OSI and TCP/IP Models: Layers, Functions, and Differences

Introduction

In the world of networking, the OSI and TCP/IP models serve as foundational frameworks that define how data flows across systems. Each model breaks down network communication into layers, with each layer performing specific functions. The OSI model, with its seven layers, provides a broad conceptual structure, while the TCP/IP model, with four layers, is more practical and widely used in internet communications. This article will explore the layers and functions of both OSI and TCP/IP models, shedding light on their structures, purposes, and the differences that set them apart.

The OSI (Open Systems Interconnection) model and the TCP/IP (Transmission Control Protocol/Internet Protocol) model both describe the functions of network protocols and services required for communication between devices on a network. While the two models have different layering schemes, they share similar functions.

The OSI model consists of seven layers: application, presentation, session, transport, network, data link, and physical. Initially, the TCP/IP model consisted of four layers: application, transport, internet, and network access. Later, it has been represented with five layers for easy comparison with the OSI model, in which the “network access layer” has been represented by data link and physical layers.

The OSI (Open System Interconnection) is developed for defining seven layers which makes two different vendors systems communicate. TCP/IP is used for internet applications mainly. The figure describes how TCP/IP layers map to OSI layers.

Figure-1 : OSI layers vs TCP IP model layers

In OSI layers, control is passed from one layer to the next, starting at Layer 7 (application layer) in one station, proceeding to the succeeding bottom layer and so on until information reaches Layer 1. Then the information is transmitted over the channel to the next station’s Layer 1 and transmitted to succeeding layers until information is delivered to Layer 7.

Each Layer in the OSI model does the job for the previous layer and also adds prefixes information termed as “header”. The data link layer adds suffixes called cyclic redundancy check as “trailer” which is used for error detection.

The brief functionality of each OSI layer is mentioned below.

Functions of OSI and TCP IP Layers

Following are functions of OSI and TCP IP layers.

Application Layer

All the applications for user reside at this layer. It includes FTP, Telnet, e-mail etc.

Presentation Layer

It formats and encrypts data to be sent across a network. Its basic task is to make application layer and network layer talk to each other by processing the data. It provides independence to the application processes from differences in data representation by way of syntax conversion.

Session Layer

It Provides control mechanism to connect two applications running on different machines. It covers basic function to establish, to maintain and to release connections.

Transport Layer

It assures reliable data communication between two hosts. It is responsible for error recovery and flow control between two hosts. It also does multiplexing and de multiplexing functionalities for transmitting and receiving host respectively. The TCP and UDP reside at transport layer of the osi model.

Network Layer

The main function is switching and routing of the packet. It adds logical addressing to do this. It also takes care of error handling, congestion control and packet sequencing.

Data Link Layer

It takes care of errors in the physical layer. It provides flow control and also does frame synchronization. ARQ is implemented to take care of retransmission in case of erroneous packets. In latest today’s technologies it provides Medium Access Control functionality, hence known as MAC. MAC takes care of resource assignment to the end devices wanted to seek connection to the network for various applications (voice, data, etc.)

Physical Layer

It mainly takes care of bit stream transmission over physical medium (cable, air etc.). It provides the hardware means of sending and receiving data through cable or wireless medium. Some wireless standards viz. wlan, wimax, zigbee, LTE defines physical layer specifications which takes care of forward error correction and modulation-demodulation functionalities as well as making the information suitable to be transmitted.

Following figure mentions logical structure of the layered protocols inside a computer on an internet. This figure is taken from RFC 1180 to describe the concept. RFC1180 describes logical structure of protocols inside a typical internet enabled device as mentioned below.

Ethernet Cable—ENET Card—ARP/IP—TCP/UDP—Network Applications

As mentioned etnernet cable is connected directly with Ethernet card which has upper layer ARP or IP. This has upper layer TCP or UDP and above all applications are running. Every device has its own Ethernet address which is 6 bytes and listens for Ethernet frames with that destination address. An IP address is 4 bytes. Ethernet uses CSMA/CD. CSMA/CD means that all devices communicate on a single medium in which only one can transmit at a time and they can all receive simultaneously. If two devices try to transmit at the same instant, the transmit collision is detected and both devices wait a random period of time before trying to transmit again. An Ethernet frame contains the destination address, source address, type field, and data.

Data flow across TCP/IP Stack

Let us understand data flow communication between TCP IP model layers of Host-A and Host-B. Let us assume host A is transmitting packet and Host B is receiving it.

• As mentioned above all the hosts receive the packets and each will compare destination address in the Ethernet header with its’ own Ethernet or MAC address of 6 bytes and accept the same if it matches otherwise does not pay attention.
• Next it will check for type field(2 byte long), if it is 0x806 then the packet will be passed to ARP module and if it is 0x800 it will be passed to IP module. ARP module responds to hosts seeking for Ethernet address based on their logical (IP) address.
• After packet is received by IP layer it will decide where the packet need to be routed based on routing table and destination IP address embedded in received packet’s IP header. IP header is usually 20 bytes.
• If the host is the final end system then IP module checks for protocol field (1 byte long) in the IP header. If it is 0x06 then the packet is passed to the TCP module and if it is 0x11 then the packet is passed to the UDP module.
• Above the TCP or UDP there is application layer, where so many applications will be usually running. Information to each application will be passed based on ‘destination port number’ field (16 bit long) embedded inside the TCP or UDP header. TCP utilizes ‘sequence number field’ (32 bit long) for reassembly of all the IP data grams received at irregular time instants in irregular orders. For example FTP has dedicated port address 23 and Telnet has dedicated port address 21, which is used to deliver the data to each.

Difference between OSI and TCP/IP Model

Let us compare osi and tcp/ip model and understand difference between OSI and TCP/IP model layers including their functions and other comparison parameters.

Following are the benefits or advantages of OSI model .

• OSI model provides clear and structured conceptual framework to understand network communication.
• OSI model’s modularity allows implementation of protocols for specific functionalities.
• It promotes interoperability between equipments from different manufactures or vendors.
• As an ISO standard it helps to establish uniformity in network design and encourages to follow common standards.
• It’s layered nature makes it easy to design, troubleshoot and upgrade individual layers without affecting others.
• OSI model remains fundamental tool to learn basics of networking.

Following are the benefits or advantages of TCP/IP model .

• It has fewer layers and streamlined structure compare to OSI model which makes it easy to implement and troubleshoot in real world networking scenarios.
• It is fundamental networking protocol suite used on internet. It is widely adopted globally to connect diverse networks.
• TCP/IP model’s flexible design allows it to scale as per increasing demands of network resources. New devices and technologies are easily incorporated without significant modifications.
• It’s direct use in practical networking including internet helps network administrators, engineers and IT professionals working in real world networks.
• TCP IP has been in use since several decades which has made it reliable and robust. It has become stable choice for network communication.

Conclusion

The OSI and TCP/IP models are essential to understanding network communication. While the OSI model offers a theoretical framework with a layered approach, the TCP/IP model provides a simplified, practical approach widely used in internet protocols. Recognizing the functions and distinctions between these models is crucial for anyone interested in networking. With this knowledge, you can better understand data flow, diagnose network issues, and appreciate the protocols enabling global connectivity.

TCP IP protocol stack development community

• Internet Architecture Board (IAB), www.iab.org/
• Internet Engineering Task Force (IETF), www.ietf.org/
• Internet Society, www.isoc.org/
• The Internet Corporation of Assigned Names and Numbers (ICANN), www.icann.org/
• Internet Research Task Force (IRTF), www.irtf.org/
• World Wide Web Consortium (W3C), www.w3.org/

TCPIP References

• RFC 1180, A TCP IP Tutorial
• RFC793, TCP
• RFC826, ARP
• RFC792,ICMP
• RFC768,UDP