Why Ethernet is Like a Polite Dinner Party

Ethernet is a local area network (LAN) technology, with networks traditionally operating within a single building and connecting devices in close proximity. At most, Ethernet devices could have only a few hundred meters of cable between them, making it impractical to connect geographically dispersed locations. However, modern advancements have increased these distances considerably, allowing Ethernet networks to span tens of kilometers.

In networking, the term protocol refers to a set of rules that govern communications. Protocols are to computers what language is to humans. Since this blog is written in English, to understand it you must be able to read English. Similarly, for two devices on a network to successfully communicate, they must both understand the same protocols.

The term Ethernet refers to the family of local-area network (LAN) protocols covered by the IEEE 802.3 standard that defines what is commonly known as the CSMA/CD protocol. Four data rates are currently defined for operation over optical fiber and twisted-pair cables:

  • 10 Mbps—10Base-T Ethernet
  • 100 Mbps—Fast Ethernet
  • 1000 Mbps
  • 10,000Mbps—Gigabit Ethernet

Ethernet is currently used for approximately 85% of the world’s LAN-connected PCs and workstations. Ethernet is the major LAN technology because of the following characteristics:

  • Is easy to understand, implement, manage, and maintain
  • Allows low-cost network implementations
  • Provides extensive topological flexibility for network installation
  • Guarantees successful interconnection and operation of standards-compliant products, regardless of manufacturer

The Ethernet protocol follows a simple set of rules that govern its basic operation. To help understand these rules, it is important to understand the basics of Ethernet terminology.

  • Medium – Ethernet devices attach to a common medium that provides a path along which the electronic signals will travel. Historically, this medium has been coaxial copper cable, but today it is more commonly a twisted pair or fiber optic cabling.
  • Segment – We refer to a single shared medium as an Ethernet segment.
  • Node – Devices that attach to that segment are stations or nodes.
  • Frame – The nodes communicate in short messages called frames, which are variably sized chunks of information.

Frames are analogous to sentences in human language. In English, we have rules for constructing our sentences: We know that each sentence must contain a subject and a predicate. The Ethernet protocol specifies a set of rules for constructing frames. There are explicit minimum and maximum lengths for frames, and a set of required pieces of information that must appear in the frame. Each frame must include, for example, both a destination address and a source address, which identify the recipient and the sender of the message. The address uniquely identifies the node, just as a name identifies a particular person. No two Ethernet devices should ever have the same address.

One of the absolute rules of the Ethernet protocol is that only one signal is allowed to be placed on a segment at a time. To ensure this rule is followed, Ethernet devices regulate communication among nodes using Carrier Sense Multiple Access/Collision Detection (CSMA/CD). While the term may seem intimidating, if we break it apart into its component concepts we will see that it describes rules very similar to those that people use in polite conversation. To help illustrate the operation of Ethernet, we will use an analogy of a dinner table conversation.

Let’s envision our Ethernet segment as a dinner table, and let several people engaged in polite conversation at the table represent the nodes. The term multiple access covers what we discussed above: When one Ethernet station transmits, all the stations on the medium hear the transmission, just as when one person at the table talks, everyone present is able to hear him or her.

Now let’s imagine that you are at the table and you have something you would like to say. At the moment, however, I am talking. Since this is a polite conversation, rather than immediately speak up and interrupt, you would wait until I finished talking before making your statement. This is the same concept described in the Ethernet protocol as carrier sense. Before a station transmits, it “listens” to the medium to determine if another station is transmitting. If the medium is quiet, the station recognizes that this is an appropriate time to transmit.

Carrier-sense multiple access gives us a good start in regulating our conversation, but there is one scenario we still need to address. Let’s go back to our dinner table analogy and imagine that there is a momentary lull in the conversation. You and I both have something we would like to add, and we both “sense the carrier” based on the silence, so we begin speaking at approximately the same time. In Ethernet terminology, a collision occurs when we both spoke at once.

In our conversation, we can handle this situation gracefully. We both hear the other speak at the same time we are speaking, so we can stop to give the other person a chance to go on. Ethernet nodes also listen to the medium while they transmit to ensure that they are the only station transmitting at that time. If the stations hear their own transmission returning in a garbled form, as would happen if some other station had begun to transmit its own message at the same time, then they know that a collision occurred.

A single Ethernet segment is sometimes called a collision domain because no two stations on the segment can transmit at the same time without causing a collision. When stations detect a collision, they cease transmission, wait a random amount of time, and attempt to transmit when they again detect silence on the medium.

This random pause and retry is an important part of the protocol. If two stations collide when transmitting once, then both will need to transmit again. At the next appropriate chance to transmit, both stations involved with the previous collision will have data ready to transmit. If they transmitted again at the first opportunity, they would most likely collide again and again indefinitely. Instead, the random delay makes it unlikely that any two stations will collide more than a few times in a row.

In my next post, we will examine some of the limitations of Ethernet that need to be overcome.

Author: David Stahl

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