Saturday, July 25, 2015

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.

Saturday, July 18, 2015

OSI model-Session Layer,Presentation Layer,Application Layer


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..

Wednesday, July 15, 2015

OSI model-Network Layer,Transport Layer


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..

Tuesday, July 14, 2015

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.


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..

Tuesday, July 7, 2015

WEIRD Things are Happening RIGHT NOW!

Underwater Waterfall

 A drastic dip in the ocean’s depth near Mauritius creates the Underwater Waterfall illusion. 

Monday, July 6, 2015



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.

  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.

Saturday, July 4, 2015



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

Thursday, July 2, 2015



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

Wednesday, July 1, 2015

Bridges in depth


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.