Working Principle of VoIP (Voice over Internet Protocol)

Let us look at very simple VoIP call. Consider two VoIP telephones connected via an IP network.In this example both VoIP telephones are connected to a local LAN. Sally’s phone has an IP address of,Bill’s phone is, the IP addresses uniquely identify the telephones. Both our phones are configured to use a widely used VoIP standard called H.323. Bill wants to talk to Sally and his phone knows the IP address of Sally’s phone. Bill lifts the handset and ‘dials’ Sally,  the phone sends a call setup request packet to Sally’s phone, Sally’s phone starts to ring, and responds to Bill’s phone with a call proceeding message.
When Sally lifts the handset the phone sends a connect message to Bill’s phone. The two phones will now exchange the data packets containing the speech. At the end of the call Bill replaces his handset and phone stops sending voice data sends a disconnect message and Sally’s phone responds with a release message. The call is now complete.  all the messages contain the Q931  ISDN  protocol.

Having introduced VoIP I will now talk about three main ‘types’ of VoIP installed in the market place today.Main ‘types’ VoIP :VoIP has broadly three main branches, which can and do overlap.
VoIP over the Internet

This is probably the best known and most publicized, talking PC to PC. Basically free telephone calls. The call is only free if both parties to the call have access to the public Internet at zero cost..

Advantage…free calls regardless of distance or length of call.
Disadvantage….often the voice quality is bad due to the lack of bandwidth available for the call.
Other factors. Have to use a PC or other computer running VoIP software.

Office to Office A large multinational company will have offices across the whole country. They have a fixed data network connecting all the offices together. This allows every computer access to every other computer in the company. By installing a VoIP Gateway in each office and connecting it to the office legacy PBX and to the data network, employees use the data network for voice calls between offices.

Advantages. Interoffice calls are free, since the company already has the bandwidth between offices. The technology is transparent to the user, and requires minimum training. The only new equipment required is a gateway at each office. Voice quality is good, because the company has control over the bandwidth.
Disadvantage.Extra bandwidth may be required between offices, which offset the savings.

Other factors… The carrier providing the interoffice bandwidth will almost certainly offer an alternative solution including management of the internal telephone traffic.

A traditional Private Branch Exchange (PBX) connects all the phones within an organization to the public telephone network. Essentially IP PBX replaces all the internal phones with VoIP telephones. The IP PBX has standard telephone trunk connections to the public telephone network. The IP PBX is a PBX with VoIP, but it also has the ability to support VoIP over the Internet and Office to Office VoIP.
Advantages.Single cable infrastructure. The technology is transparent to the user, and requires minimum training. Future proof technology.
Disadvantages Primarily useful for Greenfield sites, but can be adapted to work with existing technology.

How VoIP works :
I have made an assumption that both ends of a VoIP telephone conversation are compatible. This compatibility only happens if both ends agree to use the same protocol. All manufacturers who claim to be producing industry standard voice over IP either support SIP or H.323 protocol.

So what is H.323 ?
Over the next few years, the industry will address the bandwidth limitations by upgrading the Internet backbone to asynchronous transfer mode (ATM), the switching fabric designed to handle voice, data, and video traffic. Such network optimization will go a long way toward eliminating network congestion and the associated packet loss. The Internet industry also is tackling the problems of network reliability and sound quality on the Internet through the gradual adoption of standards. Standards-setting efforts are focusing on the three central elements of Internet telephony: the audio codec format; transport protocols; and directory services.

H.323 Call Sequence
As such, H.323 addresses the core Internet-telephony applications by defining how delay-sensitive traffic, (i.e., voice and video), gets priority transport to ensure real-time communications service over the Internet. (The H.324 specification defines the transport of voice, data, and video over regular telephony networks, while H.320 defines the protocols for transporting voice, data, and video over integrated services digital network (ISDN).

How VoIP works next level:
The call control part of H.323 sets up the parameters for the full duplex voice path between source telephone and destination telephone. I will continue with my analogies to explain how your voice gets transported across the Internet. In terms of H.323 there is a trade off between call quality and bandwidth, in general the higher the quality the greater the bandwidth required
During the call setup portion of H.323 the phones have to decide which speech encoder/decoder to use when they send the speech to the other phone, Bill and Sally both have phones that support G.723.1, G.711 and G729.
The main difference between each of these encoders is the amount of bandwidth they use, G.711 uses 64kbit/s and G.723.1 can use as little as 5.3kbit/s. Although it would seem obvious to use the encoder with the lowest bandwidth, there is a loss of quality with a lower bandwidth.. At the same time a stream of G723.1 encoded voice data starts being sent from each phone to the other phone.

How VoIP works Next level :Hear the Quality.

The performance of the speech encoders at each end, the number of packets lost on route, Latency and Jitter. I have already talked about the encoders in the previous section. I also bundle into the encoding process echo suppression. In the early days of voice calls via satellite there would be an annoying echo. As the technology improved the echo disappeared. Echo suppression is very key to good quality VoIP calls . I do not dwell on the subject since the mathematics is beyond my comprehension. Good echo suppression makes for quality calls.

Be warned that because a manufacturer has a G.723.1 encoder it may not sound the same as another manufacturer who claims to have G.723.1, quality does vary.   As a general rule the occasional lost packet will not affect too drastically the quality of a call, but lose 5 in a row and an entire word is lost and this will be a problem. So if you are going to have lost packets make sure they are only lost in a regular distributed manner. 5% lost packets distributed evenly will not result in the loss of words lose 5% of the words by clustering the packets and the effect is bad.

PROS  AND  CONS :Advantages of VoIP 

There are many advantages to be gained from implementing an IP Telephony solution within the organization. The following list aims to highlight some of the advantages of such a strategy:

  • Single network infrastructure. When installing VoIP in the office only a single cable is required to the desk, for both telephone and data. Eliminating separate telephone wiring.
  • VoIP uses “soft” switching which eliminates most of the legacy PBX equipment. Reducing the cost of installing a communications infra-structure and the maintenance cost once installed.
  • Simple upgrade path.The VoIP PBX technology is software based. It is easier to expand, upgrade and maintain than its traditional telephony counterparts.
  • Bandwidth efficiency. VoIP can compress more voice calls into available bandwidth than legacy telephony.. IP Telephony helps to eliminate wasted bandwidth by not transporting the 60% of normalspeech which is silence
  • IP – the underlying protocol is supported by most platforms and is independent of the transport protocol used. 
  • Only one physical network is required to deal with both voice/fax and data traffic instead of two physical networks. Having only one physical network has the following advantages: lower physical, equipment cost ,lower maintenance costs.

While there are many aspects of VoIP which provide considerable benefits, the technology is still very young and problems remain. The following section looks at some of the weaknesses of this technology and their consequences.

  • The Internet is not the best medium for real time communications. Individual packets can take different routes and varying delays can be encountered and packets lost in transit. Waiting for delayed packets or retransmission of lost packets can result in considerable degradation of quality. Long delays in transit can affect quality so much that the technology can become unusable, though many vendors do have solutions which aim to negate the degradation suffered due to transit delays.
  • While some standards have been set by the ITU, the technology is not fully standardized and there is no guarantee that products from different vendors will be interoperable. Some vendors are trying to resolve this problem by forming groups and making guarantees about the products in the group but this is only a partial solution vendors outwith the group cannot guarantee interoperability.
  • Heavy congestion on the network can result in considerable degradation of service as IP is not good at providing QoS (Quality of Service) guarantees. Feedback to Lucent Technologies customers reflect this worry. Major companies are planning to install IP Telephony capabilities at some point and have carried out initial investigations, however: Since only one physical network for both data and voice/fax transmissions is required, failure of the network could be catastrophic, as all communications capabilities are lost.

Many vendors offer the ability to incorporate Virtual Private Networking (VPN) with relative ease into the IP Telephony solutions they provide. This allows any transmission to be encrypted using a number of cryptographic techniques and providing security by transmitting the communications through a ‘tunnel’ which is set up using PPTP (Point-to-Point Tunneling Protocol) before commencing communications.IP Telephony allows companies to exploit Computer Telephony Integration to its full extent. The convergence of communications technologies allows greater control over communications, most vendors provide logging and accounting facilities whereby all usage can be monitored.