Tag - Carrier Sense

Collision Domain

  1. CCNA exam success depends on mastering the fundamentals, and two important fundamentals are knowing exactly what the terms “collision domain” and “broadcast domain” mean. In this free Cisco tutorial, we’ll take a look at the term “collision domain” and how a collision domain is defined.
    collision domain
    A collision domain is an area in which a collision can occur. Fair enough, but what “collision” are we talking about here? We’re talking about collisions that occur on CSMA/CD segments, or Carrier Sense Multiple Access with Collision Detection. If two hosts on an Ethernet segment transmit data at exactly the same time, the data from the two hosts will collide on the shared segment. CSMA/CD exists to lessen the chances of this happening, but collisions can still occur. To lessen the chances of collisions occurring, we may decide to create multiple, smaller collision domains.Let’s say we have four hosts on a single Ethernet segment. The entire segment is a collision domain; any data sent by one of the hosts can collide with data sent by any of the other hosts. We have one collision domain containing four devices.To create smaller collision domains, we’ll need to introduce some type of networking device into this example. Hubs and repeaters have their place as far as extending the reach of a network segment and cutting down on attenuation, but these OSI Layer One devices do nothing to define collision domains. We could connect each host into a separate port on a hub (a hub is basically a multiport repeater) and we’d still have one single collision domain with four hosts in it.The most common and most effective way to create multiple collision domains is to use a switch. If we connect each of these four hosts to their own separate switch port, we would now have four separate collision domains, each with one host; each switch port actually acts as a single collision domain, making collisions between these four hosts impossible.Passing the CCNA is all about knowing the details of how things work, and knowing CSMA/CD theory and how to define collision domains is one of the many details you’ve got to master. In the next part of this CCNA tutorial, we’ll take a look at broadcast domains, and how defining broadcast domains in the right places can dramatically cut down on unnecessary traffic on your network.
  1. A collision domain is defined as a group of hosts whose data can be involved in collisions if they transmit at the same time. If three PCs are all on the same network segment, that’s a collision domain. To many newcomers to networking, it sounds like you’d rather have one large collision domain than multiple smaller ones. That’s what I thought as well when I started! What you must keep in mind is that the smaller the collision domain, the smaller the number of hosts in the domain – and the fewer hosts we have in a collision domain, the less chance we have of collisions.And wouldn’t it be great if we could create a collision domain with only one host? Well, we can, thanks to switches!Traditional switches run at Layer 2 of the OSI model, with many new switches able to operate at both the data link (L2) and network (L3) layers. Switches help to eliminate the chance of collisions because each port on the switch is actually its own little collision domain! If we take the three PCs mentioned before and connect them each to their own switch port, they literally cannot be involved in collision with each other – and in networking, we’ll take all the guarantees we can get!While switches do help lower the number of collisions, by default they’re not much help with broadcasts.

Difference between ethernet and serial Cable


Ethernet is the most widely-installed local area network ( LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox from an earlier specification called Alohanet (for the Palo Alto Research Center Aloha network) and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless


  • Ethernet
  • Networking Resources

LANs. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD ) protocol.

Fast Ethernet or 100BASE-T provides transmission speeds up to 100 megabits per second and is typically used for LAN backbone systems, supporting workstations with 10BASE-T cards. Gigabit Ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second). 10-Gigabit Ethernet provides up to 10 billion bits per second.

Ethernet was named by Robert Metcalfe, one of its developers, for the passive substance called “luminiferous (light-transmitting) ether” that was once thought to pervade the universe, carrying light throughout. Ethernet was so- named to describe the way that cabling, also a passive medium, could similarly carry data everywhere throughout the network.

Ethernet is a protocol that controls the way data is transmitted over a local area network (LAN). It uses wires (meaning it is not a wireless technology).

The wires used for a LAN are mostly those headed by an RJ-45 jack, which is similar to the jack plugged into your telephone set, but twice as big. Some Ethernet networks use coaxial cables, but that’s rarer, and present in rather large LANs, which span over areas between buildings. If you want to see what a coaxial cable is like, look at the thick cable that links your TV antenna to your TV set.

Ethernet is by far the most popular LAN protocol used today. It is so popular that if you buy a network card to install on your machine, you will get an Ethernet card, unless you ask for something different, if of course that different protocol is available.

Ethernet has evolved over the years. Today, you can get cheap Ethernet LAN cards with speeds up to 100 Mbps; while the fastest Ethernet reaches Gbps (1 Gbps = 1000 Mbps) in speed.


Serial means one event at a time. It is usually contrasted with parallel, meaning more than one event happening at a time. In data transmission, the techniques of time division and space division are used, where time separates the transmission of individual bits of information sent serially and space (on multiple lines or paths) can be used to have multiple bits sent in parallel.

In the context of computer hardware and data transmission: serial connection, operation, and media usually indicate a simpler, slower operation and parallel indicates a faster operation. This indication doesn’t always hold since a serial medium (for example, fiber optic cable) can be much faster than a slower medium that carries multiple signals in parallel.

On your PC, the printer is usually attached through a parallel interface and cable so that it will print faster. Your keyboard and mouse are one-way devices that only require a serial interface and line. Inside your computer, much of its circuitry supports bits being moved around in parallel.

Your computer modem uses one of your PC’s serial connections or COM ports. Serial communication between your PC and the modem and other serial devices adheres to the RS-232C standard.

Conventional computers and their programs operate in a serial manner, with the computer reading a program and performing its instructions one after the other. However, some of today’s computers have multiple processors and can perform instructions in parallel.

Inside this Article

  1. Introduction to How Serial Ports Work
  2. UART Needed
  3. The Serial Connection
  1. Going With The Flow
  2. Lots More Information
  3. See all Buses articles

Considered to be one of the most basic external connections to a computer, the serial port has been an integral part of most computers for more than 20 years. Although many of the newer systems have done away with the serial port completely in favor of USB connections, most modems still use the serial port, as do some printers, PDAs and digital cameras. Few computers have more than two serial ports.

Two serial ports on the back of a PC

Essentially, serial ports provide a standard connector and protocol to let you attach devices, such as modems, to your computer. In this edition of How Stuff Works, you will learn about the difference between a parallel port and a serial port, what each pin does and what flow control is.

serial port

A port, or interface, that can be used for serial communication, in which only 1 bit is transmitted at a time.


Most serial ports on personal computers conform to the RS-232C or RS-422 standards. A serial port is a general-purpose interface that can be used for almost any type of device, including modems, mice, and printers (although most printers are connected to a parallel port).

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