IP Telephony and VoIP Tutorial

Although this is a Cisco networks dedicated blog, I decided to start a series of tutorial posts about a general technology which is not directly related to Cisco but it is a field in which Cisco is again a major player. This is IP Telephony and Voice over IP (VoIP).

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The two terms, IP Telephony and VoIP, are related around the same concept but in my opinion they are not exactly the same thing. Many people refer to these two terms interchangeably but they are not exactly the same. So, before moving on lets clarify the difference between IP Telephony and VoIP.

1. IP Telephony Vs VoIP

IP telephony has to do mainly with digital telephony systems (LAN based IP PBX systems) which use the IP protocol entirely for voice communication.

All components of the IP telephony system use digitized voice which is transferred as IP packets through an IP network (usually the LAN network).

The telephone handsets (VoIP phones) translate the analogue voice signal into digital voice (binary voice) which is transferred as IP packets from one phone to another.

The call control system is usually a software based (softswitch) server or even a hardware device like the Cisco Call Manager Express, which handles all call signaling, call routing, IP phone management etc, again using IP protocol for transport. So think about IP telephony as a bigger concept compared to VoIP.

VoIP on the other hand is a subset of IP Telephony. Basically, VoIP is the technology which is used by IP Telephony as the vehicle to transport phone calls.

VoIP is the technology in which the analogue voice signal is digitized (analog to digital conversion) and becomes binary numbers in order to be transferred by the IP protocol.

VoIP is the basis for the implementation and functionality of an IP Telephony system. VoIP can also be used by legacy TDM based PBX systems to transport voice calls over an IP WAN network or even over the Internet.

Special voice gateways are used to connect to the legacy PBX telephone system on one end and to the IP network on the other end in order to translate the TDM voice stream into IP voice packets.

So to summarize, IP Telephony is the overall concept of the modern form of voice communication which harnesses the power and features of VoIP technology in order to offer the overall experience of communicating effectively and with lots of extra features.

Now that we described the difference between IP Telephony and VoIP, let’s see more details about the two concepts:

2. More details about Voice over IP

The term VoIP or Voice over IP refers to the transfer of voice packets over networks based on Internet technology and, more specifically, the IP Protocol.

The IP protocol, on which the whole Internet is based on, was created to implement the transmission of data in the form of data packets. This means that when a data document is transferred over the Internet is cut into small IP packets and sent over the network. When the document reaches its destination, the packets are joined again thus recreating the original document.

The same logic applies if the data transferred corresponds to a voice conversation. The voice is digitized, chopped into packets of data transferred over the network via the IP protocol. At the destination the packets are rejoined to recreate the voice stream.

Here we should make clear that VoIP refers to the transfer of voice over any IP network. Such a network is the Internet of course, but when considering VoIP it does not necessarily mean that we carry voice over the Internet only. It can be any IP-based network (such as a private corporate WAN network).

3. Packet based (IP Telephony) Vs Circuit Switched Telephone Systems

IP Telephony systems are those using entirely IP packets for voice communication, as explained before. In contrast to packet switched telephone systems (those based on IP protocol), conventional telephone systems apply the logic of direct connection between the two communicating voice parties through a dedicated circuit reserved exclusively for each contact.

Thus the term Circuit switched telephone systems. In packet switched systems, however, the same communication line can be used to simultaneously pass different kinds of packets. Thus, the voice packets of one or more conversations may travel through the same route as other packets transferring data, video etc.

This is the main difference between traditional telephony which is implemented to the public switched telephone network (PSTN) and telephony implementation on IP networks (or more generally to packet switched networks).

4. Can an IP Telephony System be connected to the public telephone network

There are special voice gateways which can connect an IP Telephony system with the public switched telephone network (PSTN) or other telephone networks.

Using the voice gateway, a VoIP phone can call a legacy telephone line phone on the public telephone network and vice versa with no problems.

Basically the voice gateway translates the IP packets from the IP Telephone system into TDM voice to be transmitted over the legacy PSTN network.

Generally, regardless of the infrastructure that the IP Telephony system uses to carry out the conversation, ultimately it is a private telephone network, such as those implemented in corporate call centers, which is transparent to the public telephone network.

5. What are the benefits of IP Telephony and VoIP

The main advantages of VoIP and IP Telephony in general include:

• Single network infrastructure for data and telephony. Since the same infrastructure (communication lines and equipment) serve voice traffic and data traffic, we have significant economies of scale. Also, we achieve better management of telecommunications infrastructure.
• Maximum use of telecommunications infrastructure. The packet switched networks (e.g IP Networks) make better use of their bandwidth capacity in comparison with traditional circuit switched telephone networks since the line is not fully occupied for each call conversation therefore it can carry various data packets in addition to voice.
• Improved communication for remote workers. The use of IP telephony does not require the user to have a physical presence in the enterprise environment. If the user has an IP connection, he/she can take advantage of the features and functions of the enterprise telephone system, regardless of where the user is located.
• New services are introduced. The usage of a single infrastructure for both data and voice allows for the development of a new generation of services such as unified messaging that can contribute significantly to productivity growth.

6. Why companies are interested for IP Telephony

Since almost all companies have access to the Internet, they have already implemented their corporate networks over the IP protocol.

Thus, they are given a first class opportunity to utilize the IP network infrastructure, which includes, in addition to the communication lines, other equipment such as routers, switches, etc.

This IP network infrastructure can be used for telephony as well. Even if the IP telephony system is confined within the enterprise, the benefits are significant.

When a company uses leased circuits to connect remote branches, the use of these circuits for both IP telephony and data connectivity provides substantial benefits and cost savings to the company.

7. Is IP Telephony the most economical solution for voice communication

Like any technology infrastructure investment, usage of VoIP and IP Telephony should be treated as a medium to long term business.

According to studies, the use of packet switched networks for voice telephony is more economical than the networks that occupy the whole communication line for each conversation. And when we can serve phone calls through our corporate IP network – which in some cases is extended to different parts of the city, other cities or other countries – we certainly save money by not using the public telephone network.

When routing phone calls over our own private IP network from New York to Los Angeles and the destination call is a PSTN number in Los Angeles, the call will be charged as local in Los Angeles (it will be routed from our voice gateway in Los Angeles to the PSTN). This is an example of a toll bypass cost saving.

Companies should however consider the costs for the implementation of the IP telephony infrastructure, occurring in the increased bandwidth capacity to accommodate also voice traffic, in the extra equipment (e.g IP telephones), the additional software needed, etc.

Overall, however, in medium to long term, telephony over IP networks has proved to be much more economical than traditional telephony solutions.

Continuing our series of posts on IP Telephony and VoIP, here is Part 3 of the tutorial:

8. Is IP Telephony and VoIP Implemented Easily?

Over time, most companies have acquired the expertise to implement IP Telephony solutions, either on existing corporate networks or from scratch.

The main advantage to implementing VoIP applications is that they rely on network infrastructure which can be expanded gradually, depending on the needs of the business.

Additionally, complimentary applications have been matured as well, such as call management software, so that the implementation of solutions and their use becomes more straightforward.

9. What happens in terms of voice quality ?

Traditionally the main problem of telephony on IP networks has been the quality of the voice. Since the same network carries different data packets (documents, other voice conversations etc.) we cannot always ensure that the packets carrying the voice conversation will all get together and on time at the other end in order to carry a real-time discussion.

When you transfer a document, a web page, an email etc, we don’t care so much if one packet is delayed 1-2 seconds. In voice conversation however, delay works negatively on the quality of the voice.

A solution to this problem would be the usage of high-bandwidth lines, combined with powerful routing equipment (eg routers and large enough switches). However they cost money.

A better solution is the implementation of prioritization of voice packets with respect to other data. Gradually, as the cost of equipment and services drops, the quality of VoIP will be better and better.

Finally, we must not forget that using certain technologies (e.g voice compression, Quality of Service QoS), we can increase the efficiency of communication lines and with appropriate settings in routers we can commit certain capacity from the network for voice communication.

With that, voice transmission will be conducted as much as possible in real time, without delays and distortion.

10. Do we need special telephone handsets ?

There are special telephone handsets designed for VoIP communication that harness the potential of this technology. Such devices are available from most international manufacturers of telephony products as well as from third party manufacturers involved in related VoIP solutions.

It is worth mentioning that using special equipment we can still use normal telephone devices. You can connect for example a normal analogue telephone to a special converter which transforms the analogue signal into VoIP.

Alternatively, a company may consider the option of softphones. A softphone is essentially telephony software that is installed on a laptop or desktop computer and offer all the functionality of an IP telephone without the need for a hardware telephone device.

Of course, the use of softphones depends upon the existence of a computer. Although the first softphones presented had poor voice quality and a great network load, now the technology is fairly mature and operational.