Originally developed by Xerox in the 1970’s, Ethernet has become the defacto technology standard for LANs today. Digital, Intel, and Xerox (DIX) standardized Ethernet in 1980, with the IEEE version finalized in 1982 with the 802.3 standard. Over time Ethernet has undergone a number of changes, both with respect to how devices are connected, and the ways in which data is framed. A solid understanding of Ethernet concepts is imperative to your success on the CCNA and CCDA exams.
Media Access – CSMA/CD
The media access method used by Ethernet is the contention-based Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This name not only defines the technology, but also describes how it works. “Carrier Sense” means that different devices are listening to the media for the opportunity to transmit. “Multiple Access” describes the media as being contention-based, in that it is shared amongst many computers. “Collision Detection” is an Ethernet feature whereby systems are capable of recognizing when a collision has occurred.
When a system uses collision detection techniques, there must be a way to try and avoid the same collision from happening repeatedly. CSMA/CD handles this by having systems back off for a random period of time after a collision occurs. If these collisions continue the system back off time will increase, considerably decreasing performance. Retransmission will be attempted up to 16 times before an error message will be passed to the upper-layer protocol in use. You may be familiar with some of the distance limitations imposed on Ethernet networks (we’ll look at those shortly). Understand, however, that the limitations exist not only because of signal attenuation, but also because as distances increase, the ability of CSMA/CD to properly detect collisions decreases. This is especially true when systems at opposite ends of a network attempt to communicate at the same time, sensing the media as available.
Note: Remember that CSMA/CD is the media access method used on Ethernet networks. As such, hosts on a traditional Ethernet network “share” the media, making their transmissions susceptible to collisions. Equipment like switches and bridges help to reduce network collisions, and will be looked at in more detail in Chapter 3.
All Ethernet network adapter cards are uniquely identified by a pre-assigned hardware (or MAC) address. A MAC address is a 48-bit address represented in hexadecimal format. The first 24 bits represent what is known as the Organizationally Unique Identifier (OUI), and represents a vendor code. The last 24 bits are assigned by the vendor and act as the unique identifier (and serial number) for a particular network card.
Tip: Remember that the first 24 bits of a MAC address identify a manufacturer like Cisco, and the last 24 bits represent the unique serial number of a card. For a complete list of manufacturer OUI codes, see http://standards.ieee.org/regauth/oui/oui.txt
Hexadecimal is a numbering system that uses the numbers 0-9 and the letters A-F, where each hex digit represents 4 bits. As such, a MAC address will always be made up of 12 hex digits, in a format similar to 01-22-33-44-55-EF. The table below outlines the decimal value associates with each hexadecimal digit. Note that the highest valid hex digit is F – anything above that is not a valid character for a MAC address.
Hexadecimal to decimal conversions:
Tip: You can easily convert between decimal and hexadecimal by using the scientific calculator included with Microsoft Windows operating systems. A great online tool for converting numbers between decimal, hexadecimal, and binary easily can be found at http://www.onlineconversion.com/base.htm.
Note: During both the CCNA and CCDA exams, you will not have access to any type of calculator. As such, it is extremely important for you to be able to convert between decimal, hexadecimal, and binary numbers on your own.