The purpose of this activity is to reinforce what you have learned so far about Voice over IP technologies. Your instructor will tell you the correct answers. Your instructor may choose to have you answer these questions as part of a group discussion.
1. Match the following VoIP applications with the appropriate description.
IP local line doubling, premises IP telephony, call-center IP telephony, fax over the Internet, IP-based public phone service, enterprise toll-bypass, PC phone to PC phone:
a. Provides toll-free, company-wide voice and fax communications.
Enterprise toll-bypass
b. Allows for a sending toll-free or reduced-rate fax between fax machines at any two locations.
Fax over the Internet
c. Similar to fax over the Internet, except it transmits only voice.
PC phone to PC phone
d. Sending voice over the Internet or over new public IP networks.
IP-based public phone service
e. A customer looking at an online catalog can simply click a phone icon to talk with an agent, also known as agent-click.
Call-center IP telephony
f. Allows a single phone line to carry one or more calls, in addition to transmitting PC data.
IP local line doubling
g. PCs on an IP LAN can make calls to telephones in the same building or make outside calls by using special VoIP equipment on the premises.
Premises IP telephony
2. Describe an Internet telephone call that originates at a PC phone and terminates at a standard telephone. Be sure to indicate when the call is transmitted over the Internet and when the call is transmitted over the PSTN, and describe the difference between the two. Also, indicate when a gateway is needed.
The PC, equipped with telephony software, a microphone, and speakers, places the call over the Internet to the nearest gateway server. Because the call is being sent to a standard telephone, a gateway server is needed at the receiving end to move the call from the Internet to the PSTN. The receiving gateway server acts as a translator between the Internet and the PSTN. The gateway server sends the call via the PSTN to the receiving phone, and the call is completed.
Local Telco The
Internet
Gateway Router
Router
Figure 1-19: VoIP via a gateway
When the call is on the Internet, it is divided into packets, and the router determines the best route for each packet to take through the Internet. The packets may arrive at their destination at slightly different times depending on the path the packet takes through the network. When the call is on the PSTN, a circuit is dedicated to that transmission for the length of the call.
3. Describe the process of pulse code modulation (PCM).
One method used to translate an analog voice or fax signal into a digital signal is pulse code modulation, or PCM. PCM consists of three steps. The first step is to break up the voice/fax signal, which is smooth, and sample it to come up with discrete numbers to represent the changes in the signal. This step is called pulse amplitude modulation, or PAM. The voltage of the voice signal at each point where it sampled will determine the voltage of the new digital signal.
Figure 1-20: Pulse Amplitude Modulation (PAM) output
After the voice signal has been divided and sampled using PAM, the signal will be quantized and companded. Quantizing assigns a number to each sample that is related to the relative voltage of the sample. Companding
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The third and final step in PCM is coding. In this step, the numbers generated in the first two steps are converted to an 8-bit code, which can then be
combined with the digital transmission stream and sent across the Internet or intranet.
Figure 1-21: PCM coding results in an 8-bit code called DS0
After the digital signal has been created by PCM, it must be multiplexed to be transmitted at high speeds. The process most commonly used to multiplex a digital signal for T1 transmission is called time-division multiplexing, or TDM.
TDM takes the 8-bit “words,” or DS0 signals, generated by PCM and combines them with 23 other conversations into groups of 24 words to make one frame.
The frames are organized into precise time sequences, and marked by framing bits at each end to ensure that the bits stay within their time sequence,
resulting in a DS1 signal.
Lesson 1 Quiz
1. Internet telephony is:
a. Using the Internet for real-time data traffic.
b. Using the PSTN for real-time data traffic.
c. Using the Internet for real-time voice and video traffic.
d. Using the PSTN for real-time voice and video traffic.
2. Which of the following VoIP applications does NOT require a gateway?
a. Fax over the Internet b. PC phone to PC phone
c. IP-based public phone service d. Premise IP telephony
3. The _____ translates between the PSTN and the Internet.
a. Router
b. Telephony software c. Gateway server d. Modem 4. The Internet:
a. Is a dedicated circuit.
b. Is a packet-switched, or connectionless, network.
c. Transmits all information over the same path.
d. Is a small, managed network.
5. In a VoIP network, a router:
a. Translates between the Internet and the PSTN.
b. Decides the best path for the call to take through the network.
c. Prioritizes voice calls.
d. Ensures quality of service for VoIP calls.
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