Ethernet Fundamentals - Part 1

Ethernet is by far the most popular and widely used LAN protocol today. It was invented by Robert Metcalfe during the 70's and was developed hand in hand by DIX (DEC, Intel, Xerox). Robert Metcalfe would later on found 3Com Corporation, one of Cisco's competitors in the world of networking. Coincidentally, 3Com pertains to "Computers, Communication and Comapatibility" just a slight trivia there. This is so because Mr. Metcalfe is an advocate for compatibility of products around the industry.

The very first ethernet design as presented by Mr. Metcalfe.

What makes Ethernet a very successfull protocol is its simplicity and robustness in adapting to LAN evolution specially in terms of speed. Todays LANs are now able to transmit up to 10 Gbps. The following shows a table of IEEE 802.3 technologies.

Ethernet essentially is a broadcast protocol. Every host in the network has the ability to transmit anytime it wants. It's called Non-deterministic or others call it as opportunistic LAN. It's a first come first serve basis type of communication unlike in Token Ring networks or FDDI (Fiber Distributed Data Interface) wherein a token is needed by hosts in order to transmit one at a time. In Token Rings or FDDI, the hosts cannot transmit without the token. Transmission is in order, that is, hosts transmit taking each others turn. This is called deterministic LAN.

We can probably say that the Ethernet network is chaotic in nature. CSMA/CD takes the role of policing the Ethernet in order for the different nodes to transmit properly. However, the chaotic nature of Ethernet is only true to half-duplex transmission. The following illustrates the three methods of transmission.

Half-duplex Ethernet nowadays is a gonner. It's almost extinct because of the advent of better, cheaper and more intelligent switches. Plus, it is also driven by bandwidth-hungry applications all over the workplace increasing the demands on faster and more efficient transmissions.

Half-duplex transmission practically generates collisions on LAN. Susceptible to this are hosts that are connected in bus topologies or star topologies using hubs or muli-port repeaters. And when there are collisions, CSMA/CD goes to work. Collisions are detected on the LAN by voltage spikes. Full-duplex transmission essentially frees the LAN of collisions and hence no place for CSMA/CD to work on. The hosts on these LANs are connected on switches.

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The MAC Address

The MAC (Media Access Control) address or hardware address is burned in (hence is also called burned-in address) on every Ethernet Network Interface Card or NIC and is unique on every card. It is used by the Ethernet protocol for addressing in the Layer 2 OSI model. It is divided into two parts. The Organizational Unique Identifier (OUI), which is the first 6 HEX digits assigned by IEEE and the second 6 is assigned by the vendors themeselves to uniquely represent the NIC within the OUI. Every ethernet device will have this in its system.

Finding the MAC address or Physical address of your PC is fairly easy. Just go to the command prompt or DOS prompt and type "ipconfig /all" without the quotes.

You can also learn the MAC address of those PCs or devices which your PC recently had communication with by looking at the arp cache table using the arp -a or arp -g command in the command prompt.

Though it is not recommended, there are some ways to change the MAC address of your PC through software methods. It is important to take note that changing the MAC address can adversely affect the stability of your network. One long method is via the windows registry. You can refer to this site for the proper procedure. Or you can change it through the network card properties. But if you are not comfortable changing it, you can always try MAC spoofing such as SMAC. The SMAC software will try to hide your real MAC address from your network.

Why does anyone bother changing the MAC address anyway? Here are some of the reasons why:

1) for personal privacy since some companies do track users via their MAC addresses. However it is also a technique being used by hackers to hide themselves or to spoof other machines.
2) for performing security vulnerability and penetration testing.
3) to build high-availability netowrk solutions such as devices that run multiport NICs

If you are curious enough to find the OUIs assigned to some companies such as Cisco you can try this link: IEEE OUI and Company_ID Assignments. Just type the company name in the search box.

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Cisco and Pearson VUE Partnership

Today, I received and e-mail from Cisco announcing that Pearson VUE will be their sole vendor of Cisco certification exams worldwide effective August 1, 2007. All individual Cisco technical ceritification exams for Cisco customers and partners will need to be scheduled through Pearson VUE only and the last date to take exams through Prometric will be on July 31, 2007.

Cisco expects that by collaborating with Pearson VUE, a new standard will be created for individual information technology certifications elevating them to the level of prestigious professional certification and licensure programs. Shifting to a primary vendor vendor strategy worldwide will also allow an advance test security through the deployment of advanced biometric technologies. Cisco cited the following reasons for selecting Pearson VUE over Prometric:

• Unparalleled global reach
• Forward-thinking policies and vision of technical resource recruitment
• Ability to scale to new countries with new requirements
• Ability to meet forecasted demands
• Ability to deliver on additional investments to secure Cisco’s test delivery technology

Pearson VUE has more than 4000 test centers in 162 countries worldwide.

For more information, go to www.cisco.com and to the Pearson website.

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New CCNA 640-802 Certification

Recently, Cisco introduced a new entry-level certification and announced the updates to the popular CCNA certification. The CCENT (Cisco Certified Entry Network Technician) will provide a new point of entry to those who wants to start a career in networking. The CCENT's exam requirement will be ICND1 640-822 and the recommended course for that is ICND Part 1.

At the same time, CCNA has been revised to include a broader scope of networking topics and will be more focused on performance-based skills. CCENT will not be a prerequisite to the new CCNA level. The exam requirements will be CCNA 640-802 or ICND1 640-822 and ICND2 640-816. Examinees for the old CCNA 640-801 still has the chance to take the exam until November 6, 2007. The new curricula will be available worldwide on July 26, 2007 and exams will be initially released on August 1, 2007. For more informaton, visit www.cisco.com

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CSMA/CD Flowchart

CSMA/CD - Carrier Sense Multiple Access with Collision Detect

The following greatly simplifies the operation of CSMA/CD protocol on ethernet networks. Before any computer gets access control to the medium it follows the following pattern prior to transmission. The important thing to note about CSMA/CD is the jam signal and backoff algorithm process which happens everytime a collision is detected. They are most of the time included in the exam.

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OSI Summary

So in summary, the functions of the OSI layers are the following.... click image to enlarge.

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OSI Encapsulation Process

One of the mistakes that I 've made during the time when I was still learning OSI and its seven layers is the process of encapsulation. I have always thought that the process begins from Layer 1, the bottom layer. It was in fact the other way around. It begins on Layer 7, and works its way down to the bottom before it's de-encapsulated on its way up again on the receiving end. Here are some few illustrations behind the concept.

Protocol data Units (PDU) on every Layer of the OSI Model where every layer adds its own corresponding header.

Data Encapsulation in the Lower Level protocols where data will be encapsulated into TCP segment, IP datagram or packet, Ethernet Frame before being transformed into bytes and bits as it is sent through the physical medium.

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My very own Cisco blog...

Hi all! I've just passed my CCNA 640-801 exam with a rating of 987/1000. And I'm very proud of it hence this blog. This is my second CCNA exam, the first was the 640-507 which I can say was much different from this. The topics here are more geared to Implementation, Routing and Troubleshooting whereas back then are more concentrated to foundations like the OSI layer. The simulations are also much tougher on this exam. One must know the commands by heart, the various show commands specially and also access-lists commands.

I want to share to everyone some of what I've learned while studying for the exam, some tips about networking specifically Cisco, again hence the title. And if I encounter some more along the way, I'll be glad enough to publish it here. Thanks, hope everyone who'll visit this site will benefit from it and that I hope I could help my fellow CCNA wannabees out there...

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