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Common 100 Mbps Hardware Variations

Originally, when developed by XEROX PARC for internal use, Ethernet operated at 3 Mbps. No fundamental changes need to be made to the CSMA/CD protocol to make it operate at a higher bit rate. What does need to change are the physical layer specifications for wiring, bit encoding, inter-frame timing and propagation delay restrictions. At a higher bit rate a minimum size frame will take less time to transmit which implies a lower limit on propagation delays in order to maintain the collision detection.

Several different 100 Mbps standards have evolved that use the name Ethernet. All of them will transport Ethernet packets from interface to interface and will interoperate with 10 Mbps Ethernet by the use of a bridge. On one side of the bridge is a 100 Mbps Ethernet and on the other side is a 10 Mbps Ethernet: they cannot run on the same wire at the same time.


100BASE-T Ethernet is by far the most similar to any of the 10 Mbps Ethernet standards. The twisted pair wiring used in 10BASE-T is called "category 3" wiring which is the same type used to for telephone cable. It is of more than sufficient quality to carry 20 MHz signals (10 Mbps times 2 Hz per bit with Manchester encoding) the 100 meter maximum distance between a hub and a computer. However, it is not of sufficient quality to carry 200 MHz signals the same distance which is what would be required if one simply multiplied the clock rate by 10.

Typical offices are wired with 4 category 3 twisted pairs to the wiring closet. Two pair are used for the telephone and two pair are used for 10BASE-T. Sometimes, however, the offices are wired with two spare pairs (a total of 6 pair). In new buildings, category 5 wire may have been installed rather than the cheaper category 3 wiring.

Two different ways around this problem are both called 100BASE-T, although when actually installing the interfaces and wiring one needs to be clear on the distinctions between these two minor varients. One of them uses 4 pair category 3 wires instead of only two, and the other requires 2 pair category 5 wires.


One solution to the problem is to use higher quality cabling and to use a bit encoding that requires less than 2 Hz/bit. Category 5 twisted pair can carry a higher frequency signal the same 100 meters than category 3 twisted pair. 100 BASE-TX uses a 4B5B binary encoding rather than a Manchester encoding to encode the 100 Mbps data stream with a 125 MHz signal.

One of the pair is used for sending data and the other for receiving. Hence, one can set up such a system for full duplex communication in the case where only two interfaces are in the same collision domain: either two computers directly connected to each other with crossover cable or a computer directly connected to a switch. Full duplex communication results doubling the potential throughput, but rarely achieves more than a modest improvement in practice.


The other alternative to faster Ethernet is to use more wires rather than better wires. 100BASE-T4 uses 4 pair. One pair is always used for sending and another for receiving. The other two are used in whatever direction data is currently flowing: full duplex communication is not possible. Data is either flowing into an interface or out as one would expect in the absence of a collision.

To achieve 100 Mbps, three pair are used, each carrying a 25 MHz signal (slightly more than the 20 MHz used in 10BASE-T). Ternary signals are used instead of binary signals on each of these wires allowing for a total of 27 symbols to per MHz. 27 symbols will carry 4 bits (requiring a minimum of 16 symbols) with some redundancy. Thus one gets 4 bits per Hz on the three wires: just right for 100 Mbps at 25 MHz. It is not a particularly simple encoding, but it gets the job done with existing wiring.


In the Ball State Computer Science Department we use a mix of 10BASE-T and 100BASE-T (of the TX variety rather than the T4 variety). We have a switch located in the same room with our main computer system. This switch has 2 100 Mbps ports and 6 10 Mbps ports, each in a separate collision domain (some "switches" are really combination switch/hubs that have multiple ports per collision domain).

Our primary Unix fileserver has a full-duplex connection to one of the 100 Mbps ports and the other 100 Mbps port is connected to a pair of stacked 100 Mbps hubs that serves a lab with 16 Unix workstations. One of the 10Mbps ports connects the switch to the outside world, two are used to service departmental labs, and one is dedicated to our Netware fileserver.

Figure 1: Ethernet switch (dark gray box on bottom), and two Ethernet hubs that form the core of the Ball State Computer Science Ethernet network.

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&copy Paul Buis, Associate Professor
Computer Science Department
Ball State University
September, 1996