ble international standards. All of them use some form of layering. The number of layers, the name of each layer, the contents of each layer, and the function of each layer may differ from net- work to network. Data Communications Software Software application Host A Host B application Layer n protocol Layer n Layer n Virtual communication between peer entities at layer 4. Layer 5 protocol Layer 5 Layer 5 Physical communication between 4th layer of Layer 4 protocol Layer 4 Layer 4 host A and B Layer 3 protocol Layer 3 Layer 3 Service provided by layer 3 Layer 2 protocol Layer 2 Layer 2 Interface between layers 2 and 3 Layer 1 protocol Layer 1 Layer 1 Physical medium Figure 6.8 Protocol hierarchy. In all networks, the purpose of each layer is to offer certain services to the higher layers, shielding those layers from the details of how the provided services are actually implemented. 6.3.1.1 Protocol Each layer in one machine carries on a conversation with the corresponding layer in another machine as shown Figure 6.8. The rules and conventions used in this conversation are collectively known as the protocol of this layer. We can say that the protocol specifies the format and meaning of the infor- mation that a layer sends to the layer below. This information is received and understood by the corresponding layer at the other end if exactly the same detailed protocol specification is implemented there. With the help of its protocol each layer below provides services to the layer above it. The provided service is often specified separately from the pro- tocol specification. We could say that service specifies what the layer looks like from the point of view of the next layer above. For example, if a layer provides data transmission with or without error detection, the layer above may select which one it wants to use. How they are implemented, that is, how layers at opposite ends communicate to provide the service, in the layer is specified in the protocol specification. Introduction to Telecommunications Network Engi