group19_nammaangels:internetworking

Internetworking involves connecting two or more computer networks via gateways using a common routing technology. The result is called an internetwork. Internetworking can be extremely complex because it generally involves connecting networks that use different protocols. Internetworking is accomplished with routers, bridges, and gateways.

The most notable example of internetworking is the Internet .A network of networks based on many underlying hardware technologies, but unified by an internetworking protocol standard called the Internet Protocol Suite (TCP/IP).

The original term for an Internetwork was catenet. Internetworking started as a way to connect disparate types of networking technology, but it became widespread through the developing need to connect two or more local area networks via some sort of wide area network

Two architectural models are commonly used to describe the protocols and methods used in internetworking. The Open System Interconnection (OSI) reference model was developed under the auspices of the International Organization for Standardization (ISO) and provides a rigorous description for layering protocol functions from the underlying hardware to the software interface concepts in user applications. Internetworking is implemented in Layer 3 (Network Layer) of the model.

Hence, internetworking is a very essential and plays a vital role in the network field.

Group_19:All about error detection and correction

What is error detection and correction?Why do we need it?
We always cannot expect any data to be transmitted without getting corrupted. Due to various reasons like noise, attenuation, delay distortion etc..the data might get corrupted and the receiver gets the wrong information .To avoid this we need to detect and correct the errors.

What are the various types of errors and the various ways to detect them?
There are two types of errors-single bit error(where only one bit is corrupted) and multiple errors/burst errors(where two or more bits are corrupted).
To detect errors there are a lot of schemes developed today which uses the central concept of forward error correction in which the data bits are added with some extra bits(parity bits) , such that a relation is established between the original data bits and the parity bits.Ex: repitition schemes,parity schemes,hamming code,CRC,checksum,hashing etc.This encoded data is then transmitted through the network and the receiver checks for the errors by establishing the same relationship between the received data word and the parity bits as it had before getting transmitted.

How can errors be corrected?
There are two ways to correct errors -one is retransmission and the other is forward error correction (FEC).
a..Retransmission -In this method,the reciever asks the sender to retransmit the data which was recieved incorrectly.This is not feasible when huge amount of data transmission is to be done as it consumes a lot of time.
b..FEC -In this method ,the user tries to detect and correct the errors using various algorithms as mentioned earlier.All these algorithms involve the concept of generating the redundant bits.This is used in case of satellite communications.
Detection of errors is always easier than correction because to correct we need to know the exact position of the data being corrupted which is a little complex.

Group_19:Question and answer session(error detection and correction)

1. Are there other algorithms which can correct more than one error? (asked by sridevi)
A. Yes there are many other algorithms like the internet checksum which is actually used in the real world to detect and correct upto 4 errors. (answered by nikita)

2. You said that error correction is more difficult than error detection.Why is that so? (asked by royston)
A. Detection is any time easier because to detect errors we dont need to know the position of the error but for correction, we need to know the position at which the error occured .If we imagine huge amount of data being transferred and if a single bit error occurs ,then it becomes really difficult to find the position of the error. (answered by neha)

3. You said that CRC can only detect errors ,then what about correction? (asked by Ali)
A. Algorithm of CRC can only detect errors.To correct them CRC is usually clubbed with other algorithms which can correct errors.There are also many other error detection schemes like repetition schemes ,parity schemes,checksum etc (answered by nikita)

Group_19:Namma Angels

Team Name : Namma Angels

—Team Members : Niketha Dalmia , Neha Sharma , Chanchal Jalan , Namita Nair , Kritika Sharma
—Group A Project Chosen : HTTP/FTP proxy server
—Group B Project Chosen : Error detection and correction
—Presentation Date of Group A Project : 28 oct 2009
—Presentation Date of Group B Project : 30 sept 2009
—Group A PPT Uploaded : no
—Group B PPT Uploaded :yes
—Group A Project Report Made: no
—Group B Project Report Made : no
—Group A Project PROBLEMS FACED and SOLVED Blog Uploaded : no
—Group B Project PROBLEMS FACED and SOLVED Blog Uploaded :yes

Group_19:Problems faced and solved

problem statement:
“The algorithm of hamming code uses a generator matrix to generate the encoded data and the one used  in the code is a 4*7 generator matrix instead of a 3*4 generator matrix , which can also generate the redundant bits required to convert a dataword into a codeword . So why not the simple 3*4 matrix ?”

problems faced:
When we traced the algorithm and started writing the code ourselves using a 3*4 generator matrix (which had the 4 columns as the data bits and the three rows as the redundant bits) to generate the redundant bits and appending these at the end of the original data,the code worked perfectly !!We could never figure out why a 4*7 was used??Since there were no problems at all (it could detect the errors as well as correct them) ,we thought that we had come up with a better and much simpler code!
But as and when we tried different combinations of encoded data by putting the parity bits anywhere (which is a property of hamming code) in between or in the beginning, the code failed!!

solution :
We then realised the importance of a 4*7 generator matrix . This is used because it directly generates the 7-bit encoded data and also gives the flexibility to add the parity bits anywhere.This generator matrix has the information about the positions of each of the parity bits and also the data bits used in generating them .By just changing the position of the redundant bit in the matrix one can obtain the encoded data however they want.It works with all kinds of combinations correctly!!

Hamming code implementation in C++

The Hamming code program written by us can be found here

The complexities of matrix operations have been encapsulated in the matrix class, and all memory items are dynamically allocated.

This makes the actual Main() program with the hamming code operations easier to read. [Documentation provided]

Hamming Code: Explained

group19_angels : http proxy server

A proxy server is one that receives requests intended for another server and that acts on the behalf of the client (as the client proxy) to obtain the requested service.Basically it acts as an intermediate between the client and the server. A proxy server is often used when the client and the server are incompatible for direct connection. It may also be used for screening purposes i.e for filtering purposes ,to enable the administrator to control access to undesirable sites. The proxy server may also be used for caching purposes which enables faster access to frequently used websites.

We will host a proxy server both in Linux/Windows and for the demonstration purpose, we will host a proxy server in our home router and computer, configure a dynamic DNS (through dyndns.org) to connect to it.

Our tool of choice is Squid:

Group_19:how does internet work

Internet is simply computers connected in a network.All these computers connected in a network are assigned a unique IP address provided by the server or the router.If we connect to the Internet through an Internet Service Provider (ISP), we are assigned with a temporary IP address for the duration of your dial-in session. If we connect to the Internet from a local area network (LAN) our computer might have a permanent IP address or it might obtain a temporary one from a DHCP (Dynamic Host Configuration Protocol) server.if we have to access a web page something of the kind www.google.com ,then it connects to the domain name service (DNS)server which keeps a track of all the computer names and their corresponding IP addresses.Here google also has a unique IP address and only after DNS server matches its IP address ,the accessor gets to see the web page he/she is looking for on his/her computer.If it does not find the address of the particular computer ,the request will be forwarded to another DNS server.

Group_19:http/ftp proxy server

Squid is a caching proxy for the Web supporting HTTP, HTTPS, FTP, and more. It reduces bandwidth and improves response times by caching and reusing frequently-requested web pages. Squid has extensive access controls and makes a great server accelerator. It runs on most available operating systems, including Windows and is licensed under the GNU GPL.

Squid optimises the data flow between client and server to improve performance and caches frequently-used content to save bandwidth. Squid can also route content requests to servers in a wide variety of ways to build cache server hierarchies which optimise network throughput.