NETWORKING CONCEPTS

OSI MODEL Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s.

OSI MODEL Open System: Set of protocols that allow only 2 different systems to communicate regardless of underlying architecture OSI model is not a protocol,but a model for designing n/w architecture Purpose is to show how to facilitate communication b/w different systems without requiring changes to the logic of the underlying h/w or s/w OSI model is a layered framework for design of network systems Consist of 7 layers

OSI MODEL LAYERS

The interaction between layers in the OSI model

OSI MODEL Fig shows the layers involved when a message is sent from A to B As message travels from A to B it may pass through many nodes and intermediate nodes usually involve only first 3 layers of OSI model With a single machine each layer calls on services of layer just below it. Ie layer 3 provides services to layer 4 and uses services of layer2

OSI MODEL Between m/c layer x on one m/c communicates with layer x on other m/c and that communication is governed by agreed rules called protocols Processes on each machine that communicate at a given layer are called peer to peer processes

OSI MODEL Peer to Peer Process At physical layer communication is direct In fig device A sends stream of bits to device B(through intermediate nodes) At higher layer communication must move down through the layers through the layer on device A over to device B and then back up through layers Each layer in sending device adds its own information to message it receives from the layer just above it and passes whole package to layer below it

OSI MODEL Peer to Peer Process At layer 1 the entire package is converted to a form that can be transmitted to the receiving device At the receiving device msg is unwrapped layer by layer with each process receiving and removing the data meant for it Eg. Layer 2 removes the data meant for it and then passes the rest to layer 3.Layer 3 removes the data meant for it and passes the rest to layer 4 and so on

OSI MODEL Interface between layers Defines the information and services a layer must provide for the layer above it Organization of Layers 7 layers belong to 3 subgroups Layer 1,2,3 are n/w support layer deal with physical aspects of moving data from one device to other Eg.electrical specifications,physical connector etc

OSI MODEL Organization of Layers Layer 5,6,7.ie session presentation,application are user support layers and allow interoperability among unrelated s/w systems Layer 4-Transport layer This links two subgroups and ensures that lower layer transmission is in a form that the upper layer can use

OSI MODEL Organization of Layers Upper OSI layers are commonly implemented in s/w but lower layers are combination of h/w and s/w except physical which is mostly h/w

EXCHANGE USING OSI MODEL

D7 means data unit at layer 7 The process starts at application layer and moves from layer to layer in descending sequential order At each layer header or trailer is added to data unit Commonly trailer is added at layer 2 After data unit passes through layer 1 and it changed to some electromagnetic signal and is transported along a physical link

EXCHANGE USING OSI MODEL On destination side signal passes to layer 1 and is transformed back into digital form The data units then move backup through the OSI layers As each block reaches next high layer, header and trailer attached to it at the corresponding sending layer are removed actions appropriate to the layer are taken

EXCHANGE USING OSI MODEL When it reaches layer 7 the message is again in a form appropriate to the application and is made available to the recipient

EXCHANGE USING OSI MODEL Encapsulation A packet (header+data) at level 7 is encapsulated at level6 ie data portion at level N-1 carries whole packet from level N.This is encapsulation

EXCHANGE USING OSI MODEL Encapsulation A packet (header+data) at level 7 is encapsulated at level6 ie data portion at level N-1 carries whole packet from level N.This is encapsulation

LAYERS OF OSI MODEL Physical Layer

LAYERS OF OSI MODEL Physical Layer Deals with mechanical and electrical specifications of interface and transmission medium Responsible for movements of individual bits from one node to next

Physical Layer Concerned with Physical characteristics of interfaces and medium It defines type of medium Representation of bits Physical layer data consist of stream of bits For transmission these must be converted to electrical or optical signal Physical layer defines type of encoding(how bits are changed to signal)

Physical Layer Data rate No of bits /sec is also defined by this layer Synchronization of bits Sender and receiver must use same bit rate Line configuration Physical layer concerned with connection of devices to media Point-point configuration:2 devices connected through dedicated line Multipoint configuration:a link shared among multiple devices

Physical Layer Physical topology This defines how devices are connected to make a n/w These may be mesh,star,ring,bus etc Transmission mode Physical layer defines direction of transmission between 2 devices that may be Simplex Half duplex Full duplex

Data Link Layer

DLL Responsible for moving frames between one hop to next Responsibilities are 1.Framing DLL divides stream of bits received from n/w layer to manageable data units called frames 2.Physical addressing If frame are to be distributed to different systems on n/w the data link layer adds a header to frame to define the sender and /or receiver of the frame

Data Link Layer 2.Physical addressing if the frame is intended for a system outside the sender’s network, the receiver address is the address of the device that connects to the n/w to the next one 3. Flow control If rate at which the data is absorbed by the receiver is less than rate at which it is produced at sender the dll imposes a flow control mechanism to avoid overwhelming the receiver

Data Link Layer 4.Error Control Dll adds mechanism for to detect and retransmit damaged frame or lost frame Also adds one mechanism to recognize duplicate frames Error control is achieved by adding trailer to end of fame 5. Access control When 2 or more devices are connected to same link dll protocols determine which device has control over link at any time

Data Link Layer 4.Error Control Dll adds mechanism for to detect and retransmit damaged frame or lost frame Also adds one mechanism to recognize duplicate frames Error control is achieved by adding trailer to end of fame 5. Access control When 2 or more devices are connected to same link dll protocols determine which device has control over link at any time

Hop-to-hop delivery

Communication at all layer occurs between 2 adjacent nodes To send data from A-F, 3 partial deliveries are made 1.DLL layer at A send frame to DLL layer at B 2.DLL layer at B send frame to DLL layer at E 3.DLL layer at E send frame to DLL layer at F Frame from A-B has A is source address B is destination address Frame from B-E has B is source address E is destination address Frame from E-F has E is source address F is destination address