Today, we will learn Layered Architecture and OSI reference model. OSI reference model was developed as an open system architecture for the design of computer networks that could communicate with each other. The model partitioned the communication process into seven layers. From the bottom up, they are physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. The seven layers are for the communicating end systems. Note that the top four layers are for end-to-end communications. For a network node, it only has at lowest three layers. The network and data link layers involve interaction of peer to peer process across a single hop. One may ask, why layering architectures? A layering architecture simplifies design, implementation, and testing of networks by partitioning oval communication process into parts. Protocol in each layer can be designed separately from those in other layers. Protocol makes calls for services from the layer below. Layering also provides flexibility for modifying and evolving protocols and services without having to change layers below. Next, we discuss major functions of each layer from bottom up. Physical layer deals with the transmit of bits of our communication channel. Such as uncoupled wire pairs. The layer is concerned now with a particular choice of system parameters. It also is concerned with the procedures to set up and release the connection as well as mechanical aspects. The data link layer provides for the transfer of block of information called the frames across our transmission link that directly connects to nodes. It will insert framing information into the sequence of transmitted bits to indicate the boundary of the frames. It is concerned with how to group this into frames, how to detect big errors during digital transmission, and how to calculate checked bits for error detection. It inserts can show an address information in the header, and the check bits in the trailer, to enable recovery from transmission errors. Please note that our link includes the case where multiple nodes are connected to a broadcast medium, such as our local area network, LAN. Therefore, a media access control procedure is required to coordinate the access to the broadcast medium. Another key function is to do node to node flow control which is avoid a buffer overflow at the receiving node. Network layer provides for the transfer of packets across a communication network or multiple networks. One key aspect is that as addressing must be scalable to accommodate a very large number of network users. Another key aspect in that network codes jointly execute routing algorithms to determine the routing paths across the network. By routing protocol we mean the procedure to select paths across a network. This function makes network layer as the most complex in the reference model. The network layer is also responsible for congestion control to deal with temporary surges in packet to traffic from time to time. When two hosts are connected at two different networks, transfer of data must traverse two or more networks that are possibly differ in their internal routing and address scheme. In the networking is a part of network layer that provides transfer of package between gateway routers that are connected to intermediate network, as shown in the figure. The example in the figure illustrates a possible scenario that one host is connected to an ethernet LAN. My ATM network may connect gateway routers. Transport layer is responsible for the end to end transfer of messages from the process in a source machine to a process In the destination machine. The layer accepts message from its higher layer and prepares block of information called segments or data grams for transfer between end machines. Transporting layer provides different kind of services. One example, it may provide service that it involves reliable, error-free transfer of sequence bites or messages. And the other, it may provide a connectionless service of quick, and simple one message transfer. The session layer and the presentation layer were defined with different functions by impractice that incorporated with to the application layer. Application layer is to provide services that are frequently required by applications that involve communication, for example, HTTP protocol, domain name service, file transfer, email and other applications. As a summary, the overall communication process between two or more machines connected across one or more networks is very complex. Layering partitions related communication functions into groups that are manageable.
