Comparison of OSI and TCP/IP model

Both are based on the concept of a stack of independent protocols. Also,the functionality of layers is roughly similar.
3 concepts are central to OSI model:

1.Services-performs some services for the layer above it,tells what layer does.It defines the layer's semantics.

2.Interfaces-tells processes above it how to access it.

3.Protocols-layer's own business.

The protocols in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively easily as the technology changes.
Data Link Layer originally dealt with point-to-point networks. The committee originally expected that each country would have one network, run by govt. and using OSI protocols, so no thought was given to internetworking.
With TCP/IP the reverse was true:the protocols came first and the model was just a description of existing protocols. The only trouble was that the model did not fit any other protocol sticks.
An obvious difference between the 2 models is the number of layers:the OSI model has 7 layers and the TCP/IP has 4 layers.Both have (inter)network, transport and application layers,but other layers are different.
The OSI model supports both connectionless and connection oriented communication in the network layer.The TCP/IP model has only one mode in the network layer (connectionless)but supports both modes in the transport layer,giving the users a choice.

TCP/IP model-Application Layer and other layers and Features TCP/IP

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Application Layer

On top of the transport layer is the application layer.It contains all the higher level protocols. The early ones included virtual terminal (TELNET), FTP,and electronic mail(SMTP),Domain Name System (DNS),NNTP,HTTP.

Host-to-Network Layer
Here the host has to connect to n/w using some protocol so it can send IP packets to it.



Features of TCP/IP

1)File Transfer-FTP,RCP(Remote Copy)

2)Terminal Enuilation-Telnet & rlogin.

3)Transparent distributed file access and sharing-NFS(Network File System)

4)Remote command execution-rsh(remote shell) & rexec(remote execution) programs, users can run programs on remote computers and see the results on their own computer.

5)Remote Printing-unix command lpr provides remote printing services.



Next coming up Comparison of OSI and TCP/IP Reference Models

The Transport Layer(TCP/IP model)

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The layer above the internet layer is the transport layer.It is designed to allow peer entities on source and destination hosts to carry on a conversation just as in OSI transport layer. Two end to end transport protocols have been defined here.
The first one TCP(Transmission Control Protocol) is a reliable connection oriented protocol that allows a byte stream originating on 1 machine to be delivered without error on any other machine on internet.
TCP also handle flow control to make sure a fast server cannot swamp a slow receiver.
The second protocol,UDP,(User Datagram Protocol) is an unreliable,connectionless protocol for applications that do not want TCP's sequencing or flow control and wish to provide their own.It is widely used for one shot,client-server type request-reply queries and applications in which prompt delivery is more important than accurate delivery such as transmitting speech or video.


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TCP/IP Reference Model

This model used in the grandparent of all wide area computer n/ws,the ARPANET, and its successor,the worldwide Internet.
DoD did not want conversation to be broken off. DoD wanted connection to remain intact as long as source and destination were functioning. A flexible architecture was needed.The architecture later became known as TCP/IP Reference model,after its 2 primary protocols.

The Internet Layer

Packet switching n/w based on a connectionless internetwork layer.This layer called the Internet Layer.
The analogy is with the (snail) mail system. Most of them will be delivered to the correct address in the destination country.
The Internet layer defines an official packet format and Protocol called IP(Internet Protocol). The job of internet layer is to deliver IP packets where they are supposed to go.Packet routing is the major issue,as is avoiding congestion. TCP/IP internet layer is similar in functionality to OSI n/w layer.

OSI model-Session Layer,Presentation Layer,Application Layer

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5)The Session Layer
This layer allows users on different machines to establish sessions between them.
Session offer various services including dialog control,token management and synchronization.

6)The Presentation Layer
This layer is concerned with the syntax and semantics of the information transmitted. The presentation layer manages these abstract data structures and allows higher level data structures (eg.banking records) to be defined and exchanged.

7)Application Layer
This layer contains a variety of protocols that are commonly needed by users.One widely used application protocol is HTTP,which is the basis for the World Wide Web.


Next coming up TCP/IP Model..

OSI model-Network Layer,Transport Layer

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Network Layer
This layer controls the operation of the subnet,how packets are routed from source to destination.
If too many packets are present, such congestion is controlled by network layer.
n/w layer allows heterogeneous n/ws to be interconnected.

Transport Layer
The basic function of the transport layer is to accept data from above,split it up into smaller units if need be,pass these to n/w layer and ensure that pieces all arrive correctly at the other end.
The transport layer is a true end-to-end layer,air the way from source to destination.
The difference between layer1 through 3,which are chained and layers4 through 7,which are end-to-end.

To be continued..

OSI model

The OSI model has 7 layers:

The Physical Layer
Physical layer is concerned with transmitting raw bits over a communication channel. The design issues here largely deal with mechanical,electrical & timing interfaces and physical transmission medium.

2.The Data Link Layer
This layer transform a raw transmission facility into a line that appears free of undetected transmission errors to the network layer.In the data link layer a fast transmitter from drowning a slow receiver in data.Frequently, this flow regulation and error handling are integrated. In the data link layer, in broadcast n/w how to control access to the shared channel. A special sublayer of the data link layer, medium Access control sublayer, deals with this problem.

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Reference Models

2 important n/w architectures:
The OSI reference model
& TCP/IP reference model

Although the protocols associated with the OSI model are rarely used any more,the model itself is general and still valid,and the features at each layer are still very important.
The TCP/IP model, the model itself is not of much use but the protocol are widely used.

OSI Model
OSI model is based on ISO as a first step toward international standardization of the protocols used in various layers.It deals with connecting open system-systems that are open for communication with other systems. The OSI model has 7 layers.

To be continued..

Hub(Networking)

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Hubs/repeaters are used to connect together 2 or more Ethernet segments of Amy media type.In larger designs,signal quality begins to deteriorate as segments exceed their maximum length. Hubs provide signal amplification required to allow a segment to be extended a greater distance. A hub takes any incoming signal and repeats it out -all ports.
    Ethernet hubs are necessary in star topologies as IOBaseT.A very important fact to note about hub us that they only allow users to share Ethernet.
Hubs are mandatory in IOBaseT twisted pair Ethernet as well as Token Ring n/ws.They are also used to replace the daisy chain cabling in 10Base 5 and 10Base2 coaxial Ethernets in order to improve n/w management.

Continued
  In Token Rings,the hub is called a MAU(multi-station Access Unit).
   There are 3 types of hubs:
Passive hub: simply combines the signals of n/w segments. There is no signal processing of regeneration.

Active hub: are like passive hub except that they have electronic components that regenerate or amplify signals.

Intelligent hub: In addition to signal regeneration, intelligent hubs perform some n/w management and intelligent path selection.

Gateways

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Gateways operate at the top layers of OSI model.You select a gateway when you have to interconnect systems built on totally different communications architectures. You would use a gateway to interconnect a TCP/IP LAN to an SNA mainframe, eg.,the 2 architectures have no commonalities, so the gateway must translate the data passing between the 2 systems.
    Gateway is a computer that performs protocol conversion between different types of n/ws or applications. Eg.,a gateway can convert a TCP/IP packet to a NetWare IPX packet and vice versa,or from AppleTalk to DEC net,from SNA to AppleTalk and so on.
   Gateways function at layer 4 and above in OSI model.
An electronic mail,or messaging, gateway converts messages between 2 different messaging protocols.

Next: Hub

Routers

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Router is a device that forwards data packets from 1 LAN or WAN  to another. Based on routing tables and routing protocols, routers read the n/w address in each transmitted frame and make a decision on how to send it based on the mostexpidient route(traffic load,line costs,speed,bad lines etc).Routers work at layer 3 whereas bridges and switches work at layer 2.
Multiprotocol routers support several protocols as IP,IPX,AppleTalk and DECnet.
Router functions can also be implemented by adding routing s/w to a file server.NetWare,Operating System includes routing s/w.
In older Novell terminology,a router is a n/w layer bridge.Routers also used to be called gateways.

Brouter-is a router that can also bridge.If the brouter does not support the protocol of the packet it is using or cannot deliver the packet based on protocol information, it bridges the packet using the physical address.


Next: Gateways

Bridges in depth

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Bridges are also called store and forward devices because they look at the whole Ethernet packet before making filtering or forwarding decisions. If the segment are same,the packet is dropped (filtered).If the segments are different then the packet is forwarded to the correct segment.
A Spanning Tree Algorithm is a software standard for describing how switches and bridges can communicate to avoid n/w loops.
    We cannot use bridge to connect LANs of different type and protocols because each n/w type uses different physical addressing.

Transparent Bridge-in which the host stations are unaware of their existence in the n/w.
Ethernet uses this type of bridge ,also called as adaptive bridge.

More about Bridge..

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Bridge cont'd

Bridge is a device that connects 2 LAN segments together, which may be of similar or dissimilar types, such as Ethernet and Token Ring.
    A bridge is inserted into a n/w to segment it and keep traffic contained within the segments to improve performance.
   Bridges learn from experience and build and maintain address tables.By monitoring which station acknowledge receipt of address, they learn which nodes belong to the segment.
    Bridges work at the data link layer(MAC) whereas routers work at the n/w layer.
    Bridges are protocol independent, routers are protocol dependent.
Bridges are faster than routers.Bridges with more than 2 ports perform a switching function.

Next:Bridges continued

Repeater,Bridge

Repeater

Repeater is a communications device that amplifies the data signal in order to extend the transmission distance. Repeaters work at layer 1 of the OSI model.

Bridges

Bridges connect 2 n/ws that use the same technology (eg. Ethernet and ARCNET).
The use of bridge increases maximum possible size of n/w.
Unlike a repeater which simply passes all signals it receives ,a bridge selectively determine the appropriate segment to which it should pass a signal. It does this by reading the address of all the signals it receives.The bridge reads the physical location of the source and destination computer from this address.

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Network Connecting Devices

Previous:Wireless N/ws

Network Connecting Devices

1)Network Interface Card
2)Repeaters
3)Bridges
4)Routers
5)Gateways

Network Interface Card(NIC)
It is also known as Network adapter.It is a printed circuit board that plugs into both the clients and servers and controls the exchange of data between them.
The network adapter provides service at the data link level of the n/w,which is also known as the access method (OSI layers 1 and 2).The most common n/w adapters are Ethernet and Token Ring.The Ethernet adapter ,sometimes is built into the motherboard eg.Local Talk.
A transmission medium ,interconnects all the adapters in the network.

Wireless Networks cont'd

Previous: Wireless LANs

3)Wireless WANs
The radio n/w used for cellular telephones is an example of a low bandwidth wireless system.This system has gone through 3 generations-
1)First generation-analog & voice only
2)Second generation-digital & for voice only
3)Third generation-digital & is for both voice & data

Wireless LANs can operate at 50 Mbps.
Cellular system operate at about 1 Mbps.
Wireless internet service is often called local multipoint distribution service eg.mts
Almost all wireless n/ws hook up to the wired n/w at some point to provide access to files,databases & the internet.
Many n/ws exist in the world,often with different h/w & s/w.
The fulfillment of this desire requires that different & frequently incompatible n/ws be connected by means of machines called gateways to make the connection and provide the necessary translation, both in terms of h/w and s/w called internetwork or internet.
If the system within the gray area contains only routers it is a subnet.
If it contains both routers & hosts it is a WAN.
As an analogy,the telephone system consists of telephone switching offices connected to one another by high speed lines & to houses & business by low speed lines.
These lines and equipment,owned and managed by telephone company form the subnet of telephone system.

Next:Network Connecting Devices

Wireless LANs

Previous: Wireless Networks

2)In Wireless LANs,if the systems are close enough, they can communicate directly with one another in a peer to peer configuration. Wireless LANs are common in small offices and homes.There is a standard for wireless LANs called IEEE 802.11
Wireless LANs are systems in which every computer has a radio modem and antenna with which it can communicate with other system.

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Wireless Networks

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Wireless n/w can be divided into 3 main categories-

1)System interconnection
2)Wireless LANs
3)Wireless WANs

System interconnection
It is all about interconnecting the components of a computer using short-range radio.
Some companies got together to design a short-range wireless n/w called Bluetooth to connect these components without wires.
Bluetooth also allows digital cameras,headsets,scanners and other devices to connect to a computer by merely being brought within range.
System interconnection n/w use the master slave paradigm. The system unit is normally the master,talking to the mouse,keyboard as slaves.
The master tells the slaves what addresses to use,when they can broadcast ,how long they can transmit, what frequencies they can use etc.


Next: Wireless LANs

Wide Area Network cont'd

Previous:WAN

The collection of communication lines and routers form the subnet.When a packet is sent from 1 router to another via 1 or more intermediate routers,the packet is received at each intermediate router in its entirety, stored until required output line is free and then forwarded.A subnet organized according to this principal is called a store and forward packet switched subnet.
When packet are small and all the same size,they are often called cells.
Packet travelling bearing number in the sequence.
Not all WANs are packet switched. A second possibility for a WAN is a satellite system.
Each router has an antenna through which it can send and receive.All routers can hear the output from satellites.


Next: Wireless Network

Wide Area Network or WAN

WAN spans a large geographical area,often a country or continent. The job of the subnet is to carry messages from host to host,just as telephone system carries words from speaker to listener.
    In most wide area n/ws,the subnet consists of 2 distinct components : transmission lines & switching elements.
Transmission lines move bits between machines. They can be made of copper wire,optical fiber or even radio links.
When data arrive on an incoming line,the switching element must choose an outgoing line on which to forward them.
Switching element is also called router.


Next:WAN contn'd