Physical Layer - Network Cables

In this modern world we make use of network in everyday life. But do you know how the network cables looks like or how many types of network cables are there?

Here we go...

We have many different types of cables they are :-

1. UTP - Unshielded twisted pair cable

2. Coaxial cable design

3. STP - Shielded twisted pair cable

We have copper media connectors such as shown below :-

First is 110 punch block, second RJ45 UTP Plugs which are for the Local area network and the third RJ45 UTP socket.

We need to take care while using copper cables. These are the safety measures to follow while dealing with copper cables.

HTTP Protocol

 HTTP : HyperText Transfer Protocol.

It is a web layer protocol. It is based on client server model.

Client : Browser that requests, receives, “displays” Web objects.

Server : Web server sends objects in response to requests.

See the figure below :-

We have two versions of http protocol one is HTTP 1.0 and 

HTTP 1.1

HTTP  is stateless which means it does not  maintain any information.

For example it does not maintain any information like how many times clients request the same object.

We have two types of HTTP messages :-

1. Request Message 

2. Response Message

Request message contains a request line and header lines :-

Response message  contains a status line and a header lines and the data :-

So this is how the browser works when we type the URL.

Living in a Network Centric World

Being in a network centric  world has got a lot of advantages than one can imagine.
See the figure below you will get a rough picture of network world:-

Network has an impact on daily life.
1. Instant messaging - A real time communication between two or more people based on typed text.

2. Weblogs - Web pages created by an individual.

3.  Pod casting - Websites that contain audio files for downloading.

The way we work also changes with the network.



With network we can have entertainment such as :-

This is how our daily life goes with the network and it developing everyday.

DNS - Domain Name System

Domain Name System (DNS) translates hostnames to IP addresses, it fulfills a critical role in the Internet infrastructure.
What happens is client sends query  to its local DNS server  and receives a response back.
Figure below explains the working of DNS :-

Lets look at nslookup.


nslookup is the most basic operation which allows the host to query any specified DNS server for a DNS record, which means nslookup sends a DNS query to the specified DNS server ,receives a DNS reply from that same DNS server , and displays the results.


See the figure below:-

Lets look at ipconfig.


ipconfig for Windows is the most useful utilities in your host , especially in network issues.
ipconfig can be used to show your current TCP/IP information , including your address , DNS server addresses , adapater type and so on.
To do ipconfig you need to go to command in the start menu and then type ipconfig.
ipconfig/all gives the complete information.


See the figure below:-
This is ipconfig :-

This is ipconfig/all :-

P2P File Sharing

Has any one thought of how does peer to peer file sharing takes place here we go...

We have many peer to peer file sharing sites such as torrents which accounts for more traffic than any other application including the web on the internet.

A peer is nothing but a PC connected to the centralized server.

There may be as many peers as possible. Let first tell you how does it works.

Let us take an example of a person X who searches for a particular file and when get it file transfer takes place from a person say Y who has the required file.

See the figure below:-

Here is the working :-

1) Lets say a person X wants a file named "Book on Web".

2) Now what X will do is he will connect to the internet and check in the centralized server as to which peer having this file is available.

3) Now X can see many peers having this particular file so X can from a particular peer so that X can download at good speed.

4) If X request from the peers say Y, a direct  TCP connection is established  between X's PC and Y's PC and the file is sent from Y to X.

Now lets see what happen when peer Y disconnects from the internet?

When Y disconnects when the download or the file transfer is taking place then the remainder of the file is obtained from another peer that has the file.

The Conditional GET

Web caching can reduce response times,but we have a problem that is the copy of an object residing in the cache may be stale. Which i mean to say object housed in the Web cache may have been modified since the copy was cached at the client. Hence we have the conditional GET.

Conditional GET works as follows:-

1) The request message uses the GET method.

2) The request message includes an IF-Modified-Since: header line.

You may ask how does the conditional get looks like,

Look at the example below

HTTP/1.1 200 OK

Date: Mon, 7 Jul 2008 18:12:15

Server: Apache/1.4

Last-Modified: Wed, 6 Jun 2008 10:29:46

Content-Type: image/gif

(data,data,data, data,data,data...) 

Importantly, the cache also stores the last-modified date along with the object.

There are chances that object may have been modified at the web server so cache will perform an up -to-date check by issuing a conditional GET.

If any modifications are there then the updated object is sent in the response message otherwise unmodified object is sent in the response message.
  

Web Caching

Web Cache also called a proxy server. The Server has its own storage and keeps copies of recently requested objects in this storage.


Figure below shows an example.

Note that a cache is both a server and a client at the same time.
Advantages of web cache are:-
1) Web cache can substantially reduce the response time for the client request.
2) Web cache can substantially reduce traffic on an institutional's access link to the Internet.
     By reducing traffic ,the institution does not have to upgrade bandwidth , there by reducing                       
     costs.


Usually Web Cache is installed by Internet Service provider. 


Figure below shows cache added to the institutional network.

These are some of the features of Web Caching.
Please refer Conditional GET post for more information regarding  if the requested contents in the cache are up to date.