CertiGuide to Network+  9  Chapter 0010:  ISO, OSI, and IEEE Standards       9  III  IEEE 802.x Standards

802.2 Logical Link Control 1 802.4 Token Bus Network

Derived from the work at Xerox PARC, this group defined the manner that twisted pair, fiber optic Ethernet and coax cable operates in the CSMA/CD (Carrier Sense Multiple Access/Collision Detect) network traffic management format, or access method.

This applies to Ethernet, Fast Ethernet and Gigabit Ethernet – the standards you’ll probably encounter most frequently out in the real world. As noted in the cabling chapter, these varieties of Ethernet can run over various physical cable topologies, with Fast Ethernet and Gigabit Ethernet typically running over UTP in a LAN environment, and Ethernet running over UTP or coax cable.

What’s the deal with Ethernet’s CSMA/CD? The simple story for CSMA/CD is similar to a railroad crossing. The old Stop, Look, Listen applies here. Before a host computer’s network interface card (NIC) transmits, it waits for a quiet period.

Then it dashes the data off. Since it takes time for data to travel down a wire, another NIC may have transmitted as well, not knowing of the data from the other NIC.

When this happens a collision occurs. This causes a rise in voltage on the cable, since both transmitted. In this event, the voltage increase indicates Collision Detection. Each NIC waits a random amount of time and re-transmits the data, because collisions on a network cable is much like a collision on a road – two entities run into each other and stop in their tracks.

Some years ago, communications engineers assumed that fiber would replace copper all the way to the end device an employee or homeowner would be using. That was a safe assumption because nobody thought copper could be made to work at speeds as fast as it does today. It was assumed that a regular dial-up phone line would “hit the ceiling” at 2400 bits per second (baud). In one fashion, it is true that a dial-up phone line does hit that technical limit, and no thought was given to concepts such as data compression, or using a signaling technique called phase shifting to cram more data down the same ‘pipe’.

Similarly, in the late 1980’s no one saw Fast Ethernet (100BaseT) flowing down unshielded twisted pair, either. Today that is common using Category 5 cable, and the 802.3 standard is still flourishing. Sometimes, the old adage “Where there is a will, there is a way” holds very true. The late 1990’s saw the release of Gigabit Ethernet (1000BaseT), and it has become commonplace enough to see prices dropping into the commodity range. For example, in early 2001, Gigabit Ethernet Network Interface Cards dropped under $100 retail for single units. Not only did engineers find a way to make Gigabit Ethernet run over unshielded copper; they found a way to make it work with Category 5 cable, as mentioned in the chapter on LAN Cabling.

802.2 Logical Link Control 1 802.4 Token Bus Network