Layman View of OSI Reference Model (By Diganta Deka, Former Student, Proprietor-Trainer East India Technologies, Guwahati. ), diganta. [email protected] com When we talk about computer networks and how computers operate in networks, things do not remain as simple as it seems to be. Here numerous protocols work in tandem for the sake of data integrity during the course of data transfer. To make things worse there are a lot of vendors in the market who provide and produce great number of network hardware and push us to use them.
Therefore to provide a solution and to make all these devices communicate in 1984, The International Organization for Standardization (ISO) developed the Open Systems Interconnections (OSI) reference model. Basically it described how information is transferred from one networking component to another, from the point when a user enters information using a mouse or a keyboard to when that information is converted to electrical or light signals transferred along a piece of wire or radio waves through the air and vice versa.
So we can say, OSI reference model provides a foundation to use when you are considering what happens between the network components when they talk with each other. ISO developed a seven layer model which allowed vendors and network administrators to understand a stable data transfer and also provided guidelines for implementing new networking standards. Table-1 below briefly describes these layers, their associated services and protocols supported. Layer| Function| Service Description| | Application| The Application layer provides an interface between the communication software and any other application that needs to communicate outside the computer on which the application resides. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail. | 6| Presentation| The Presentation layer translates data from the Application layer into an intermediary format, such as ASCII text, EBCDIC text, binary, BCD, JPEG etc.
This layer also provides services such as data encryption, and data compression. | 5| Session| The Session layer allows two applications on different computers to establish, use, and end a session. This includes the control and management of multiple bidirectional messages so that the application can be notified if only some of a series of messages are completed. This allows Presentation layer to have a seamless view of an incoming stream of data. | 4| Transport| The Transport layer defines the end-to-end transmission of data between nodes (e. . PCs), including flow control and error recognition and recovery. It also repackages long messages when necessary into smaller packets for transmission and, at the receiving end, rebuilds packets into the original message. | 3| Network| The Network layer provides routing, logical network addressing, path determination, media/framing translation, frame fragmentation, and congestion signaling/control. The routing concepts define how devices route or forward packets to their final destination.
Logical addressing defines how each device can have an address that can be used by the routing process. Path determination refers to the work done by which all possible routes are learned, but the best route is chosen for use. | 2| Data Link| The Data Link layer packages raw bits from the Physical layer into frames (logical, structured packets). This layer specifies the device addressing, topology and media access, bit/byte synchronization, flow control, and error detection/recovery services associated with sending frames of data over a physical link. 1| Physical| The Physical layer specifies how bit streams are to be transmitted over a physical link in the network. This includes physical characteristics of transmission medium, including pins, use of pins, electrical, current, encoding, light modulation and the rules for how to activate and deactivate the use of the physical medium. | Table-1 Within the network, data traverses a network stack using a process called encapsulation. When an application requires communications resources, the message is handed over to the network stack’s application layer.
There, the message is processed into a Protocol Data Unit (PDU). Application specific header information is wrapped around the data. The resultant PDU is then passed to the stack’s presentation layer, where additional header information is added. Each subsequent layer in the source system’s network stack adds service-specific header information to the PDU, until a fully formatted data frame is presented to the physical layer for transmission onto the communications medium.
This encapsulation process is illustrated in Figure 1. Encapsulation Process Fig-1 The real data transfer process starts from the Transport Layer, where the data is segmented and hence the data is termed as segment in transport layer. When the segments reach the network layer each segment is broken into packets. As these packets are transferred to Data Link Layer each packet is again broken into frames and in Physical Layer the frames converts into Bits. When the bit stream reaches the destination system, the reverse occurs.
Each layer in the destination system’s network stack analyzes and then strips its associated header information from the message. The resultant PDU is then passed to the next higher layer, until the original message is presented to the destination application. Typically, a communication layer does not process, or alter the PDU’s content as generated by an adjacent layer. Information exchange only occurs between peer OSI layers. The peer relationship between OSI layers is illustrated in Figure 2.
Illustrated Peer Relationship between OSI Layers Bit Stream Frame Packet Segment Data Stream Data Stream Data Stream Bit Stream Frame Packet Segment Data Stream Data Stream Data Stream Figure-2 Till now we have been talking about the concept behind the OSI reference model, but if we do not discuss about the protocols (protocol is an agreed set of rules by which devices in a network or number of networks can successfully communicate with each other) and devices that work in each layer of the model then the discussion would remain incomplete.
Table-2 and Table-3 respectively represent the devices and the protocols used in various layers in the OSI Reference Model. Devices and the Layers at Which They Function Layer| Name of the layer| Devices| 3| Network| Router, Layer3 Switch| 2| Data Link| Switch, Bridge and NIC| 1| Physical| Hub, Repeater| Table-2 Protocols and the Layers at Which They Function Name of the Layers| Protocols| Application| FTP, Telnet, NFS, HTTP, TFTP, DHCP, DNS, VoIP, SNMP, POP3, SMTP| Presentation| ASCII, HTML, JPEG| Session| SQL, RPC|
Transport| TCP, UDP, SPX| Network| IP, X. 25, IPX, Q. 931| Data Link| HDLC, Ethernet, LLC, Frame Relay, ATM, Q. 921, FDDI| Physical| RS-232, V. 35, 10bT, RJ45, G. 703/G. 704| Table-3 In the field of networking there very few people who is not aware of OSI reference model and it is impossible to imagine the world of Information Technology without this model. Over the years we have come across so many network hardware, software or operating systems but everything is manufactured keeping OSI reference model as a benchmark.