Cisco ccnp ONT – Optimizing Converged Cisco Networks : 642-845 Exam

“Optimizing Converged Cisco Networks”, also known as 642-845 exam, is a Cisco certification.
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QUESTION NO: 4
Certifyme.com is rolling out an H.323 VOIP network using Cisco devices. Which IOS
feature provides dial plan scalability and bandwidth management for H.323 VoIP
implementations?

A. Digital Signal Processors
B. Call Routing
C. Gatekeeper
D. Call Admission Control
E. None of the above
Answer: C
Explanation:
Enterprise voice implementations use components such as gateways, gatekeepers, Cisco
Unified CallManager, and IP phones. Cisco Unified CallManager offers PBX-like
features to IP phones. Gateways interconnect traditional telephony systems, such as
analog or digital phones, PBXs, or the public switched telephone network (PSTN) to the
IP telephony solution. Gatekeepers can be used for scalability of dial plans and for
bandwidth management when using the H.323 protocol.

QUESTION NO: 5
A Cisco router is being used as a VOIP gateway to convert voice signals in the
Certifyme.com network. What steps are taken when a router converts a voice signal from
analog to digital form? (Select two)

A. Quantization
B. Serialization
C. Packetization
D. Sampling
Answer: A, D
Explanation:
Step 1 Sampling: The analog signal is sampled periodically. The output of the sampling
is a pulse amplitude modulation (PAM) signal.
Step 2 Quantization: The PAM signal is matched to a segmented scale. This scale
measures the amplitude (height) of the PAM signal.
Step 3 Encoding: The matched scale value is represented in binary format.
Step 4 Compression: Optionally, voice samples can be compressed to reduce bandwidth
requirements. Analog-to-digital conversion is done by digital signal processors (DSPs),
which are located on the voice interface cards. The conversion is needed for calls
received on analog lines, which are then sent out to a packet network or to a digital voice
interface.
QUESTION NO: 6
You need to implement the proper IOS tools to ensure that VOIP works over the
Certifyme.com network. Which queuing and compression mechanisms are needed to
effectively use the available bandwidth for voice traffic? (Select two)

A. Priority Queuing (PQ) or Custom Queuing (CQ)
B. Real-Time Transport Protocol (RTP) header compression
C. Low Latency Queuing (LLQ)
D. Class-Based Weighted Fair Queuing (CBWFQ)
E. TCP header compression
F. UDP header compression
Answer: D, E
Explanation:
1. Class-based weighted fair queuing (CBWFQ) extends the standard WFQ functionality
to provide support for user-defined traffic classes. By using CBWFQ, network managers
can define traffic classes based on several match criteria, including protocols, access
control lists (ACLs), and input interfaces. A FIFO queue is reserved for each class, and
traffic belonging to a class is directed to the queue for that class. More than one IP flow,
or “conversation”, can belong to a class.

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Once a class has been defined according to its match criteria, the characteristics can be
assigned to the class. To characterize a class, assign the bandwidth and maximum packet
limit. The bandwidth assigned to a class is the guaranteed bandwidth given to the class
during congestion.

CBWFQ assigns a weight to each configured class instead of each flow. This weight is
proportional to the bandwidth configured for each class. Weight is equal to the interface
bandwidth divided by the class bandwidth. Therefore, a class with a higher bandwidth
value will have a lower weight.

By default, the total amount of bandwidth allocated for all classes must not exceed 75
percent of the available bandwidth on the interface. The other 25 percent is used for
control and routing traffic.

The queue limit must also be specified for the class. The specification is the maximum
number of packets allowed to accumulate in the queue for the class. Packets belonging to
a class are subject to the bandwidth and queue limits that are configured for the class.
2. TCP/IP header compression subscribes to the Van Jacobson Algorithm defined in RFC
1144. TCP/IP header compression lowers the overhead generated by the
disproportionately large TCP/IP headers as they are transmitted across the WAN. TCP/IP
header compression is protocol-specific and only compresses the TCP/IP header. The
Layer 2 header is still intact and a packet with a compressed TCP/IP header can still
travel across a WAN link.
TCP/IP header compression is beneficial on small packets with few bytes of data such as
Telnet. Cisco’s header compression supports Frame Relay and dial-on-demand WAN link
protocols. Because of processing overhead, header compression is generally used at
lower speeds, such as 64 kbps links.

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