Ethernet LANs behave slightly differently depending on whether the LAN has mostly modern devices, in particular, LAN switches rather than 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 during this chapter looks at these basic differences.
Sending in Modern Ethernet LANs Using Full Duplex Modern Ethernet LANs use a variety of Ethernet physical standards, but with standard Ethernet frames which will flow over any of those sorts of physical links. Each individual link can run at a special speed, but each link allows the attached nodes to send the bits within the frame to the next node. they need to 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 2-17 shows an example during which PC1 sends an Ethernet frame to PC2.
Following the steps within the figure:
– PC1 builds and sends the original Ethernet frame, using its own MAC address because 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 2-17 uses full duplex on each link, but the concept could be difficult to see. Full-duplex means that the NIC or switch port has no half-duplex restrictions. So, to know full duplex, you would like to know half duplex, as follows:
Half duplex: The device must wait to send if it’s currently receiving a frame; in other words, it cannot send and receive at the same time.
Full duplex: The device doesn’t need 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 an equivalent instant in time. for instance , in Figure 2-17, PC1 and PC2 could send frames to each other simultaneously, in both directions, without any half-duplex restrictions.
Using Half Duplex with LAN Hubs
To understand the need for half-duplex logic in some cases, you’ve got to know a little about an older sort of networking device called a LAN hub. When the IEEE first introduced 10BASE-T in 1990, the Ethernet didn’t yet include LAN switches. rather than switches, vendors created LAN hubs. The LAN hub provided a number of RJ-45 ports as an area to connect links to PCs, just like a LAN switch, but it used different rules for forwarding data.
LAN hubs forward data using physical layer standards, and are therefore considered to be Layer 1 devices. When an electrical signal comes in one hub port, the hub repeats that electrical signal out all other ports (except the incoming port). By doing so, the data reaches all the rest of the nodes connected to the hub, therefore the data hopefully reaches the correct destination. The hub has no concept of Ethernet frames, of addresses, and so on. The downside of using LAN hubs is that if two or more devices transmitted a signal at the same instant, the electrical signal collides and becomes garbled. The hub repeats all received electrical signals, albeit it receives multiple signals at the same time. for instance , Figure 2-18 shows the idea, with PCs Archie and Bob sending an electrical signal at the same instant of your time (at Steps 1A and 1B) and therefore the hub repeating both electrical signals out toward Larry on the left (Step 2).
If you replace the hub in Figure 2-18 with a LAN switch, the switch prevents the collision on the left. The switch operates as a Layer 2 device, meaning that it’s at the data-link header and trailer. A switch would check out the MAC addresses, and even if the switch needed to forward both frames to Larry on the left, the switch would send one frame and queue the opposite frame until the first frame was finished.
Now back to the difficulty created by the hub’s logic collisions. to stop these collisions, the Ethernet nodes must use half-duplex logic rather than full-duplex logic. a problem occurs only two or more devices send at the same time; half-duplex logic tells the nodes that if somebody else is sending, wait before sending.
For example, back in Figure 2-18, imagine that Archie began sending his frame early enough so that Bob received the first bits of that frame before Bob tried to send his own frame. Bob, at Step 1B, would notice that he was receiving a frame from somebody else , and using half-duplex logic, would simply wait to send the frame listed at Step 1B.
Nodes that use half-duplex logic actually use a relatively well-known algorithm called carrier sense multiple access with collision detection (CSMA/CD). The algorithm takes care of the obvious cases but also the cases caused by unfortunate timing. for instance , two nodes could check for an incoming frame at the exact same instant, both realize that no other node is sending, and both send their frames at the exact same instant, causing a collision. CSMA/CD covers these cases also , as follows:
Step 1. a device with a frame to send listens until the Ethernet isn’t busy.
Step 2. When the Ethernet isn’t busy, the sender begins sending the frame.
Step 3. The sender listens while sending to discover whether a collision occurs; collisions could be caused by many reasons, including unfortunate timing. If a collision occurs, all currently sending nodes do the following:
A. They send a jamming signal that tells all nodes that a collision happened.
B. They independently choose a random time to wait before trying again, to avoid unfortunate timing.
C. the next attempt starts again at Step 1.
Although latest LANs don’t often use hubs, and thus don’t need to use half duplex, enough old hubs still exist in enterprise networks so that you need to be able to understand duplex issues. Each NIC and switch port has a duplex setting. For all links between PCs and switches, or between switches, use full duplex. However, for any link connected to a LAN hub, the connected LAN switch and NIC port should use half-duplex. Note that the hub itself doesn’t use half-duplex logic, instead just repeating incoming signals out every other port. Figure 2-19 shows an example, with full-duplex links on the left and one LAN hub on the right. The hub then requires SW2’s F0/2 interface to use half-duplex logic, along with the PCs connected to the hub.
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