Group06_Bandwidth Brothers : Go Back N ARQ Pt.1

To improve the efficiency of transmission, multiple frames must be in transition while waiting for acknowledgment. In other words, we need to let more than one frame be outstanding(explained in the third paragraph) to keep the channel busy while the sender is waiting for acknowledgment. We have two protocols for performing the above operation, one of them being Go Back N Automatic Repeat Request(ARQ). In this protocol we can send several frames before receiving acknowledgments; we keep a copy of these frames until the acknowledgments arrive.

Sequence Numbers
Frames from a sending station are numbered sequentially. However, because we need to include the sequence number of each frame in the header, we need to set a limit. If the header of the frame allows m bits for the sequence number, the sequence numbers range from 0 to [(2^m) - 1]. For example, if m is 4, the sequence numbers are 0 through 15 inclusive. However, we can repeat the sequence. In other words we can say that the sequence numbers are modulo(2m).

Sliding Window
In this protocol we use an abstract concept called as Sliding Window, that defines the range of sequence numbers that concerns the sender and receiver. The range which concerns the sender is called the send sliding window; the range that concerns the receiver is called the receive sliding window.The maximum size of the window is [(2^m) - 1]. The window at any time divides the possible sequence numbers into four regions. The first region, from the far left to the left wall of the window, defines the sequence numbers belonging to frames that are already acknowledged. The second region, defines the range of sequence numbers belonging to the frames that are sent and have an unknown status. The sender needs to wait to find out if these frames have been received or were lost. These are known as Outstanding Frames. The third region defines the range of sequence numbers for frames that can be sent. Finally, the fourth region defines sequence numbers that cannot be used until the window slides. We use three variables to define its size and location at any given time. We call these variables S(f)(send window, the first outstanding frame), S(n) (send window, the next frame to be sent), and S(size) (send window, size). The variable S(f) defines the sequence number of the first outstanding frame. The variable S(n) holds the sequence number that will be assigned to the next frame to be sent. Finally, the variable S(size) defines the size of the window. The receive window makes sure that the correct data frames are received and that the correct acknowledgments are sent. The size of the receive window is always 1. The receiver is always looking for the arrival of a specific frame. Any frame arriving out of order is discarded and needs to be resent.

There can be a timer for each frame that is sent. We have used only one timer in our code, reason being the timer for the first outstanding frame always expires first and thus we send all outstanding frames when this timer expires.

Go Back N ARQ V/s Stop and Wait ARQ
We can say that the Stop and Wait ARQ Protocol is actually a Go Back N ARQ in which there are only two sequence numbers and the send window size is 1. In other words, m = 1, [(2^m) - 1] = 1. In Go Back N ARQ, we said that the addition is mod(2m); in Stop and Wait ARQ it is 2, which is the same as 2m when m = 1.

Group06_Bandwidth Brothers : Video

Since many raised questions about repeaters, we thought of sharing this video with the class. The above video shows the application of repeaters, which made the 100 metered range bluetooth signal travel a mile.

Group06_Bandwidth Brothers : Grp A project problem-faced-and-solved blog

The main problem we faced during our demonstration was :

One pc running Windows XP did not recognize the other pc running Windows Vista when connected by a cross-crimped cable.

SOLUTION : Both the computers were supposed to have the same Network Work-group Name, which Windows XP by default will set it to MS-HOME and Windows Vista will set it to WORKGROUP. So we changed the work-group name in the Windows XP machine to WORKGROUP and configured the Ethernet adapter in both the computers manually.

Group06_Bandwidth Brothers : Mandatory Blog

Team Name : Bandwidth Brothers

Team Members : Hitesh M.P (IS034), Janardhan.C (IS035), Mandeep R.J (IS140), Vikram Singh (IS142)

Group A Project Chosen : Transmission Media

Group B Project Chosen :Go Back and Wait ARQ

Presentation Date of Group A Project : Completed, gave on September 9 2009

Presentation Date of Group B Project : Not Completed, as on October 1 2009

Group A PPT Uploaded : Yes

Group B PPT Uploaded : No

Group A Project Report Made : No

Group B Project Report Made : No

Group A Project problems-faced-and-solved blog uploaded : Yes

Group B Project problems-faced-and-solved blog uploaded : No

Transmission Media PPT

You can find the ppt here

Group06_Bandwidth Brothers: Understanding Computer Networks

1.Networking is a practice of linking various devices together with necessary hardware and software so as to facilitate communications between them. If the devices are computers, then it is called computer networking. Mostly data is transmitted or received between them. The technologies used to transmit can be broadly classified as Wired and Wireless. Wired includes popular technologies such as Telephone Cables, Ethernet Cables, Optical Fibres etc. Wireless includes Bluetooth, Infrared, Wifi, Wimax, 3G, gsm, gprs etc. Most popular type of computer networks are LAN,WAN,VPN and Internet. However there are other computer networks such as CAN,MAN,etc. Finally we need basic hardware equipments such as Network Interface Cards (NICs), bridges, hubs, switches and routers in order to facilitate computer networking. Data is transmitted between the computers through wires.

2.Internet is considered as “baap” of all computer networks. In this network, all the computers participating have a unique address(such as the unique home address) known as IP (Internet Protocol) Adressses. This uniquely naming all the computers using numbers or hexadecimal chareters forms the basic stage in TCP/IP (the language with which the computers in Internet talk). It all happens when you order an internet connection to your home. The ISP (Airtel or BSNL) will provide you with the basic CPE(Consumer Premise Equipment) which will help you to connect to the Internet. When you type in a web address and hit enter in the browser, your request gets processed and sent into the DNS server which maintains the lookup tables for each web address. This lookup tables will then search for your entry and then the IP address will be retreived and the DNS server will then send a request to that web address. The information received from the web address will be forwarded to the client who requested that web address page. This is the general procedure followed while retreiving a world wide web address. There are many other protocols which use internet as the transmitting medium such a IRC, HTTP, FTP, Bittorrent etc.

Group06_Bandwidth Brothers: Transmission Media

Transmission media is the state which propagates energy waves. The state may be solid,liquid or gas. Our team will stress more on transmission media in networking as it is will be more useful for our subject. In our talk we would like to show visually the media type categories if possible and talk about it. The three main media type categories according to us are copper cable ( like coaxial cable), wireless ( such as microwaves used in LAN,infrared) and finally fibre optics (most expensive among the three). According to us none of them can be considered “the best” cause each has different use in different situation. So according to the need or situation we have to choose the best media. Also suppose if one fails we must have another to back upon, cause if events such as earthquake occurs if one media fails to work the other must be ready to use.We will be focussing mainly on the five traditionally used transmission media and explain everything related to it in detail. We would also rate the performance of they types of transmission media after thorough analysis. We would also like to download applications of transmission media and give a small demonstration. The basic understanding of transmission media will provide a good base for students studying computer communication. So to make everyone understand the basics of transmission media would be our team’s main aim. The five main transmission media which we will be focussing is twisted pair(used for voice applications), coaxial cable, microwave, satellite and finally fibre.