Network Protocols : Basic Overview

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Network communication is quite a complex task. In order to understand and implement this complex process easily, they have been broken down into layers. Each layer represents a category of related tasks. A protocol stack is an implementation of this layered protocol architecture.

The protocols and services associated with the protocol stack interact to prepare, transmit, and receive network data. They each have their own duties like drones in a hive. Each layer has their own tasks and duties.

If Machine A needs to send a message to Machine B, it will split up the message into packets, or frames, which would include header and trailer information aside from bits of the data itself. That would be likened to a letter, where the data is the actual letter inside the envelop itself, and the header and trailer bits would be the address, the stamp and the return address. The frame would have also error checking information to ensure that what is sent is correct. If an error is detected, it will be retransmitted.

How do two diverse computer systems communicate?

In order for two computers to communicate, they must run compatible protocol stacks. That is like saying, in order for two person to communicate, they must understand a common language, be it English, one or the other’s native language, or sign language; it doesn’t matter as long as both can understand it. This is because each layer in a computer must interact with the corresponding layer in the other computer’s protocol stack. TCP/IP is a protocol stack that is available for almost all computers and operating systems. If a Macintosh and a UNIX workstation can run TCP/IP, the Mac can then access files on the UNIX workstation and vice versa.

OSI Physical Layer Concept

The physical layer is the lowest layer in the OSI Model. It controls the way unstructured, raw, bit-stream data (i.e. the stream of 0’s and 1’s) is sent and received over the physical medium. It describes the electrical or optical, mechanical and functional interfaces to the physical network medium.

This layer defines how the cable is attached to the network adapter card. For example, it defines how many pins the connector has and what each pin is used for. It describes the topology used to connect computers together like Token Ring, where the station must get a token before it is allowed to transmit data (sort of like taking turns), Ethernet, where everyone listens to the network and when they are sure that nobody is transmitting, take their chance to send out data (more random way of trying your luck) or others. It also defines which transmission technique will be used to send data over the network cable.

In a nutshell, it just takes cares of the pulses of 1’s and 0’s that passes through the wire when it is either transmitting or receiving.

Data Link Layer

The data link layer provides error-free transfer of data frames from one computer to another over the physical layer. The layers above this layer can assume virtually error free transmission over the network.

Real messages does not consist of single bits, but of meaningful groups of bits. The Data Link layer receives messages, called frames, from the upper layers. The primary function of the Data Link layer is to disassemble these frames into bits for transmission, and then on the receiving end, to reconstruct the frames from the bits received.

The exact format of the frames depend on the network topology. For example, an Ethernet frame is different from a Token Ring frame. These frames contain source and destination addresses so that the sending and receiving computers can recognize and retrieve their own frames on the network.

The Data Link Layer controls functions that include managing media access, delimiting frames, checking frame errors and recognizing frame addresses. It also establishes and maintains links between communicating devices, controlling frame traffic, sequencing frames, and acknowledging them when they arrive.

A very simple data frame

Network Layer

Layer 3, the Network Layer, is responsible for addressing messages and translating logical addresses and names into physical addresses. For example, it may not be possible to remember the physical address of say, Barbara’s machine that you may want to send a message to. This is because the format of physical address could be 00-00-E8-CF-FB-33 or some other hard to remember series of numbers and letters like that. So the Network Layer takes care of translating an easier to remember machine name (logical address) like barbara_pc to machine/physical address.

Not only that, but this layer also determines the route from the source to the destination computer based on network conditions, priority of service and other factors. It also manages traffic problems, such as switching, routing, and controlling the congestion of data packets, on the network.

The Network Layer bundles small data frames together for transmission across the network. It also restructures large frames into smaller packets. On the receiving end, the Network Layer reassembles the data packets into their original frame structure.

Transport Layer

This layer makes sure that messages are delivered in a reliable manner in the order in which they were sent and that there is no loss or duplication. It removes the concern for higher layer protocols about data transfer between the higher layer and its peers.

However ‘reliable’ does not mean that errors cannot occur; instead, it means that if errors occur, they are detected. If errors such as loss of data are detected, the Transport layer either requests retransmission or notifies upper-payer protocols so that they can take corrective action.

Functions of the transport layer includes the following :

• Accepting messages from the layer above and, if necessary, splitting them into frames
• Providing reliable, end-to-end message delivery with acknowledgements
• Instructing the transmitting computer not to transmit when no receive buffers are available
• Multiplexing several process-to-process message streams or sessions onto one logical link and keeping track of which messages belong to which sessions

Session Layer

The Session layer establishes a communications session between processes running on different computers and can support message-mode data transfer. In short, this layer manages dialogs between two computers by establishing, managing, and terminating communications. This can take three forms :

• Simplex Dialogs – These are only one way data transfer, like a fire alarm which sends signal to the fire station but cannot (and need not) send messages out. Another example would be television which only receives broadcast and does not send out (although now with web tv…)
• Half-duplex – This is a two way data transfer, but only one direction at a time, like CB radio operators they have to say "over" so that the other party can talk.
• Full-duplex – This permits two-way simultaneous transfer by providing each device with its own separate communication channel. Voice telephones are full duplex devices, and either party to a conversation can talk at any time.

Connection Establishment

This phase establishes the parameters for the communication session. The requesting service (requesting machine) requests initiation of a service. During the setup process, communication is established and rules are agreed upon. It involves tasks such as

• Specification of required services
• User login authentication and other security procedures
• Negotiation of protocols and protocol parameters
• Notification of connection Ids
• Establishment of dialog control, as well as acknowledgement of numbering and retransmission procedures

Data Transfer

Because of rules agreed upon during setup, each party knows what to expect. Communication is therefore efficient, and errors are easy to detect. Besides exchanging data, these devices exchange acknowledgements and other control data that manage the dialog.

The Session layer also can incorporate protocols to resume dialogs that have been interrupted. The devices can recognize a lost connection whenever the connection has not been formally released because acknowledgement was not received or data transmission was interrupted.

Connection release

When the session is completed, the dialog is terminated in an orderly fashion so that communication can be shut down and resources can be released back to the services provider.

Presentation Layer

The presentation layer serves as a data translator for the network. This layer deals with the syntax, or grammatical rules, needed for communication between two computers. It converts system-specific data from the Application layer into a common, machine-independent format that will support a more standardized design for lower protocol layer. This just means that the 1’s and 0’s sent and received across the wire is translated into meaningful bits of characters that the application understand, like an email message for example.

Other functions of this layer include also data conversion, data compression as well as data encryption.

Application Layer

The Application layer of the OSI Reference model is concerned with providing services on the network, including file services, print services, e-mail services, and database services among others. It further provides the following functions :

• Resource sharing and device redirection
• Remote file access
• Inter-process communication support
• Remote procedure call support
• Network Management
• Directory services
• Electronic messaging
• Simulation of virtual terminals (e.g. telnet)

If you have any comments, please feel free to email me at foogy@geocities.com
Copyrighted 1997 Lim Pei Mun J
Last updated : 13 May 1998

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