Networking Reviews

TCP/IP Model vs OSI Model

Admin — Wed, 04 Jun 2008 15:15:49 +0000

We have talked in other posts about the OSI model, which is the de-facto standard model for describing and implementing IP communication networks. You might be thinking that the OSI model was the original standard created when the first computer networks started to appear, but this is wrong. The TCP/IP model is older than the OSI, despite that the OSI is the most well known model today.

The TCP/IP model originated by the U.S Department of Defense in the 1970s. It is very similar to the OSI Model, but with a simpler structure, as shown below:

As you can see from the diagram above, the TCP/IP model matches the OSI up to the Transport Layer. The top three layers of the OSI (Session, Presentation, Application) are represented just by one layer on the TCP/IP model (the Application Layer).

The strength of the TCP/IP model compared to the OSI is that its simpler and is completely aligned with the TCP/IP protocol stack. The weakness of the TCP/IP model compared to the OSI is that its mostly network-centric and does not describe the application interaction in much detail.

OSI Layer 7: Application Layer

Admin — Wed, 05 Mar 2008 18:05:34 +0000

The Application Layer is the one which interacts with the user. A Software application that implements a communicating component, uses the OSI Application Layer to establish the application’s communication. For example a word processor that does not have communications capabilities, would not be concerned with the OSI Application Layer. On the other hand, an application with communication capabilities (e.g a Web Browser), has to implement the OSI Application Layer standards.

Examples of Layer 7 Applications
• Telnet
• FTP
• HTTP
• SMTP
• WWW Browsers
• SNMP

OSI Layer 6: Presentation Layer

Admin — Wed, 05 Mar 2008 18:03:38 +0000

The main function of this layer is to define data formats, coding, and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Encryption and compression of Data are also defined as Presentation layer functions.

Some popular implementations of the Presentation Layer are well-known standards such as QuickTime and MPEG, Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF).

Examples of Layer 6 Standards
• ASCII
• EBCDIC
• TIFF
• GIF
• JPEG
• MPEG
• MIDI

OSI Layer 5: Session Layer

Admin — Wed, 05 Mar 2008 17:51:06 +0000

The Session Layer starts, controls, and manages communication conversations (sessions). Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. The session layer creates ways to imply which flows are part of the same session and which flows must complete before any are considered complete.

Examples of Layer 5 Specifications and Protocols
• RPC
• Session Control Protocol (SCP)
• SQL
• NFS
• NetBios names

OSI Layer 4: Transport Layer

Admin — Wed, 05 Mar 2008 17:50:07 +0000

The Transport Layer (e.g TCP) provides reliable delivery of Data by using error checking, acknowledgments, flow control, and sequence checking. Multiplexing of incoming data for different flows to applications on the same host (for example, TCP ports) is also performed. Flow control in this layer ensures that the transmitting device does not send more data than the receiving device can process.

Examples of Layer 4 Specifications and Protocols
• TCP
• UDP
• SPX
• Port Numbers (e.g 80 for HTTP)

OSI Layer 3: Network Layer

Admin — Wed, 05 Mar 2008 17:47:36 +0000

This Layer defines logical addressing (e.g IP address) in order to identify each endpoint in a network and provide end-to-end delivery of packets. Think of the Network Layer as the postal service in a country. When you want to send a letter to a friend, you just need to know the destination postal address (e.g destination IP address) and you send the letter. The postal service (e.g the Network Layer) will determine the path to the destination (e.g IP Path Determination) and take care of the delivery (e.g using IP Routing). The two endpoints do not need to know the exact path taken for the delivery of the letter.

This Layer also encapsulates data into Packets, and defines how to fragment a packet into smaller pieces to accommodate media with smaller Maximum Transmission Units. An IP Router works in this Layer, since it can use the IP address information to route packets.

Examples of Layer 3 Specifications and Protocols
• IP
• IPX
• AppleTalk

OSI Layer 2: Data Link Layer

Admin — Wed, 05 Mar 2008 17:44:48 +0000

The Data Link Layer provides error detection and error recovery of the data flowing across the physical network link. The Specifications in this OSI Layer define network and protocol characteristics such as Physical addressing, network topology, error notification, flow control etc. The physical addressing defined in this layer determines how devices are addressed on the data link layer. An example of physical addressing is the MAC address which is hard-coded in all Ethernet Network Interface Cards (NIC). Each Ethernet NIC that exist in the world has a unique MAC address. The Data Link Layer also combines bits into Frames. Each Frame has a Layer 2 Header which includes the Source and Destination MAC addresses among other fields.

Examples of Layer 2 Specifications and Protocols
• IEEE 802.3 Ethernet
• HDLC
• Frame Relay
• ATM
• PPP

OSI Layer 1: Physical Layer

Admin — Tue, 04 Mar 2008 20:30:11 +0000

The Physical Layer defines the Electrical, Mechanical, and functional specifications of the physical link used to connect the communication device on the network. This layer defines characteristics like voltage levels, data rates, maximum transmission distances, connectors etc.

The specifications in this Layer can be categorized as LAN or WAN:

LAN Specifications Examples:
• Ethernet
• FDDI
• Token Ring

WAN Specifications Examples:
• V24
• V35
• X21
• EIA/TIA-232