Contention-ratio and priority
Nowadays almost everyone has become accustomed to the packages offered by internet providers for their broadband. However, too many people judge their broadband service by the download speed (‘bandwidth’) or even solely on the ‘brand’ of the service provider. This is an error for two reasons. Firstly, for VoIP, upload is as important as download speed (which in an ADSL connection is always higher than the upload speed). Secondly, large ISPs often offer broadband packages at low prices because they share network resources between more customers. Concepts like contention-ratio and
priority are important as they measure how close we are to having a network circuit fully dedicated to our phone call. Although it is possible to have a perfectly good VoIP call even with high contention, generally in this case the user will experience very variable quality of audio throughout the day.
Voice and data
Unfortunately, audio packets (digital packets used to transfer audio signals over an IP network) must contend for network resources with data packets (digital packets transferring information such as emails, web browsing, files, etc.). Very often the amount of data transferred over an office LAN is far superior to that required for standard phone calls. An uncompressed audio conversation requires only about 80 Kbps contrary to the many Mpbs used for emails, data files and web access. The effect of having voice and
data flowing on the same broadband connection is that it becomes difficult to maintain a continuous flow of audio packets. When we access the web or receive emails we can tolerate a delay of 1 or 2 seconds. However, the flow stream required for a good quality audio conversation must be stable and any delay above 50 ms will be perceived negatively by the user. A solution to this problem is to
allocate a dedicated broadband connection for voice and a second one for data. This is often the best option as it enables a company to allocate a better
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performing connection for voice and another with more bandwidth for data. However, this often means doubling connectivity costs. Therefore, in instances in which only one broadband line is available, it is important to make sure the service provider offers QoS (Quality of Service) which gives higher priority to voice traffic over data.
VoIP carrier
A VoIP carrier is a telephone service provider responsible for providing the connection between the person calling and the destination. It is a misconception to think that all VoIP calls are free (although some of them are). Let’s take Skype as an example of VoIP service provider for residential customers. When a user calls another Skype user there is no cost to establish the call, regardless of the location of each user. However, when a Skype user calls a destination on the PSTN or mobile network the service provider (i.e. Skype) will charge for the call. There are many VoIP service
providers who are able to offer these services to businesses connecting a company’s phone system to the PSTN network. Almost all use an open standard called Session Initiation Protocol (SIP) and so they are often referred to as SIP carriers.
Market & technologies
VoIP telephone systems can be grouped in the following categories: 1) Premise-based PBX: the phone
system is located at the client’s office. This includes larger PBX manufacturers who have always dominated the market. These systems offer the option to connect traditional lines (analogue and ISDN) as well as VoIP lines. These can be further divided into: Hybrid Hardware PBX where most of the ‘intelligence’ of the PBX is contained into a proprietary hardware appliance operating as central unit for the telephony network; and Software PBX, where solutions make use of existing IT equipment (data server, PC, switches and router) transforming an office LAN into a feature-rich telephony network.
2) Hosted or IP Centrex PBX:
the ‘brain’ of the system is located at the service provider data centre. These solutions are fully reliant on a broadband connection. Ideally, without the limitations affecting a broadband connection described above, hosted solutions offer a good compromise between services, flexibility and price. They are often the preferred option by SOHO businesses because of their affordability. However, at present the level
of service offered on broadband lines is very variable depending on geographical location and provider. Therefore, premise-based solutions that can offer full redundancy and interoperability with legacy systems are still the preferred route for the majority of businesses. In this respect, software PBXs offer added flexibility on the underlying hardware equipment used.
Future trends
Undoubtedly the whole telephony market is fast approaching an era dominated by IP based technologies. The advantages offered by VoIP are so important that they cannot be underestimated. We can look forward to a time when broadband services will be uniform across the whole country. In the meantime, the best approach remains to adopt a technology that can help businesses migrate from traditional lines to a fully IP-based world. Although there is a strong industry
push for ‘cloud’ based technologies at present, this process is driven primarily by technology suppliers rather than customers. A great number of companies (primarily large SMEs) are still keen to retain strategic services such as telephony in-house. The freedom of choosing service providers, as well as the need to keep tight control over hardware equipment makes a premise-based solution still the preferred route for many larger organisations and this trend will most probably continue unshaken. However, the flexibility offered by ‘The Cloud’ cannot be denied and the debate between premise-based systems and hosted solutions will no doubt continue.
NETCOMMS europe Volume I, Issue 4 2011 17
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