Understanding Frame Types, Especially Ethernet

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Contents 1 Understanding Frame Types, Especially Ethernet 2 Remembering the Ancestors 3 Ethernet’s Entrance 4 Considerations with Ethernet 5 Ethernet Media Table

[edit section] Understanding Frame Types, Especially Ethernet

Frame technologies are the medium between the physical electric signals and the higher-level logical packets that drive networking technology. Through the years, we have seen a number of prominent frame technologies.

Ethernet (802.3) is the frame technology standard that drives most networks today and probably the one that you are most familiar with. Understanding Ethernet is a key to doing well on the Network+ exam, yet there is actually not much information that you need to know about Ethernet. Instead, it is more important that you understand how Ethernet works and the methods it employs in controlling traffic “on the wire.” Remember that Ethernet is not a protocol as it operates at the Data Link layer of the OSI model.

[edit section] Remembering the Ancestors

Before you can understand (and appreciate) Ethernet, it is important to backtrack a bit through the predecessors of the Ethernet standard.

Typically, when networks are illustrated to “laypeople,” they are demonstrated as computers that appear to be linked or “daisy-chained” to each other. This in fact was one of the earliest networking concepts, the bus topology. That topology is inexorably linked to the 802.4 Token Bus standard, which defined the way in which data would move across a token bus network. The standard stipulates that data would be treated like a token – that is, data would move through the network, passing from one node to the next until it reached its destination. You can imagine some of the problems incurred with this – for example, for a frame to travel across the network, it would have to “pass through” every node between two communicating nodes. This mode of communication made impossible the idea of information privacy; in addition, the bus technology is infamous for the large amount of traffic produced. Don’t forget that bus networks require “terminators” on the ends on networks to “end” token travel.

The token ring (802.5), though an improvement on the bus design was only a slight improvement at best. In the ring formulation, a “token” still had to be passed from node to node – the only difference was that in the token ring, there was no need for terminators necessarily, but in practice, very few ring networks literally daisy-chain all of their nodes together. In fact, in many cases, the terminators are still employed. Still, the ring was promoted as an improvement to the bus standard and a competitor to the rising Ethernet standard.

[edit section] Ethernet’s Entrance

Yet, in the end, Ethernet prevailed. The key difference between Ethernet and the aforementioned two standards was that Ethernet featured a seemingly counter-intuitive and problematic approach to handling network traffic. In the Ethernet standard, traffic is not “passed” on a token. Instead, information is sent almost haphazardly along the wire without regard to the status of other packets. In the token formulation, the transfer of information can be schematically controlled because the passing of tokens implies that no collisions occur. Ethernet, in contrast, features a system called CSMA/CD (Carrier Sense Multiple Access with Collision Detection) that allows Ethernet to automatically detect and fix collisions in frame communication. The basic principle behind this system can be conveyed in three steps:

Determine if frame is ready for transmission (if wire is “idle”); wait until it becomes open

Send frame

Run collision detection procedure if collision occurs

Note About How Ethernet Works: Notice that there is no “passing” of tokens through nodes in the Ethernet standard. So, it is very possible that collisions occur. If they do occur, Ethernet simply waits a random “back off” period before reattempting transmission. After too many failures, the attempts stop and the transmission itself is deemed a failure.

Also be aware: Ethernet has a feature called “promiscuous mode” in which nodes can receive all frames of information and not just those passed along to those specific computers. This can be defeated by using switching.

Now, critics of the time charged that the Ethernet system was inefficient, that failing to prevent collisions would create excess network traffic, and that the Ethernet standard would be inherently “slow” in nature. All of these concerns however proved to be unnecessary as the Ethernet standard is still dominant today and has been since the early 1990’s.

[edit section] Considerations with Ethernet

As with any networking technology, Ethernet has special facets and features you must take into account when troubleshooting it and dealing with it. One of the most important features of Ethernet today is the ability to “auto-negotiate” network speed and duplex mode. Duplex mode refers to whether the traffic is “one-way” (talk or listen) or “two-way” (talk and listen). Half duplex is akin to a one-way radio. A node can transmit or receieve, but not do both at the same time. Full duplex allows for two-way communication, a node can both transmit and receive at the same time. Auto-negotiation allows Ethernet devices to determine which mode to use. In addition, Ethernet speeds vary depending on medium and the router or gateway employed; below is a table of Ethernet media and information you should know concerning them.

[edit section] Ethernet Media Table

Name/Max Length of Cable	Type	Speed	Use
10Base5 / 500 m.	Special coaxial cable, needs “vampire taps” (cut into wire to read), SHARED MEDIUM (similar to bus)	10 Mbps	Oldest technology and media; rarely employed today
10Base2 (“ThinNet”) / 185 m.	Coaxial cable with BNC connector. SHARED MEDIUM	10 Mbps	Also old and rarely used, remember that it requires special circular BNC connector, similar to “cable TV connector”
10BaseT / 100 m.	Twisted Pair wire with EXCLUSIVE MEDIA (connected to hubs rather than to other nodes), RJ-45	10 Mbps	Used RJ-45 connectors (look like large phone line connector)
100BaseTX (“Fast Ethernet”) / 100 m.	Twisted pair wire, EXCLUSIVE MEDIA, RJ-45	100 Mbps	An improvement on speed of 10BaseT, capable of auto-negotiation of speed
1000BaseT (“Gigabit Ethernet”) / 100 m.	Twisted pair wire, EXCLUSIVE MEDIA, RJ-45	1000 Mbps	Another speed improvement
Faster Speed/Other media Ethernet (10GBASE-SR, 10GBASE-CX4, etc) / 2000+ m.	Uses next-generation fiber optic cabling to achieve 10+ Gbps speeds	10+ Gbps	Another speed improvement and change in connectors, cabling

Note for the exam: You may see options such as “100BaseFX” – this simply means it is the same as 100BaseTX, but with a fiber optic connection (media) and an SC or ST connector.