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VoIp Usability 

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Usability

Usability consists of several different subtopics but the only mesurand is the consumer satisfaction. The user want to have a high available, cheap and secure system that allows to communicate with a high speech quality. Easy useage and installation and integration into existing networks are othe impotand points of the VoIp usability.
For the users the over all speach quality is one of the most importanr things. This quality is influenced by the delay, the quality of the used codecs and several system depanding components. There are a lot of possible network configurations. Each of them has its own assets and drawbacks. Which one is chosen depends on the system requirements.

Network configurations

LAN:

The use of VoIp in a closed LAN is quite easy und has the highest quality. There is no need of additional components and  the installation of existing software tools is quite easy.

Over the internet:

Using VoIp over the internet is easy when both PC's are directly connetced to the net. There are some security problems because there is no easy way to implement a firewall that can handle H.323. Using VoIp over the internet and  one of the PC's is member of a LAN and connected to the web via a gateway is a little bit mor complex. The PC that is member of the LAN can connect to the other but without a Gateway that supports H.323 it is not possible for the other PC to set up an connection.
Sometimes security is a must. In this case the security problem can be solved by using a complex application based firewall that is hard to implement or set up a virtual private network. A VPN is used to encrypt the data that is sent over the network and to make a PC that is connected to the LAN via the internet to a member of the LAN so that it should be possible to set up a VoIp connection. This causes an overhad for each sent package and more bandwidth is needed. This will probably decrease amount of available data and the quality of the transmitted speech.

Combinig Public Switched Telefone Networks and VoIp:

An interesting way is combining the public switched telefone network via a gateway with a LAN. A gateway can be used to provide a highly transparent access from and to the PSTN

Speech quality

The used codecs in the gateway and the terminal pc, packet loss in the LAN and  the quality of the loudspeakers and microphones will influence the speech quality.