ccna 200-301 network simulator toolstation clearance

ccna 200-301 network simulator toolstation clearance.LOU Chapter 2 Network Interconnection and Reference Models*** I02It is because of this shortcoming of hubs that they cannot be used alone in larger networks (usually with devices such as switches that share a small portion of the network traffic load), just as there cannot be a single lane in the center of a large city, because the larger the network, the greater the chance of a network collision phenomenon. Because of this, the data transmission efficiency of the hub is relatively low, because it can only have one direction of data transmission at the same time, which is also known as "half-duplex" method. The most common devices that work in half-duplex are walkie-talkies, where you can talk but not answer when you press the talk button, and answer but not talk when you release the talk button. The telephone in our lives works in "duplex", where you can talk and listen at the same time, while the radio works in "simplex", where you can only ever answer and transmit in one direction. If a hub is to be used as a single connection device in a network, it is best to have a network size of less than 10 units and a hub bandwidth of 10/100Mb/s or more.In addition to the shared bandwidth deficiency of the hub, there is another aspect of the deficiency that must be considered, that is, its broadcast mode of operation. Because the hub belongs to the physical layer of the OSI seven-layer model, basically does not have the ability to "intelligence", let alone "learning" function. It also does not have the MAC address table of the switch, so it sends data without targeting, but in a broadcast manner. That is, when it sends data to a node, instead of sending the data directly to the destination node, it sends the packet to all the nodes connected to the hub. As shown in Figure 2-3-2 ,the source host PGA needs to send data to the destination host PC-B, PC-A sends the packet to the hub, the hub receives the packet from port 1, then copies the packet and amplifies the signal and sends it out from ports 2, 3, and 4, even though PC.C and PC-D are not the destination host, they will also receive the packet sent from PC-A to PC-B.Two concepts are introduced here: CollisionDomain and BroadcastDomain. A conflict arises when two bits are transmitted simultaneously on the same medium. The so-called collision domain is the range affected by sending data to a single destination (that is, a unicast frame; what kind of frame is a unicast frame is explained in Chapter 3), as in Figure 2-3-2, PC-A sends data to the destination host PC-B, as a result, the hub will forward the packet to all ports except the receiving port, and PC-C and PC.D also receive the data, they belong to the same The so-called broadcast domain is the range affected by sending data to an unspecified destination (that is, a broadcast frame or multicast frame; what kind of frame is a broadcast frame or multicast frame is explained in Chapter 3), as in Figure 2-3-2, PC-A sends a broadcast packet, and the hub will forward that broadcast packet to all ports except the receiving port, and all devices on the hub belong to the same broadcast domain. In conclusion, there is only one broadcast domain, one conflict domain, in all networks interconnected through hubs (no matter how many hubs there are).This broadcast type of sending data has two shortcomings: firstly, the user packets are sent to all nodes, which is likely to be insecure and some people with ulterior motives can easily intercept other people's packets; secondly, since all packets are sent to all nodes at the same time, coupled with the shared bandwidth approach described above, it is more likely to cause network congestion and reduce network execution efficiency.2.3.2 Data link layer . **1.Function of DataLink Layer The main function of the DataLink Layer is how to carry out reliable transmission of data over unreliable physical lines. The DataLink Layer accomplishes reliable data communication between neighboring nodes in the network. To ensure reliable data transmission, the sender encapsulates the user data into frames (Frames) and transmits each frame in sequence. Due to the unreliability of the physical line, the data frames sent by the sender may beDue to the unreliability of the physical line, the data frames sent by the sender may be erroneous or lost on the line, resulting in the receiver not receiving the data frames correctly. To ensure that the receiver can determine the correctness of the received data, the sender calculates a CRC (cyclic redundancy check) for each data chunk and adds the CRC to the frame so that the receiver can determine the correctness of the data reception by recalculating the CRC. Once the receiverside finds an error in the received data, the sender must retransmit the frame. However, multiple transmissions of the same frame may also result in the receiver receiving duplicate frames. For example, if the receiver's "acknowledgement frame" to the sender is corrupted, the sender will also retransmit the previous frame, and the receiver may receive a duplicate frame. The data link layer must solve the problems caused by corrupted, lost, and duplicated frames.Another problem that the data link layer has to solve is to prevent the high-speed sender from "swamping" the low-speed receiver with data. Therefore, some kind of information flow control mechanism is needed to let the sender know how much cache space is currently available on the receiver side. For ease of control, flow control is often implemented in conjunction with error handling.