Author: John
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High Availability and Failover options for SIP and Asterisk
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Overview What’s the disaster we are trying to avoid? The assumed scenario is this: Some kind of centralised VoIP service is being offered to a number of users; the service operates on servers located at a data centre or office and the users each have a SIP client device, such as an IP phone, that connects to the centralised service over the Internet or over the company network. That is the typical setup for an Internet…
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SIP Subscribe/Notify and Asterisk Hints Explained
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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 in the SIP protocol document RFC 3265. Basically, it works like this: If client device A wants to be informed of changes to the status of device B, it sends a SUBSCRIBE…
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About IP Packet Communications – TCP and UDP
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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 which we do not attempt to explain in detail here. However, there are some important common factors about the way they…
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What is NAT?
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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 the PC’s on the private side of the NAT device initiates a connection with a server on the Internet, then it…
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Configuring and Using SIP Domains in Asterisk
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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 define the location of the device on the Internet or within the scope of the network infrastructure that is being used.…
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Asterisk SLA (Shared Line Appearances) – Part 4
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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 is certainly not the only reason for using SLA. Visual representation of call activity: SLA provides users with clear visual indications…
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Asterisk SLA (Shared Line Appearances) – Part 3
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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 in the SIP protocol document RFC3265. Basically, it works like this: If client device A wants to be informed of changes…
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Asterisk SLA (Shared Line Appearances) – Part 2
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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 will then gradually improve the dial plan to make it handle a broader range of user actions. All examples are based…
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Asterisk SLA (Shared Line Appearances) – Part 1
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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 the start: Asterisk Server or Asterisk PBX refers to the Asterisk server that is being used as a PBX or Business…
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Why bother with SLA on Asterisk?
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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 suppliers are very familiar with their principle working concept – Shared Line Appearances. Existing users will often be quite resistant to…
