Basics Of VoIP - An IP TutorialThis module gives you a basic IP Tutorial in order to further understand VoIP, focusing on what IP is, IP Addresses, the OSI model, and some more detailed analysis. IP (Internet Protocol) is one of the most flexible protocols man has developed, in that it is capable of handling many different applications and media, and allows them to be carried across different networks to communicate in new ways. The following IP tutorial will give you an insight into some of the key characteristics of this protocol. Before understanding the nature of IP packets, and how this data is transported, this IP tutorial first look at the model that sits behind IP – the Open Systems Interconnection model (OSI). This model provides the blocks on which most protocols are developed. For the purpose of this IP Tutorial, the OSI model is shown below :
Open Systems Interconnection Model The OSI model is a layered approach to explaining the communication between different machines. This IP tutorial will now explain the seven different layers of the model:
1. The Application Layer – the top of the OSI model, this includes applications such as web browsing and e-mail, ie the physical end-user applications 2. The Presentation Layer – this layer is concerned with the interaction of two different applications, and ensuring they can inter-communicate, both in terms of the format and presentation of the data. 3. The Session Layer – this layer initiates, manages and terminates any session between the applications in (1) above. For example, with a phone call over VoIP, this layer manages the setting up of the call, the management of the call itself, and termination of that call at the end of the conversation. 4. The Transport Layer – this layer ensures reliability of the transport of the data over the network, for example checking for errors, re-transmitting where necessary, as well as end to end acknowledgements. In some instances, it may also handle congestion on the network. 5. The Network Layer – here, the layer allows for the addressing which allows one system on one network to find another system on another network. In the case of transmission via the internet, this layer therefore concerns itself with IP Addressing (see later notes). IP Routers are used to find the most logical and efficient means of sending IP packets from one IP Address to another based on the information contained within that IP Address. 6. The Data Link Layer – this layer has its own scheme of addressing, and allows transport across the physical layer (see (7) below). It provides the functional means of transferring data between the two different networks. 7. The Physical Layer – this layer is the core of IP, and is concerned with creating the 1’s and 0’s that are transmitted across the networks. It is the most basic network layer, providing the means of transmitting the raw bits. A good analogy would be in posting a letter – the physical layer would be the pen and paper used to write the letter. Now you have an understanding of the OSI model, we can move on to have a closer look at Internet Protocol itself.
So What Is IP? As mentioned, the next part of this IP tutorial focuses on a definition of IP itself. Internet Protocol is what is known as a connectionless protocol, in that one device, eg an IP phone, can send packetised data to any other device on any other network that it hasn’t necessarily communicated with before. This is unlike the Public Switched Telephone Network (PSTN) in that the physical set-up of a circuit needs to be made before the two devices can communicate. Furthermore, the networks being used to connect the two devices can be very different. For example, an Ethernet network can communicate with a WiFi or other wireless network. You can run IP into your business or home through any means you like, such as broadband, WiFi or mobile networks. Obviously, this has had a huge impact of the development and use of IP, and is at the core of the major growth of IP over the past decade. Many people use IP on a daily basis without even considering that they are doing so. The drawback of IP is that it is not a totally reliable service, and works on the basis of best efforts delivery. When a media stream, such as a phone call, is broken up into many separate packets (and sometimes sent over the internet via different routes), there can be a number of potential problems : - Packet loss, whereby individual packets do not reach their destination, making for incomplete media streams - Packet delay, meaning packet A may arrive after packet B, leading to a confused stream of information (such as a jittery conversation) - Packet data corruption – leading to problems at the far end The problems above are confounded on a telephone conversation when the call does not pass over a network with Quality of Service (QoS). With QoS, voice, as a time-crucial media, is prioritized when sent over the network, meaning these packets are sent with first priority to ensure quality of the call. Without this, calls can be poor quality to the end users and frustrating to listen to! IP Addressing This part of the IP tutorial refers to the unique address that a device such as a phone or PC that are connected to the Internet are assigned, known as the IP Address. It is not only end-user devices, but any device such as an IP route or IP Gateway. This means that regardless of where you connect your phone or PC to the Internet, you can communicate with any other device on any other network. In simple terms for this IP tutorial, the IP Address is similar to the postal analogy, whereby each person posting and receiving a letter has their own unique address and postal code that allow the letter to arrive at its destination. Many different protocols exist, and each has its own different addressing scheme. Logical addressing is similar to a postal address, as it has a number of different layers. IP addresses are written in numerical format, separated by dots. For example, 121.13.6.158. They can be anything up to 21 bits of data. An IP address can appear to be shared by many devices, either because they form part of a ‘shared hosting web server’ or because a ‘proxy server’ acts as an intermediary agent on behalf of its customers. When using a VoIP phone via your PC, it is not necessary to know the full IP address of the person you are looking to call, especially when you are a member of a community such as Skype. These services allow you to identify those with whom you want to communicate, ie your user group, by means of their names. The program itself uses their Skype name as a proxy for the IP address they are using to connect to the internet, and therefore allow you to identify and contact them easily. This has provided you with a fairly basic IP tutorial, but hopefully it should give a reasonable enough understanding of IP in order to move to the next section.
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