ccna present cost

ccna present cost.Exam Essentials Flow control aims to avoid the source device from sending more information than the target device can handle.Ready/not ready signalingWhen using the ready/not-ready signal, the target device sends a not-ready signal to the source device to stop transmitting data when it receives more traffic than it can process. The target device sends the ready signal to the source device when it is able to process the data sent by the source device. Once the ready signal is received, the source device can continue to send data.There are two problems with using the ready/not ready signal to implement flow control. First, when the buffer is full, the target device will send the not-ready signal to the source device. The source device will continue to send information to the target device while the message is traveling to the source device, and the target device will have to discard the information because the buffer is full. Second, after the target device is ready to receive more information, it must first send the ready signal to the source device, which will have to wait for this signal to be received before continuing to send information. This increases the delay in message transmission. Given these two inefficiencies of the ready/not ready signal, it is usually not used to implement flow control. This process is sometimes referred to as stop/start, where transmission is stopped for a period of time and then restarted.Windowing TechniquesThe windowing technique is a much more complex method of traffic control than the ready/not-ready signal. When using windowing techniques, a window size is defined which specifies how much data (often called a data segment at the transport layer) the source device must wait for the target device to 22 Part 1 Introduction to NetworkingAcknowledgement (ACK) O receive an ACK, the source device can then send the next batch of data (up to the value defined by the window size).The windowing technique implements two functions. One is window size-based traffic control. In many protocol implementations, the window size is pre-negotiated dynamically and can be renegotiated over the lifetime of the connection. This ensures that the optimal window size that does not allow the target device to discard any information can be used. Second, with the windowing technique, the target device is able to indicate to the source device what information it has received. This lets the source device know if any data was lost on the way to the target device and in turn retransmits the lost messages. This gives reliability to the connection, while being more efficient than when using ready/not ready signals. Given these advantages, most connection-oriented transport protocols (such as TCP in the TCP/IP stack) use windowing techniques to implement flow control.The window size chosen for a connection determines how many data segments (or bytes) the source device must wait for after sending ACK, which affects the efficiency and throughput of the connection. Figure 2.2 illustrates the importance of window size. In the top half of the figure, the connection uses a window size of lo In this case, the source device sends a data segment containing a sequence number (here 1) and then waits for the target device to acknowledge it. The way in which the target device sends the ACK varies with the transport protocol: it may send a list of sequence numbers indicating which data segments were received, or it may send back a sequence number indicating which segment it expects to receive next. Here, the ACK sent by the target device contains the number 2, which tells the source device that it can then send data segment 2. Again, since the window size is 1, the target device will use an acknowledgement to answer immediately upon receipt of this data segment, indicating that it was received. In this example, the acknowledgement sent back by the target device contains the number 3, indicating that it is ready to continue with data segment 3. And so on.As you can see, with a window size of 1, the flow control process is not very efficient. Here is an example with a window size of 3, as shown in the bottom half of Figure 2.2. With a window size of 3, the source device can send three data segments at once, and then after waiting for ACKo to send these segments (each with a unique sequence number: 1, 2, and 3), the source device must wait for an acknowledgement. Here, the target device sends back an ACK containing the number 4, indicating that it expects to receive the 4th data segment next. Next, the source device sends data segments 4, 5, and 6, and waits for an acknowledgement from the target device. In this example, the window used is larger and therefore more efficient: an acknowledgement is required only after every 3 data segments have been sent. Thus, the larger the window, the more efficient the message transfer.