Ethernet LANs behave slightly differently depending on whether the LAN has mostly modern devices, in particular, LAN switches instead of some older LAN devices called LAN hubs. Basically, the use of more modern switches allows the use of full-duplex logic, which is much faster and simpler than half-duplex logic, which is required when using hubs. The final topic in this chapter looks at these basic differences.
Modern Ethernet LANs use a variety of Ethernet physical standards, but with standard Ethernet frames that can flow over any of these types of physical links. Each individual link can run at a different speed, but each link allows the attached nodes to send the bits in the frame to the next node. They must work together to deliver the data from the sending Ethernet node to the destination node.
The process is relatively simple, on purpose; the simplicity lets each device send a large number of frames per second. Figure 2-21 shows an example in which PC1 sends an Ethernet frame to PC2.
Figure 2-21 Example of Sending Data in a Modern Ethernet LAN
Following the steps in the figure:
PC1 builds and sends the original Ethernet frame, using its own MAC address as the source address and PC2’s MAC address as the destination address.
Switch SW1 receives and forwards the Ethernet frame out its G0/1 interface (short for Gigabit interface 0/1) to SW2.
Switch SW2 receives and forwards the Ethernet frame out its F0/2 interface (short for Fast Ethernet interface 0/2) to PC2.
PC2 receives the frame, recognizes the destination MAC address as its own, and processes the frame.
The Ethernet network in Figure 2-21 uses full duplex on each link, but the concept might be difficult to see.
Full duplex means that the NIC or switch port has no half-duplex restrictions. So, to understand full duplex, you need to understand half duplex, as follows:
Half duplex: The device must wait to send if it is currently receiving a frame; in other words, it cannot send and receive at the same time.
Full duplex: The device does not have to wait before sending; it can send and receive at the same time.
So, with all PCs and LAN switches, and no LAN hubs, all the nodes can use full duplex. All nodes can send and receive on their port at the same instant in time. For example, in Figure 2-21, PC1 and PC2 could send frames to each other simultaneously, in both directions, without any half-duplex restrictions.
To understand the need for half-duplex logic in some cases, you have to understand a little about an older type of networking device called a LAN hub. When the IEEE first introduced 10BASE-T in 1990, Ethernet switches did not exist yet; instead, networks used a device called a LAN hub. Like a switch, a LAN hub provided a number of RJ-45 ports as a place to connect links to PCs; however, hubs used different rules for forwarding data.
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