Ethernet is best understood by first considering the early DIX Ethernet specifications, called 10BASE5 and 10BASE2. These two Ethernet specifications defined the details of the physical layer of early Ethernet networks. (10BASE2 and 10BASE5 differ in the cabling details, but for the discussion included in this chapter, you can consider them as behaving identically.) With these two specifications, the network engineer installs a series of coaxial cables, connecting each device on the Ethernet network—there is no hub, switch, or wiring panel. The Ethernet consists solely of the collective Ethernet cards in the computers and the cabling. The series of cables creates an electrical bus that is shared among all devices on the Ethernet. When a computer wants to send some bits to another computer on the bus, it sends an electrical signal, and the electricity propagates to all devices on the Ethernet.
Because it is a single bus, if two or more signals were sent at the same time, the two would overlap and collide, making both signals unintelligible. So, not surprisingly, Ethernet also defined a specification for how to ensure that only one device sends traffic on the Ethernet at one time—otherwise, the Ethernet would have been unusable. The algorithm, known as the carrier sense multiple access with collision detection (CSMA/CD) algorithm, defines how the bus is accessed. In human terms, CSMA/CD is similar to what happens in a meeting room with many attendees. Some people talk much of the time. Some do not talk, but they listen. Others talk occasionally. Being humans, it's hard to understand what two people are saying at the same time, so generally, one person is talking and the rest are listening. Imagine that Bob and Larry both want to reply to the current speaker's comments. As soon as the speaker takes a breath, Bob and Larry might both try to speak. If Larry hears Bob's voice before Larry actually makes a noise, Larry might stop and let Bob speak. Or, maybe they both start at almost the same time, so they talk over each other and many others in the room can't hear what was said. Then there's the proverbial "Excuse me, you talk next," and eventually Larry or Bob talks. Or, in some cases, another person jumps in and talks while Larry and Bob are both backing off. These "rules" are based on your culture; CSMA/CD is based on Ethernet protocol specifications and achieves the same type of goal.
Figure 3-4 shows the basic logic of an old Ethernet 10BASE2 network, which literally uses a single electrical bus, created with coaxial cable and Ethernet cards.
Figure 3-4 Small Ethernet 10BASE2 Network
10BASE2, Single Bus
Solid Lines Represent* Co-Ax Cable
The solid lines in the figure represent the physical network cabling. The dashed lines with arrows represent the path that Larry's transmitted frame takes. Larry sends a signal across out his Ethernet card onto the cable, and both Bob and Archie receive the signal. The cabling creates a physical electrical bus, meaning that the transmitted signal is received by all stations on the LAN. Just like a school bus stops at everyone's house along a route, the electrical signal on a 10BASE2 or 10BASE5 network is propagated to each station on the LAN.
Because the transmitted electrical signal travels along the entire length of the bus, when two stations send at the same time, a collision occurs. The collision first occurs on the wire, and then some time elapses before the sending stations hear the collision—so technically, the stations send a few more bits before they actually notice the collision. CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this:
1. A device with a frame to send listens until the Ethernet is not busy.
2. When the Ethernet is not busy, the sender begins sending the frame.
3. The sender listens to make sure that no collision occurred.
4. Once the senders hear the collision, they each send a jamming signal, to ensure that all stations recognize the collision.
5. After the jamming is complete, each sender randomizes a timer and waits that long.
6. When each timer expires, the process starts over with Step 1.
So, all devices on the Ethernet need to use CSMA/CD to avoid collisions and to recover when inadvertent collisions occur.
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