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Standards and Layered Approaches

 

Network Protocols:

    ‘Protocol’ - as defined in

    The Oxford English Reference Dictionary, 1996.

    "Formality and etiquette ‘s observed. The rules and formalities of any procedure. A

    formal statement of a transaction. A set of rules governing the exchange or transmission of data electrically between devices.

     

What is a network protocol?

• A protocol is a formally set of conventions, agreements, or procedures, etc., that govern the way in which two entities co-operate in order to exchange infomation.

• A protocol describes the allowed interaction between entities - the devices.

• In other words, it will control the transmission of data between devices that wish to understand each other and exchange information.

• Consider the use of an interpreter between two people speaking different languages.

• A protocol is a method of organising conununications between two entities.

• It establishes a set of rules and conventions that will allow the communication.

• Both sides in the conversation will obey the agreed protocol.

Whatever the protocol, it must be:

    - complete

    - robust

    - efficient

     

Why use a Protocol?

• For ease of communication - knowing exactly what to do, and what is expected leads to an easier dialogue.

• Faster conununication - a basic knowledge of the rules means that there is less (or no) requirement for further negotiation.

• Robust communications - understanding of rules places you on firmer ground, you can better cope with any problems.

• Secure communications - security rules and aspects are incorporated in the protocol - sound transmission.

 

Open Standards

• Standards are important - we need to ensure both hardware and software produced by different vendors can work together.

• Without agreed standards, sharing would be difficult.

• Old systems and protocols were proprietary - they belonged to each originating vendor.

• Companies (wrongly) hoped that this would better ‘tie’ users to ‘their’ systems,

• Technology advancements - resulted in freeing of users.

• Nowadays the idea is of ‘open’ protocols.

• Published agreed protocols can now be implemented by many companies.

• Protocols will be based on consolidated results of technology & user experiences - aimed at producing optimum results for the good of the community.

• Companies can now produce improved and more ‘open’ products, and subsequently can retain their customers through better service.

Two Types of Standards

1. Formal;
2. Defacto.

Formal standards:-

• These are typically developed by an official industry or government body.

• Major problem is that they take years to develop and implement.

• Technology moves faster than the development of the standard.

De facto standards:-

• These standards typically emerge within the market place itself

• They tend to be supported by several main-stream vendors - adding credence to their value.

• They are not necessarily immune to corporate politics or the influence of a national government.

International Organisation for Standardisation (ISO)

ISO - This is a United Nations Agency responsible for the standardisation of products across a very broad field, in particular, making recommendations about data communication interfaces.

• Members of the International Telecommunications Union (ITU), who make technical recommendations about telephone and data conununications world-wide.

• They developed the Open Systems Intercommunication Reference Model.

• Model aims to act as a logical framework of protocols.

• Model acts to facilitate computer-to-computer communications.

• Ultimately facilitating network interconnection.

The OSI model has seven layers of modeling. Typically the layers get more sophisticated as you go up the mode. -not surprisingly the higher levels are the levels that the user is more likely to be involved with. Each layer is dependent upon the layer above or below. Individually the operations undertaken by each layer cannot compete with the power and co-ordination of the joint efforts of all the layers. It should be noted however, that depending on the configuration of the data communications equipment some layers may not be required. Ultimately the layering promotes a form of commonality between different equipment.

 

The layers are:-

          7) Application

          6) Presentation

          5) Session

          4) Transport

          3) Network

          2) Data Link

          1) Physical

           

OSI: Why the Layers?

The OSI model attempts co-ordinate the development of standards at all levels of communication.

• Layers allow for the increased independence between the great variety of complicated operations involved in data communications.

• The OSI model organises the communication processes into seven different categories (layers).

• Layered structures prepare the data for transportation over a network.

• Data are ‘chunked’ together into packets, and passed through the different layers.

• Each layer will add its own information (control data) to the transporting data packet (msg).

• Each layer thinks that it is communicating with its corresponding layer at destination.

• Each layer will accept a message from the layer above, add its own control data, and forward message to layer below.

On receiving end the process is reversed - control data are stripped off and message passed above.

The OSI Model brings assurance that hardware and software components from different vendors will be ‘compatible’.

Manufacturers now have a common framework to adhere to:-

• Layers 7 -4 deal with the end-to-end communications, form message source to message destination point.

• Layers 3 - 1 deal with the network access issues.

• Physical (1) Lowest layer, will send the data bits over the data channel. Responsible for transmission of a raw bit stream.

• Data Link (2) Puts the bits into frames and provides error free delivery. Uses error-detecting codes, & host-to-host control messages to enforce reliable transmission.

Network (3) Routes the frames over the network. Also concerned with congestion.

• Transport (4) Deals with multiple sessions, and end-to-end performance. Provides host-to-communications, and hides complex networks details from session layer.

 

Connection Methods

Connection-Oriented:

• This connection method requires a pair of devices (computers) to have already established connection prior to delivery of data.(Consider telephone call).

• The Network Layer establishes the virtual route, connection is established between sender and receiver, then the data are sent down the route.

• All the data packets belonging to one message will then be received in correct order of sending.

Connectionless:

• This connection method has the ability to send data to any other computer at any time. (Consider posting a letter).

• Each data packet is treated independently and will travel through the network.

• Individual data packets (from one message) may each take different routes to end node, where data packets will then be re-ordered.

Transmission Control Protocol (TCP)

• This provides application programs access to connection-oriented communication services.

• It does offer a reliable and flow controlled data delivery.

 

Internet Protocol (IP)

• This protocol defines format of data packets for use on TCP/IP Internet.

• It offers a mechanism for routing packets to their destination.

 

Transmission Control Protocol / Internet Protocol (TCP/IP)

This is a connection oriented transport layer protocol that also uses the connectionless services of the Internet Protocol (IP).

Why is TCP/IP needed?

Growing trend for world-wide demand for inter-networking products. Networking technology vendors forced to modify protocols to accommodate growing inter-networking trend. It was designed by the Department of Defense (DoD) in the 70s. With the connectionless service facilities a link does not have to be established prior to the transmission. Protocol facilitates the sending of many data packets to multiple destinations, (e.g., Email).

This protocol facilitates the following services:

Telnet:

• This allows computer users to login into other computers via the Internet.

• Users should have an authorised account and password.

• Can be used to access world-wide public databases, libraries and other services.

 

FTP (File Transfer Protocol):

• This allows a computer user to send and receive files over the Internet.

• FTP allows you to access world-wide public file archives.

• Program called ‘Archie’ can tell you address of all file archives that have copy of FTP.

 

 

Simple Mail Transport Protocol (SMTP

(Application layer protocol)

• This protocol is used to transfer electronic mail from one computer to another across the Internet.

• It is considered the defacto standard on the Internet.

• A mail transfer program contacts the remote machine.

• A connection for communication is formed.

• SMTP is responsible for:-

      -~ ensuring sender identifies themselves; recipient details are specified;

      — Email message is then transferred.

       

Simple Network Management Protocol (SNMP)

This is an industry standard protocol used for monitoring and configuring network devices.

• It specifies how a network management station is allowed to conununicate with agent software situated in a remote device (e.g., routers).

• Data are passed to the network management console via the SNMP agent.

• SNMP also facilitates the collection of, and evaluation of network performance data.

• The SNMP provides a full set of rules describing how TCPJIP networks will undertake basic maintenance and system management operations.

 

 

 

Application

Layer 7

• Details how application uses network.

• Provides end user access to network.
• The protocol states how an application program on one machine makes a request, and the other

machine responds.

• The layer allows a program to access the OSI environment.

• Also contains management functions and mechanisms to support the sharing of applications.

• Monitoring, network management statistics and diagnostics reside here.
Presentation Layer 6

• This offers a protocol that states how to represent the data.

• It defines the presentation format of the data being transported.

• The layer considers protocol conversion between different formats.

• Application programs are offered this set of ‘conversion’ services here.

• Examples include, data encryption & compression.
Session Layer 5

• This layer provides a mechanism for controlling peer-to-peer dialogue.
• Example. Consider PC communicating with mainframe - this layer provides the s/w to facilitate this

type of connection.
• Login, logout, and password verification procedures are undertaken here.
• The Session layer will initiate, maintain and terminate each l9gical session between two

machines.
Transport Layer 4

• Sometimes described as host-to-host or end-to-end layer.
• This layer provides a reliable mechanism to facilitate data exchange between computers
• It regulates flow of data, e.g., where one machine might send packets of data quicker that another

machine might receive sent data.

• Protocol here is complex, but ultimately aims to maintain and terminate logical connection.

• It also aims to ensure reliable data transfer.

• The layer will generate end-user address, split packets of data into smaller packets.

• Data checks are assigned to packets, to ensure split data packets can be reformed in their correct

order on completion.
Network Layer 3

• This protocol will specify how many packets of data are addressed.
• It also controls how many packets are routed across the network.

• The network layer will establish a virtual route between two nodes (routers function at this point).
• This layer also provides supervisory control over layers 1 & 2.
Data Link Layer 2

• This protocol specifies how data are orgainsed into frames.

• How to transmit the data over the network.

• It has to deal with both communications requirements and requirements of the user.
• The data link layer attempts to make the physical link reliable.
• It can activate, maintain and deactivate the link.
• The layer is also responsible for error detection and control.
• All the higher level layers can assume error free transmission if there is a fully functional data link layer

protocol in place.
Physical Layer 1

• This is the lowest layer in the OSI Model, it corresponds to the basic network hardware. (E.g., repeater operate here),

• It is concerned with sending data bits over a communications circuit.

• It deals with mechanical aspects such as connectors used to join two devices.
• Electrical characteristics such as dealing with voltage levels, and voltage timing - e.g., to establish data

transfer and distance.
• Functional characteristics will apply meaning to the receipt of the various signals - e.g., modulation &

demodulation.
• Procedural characteristics such as specifying sequence of transmission.
• The layer will specify the physical medium used in the network, e.g., cabling and addressing functions

necessary to send data bits down the specific medium.

  

A Server is a device unit (computer) at one node on a network that provides a specific type of service to all permitted network users. The server computer can be anything from a PC to a supercomputer, it will perform a number of functions for its client computers. One server computer can support many client computers (according to the configurations). Functions will range for example, from supporting storage of data through to access to software applications.

 

The file server stores user files. This server is normally a dedicated PC with a reasonably large sized hard disk for storing the data and applications to be shared by the network users. It may contain for example, a master file holding customer records, and applications such as MS Word and MS Access. If access is required to the files or applications, a user will locally make a request to the server. The file server will then ensure the relevant software and data are downloaded from the file server to the user’s RAM. Benefits allow any authorised user to access the same set of shared files and applications available on the shared disk. Novel’s NDS provides access mechanisms that restricts access to shared resources according to authority and privileges assigned to a specific user or user group.

  

The print server provides printing facilities. This server is typically housed in the same dedicated PC as the file server, although it could be similarly housed in it own dedicated PC. It could also be housed in a ‘black box’, a device that performs all the functions of a printer server, but at a lower cost than a dedicated PC. Where the printer server functions are housed separately from the LAN server, the overall efficiency of the network might be improved. The print server will handle the print jobs and controls at least one printer. It may be necessaiy for the server to spool print jobs, here the server will save all the print jobs to disk until the requested printer is available for printing. The print server will then take responsibility for routing the print job to the printer.

The communications server will facilitate access to other networks. The communications server controls access to external areas - e.g., other LANs. Control will be accomplished for example, via use of modems or via access to an ISDN line. Three types of specialist communication servers include fax servers, modem servers and access servers (e.g. gateway).

 The client computer (e.g., a PC or workstation on network) will request the help of a server (or many servers) to undertake a particular function. Typical function requests might include printing or remotely accessing another network.

 

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