Group11_DNAA:Presentation of Part B project.

group11_Dnaa:Stop and Wait ARQ

Stop-and-wait ARQ is the simplest kind of automatic repeat-request (ARQ) method. A stop-and-wait ARQ sender sends one frame at a time. After sending each frame, the sender doesn’t send any further frames until it receives an ACK (acknowledgement) signal. After receiving a good frame, the receiver sends an ACK. If the ACK does not reach the sender before a certain time, known as the timeout, the sender sends the same frame again.

Typically the transmitter adds a redundancy check number to the end of each frame. The receiver uses the redundancy check number to check for possible damage. If the receiver sees that the frame is good, it sends an ACK. If the receiver sees that the frame is damaged, the receiver discards it and does not send an ACK — pretending that the frame was completely lost, not merely damaged.

One problem is where the ACK sent by the receiver is damaged or lost. In this case, the sender doesn’t receive the ACK, times out, and sends the frame again. Now the receiver has two copies of the same frame, and doesn’t know if the second one is a duplicate frame or the next frame of the sequence carrying identical data.

Another problem is when the transmission medium has such a long latency that the sender’s timeout runs out before the frame reaches the receiver. In this case the sender resends the same packet. Eventually the receiver gets two copies of the same frame, and sends an ACK for each one. The sender, waiting for a single ACK, receives two ACKs, which may cause problems if it assumes that the second ACK is for the next frame in the sequence.

To avoid these problems, the most common solution is to define a 1 bit sequence number in the header of the frame. This sequence number alternates (from 0 to 1) in subsequent frames. When the receiver sends an ACK, it includes the sequence number of the next packet it expects. This way, the receiver can detect duplicated frames by checking if the frame sequence numbers alternate. If two subsequent frames have the same sequence number, they are duplicates, and the second frame is discarded. Similarly, if two subsequent ACKs reference the same sequence number, they are acknowledging the same frame.

group11_dnaa:Interactions blog

1) What is auto negotiation?

• Auto negotiation is an Ethernet procedure by which two connected devices choose common transmission parameters, such as speed and duplex mode. In this process, the connected devices first share their capabilities as for these parameters and then choose the fastest transmission mode they both support.
- Answered by Archana.H.V.

2) Why is the whole message not packed in a single frame and sent?

• If the frame is very large flow and error control will be inefficient .When a message is carried in one very large frame, even a single-bit error would require the retransmission of the whole message. When a message is divided into smaller frame, a single-bit error affects only that small frame.
- Answered by Dhanusha.

3) Explain hop-to-hop delivery by data link layer.
• Hop-to-hop deliver is also called node to node delivery. A node can be either a system or a router connected to the network. Hop-to-hop delivery involves delivery of frames from one node to another node.
- Answered by Nishita.M.

4) If data link layer is reliable and has flow and error control, do we need this at the transport layer also?
• Yes, because reliability at data link layer is between two nodes but we need reliability between two ends. Since network layer in the internet is unreliable, we have to implement reliability at transport layer.
- Answered by Ananya.B.Naik.

5) How is data transmitted in physical layer?
• Data is transmitted in the form of analog signals.
- Answered by Archana.H.V.

6) What is the need for connectionless transfer?
• Connectionless transfer is required when data transfer is simple. It’s needed where reliability is not a major issue and speed is of high importance. E.g. online games.
- Answered by Ananya.B.Naik.

7) In routers which part is responsible for determining the path of the packets?
• Routing algorithms are responsible for determining the path of the packets.
- Answered by Mam.

group11_dnaa: group A problem faced and solved

The main problem faced by us during our demonstration was the first tool that we tried to implement for demonstration was a single interface tool called Wireshark wherein it showed the different protocols operating only on a single system and works only in windows operating system.
This problem was solved after finding out another tool called ether ape that operates only on Linux operating system. It is a multiple interface tool showing all the protocols connected to the nearby LAN.

group11_dnaa:Mandatory blog

Team Name: DNAA
Team Members: Archana.H.V (IS012), Nishita.M (IS061), Ananya.B.Naik (IS007), Dhanusha (IS025)
Group A Project Chosen: Protocol stack and addressing
Group B Project Chosen: Stop and Wait ARQ
Presentation Date of Group A Project: Completed, gave on September 16 2009
Presentation Date of Group B Project: Not Completed, as on November 04 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

group11_DNAA: Protocol stack and addressing video

group11_dnaa:Stop and Wait ARQ

Stop and wait protocol is found in data link layer and helps in dataflow control.

In data link layer there are two types of channels:
Noiseless channel: these are error free channels.
Noisy channel: These are error creating channels.

Stop and wait protocol is noiseless channel protocol. It is unidirectional in the sense that the data frames travel from one node, called sender to another node, called the receiver. Although special frames, called acknowledgement and negative acknowledgement can flow in opposite direction for flow and error control purpose, data flow in only one direction.

The data link layer at the sender site gets the data from its network layer, makes a frame out of the data, and sends it. The data link layer at the receiver site receives a frame from its physical layer, extracts the data from the frame, and delivers the data to its network layer. The sender site cannot send a frame until its network layer has a data packet to send. The receiver site cannot deliver a data packet to its network layer until a frame arrives. If data frames arrive at the receiver site faster than they can be processed, the frames must be stored until their use. Normally the receiver does not have enough storage space especially if it is receiving data from many sources. This may result in either the denial or discarding of frames. To prevent the receiver from becoming overwhelmed with frames stop and wait protocol is used. Now sender sends one frame, stops until it receives confirmation from the receiver, and then sends the next frame.

A stop and wait protocol transmits a Protocol Data Unit (PDU) of information and then waits for a response. The receiver receives each PDU and sends an Acknowledgement (ACK) PDU if a data PDU is received correctly, and a Negative Acknowledgement (NACK) PDU if the data was not received.
Under normal transmission the sender will receive an ACK for the data and then commence transmission of the next data block. For a long delay link, the sender may have to wait for this response. While it is waiting the sender is said to be in the “stop” state and is unable to send further data.

When PDUs are lost, the receiver will not normally be able to identify the loss. The transmitter must then rely upon a timer to detect the lack of a response.

group11_DNAA:protocol stack and addressing

Group11_DNAA:Protocol stack and addressing

Transport layer

The transport layer is fourth and middle layer of the OSI Reference Model protocol stack. This layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. It can keep track of the segments and retransmit those that fail.

The transport layer acts as a link between the applications at the higher layers, and the concrete functions of the bottom layers. Its overall job is to provide the necessary functions to enable communication between software application processes on different computers.The transport layer is responsible for providing a means by which all different applications can all send and receive data using the same lower-layer protocol implementation. Thus, the transport layer is said to be responsible for end-to-end or host-to-host transport.
For transmission, the transport layer protocol must keep track of what data comes from each application, then combine this data into a single flow of data to send to the lower layers. The device receiving information must reverse these operations, splitting data and funneling it to the appropriate recipient processes. The transport layer is also responsible for defining the means by which potentially large amounts of application data are divided into smaller blocks for transmission.

The transport layer has three main responsibilities in terms of the exchange of data between systems. These include:

• Data segmentation.

Data segmentation is the process by which the Transport layer uniquely handles all data passed to and from different upper-level applications. This is usually implemented in the form of source and destination port numbers that are defined within a particular application.

• Establishment of end-to-end connections between hosts.

Connection oriented sessions: When a connection-oriented session is established between systems, acknowledgements are returned to the sender as proof that segments reached their destination. If an acknowledgement is not received, the associated data will be resent.

There are three main phases to a connection-oriented session. These include:

• Call Setup. When a connection is being established, a path known as a virtual circuit is created between the sender and receiver.
• Data Transfer. Once the path is created, data is transmitted sequentially to the receiver.
• Call Termination. When an established connection is no longer required, the virtual circuit is terminated.

Connectionless sessions: Connectionless sessions communicate without receipt acknowledgements or sequence numbers. Connectionless protocols don’t have any reliability mechanisms built in, since they’re mainly built for speed.

• Flow control

Using flow-control mechanisms to ensure that data is sent at rates that the receiver can handle.In network environments, systems use a portion of memory referred to as buffer space to hold data that has been received more quickly than they can process it. However, once this buffer space fills, systems run the risk of dropping data that they can’t find room for. To account for this, the Transport layer on the receiving machine will pass status information to the sender, asking it to stop sending segments if its buffers become full. Once buffer space becomes available, another message is passed to the sender allowing it to resume transmission.

Examples of transport layer protocols:

• Transmission Control Protocol (TCP) Connection-oriented sessions.
• User Datagram Protocol (UDP) Connectionless sessions.