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An introduction to network layers

Telecom engineers think of networks as being built up in layers, following something called the OSI 7-layer model (shown in the graphic) and it's useful to understand them when you are comparing telco data offerings.

Only the three lowest layers in the diagram, labeled Media Layers, are relevant to the telecom network, the four layers above, labeled Host Layers, are implemented in the user's equipment.

The physical layer (layer 1) specifies how to identify the 0s and 1s by varying the amplitude, frequency or phase of electricity, light or radio waves on a wire, fiber or the airwaves, as well as how to distinguish between data and signalling information. SONET fiber only specifies layer 1.

The data link layer (layer 2) specifies how the two ends of a single wire, fiber or radio link will know what to do with the data. Ethernet and WiFi are Layer 2 protocols that also specify layer 1. They structure the data into 'frames' and can resolve conflicts from multiple terminals on a link. SONET requires the telco to add a layer 2 protocol, usually Frame Relay. Telco broadband often uses asynchronous transfer mode (ATM) at layer 2, which has very small 'cells' instead of frames.

Layer 3, the network layer, specifies how 'packets' of data are transferred across the whole network. The most common layer 3 protocol is Internet Protocol or IP. This runs 'on top of' a layer 2 protocol that puts the IP packets inside frames (or cells) for each link in the network. The packets are handed over unchanged, for example from your WiFi to the telco's broadband.

Layer 4, the transport layer, is the first layer implemented in the user's equipment, it ensures that the packets combine into usable information and requests re-transmissions if necessary. For basic Internet connections, this is Transmission Control Protocol, TCP.

The layers above have little to do with the network: the session layer, for example http, sets up, takes down and clears communication sessions; the presentation layer makes sure the application can make sense of the data; and the application layer is the program that makes use of the information, such as your web-browser or email client.