The Network, Data Link, and Physical Layers
Introduction
The OSI (Open Systems Interconnection) model has served as a reference for the most basic elements of computer networking since its inception in 1984. Based on a proposal created by the International Standards Organization (ISO), the OSI model was developed “to provide a set of design standards for equipment manufacturers so they could communicate with each other” (Miller, 2001). It consists of seven layers, which, from top to bottom, are the following: Application, Presentation, Session, Transport, Network, Data Link, and Physical.
I have learned, through the research I conducted, that the three upper layers perform the application-specific functions such as connection management, encryption, and data formatting. The four bottom layers, on the other hand, provide network-specific functions such as flow controls, addressing, and routing.
In this paper, I will discuss the three bottom layers, namely the Network, Data Link, and Physical layers. The Network layer is responsible for routing information in the network. The Data Link layer is responsible for providing an error control mechanism. And the Physical layer is responsible for connecting the entity to the transmission media (Miller).
The Network Layer
The Network layer is responsible for controlling a sub-net’s operation. It also provides accounting, congestion control, and routing. It is capable of providing services that are either connection-oriented or connectionless (Miller).
The main design issue in this layer is the determination of how packets should be routed from their source to their destination. One way to determine this is by using static tables within the network, which seldom change. Another is by determining the route at the beginning of each conversion. A third method is to make routes dynamic, that is, a new route is determined for each packet in accordance to the current network load. The Network layer enables the interconnection of heterogeneous networks. All of the routers in the network operate at this level, and even the IP protocol can be found in this layer.
This layer controls congestion when the number of packets in the sub-net becomes excessive. It also performs an accounting function by ensuring that the number of bits sent is the same number of bits received. In addition, the Network layer can contain controls for the data flow, segmenting and sequencing, logical channels, and network connections (Miller). Examples of Network layer protocols are IPX, which is usually used with SPX in Windows and Novel Networks; and IP, which is usually used with TCP (Lowe, 2011).
The Data Link Layer
The Data Link layer provides the interface between the software that runs on the computer and the physical media (Bhardwaj, 2007). It is mainly responsible for transforming raw transmission into a line in the network layer that is free of transmission errors (Miller). It breaks down the sender’s input data into data frames, which it then sequentially transmits. After transmission, it processes the acknowledgement frames that the receiver sends back (Miller).
The data frames contain the source and destination addresses of each network card that is attached to the network cable. Some examples of LAN data link protocols are ARCnet, Token Ring, and Ethernet. On the other hand, examples of data link protocols that are used for connecting to the Internet would include the Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP).
The data blocks sent by the Data Link layer contain the necessary flow control, detection/correction error control, and synchronization. These data flow controls are responsible for about 70% of all error handling. With the physical layer of the OSI model merely accepting and transmitting a stream of bits without any regard to the structure’s meaning, it is the Data Link layer’s responsibility to recognize and create frame boundaries through the attachment of special bit patterns at the beginning and end of the frame (Miller). In addition, the Data Link layer uses encryption to ensure that the message is protected during its transmission from one network node to another. Each node then decrypts the message it receives and encrypts the message again before transmitting it to the next node (Miller).
Physical Layer
The physical layer of the OSI model is responsible for transmitting raw bits through a communication channel. The design considerations in this layer deal mainly with the procedural, functional, electrical, and mechanical interface. The main design issue is ensuring that when a side sends a 1 bit, that the other side also receives a 1 bit and not a 0 bit. One of the things that must also be determined is the number of volts needed for the representation of a 1 and the number of volts needed for the representation of a 0. Other considerations would be the number of pins in the network connector and the purpose of each pin; how to establish the initial connection and how to dissolve it once the transmission is complete; whether to enable transmission in both directions; and the number of microseconds that a bit lasts (Miller).
The physical layer describes the transmission media to be some type of cabling system. In addition, it describes the network topology and the distribution of the transmission media. Examples include the ring, star, and bus topologies. There are also two categories of physical layer implementations, namely WAN and LAN (Cisco Systems, Inc., 2004)
References
Bhardwaj, P. K. (2007). A+, network+, security+ exams in a nutshell. Sebastopol, CA: O’Reily
Media, Inc.
Cisco Systems, Inc. (2004). Internetworking technologies handbook (4th ed.). Indianapolis, IN:
Cisco Press
Lowe, D. (2011). Networking for dummies. Hoboken, NJ: Wiley & Sons
Miller, R. L. (2001). The OSI model: An overview. Retrieved from
http://www.sans.org/reading_room/whitepapers/standards/osi-model-overview_543
