOIP on a wireless LAN. How many simultaneous WLAN
VOIP calls can be supported by an AP with Quality of Service enabled, assuming
that the G.711 codec is used?
E. None without proxy ARP enabled
The following network capacity guidelines apply to sizing the Wireless IP Telephony network:
No more than 7 concurrent G.711 calls per AP.
No more than 8 concurrent G.729 calls per AP.
QUESTION NO 2: What is eDCA?
A. The difference in the delay used by 802.11 management frames, and data frames
B. The time taken between the when a channel becomes free and a radio tries to send a frame
C. The standard 802.11 contention mechanism
D. A mechanism for adjusting the random backoff of WLAN traffic based in traffic classification
E. An authentication type for handheld devices
EDCA (Enhanced Distributed Channel Access) was specified in the 802.11e draft. EDCA, also known as prioritized DCF, improves on DCF by giving higher-priority traffic an advantage during contention. Instead of waiting the normal period before transmitting after the back-off period expires, higher-priority traffic can attempt to transmit only after a PIFS (point coordination function interframe space) period and associated back-off time. Using the EDCA scheme, nodes that offer high-priority traffic, an example being VoIP phones, have a higher probability of gaining channel access than the nodes offering lower-priority traffic, such as PC downloads.
QUESTION NO: 3 The TestKing network is utilizing Voice over Wireless LANs to provide for a mobile workforce. How would you design frequency overlap for voice over WLAN versus
802.11 for data only?
A. You would ensure that all areas where an 802.11 voice call could be initiated is covered by at least two RF infrastructure devices.
B. You would configure all the RF instrastructure devices to select optimal channels as required.
C. You would ensure each cell is at least 20% overlapped by second RF infrastructure device.
D. You would ensure all infrastructure RF devices were set to maximum power.
E. None of the above.
The critical components in the wireless network are the access points (APs) that provide the “hot spots” or wireless links to the network. Cisco requires that CiscoIOS is running on the APs that support voice calls since Cisco IOS provides features for managing voice traffic. The AP has a transmission range or coverage area that depends on its type of antenna and transmission power. The access point coverage range generally varies from 500 to 1000 feet. To provide effective coverage, access points need a range overlap of approximately 20 percent to allow uninterrupted connections as phone users roam from one access point to another.
QUESTION NO: 4
The TestKing network plans on using VOIP phones over the Wireless data network.
When deploying a low latency wireless network, what are the key guidelines that
should be maintained?
A. The access points requirements.
B. Use fixed channels, static WEP keys, all AP on the same channel.
C. Dynamic channels, diversity antenna, overlapping channels with more than 20% RSSI
D. Use fixed channels, diversity antenna, same transmit power on phone as the AP, overlapping channels have less than 20% RSSI.
E. Use fixed channels, CCKM, all AP on the channel, diversity antennas.
Recommended Environment for A Low Latency, VOIP network:
Deploy a minimum of two APs on non-overlapping channels, with a Received Signal
Strength Indicator (RSSI) that is greater than 35 at all times in the phone’s site survey
Deploy no more than one AP per overlapping channel set, with a received signal strength
indicator (RSSI) that is greater than 35.
Although APs might appear to have an RSSI that is less than 35 (on overlapping APs),
this situation can still cause interference and should be minimized as much as possible.
(This interference or noise will degrade voice quality.)
Noise is additive. Having three extra APs on the same channel, all with low RSSI, can be
as harmful as a single extra AP with a higher RSSI.
Figure2-1 shows a typical deployment, with a 15% to 20% overlap of a given AP’s cell
from each of the adjoining cells. This configuration provides almost complete redundancy
throughout the cell, thus complying with the above requirements.
Figure2-1 Cell Overlap Guidelines
Two of the APs (including the one with which the wireless phone is associated) must have an RSSI that is greater than 35 (which is equivalent to a receiver threshold of -67 decibels per milliwatt) and a channel utilization QoS Basis Service Set (QBSS) load that is less than 45. This requirement provides for smoother roaming and a backup AP if one of the APs suddenly becomes unavailable or busy.
The QBSS load represents the percentage of time that the channel is in use by the AP. The overall channel load might be much higher than the QBSS load because several APs could be sharing the same RF channel and background or environmental noise could add to the load too. The Cisco7920 Wireless IP Phone uses the QBSS load in its roaming algorithm. The measured QBSS load will vary, depending on the time of day when you perform the site survey. For example, at night (when the network is largely idle), the QBSS load will usually be very low. Therefore, you should perform the site survey during peak hours. You can reduce the QBSS load by adding APs as needed.
Maintain at least 11Mbps of available link speed at all times for data clients as well as voice clients.
Maintain an AP coverage overlap of at least 15% to 20%.
In certain situations, data rates below 11 Mbps must be enabled for legacy devices. This lower speed will affect voice quality and the RF environment, and it is not the recommended setting. If you have to enable both 11Mbps and 2Mbps, these low speeds will reduce the number of simultaneous calls that each AP can handle and will also increase the overlap because they will extend the range of the APs.
Maintain a packet error rate (PER) no higher than 1% (or a success rate of 99%).
Maintain a minimum signal-to-noise ratio (SNR) of 25dB (see Figure2-2). Figure2-2 Signal-to-Noise Ratio Try to use the same transmit power on the AP and on the phones. If the transmit power of the APs varies, set the transmit power of the phones to the highest transmit power of the APs.
All AP antennas must use diversity.
APs in an optimal setting can handle seven G.711 or eight G.729 concurrent phone calls. If more concurrent phone calls are needed in a single location (a high usage area, for example), plan to have load-balancing APs available during the site survey. Overlapped basic service sets (BSSs, or APs sharing the same RF channel) reduce the number of concurrent phone calls per AP.
QUESTION NO: 5 Within the TestKing WLAN, fast secure roaming needs to be implemented to support wireless VOIP. What components are necessary when implementing fast secure L3 roaming?
A. AP, clients, WLSE
B. AP, CCX clients
C. AP, CCX clients WLSE
D. AP and clients
E. AP, CCX clients, WLSM
Access points in many wireless LANs serve mobile client devices that roam from access point to access point throughout the installation. Some applications running on client devices require fast reassociation when they roam to a different access point. Voice applications, for example, require seamless roaming to prevent delays and gaps in conversation. During normal operation, LEAP-enabled client devices mutually authenticate with a new access point by performing a complete LEAP authentication, including communication with the main RADIUS server, as in Figure11-1. Figure11-1 Client Authentication Using a RADIUS Server (Normal operation) When you configure your wireless LAN for fast, secure roaming, however, LEAP-enabled client devices roam from one access point to another without involving the main server. Using Cisco Centralized Key Management (CCKM), an access point configured to provide Wireless Domain Services (WDS) takes the place of the RADIUS server and authenticates the client so quickly that there is no perceptible delay in voice or other time-sensitive applications. Figure11-2 shows client authentication using CCKM. Figure11-2 Client Reassociation Using CCKM and a WDS Access Point The WDS access point maintains a cache of credentials for CCKM-capable client devices on your wireless LAN. When a CCKM-capable client roams from one access point to another, the client sends a reassociation request to the new access point, and the new access point relays the request to the WDS access point. The WDS access point forwards the client’s credentials to the new access point, and the new access point sends the reassociation response to the client. Only two packets pass between the client and the new access point, greatly shortening the reassociation time. The client also uses the reassociation response to generate the unicast key. Since the AP acts as the WDS, only a CCKM client and AP is required to configure the fast secure L3 roaming feature.
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