1. Introduction to Networking
Voice Over IP

2. Voice Over IP
Voice over IP (VoIP) is a protocol that sends voice data over an IP network
Instead of connecting your office to a phone network and a data network, you can have a single network for sending both voice and data traffic
VoIP is a protocol optimized for the transmission of voice data (telephone calls) through a packet-switched IP network. It routes phone calls through an IP network, including the Internet, instead of through the public switched telephone system (PSTN)
However, VoIP solutions are typically integrated with the PSTN to allow VoIP customers to call any phone on the PSTN, and to allow phones on the PSTN to call phones connected to the VoIP network

3. VoIP Implementation
The regular analog voice network is called the public switch telephone network (PSTN)
PSTN will transmit the analog data directly, using circuit switching, which maintains a direct connection
Voice data generated using VoIP will be broken down into packets and transmitted through your digital data network, the same we use for computers. We call this type of routed network a packet switching network
Analog phone calls are converted to digital packets, routed through the data network, and then converted back into analog voice data

4. VoIP Device Implementation
Obtaining VoIP service can be done in the following ways:
Using an analog telephone adapter to connect the existing analog phone system to a VoIP network. Analog signals are converted to digital signals, then encapsulated into IP packets for transmission on the VoIP network.
Using a VoIP phone that is capable of sending and receiving digital voice signals that are already formatted for the VoIP network. When using VoIP phones, you may need to connect the phones to special switches with Power over Ethernet (PoE) capabilities. PoE supplies power to the VoIP phone through the Ethernet cable, which is the same cable used for transmitting data signals. PoE requires Cat5 or higher cabling.
Running special software that allows a computer to send and receive VoIP calls. The software converts the input from the computer's microphone into VoIP data. It also converts received VoIP packets into audio data that plays through the computer's speakers.

5. VoIP Protocols
There are several protocols that make this work. First is the real time transport protocol (RTP). It contains the actual voice data stream
Additional protocols are required to set up, maintain and terminate calls. H.323 is a widely used VoIP protocol that provides communication sessions on an IP network
The session initiation protocol (SIP) is also a widely used option for controlling communication sessions
The media gateway control protocol (MGCP) is used to manage the VoIP interface with the PSTN
In general, we can split the protocols into two - the voice carrier stream that carries the actual voice (RTR) and the call control signaling to set up, maintain, teardown and redirect the call (H.323, SIP and MGCP)

6. VoIP Advantages
Using an IP network for voice has the following advantages:
Administration is simplified because you maintain a single network for both data and voice instead of using a separate infrastructure for voice-only traffic
Costs are typically lower than long-distance costs over the PSTN
Adding additional phone lines is easier and less expensive than adding lines from the PSTN
Because VoIP packets are regular IP packets, encryption is easily added to VoIP data—something that is difficult to accomplish for traditional PSTN calls

7. VoIP Disadvantages (Part 1)
VoIP is a great solution for businesses to control long term costs, but there are some disadvantages when comparing it to an analog solution. Fortunately, some of these disadvantages can be mitigated
Initial cost. It is going to be more expensive initially to set up a VoIP solution than a traditional PSTN solution
Delay (or latency). Some latency is introduced if the signal needs to be converted to and from a digital format. In addition, latency is introduced as packets are routed between devices. Delays cause long pauses between speaking and receiving, and can result in callers continually interrupting each other. International standards call for a delay of 150 ms or less
Jitter is a variation in the delay of individual packets. Jitter causes strange sound effects as the delay of packets fluctuates. This is caused by dynamically changing network conditions changing the amount of time data takes during routing

8. VoIP Disadvantages (Part 2)
More disadvantages:
Packet loss. This occurs when packets do not arrive at all. Packet loss causes drop-outs in the conversation. Most VoIP implementations attempt to reduce latency by using the UDP protocol instead of TCP. However, because UDP packets aren't acknowledged, it is possible for them to be lost in transmission. Because voice data is time sensitive, any information that is lost will not be retransmitted. Retransmission would cause irregularities in the words being spoken. Voice traffic is very sensitive to packet loss. Even a 1% loss of packets will be noticeable
Echo. This occurs when you hear your own voice in the telephone receiver while you are talking. Excessive delay can cause unacceptable levels of echo
Power loss. Power loss at your local facility or at any point in the IP network can disrupt phone service. With regular phone service, power to the phone line is supplied separately from the electrical power to the building, and you might be able to make calls when the power is out. With VoIP, a power disruption will typically prevent you from making phone calls. In addition, without some type of backup power source, an outage could also take down the routers that are needed to put voice traffic onto the data network

9. Quality of Service
The term quality of service (QoS) describes the mechanisms that can be implemented with VoIP to help ensure call quality, reduce latency, reduce jitter, and prevent packet loss. When you sign up for VoIP service, you will typically get a guaranteed level of service from the provider, however you will also need to take steps on your own internal network to ensure call quality as well
For example, most implementations will use VLANs or even an entire redundant network infrastructure dedicated to VoIP traffic. Otherwise, peaks in traditional network traffic might adversely impact VoIP traffic

10. Unified Communication Devices
Many organizations use unified communication devices to provide voice services. These are appliances that plug directly into your network and provide a variety of communication services, including:
Voice
Voic
Instant Messaging
Presence Information (Identifies if a user is online and available or not)
Video Conferencing
Faxing
Web Conferencing and Desktop Sharing