2.1 The Open System Interconnection (OSI) Standard

The Open Systems Interconnection (OSI) model is another standard protocol architecture which was specified by the International Organization for Standardization (ISO).

OSI is more of a theoretical model that has not been widely used compared to the Internet protocol stack standard.

Although most communication technologies and protocols are not designed to meet the OSI model, they are described in terms of how they fit into its layers.

The OSI Reference model is composed of seven layers (as illustrated in Figure 2.1) with each layer:

		Responsible for a specific communication task and
		Supports the layer above it.

The level of abstraction increases as we get higher in the layer stack.

		The first layer (the physical layer) deals with the low-level signaling and hardware issues related to the communication process.
		The higher the layer in the stack, the more it deals with the logical issues related to the communication with the highest layer (the application layer) dealing with high-level applications.

2.1 The Open System Interconnection (OSI) Standard

The seven layers of the OSI model are outlined below:

Physical Layer:

		Responsible for interfacing directly with the communication media (e.g. unshielded twisted pair cables).
		It sends and receives signals representing data as bits over the transmission media.

Data Link Layer:

		Responsible for the physical transmission of data from one node to another.
		This layer defines the data frame of the network, i.e. the format with which the data will physically be transmitted on the network which includes checksums (for error detection), source and destination addresses and data.
		This layer translates messages received from upper layers into data frames and adds new headers to the data. Such headers include the hardware addresses of the source and destination and checksums of the data.
		The data link layer also builds data frames from 0s and 1s forwarded by the physical layer which the latter received from the communication medium. Bits in the data frame are summed by the data link layer to detect any errors that could have occurred during transmission.

2.1 The Open System Interconnection (OSI) Standard

The Network Layer:

		Responsible for routing data from one node to another with the nodes possibly belonging to different networks.
		The network layer manages the addressing of nodes usually with a two-part address, one part being a network id and the other part is a node id.
		In this layer, routing tables are maintained and used to determine the best path for moving data between nodes.

The Transport Layer:

		Provides end-to-end connection-oriented reliable service and ensure the integrity of data transmission.
		This layer hides network-dependent information from higher layers.
		It divides data received from upper layers into segments and uses flow control mechanisms to control the rate at which data is sent from the source to the final destination.
		An acknowledgment and retransmission mechanism is used to ensure the successful delivery of data.

2.1 The Open System Interconnection (OSI) Standard

The Session Layer:

Responsible for controlling communication sessions between application and in practice it is often combined with Transport Layer.

The Presentation Layer:

		Concerned with data representation issues including data formatting, compression and encryption.
		Standards include MPEG, MIDI, TIFF, JPEG and ASCII.

The Application Layer:

		Responsible for providing network services to user applications.
		Such network applications include FTP, Telnet and DNS.

2.1 The Open System Interconnection (OSI) Standard