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Mark Pearl

 

After reading this chapter you should be able to

  • Identify organizations that set standards for networking
  • Describe the purpose of the OSI model and each of its layers
  • Explain specific functions belonging to each OSI model layer
  • Understand how two network nodes communicate through the OSI model
  • Discuss the structure and purpose of data packets and frames
  • Describe the two types of addressing covered by the OSI model

Networking Standards Organizations

What are standards?

  • Standards are documented agreements containing technical specification or other precise criteria that stipulate how a particular product or service should be designed or performed.
  • Many different industries use standards to ensure that products, processes and services suit their purposes.
  • Without standards, it would be very difficult to design a network because you would not be certain that software or hardware from different manufacturers would work together.

There are a ton of different standards organizations including the following…

  • ANSI – American National Standards Institute
  • IEEE – Institute of Electrical and Electronics Engineers
  • ISO – International Organization for Standardization
  • ISOC – Internet Society

The OSI Model

OSI stands for Open Systems Interconnection model. The OSI model was developed by ISO during the 1980’s

It divides network communications into 7 layers:

  1. Physical
  2. Data Link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

Useful sentence to remember the first letter of every layer is as follows: “Programmers Dare Not Throw Salty Pretzels Away”

At each layer protocols provides services unique to that layer.

The OSI model is a theoretical representation of what happens between two nodes communicating on a network. It does not prescribe the type of hardware or software that should support each layer. Nor does it describe how software programs interact with other software programs or humans. Every process that occurs during network communications can be associated with a layer of the OSI model.

Below is an illustration of the path that data takes from one computer to another through the OSI model.

osi_model

 

Application Provides interface between software applications and network for interpreting applications requests and requirements
Presentation Allows hosts and applications to use a common language, performs data formatting, encryption, and congestion
Session Establishes, maintains, and terminates user connections
Transport Ensures accurate delivery of data through flow control, segmentation and reassembly, error correction, and acknowledgement
Network Establishes network connections, translates network addresses into their physical counterparts and determines routing
Data link Packages data in frames appropriate to network transmission method
Physical Manages signalling to and from physical network connections

 

Application Layer
  • Facilitates communication between software applications and lower-layer network services so that the network can interpret an application’s request and in turn the application can interpret data sent from the network.
  • An example of an application level protocol would be HTTP (Hypertext Transfer Protocol)
Presentation Layer
  • Protocols at the Presentation layer accept Application layer data and format it so that one type of application and host can understand data from another type of application and host.
  • The presentation layer serves as a translator.
  • It understands formats of data being transported over the network like Image and Sound formats (mp3, jpeg, etc.)
  • It also manages data encryption and decryption
Session Layer
  • Protocols on the session layer coordinate and maintain communications between two nodes on the network
  • Session refers to a connection for on-going data exchange between two parties
  • Historically it was used in context of communication between  terminal and mainframe communications where a device had little if any processing or disk capacity and the host would supply it with software and processing services.
  • Today the term session is used in the context of a connection between a remote client and an access server or between a Web browser client and a Web Server

Some of the functions include establishing and

  • Keeping alive the communications link for the duration of the session
  • Keeping the communication secure
  • Synchronizing the dialog between the two nodes
  • Figuring out when to restart communication if it has been cut off
  • Monitors the identification of session participants, ensuring only authorized nodes can access the session
Transport Layer
  • Protocols accept data from the Session layer and manage end-to-end delivery of data
  • They ensure data is transferred from point A to point B
  • Without Transport layer services, data could not be verified or interpreted by its recipient
  • Transport layer protocols also handle flow control (determines the appropriate rate of transmission)
  • Transport layer protocols break large data units received from the session layer into smaller units called segments

There are dozens of transport layer protocols that exist, but most modern websites rely on only a few, typically TCP

There are different types of Transport Layer Protocols including the following

Connection oriented protocols

  • Ensure that data arrives exactly as it was sent
  • TCP is connection oriented
  • To ensure data integrity, they use a Checksum which allows the receiving node determine if the data was received correctly

Connectionless protocols

  • More efficient that connection oriented protocols
  • Make no effort to ensure data delivery
  • Useful when data needs to be transferred quickly (i.e. video live streaming)
Network Layer
  • Primary function is to translate network addresses into their physical counterparts and decide how to route data from the sender to the receiver
  • Network layer protocols accept the Transport layer segments and add logical addressing information in a network header – at this point the data unit becomes a packet
  • Network layer protocols also determine the path from point A to point B on another network
  • Because the network layer handles routing, routers – the devices that connect network segments and direct data – belong in the Network layer
Data Link Layer
  • The primary purpose is to divide data received from the Network layer into distinct frames that can then be transmitted by the physical layer
  • A frame is a structured package for moving data that includes not only the raw data but also the sender’s and receiver’s network addresses, and error checking and control information
  • The data link protocols find out information that has been dropped and requests it to be sent again
  • The data link layer controls the flow of information, allowing the NIC to process data without error

The data link layer is divided up into two sub layers

  1. LLC – logical Link Control – provides an interface to the Network layer protocols, manages flow control and issues requests for transmission for data that has suffered errors
  2. MAC – media access control - manages access to the physical medium

The MAC address is also known as a Data Link layer address or a hardware address

Physical Layer
  • This is the lowest layer of the OSI model
  • Protocols at the physical layer accepts frames from the Data Link layer and generate signals as changes in voltage at the NIC
  • When receiving data, physical layer protocols detect and accept signals, which they pass on to the Data Link Layer
  • Physical layer protocols also set the data transmission rate and monitor data error rates
  • Simple connectivity devises such as hubs and repeaters operate ate the Physical layer
  • NICS operate at both the physical layer and at the Data link layer
  • Cables etc. operate at the physical layer

Applying the OSI Model

At each layer of the OSI model, some information is added to the original data. After it has followed the path from the application layer to the physical layer, data is significantly transformed.

Frame specifications

By far the most popular type of networking technology in use today is Ethernet, which uses Ethernet frames.

Ethernet was developed at Xerox in the early 1970’s and has been improved on during the years.

There are four different types of Ethernet frames. The most popular form of Ethernet is characterized by the unique way in which devices share a common transmission channel, described in the IEEE 802.3 standard.

There is another type of networking technology called token ring – this is described in the IEEE 802.5 standard

Most LAN’s do not support more than one frame type because devices cannot support more than one frame type per physical interface, or NIC.

IEEE Networking Specifications

Standard Name Topic
802.1 Internetworking Routing, bridging, and network to network communication
802.2 Logical Link Control Error and flow control over data frames
802.3 Ethernet LAN All forms of Ethernet media and interfaces
802.5 Token ring LAN All forms of token ring media and interfaces
802.11 Wireless Networks Standards for wireless networking for many different broadcast frequencies and usage techniques
802.15 Wireless personal area networks The coexistence of wireless personal area networks \with other wireless devices in unlicensed frequency bands
802.16 Broadband wireless metropolitan area networks The atmospheric interface and related functions associated with broadband wireless connectivity, also known as WiMax
802.17 Resilient packet rings Access method, physical layer specifications, and management of shared packet-based transmission on resilient rings
8702.20 Mobile broadband wireless network Packet handling and other specifications for multi vendor, mobile high-speed wireless transmission
802.22 Wireless regional area networks (WRAN) Wireless broadcast style network to operate in the UHF / VHF frequency bands formerly used for TV channels
Posted on Saturday, January 7, 2012 1:20 PM UNISA COS 2626 Networks | Back to top


Comments on this post: Computer Networks UNISA – Chap 2 - Networking Standards and the OSI Models

# re: Computer Networks UNISA – Chap 2 - Networking Standards and the OSI Models
Requesting Gravatar...
Please could you forward me more information on this course and duration.
Left by ottilia Lindani Mathe on Jan 11, 2012 8:06 AM

# re: Computer Networks UNISA – Chap 2 - Networking Standards and the OSI Models
Requesting Gravatar...
@Ottilia - it is a course offered by Unisa... www.Unisa.ac.za, this poarticular module lasts one semester - but in reality it is the N+ course, so unless you are wanting to do an undergraduate degree I would suggest you rather do the N+ certification.
Left by Mark Pearl on Jan 11, 2012 8:56 AM

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