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About IP Packet Communications – TCP and UDP

February 20, 2009

Devices communicate data to other devices on the Internet using certain recognised protocols that involve routing and transmission of packets of data to a remote device identified by an IP addresses. The most widely used protocols on the Internet are TCP and UDP. The differences between these two protocols are of a highly technical nature [...]

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What is NAT?

February 20, 2009

What is NAT? NAT stands for Network Address Translation. Many devices sold as Firewalls or Routers are actually combined Firewall, Router and NAT device in one box. NAT is the mechanism that allows you to have many PC’s on your LAN all connected to the Internet through a single external IP address. When one of [...]

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What does it cost to make calls with a VoIP phone?

February 13, 2009

How can you make calls for free with VoIP? The cost of a telephone call Just in case you are wondering, you can’t buy a VoIP phone, plug it in, set it up and start calling any number you like for free. It is not that simple – in fact, when you start looking into [...]

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Configuring and Using SIP Domains in Asterisk

January 28, 2009

What are SIP Domains? SIP requests delivered to a Proxy server or other SIP device must contain details of valid destinations and end-points that are to be reached or which are to receive responses. The address of a SIP device is generally referred to as its URI (Uniform Resource Identifier) – it has to uniquely [...]

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Asterisk SLA (Shared Line Appearances) – Part 4

January 14, 2009

Link to part 3 What is good and what is bad about the Asterisk implementation of SLA? Benefits of Asterisk Shared Line Appearances Replacement of legacy systems: The main reason for wanting to configure Shared Line Appearances in Asterisk is because the Asterisk system is replacing a traditional key and lamp telephone system. However, this [...]

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Asterisk SLA (Shared Line Appearances) – Part 3

January 14, 2009

Link to part 2 EXTENSIONS.CONF: Setting the Hints used by SLA The SIP SUBSCRIBE/NOTIFY mechanism – what it is and how it works The SIP protocol includes a standardised mechanism to allow any SIP client (an IP phone being an example of a SIP client) to monitor the state of another device. Details are provided [...]

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Asterisk SLA (Shared Line Appearances) – Part 2

January 14, 2009

Link to part 1 About the examples and conf file samples used in this article The following sections will guide you through the setup of a small Asterisk system using shared line appearances. Each step is explained to show how it works. The dial plan examples will start with a fairly simple configuration and we [...]

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Asterisk SLA (Shared Line Appearances) – Part 1

January 14, 2009

Terminology and SLA Concepts Terminology We have attempted to use the correct terminology (as recognised throughout the telephony industry) within the explanations on this web page. However, as it is so important to be clear about the meaning of the terminology used here, it seemed like a good idea to set it out explicitly at [...]

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What is DISA and how is it relevant to SLA?

January 12, 2009

What is DISA? DISA (Direct Inward System Access) is a mechanism available on some PBX’s that permits inbound calls to be answered and immediately presented with system dial tone. The caller is then able to dial a number that the PBX uses to decide how to forward the call. It is like the caller is [...]

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Why bother with SLA on Asterisk?

January 12, 2009

Familiarity There are, in the business world, a huge number of legacy telephone systems that use a “key and lamp” principle of operation – so-called key telephone systems. Many small business phone systems available in the market today still use this principle of operation. They are extremely ubiquitous and a large number of users and [...]

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