Networking Protocols

In a Nutshell - CIW Course Section 1, Part C, Chapter 2

Protocols

The OSI/RM (Open Systems Interconnection Reference Model) is not a protocol. It is a standard providing the concepts that most protocols adhere to. The OSI/RM comprise seven layers as shown below. It is essential to know this model well.

Layer	Layer Number	Description
Application	7	The interface to the end user in an OSI environment - supports file transfers, network management and other services
Presentation	6	Responsible for providing useful transformations on data to support a standardised application interface and general communications services. For example, it converts text from American Standard Code for Information Exchange (ASCII) format to Extended Binary Coded Decimal Interchange Code (EBCDIC).
Session	5	Establishes, manages and terminates connections (sessions) between cooperating applications. Also adds traffic flow information.
Transport	4	Provides reliable, transparent transport between end points (the source and destination hosts). Also supports end-to-end error recovery and flow control.
Network	3	Organises data into datagrams, or packets. Forwards and routes datagrams to their destinations.
Data Link	2	Provides reliable data transfer across the physical link. Frames are transmitted with the necessary synchronisation, error control, and flow control. In short, it prepares the information so that it can be placed on the transmission medium, such as copper wire. In the IEEE 802 series of LAN standards, the data link layer is divided into two sublayers: The Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. The LLC is responsible for error and flow control, and the MAC layer is responsible for placing the data on the transmission medium.
Physical	1	Concerned with transmission of unstructured bit stream over a physical link. Responsible for the mechanical, electrical and procedural characteristics to establish, maintain and deactivate the physical link.

A packet is a fixed piece of information sent across the network. It consists of three elements: A Header, The Data, A Trailer. The packer creation process begins with layer 7 of the OSI/RM model and, as the packet traverses the model, each layer adds it's own header snippet. At the receiving end, as the packet traverse the OSI model each layers strips out it's header. This allows each layer at the source and destination to communicate.

Some protocols require that a session be established before communication can begin. These are known as Stateful. Others that do not require a session are known as connectionless or stateless.

CRC A Cyclic Redundancy Check is a mathematical calculation that allows the receiving computer to verify that the received packet is valid.

Transport Layer Protocols

The transport layer provides reliable data delivery. Protocols used at this layer include:

Transmission Control Protocol (TCP)
Sequenced Packet Exchange Protocol (SPX)
NWLink
Appletalk Transaction Protocol (ATP)
NetBEUI

Network Layer Protocols

Network Protocols provide addresses used on a network and rules for particular networks. Network protocols include:

Internet Protocol (IP)
Internetwork Packet Exchange (IPX)
NWLink
Datagram Delivery Protocol (DDP)
NetBEUI
X.25
Ethernet

TCP/IP