Group16_QuadMasters:Selective Repeat ARQ

Selective Repeat ARQ is a specific instance of the Automatic Repeat-reQuest (ARQ) Protocol. It may be utilized as a protocol for the delivery and acknowledgement of message units, or it may be utilized as a protocol for the delivery of subdivided message sub-units.

Performance of Selective Repeat ARQ :

Efficiency of Selective Repeat ARQ:

It is the most efficient of all the ARQ protocols

that have been discussed.

The probability of a frame being received in good order is

1 – Pf

Thus the average number of times a frame has to be sent

is the reciprocal of the probability, i.e., 1/(1 – Pf)

the  average transmission is :      tSR  =Tf/ 1-Pf

When used as the protocol for the delivery of subdivided messages it works somewhat differently. In non-continuous channels where messages may be variable in length, standard ARQ or Hybrid ARQ protocols may treat the message as a single unit. Alternately selective retransmission may be employed in conjunction with the basic ARQ mechanism where the message is first subdivided into sub-blocks (typically of fixed length). The original variable length message is thus represented as a concatenation of a variable number of sub-blocks. While in standard ARQ the message as a whole is either acknowledged (ACKed) or negatively acknowledged (NAKed), in ARQ with selective transmission the NAKed response would additionally carry a bit flag indicating the identity of each sub-block successfully received. In ARQ with selective retransmission of sub-divided messages each retransmission diminishes in length, needing to only contain the sub-blocks that were NAKed.

In most channel models with variable length messages, the probability of error-free reception diminishes in inverse proportion with increasing message length. In other words it’s easier to receive a short message than a longer message. Therefore standard ARQ techniques involving variable length messages have increased difficulty delivering longer messages, as each repeat is the full length. Selective retransmission applied to variable length messages completely eliminates the difficulty in delivering longer messages, as successfully delivered sub-blocks are retained after each transmission, and the number of outstanding sub-blocks in following transmissions diminishes.

Group16_QuadMasters:Selective Repeat ARQ

Selective Repeat ARQ is a specific instance of the Automatic Repeat-request (ARQ) Protocol. It may be utilized as a protocol for the delivery and acknowledgment of message units, or it may be utilized as a protocol for the delivery of subdivided message sub-units. When used as the protocol for the delivery of messages, the sending process continues to send a number of frames specified by a window size even after a frame loss. Unlike Go-Back-N ARQ, the receiving process will continue to accept and acknowledge frames sent after an initial error.

The receiver process keeps track of the sequence number of the earliest frame it has not received, and sends that number with every ACK it sends. If a frame from the sender does not reach the receiver, the sender continues to send subsequent frames until it has emptied its window. The receiver continues to fill its receiving window with the subsequent frames, replying each time with an ACK containing the sequence number of the earliest missing frame. Once the sender has sent all the frames in its window, it re-sends the frame number given by the ACKs, and then continues where it left off. The size of the sending and receiving windows must be equal, and half the maximum sequence number (assuming that sequence numbers are numbered from 0 to n-1) to avoid miscommunication in all cases of packets being dropped. The sender moves its window for every packet that is acknowledged.

• Transmitter’s rule of action
1) Keeps transmitting frames continuously.
2) If any frame is not acknowledged Tout after the frame is transmitted (or receives NACK), assume that the frame is lost and retransmit only the lost frame.

• Receiver’s rule of action
1) Does the same action as in the Stop-and-Wait ARQ, but never discard all frames delivered without errors and has the capability to reorder them in the correct order.
2) Sequence numbers are required to identify different frames.

Group16_QuadMasters:Remote Login

Advantages/Applications of Remote Login:

1) One of the most common uses of desktop sharing is for real-time collaboration. Real-time collaboration is when two or more coworkers, classmates, colleagues or friends use desktop sharing software to simultaneously work on the same project, share files and work through problems as a team.

2) Desktop sharing allows two or more users to work on the same files even when they’re not in the same location. One of the most useful applications of real-time collaboration is for remote technical support.

3) A helpful feature of many desktop sharing programs is real-time annotating of documents. Whoever is in control of the host computer has access to the drawing and annotating tools, whether it’s a remote user or the person sitting at the host computer. Common annotating tools are a highlighter, a freehand drawing tool and a pointer.

Disadvantages of Remote Login:

The major disadvantage of the Remote Login would be security problems. A microcomputer connected to a broadcast LAN can easily eavesdrop using snooper software to capture a login name and the corresponding password (even if it is encrypted). To overcome these disadvantages we use more secure tools like Putty, SSH, Ultra VNC, Team Viewer etc for remote login.

Group16_QuadMasters:Remote Login

Remote Access to the Ubuntu Linux Desktop

Ubuntu Linux provides remote desktop access. This provides two extremely useful features. Firstly it enables you or another person to view and interact with your desktop environment from another computer system either on the same network or over the internet. This is useful if you need to work on your computer when you are away from your desk while traveling etc. It is also useful in situations where a co-worker or IT support technician needs access to your desktop to resolve a problem.

Secondly, in addition to providing access to your primary desktop it also enables you to create multiple desktops and connect to them remotely.

The Ubuntu remote desktop functionality is based on technology called Virtual Network Computing (VNC) and in this chapter we will cover the key aspects of configuring and using remote desktops with Ubuntu Linux. It is important to note that there are both secure and insecure ways to access a remote desktop. Although both approaches will be covered, the secure method is strongly recommended when accessing remote desktops over an internet or other insecure connection.

Activating Remote Desktop Access:

The first step in setting up remote desktop access is to activate it and define some basic security settings. These settings are configured in the Remote Desktop Preferences dialog. To access this dialog, open the desktop System menu, select Preferences and click on Remote Desktop.

The Remote Desktop Preferences dialog consists of two panels. The General settings panel is displayed by default and provides the following configuration options:

1) Allow others to view your desktop – Activates remote desktop access for viewing purposes.

2) Allow other users to control your desktop – Allows users accessing your remote desktop to control the desktop. In other words the remote user can do anything to your desktop that they want using their mouse and keyboard as if they were sitting physically at the local system. When this option is disabled, the remote user can see what is happening on the desktop, but is unable to interact with it using the keyboard and mouse.

3) Ask for confirmation – When selected, this option causes a dialog to appear warning you of an attempt by a remote user to connect and prompting you to confirm or deny the connection. If you are likely to want to log in remotely you will need to turn this off since you will not be at the local system to accept your own connection.

4) Require the user to enter this password – Specifies a password which must be entered by the remote user to access your desktop. It is strongly advised that you select this option and specify a password.

Finally this screen specifies the command to run on the remote system to access the desktop.