CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [401]
Although a topology describes the method by which systems in a network connect, the topology alone doesn’t describe all of the features necessary to make a cabling system work. The term bus topology, for example, describes a network that consists of some number of machines connected to the network via the same piece of cable. Notice that this definition leaves a lot of questions unanswered. What is the cable made of? How long can it be? How do the machines decide which machine should send data at a specific moment? A network based on a bus topology can answer these questions in a number of different ways.
Most techs make a clear distinction between the logical topology of a network—how the network is laid out on paper, with nice straight lines and boxes—and the physical topology. The physical topology describes the typically messy computer network, with cables running diagonally through the ceiling space or snaking their way through walls. If someone describes the topology of a particular network, make sure you understand whether they’re talking about the logical or physical topology.
Over the years, manufacturers and standards bodies created several specific network technologies based on different topologies. A network technology is a practical application of a topology and other critical technologies to provide a method to get data from one computer to another on a network.
Essentials
Packets/Frames and NICs
Data is moved from one PC to another in discrete chunks called packets or frames. The terms packet and frame are interchangeable. Every NIC in the world has a built-in identifier, a binary address unique to that single network card, called a media access control (MAC) address. You read that right—every network card in the world has its own unique MAC address! The MAC address is 48 bits long, providing more than 281 trillion MAC addresses, so there are plenty of MAC addresses to go around. MAC addresses may be binary, but we represent them by using 12 hexadecimal characters. These MAC addresses are burned into every NIC, and some NIC makers print the MAC address on the card. Figure 23-3 shows the System Information utility description of a NIC, with the MAC address highlighted.
* * *
NOTE Even though MAC addresses are embedded into the NIC, some NICs allow you to change the MAC address on the NIC. This is rarely done.
Hey! I thought we were talking about packets? Well, we are, but you need to understand MAC addresses to understand packets. The many varieties of packets share certain common features (Figure 23-4). First, packets contain the MAC address of the network card to which the data is being sent. Second, they have the MAC address of the network card that sent the data. Third is the data itself (at this point, we have no idea what the data is—certain software handles that question), which can vary in size depending on the type of frame. Finally, some type of data check—such as a cyclic redundancy check
Figure 23-3 MAC address
Figure 23-4 Generic packet/frame
(CRC)—is performed and information is stored in the packet to enable the receiving network card to verify if the data was received in good order.
This discussion of packets raises the question, how big is the packet? Or more specifically, how much data do you put into each packet? How do you ensure that the receiving PC understands the way the data was broken down by the sending machine and can thus put the pieces back together? The problem in answering these questions is that they encompass so many items. When the first networks were created, everything from the frames to the connectors to the type of cable had to be invented from scratch.
To make a successful network, you need the sending and receiving PCs to use the same hardware protocol. A hardware protocol defines many aspects of a network, from the topology, to the packet type, to the cabling