OSI Architecture

ISO defines a common way to connect computer by the architecture called Open System Interconnection(OSI) architecture.

          Network functionality is divided into seven layers.

Organization of the layers

The 7 layers can be grouped into 3 subgroups

1.     Network Support Layers

Layers 1,2,3 - Physical, Data link and Network are the network support layers. They deal with the physical aspects of moving data from one device to another such as electrical specifications, physical addressing, transport timing and reliability.

2.     Transport Layer

Layer4, transport layer, ensures end-to-end reliable data transmission on a single link.

3.     User Support Layers

Layers 5,6,7 – Session, presentation and application are the user support layers. They allow interoperability among unrelated software systems

An Data exchange using the OSI model

Functions of the Layers

1.     Physical Layer

The physical layer coordinates the functions required to transmit a bit stream over a physical medium.

The physical layer is concerned with the following:

Ø  Physical characteristics of interfaces and media - The physical layer defines the characteristics of the interface between the devices and the transmission medium.

Ø  Representation of bits - To transmit the stream of bits, it must be encoded to signals. The physical layer defines the type of encoding.

Ø  Data Rate or Transmission rate - The number of bits sent each second – is also defined by the physical layer.

Ø  Synchronization of bits - The sender and receiver must be synchronized at the bit level. Their clocks must be synchronized.

Ø  Line Configuration - In a point-to-point configuration, two devices are connected together through a dedicated link. In a multipoint configuration, a link is shared between several devices.

Ø  Physical Topology - The physical topology defines how devices are connected to make a network. Devices can be connected using a mesh, bus, star or ring topology.

Ø  Transmission Mode - The physical layer also defines the direction of transmission between two devices: simplex, half-duplex or full-duplex.

2. Data Link Layer

          It is responsible for transmitting frames from one node to next node.

The other responsibilities of this layer are

Ø  Framing - Divides the stream of bits received into data units called frames.

Ø  Physical addressing – If frames are to be distributed to different systems on the n/w , data link layer adds a header to the frame to define the sender and receiver.

Ø  Flow control- If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender ,the Data link layer imposes a flow ctrl mechanism.

Ø  Error control- Used for detecting and retransmitting damaged or lost frames and to prevent duplication of frames. This is achieved through a trailer added at the end of the frame.

Ø  Access control -Used to determine which device has control over the link at any given time.

3. NETWORK LAYER

This layer is responsible for the delivery of packets from source to destination.

It is mainly required, when it is necessary to send information from one network to another.

The other responsibilities of this layer are

Ø  Logical addressing - If a packet passes the n/w boundary, we need another addressing system for source and destination called logical address.

Ø  Routing – The devices which connects various networks called routers are responsible for delivering packets to final destination.

4. TRANSPORT LAYER

Ø  It is responsible for Process to Process delivery.

Ø  It also ensures whether the message arrives in order or not.

The other responsibilities of this layer are

Ø  Port addressing - The header in this must therefore include a address called port address. This layer gets the entire message to the correct process on that computer.

Ø  Segmentation and reassembly - The message is divided into segments and each segment is assigned a sequence number. These numbers are arranged correctly on the arrival side by this layer.

Ø  Connection control - This can either be connectionless or connection-oriented. The connectionless treats each segment as a individual packet and delivers to the destination. The connection-oriented makes connection on the destination side before the delivery. After the delivery the termination will be terminated.

Ø  Flow and error control - Similar to data link layer, but process to process take place.

5.SESSION LAYER

This layer establishes, manages and terminates connections between applications.

The other responsibilities of this layer are

Ø  Dialog control - This session allows two systems to enter into a dialog either in half duplex or full duplex.

Ø  Synchronization-This allows to add checkpoints into a stream of data.

6.PRESENTATION LAYER

 It is concerned with the syntax and semantics of information

exchanged between two systems.

The other responsibilities of this layer are

Ø  Translation – Different computers use different encoding system, this layer is responsible for interoperability between these different encoding methods. It will change the message into some common format.

Ø  Encryption and decryption-It means that sender transforms the original information to another form and sends the resulting message   over the n/w. and vice versa.

Ø  Compression and expansion-Compression reduces the number of bits contained in the information particularly in text, audio and video.

7             APPLICATION LAYER

This layer enables the user to access the n/w. This allows the user to log on to remote user.

The other responsibilities of this layer are

Ø  FTAM(file transfer,access,mgmt) - Allows user to access files  in a remote host.

Ø  Mail services - Provides email forwarding and storage.

Ø  Directory services - Provides database sources to access information about various sources and objects.

The interaction between layers in the OSI model

Summary of layers

PHYSICAL LINKS

Guided Media

Guided media conduct signals from one device to another include Twisted-pair cable, Coaxial Cable and Fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium.

Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a glass cable that accepts and transports signals in the form of light.

Twisted Pair Cable

A twisted pair consists of two conductors (normally copper) each with its own plastic insulation, twisted together.

Ø  One of the wires is used to carry signals to the receiver

Ø  Other is used as ground reference

Interference and cross talk may affect both the wires and create unwanted signals, if the two wires are parallel.

By twisting the pair, a balance is maintained. Suppose in one twist one wire is closer to noise and the other is farther in the next twist the reverse is true. Twisting makes it probable that both wires are equally affected by external influences.

Twisted Pair Cable comes into two forms:

Ø  Unshielded 

Ø  Shielded

Unshielded versus shielded Twisted-Pair Cable

Ø  Shielded Twisted-Pair (STP) Cable has a metal foil or braided-mesh covering that encases each pair of insulated conductors.

Ø  Metal casing improves that quality of cable by preventing the penetration of noise or cross talk.

Ø  It is more expensive. The following figure shows the difference between UTP and STP

Applications

Ø  Twisted Pair cables are used in telephone lines to provide voice and data channels.

Ø  Local area networks also use twisted pair cables.

Connectors

The most common UTP connector is RJ45.

Coaxial Cable

Coaxial cable (coax) carries signals of higher frequency ranges than twisted pair cable.

Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, and with outer conductor of metal foil.

The outer metallic wrapping serves both as a shield against noise and as the second conductor and the whole cable is protected by a plastic cover.

Categories of coaxial cables

Category	Impedance	Use
RG-59	75	Cable TV
RG-58	50	Thin Ethernet
RG-11	50	Thick Ethernet

Applications

Ø  It is used in analog and digital telephone networks

Ø  It is also used in Cable TV networks

Ø  It is used in Ethernet LAN

Connectors

Ø  BNC connector – to connect the end of the cable to a device

Ø  BNC T -  to branch out network connection to computer

Ø  BNC terminator - at the end of the cable to prevent the reflection of the signal.