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100BASE-TX

Exhibit 1-2-5 summarizes the attributes of 100BASE-TX. It operates over two pair of Category 5 UTP or STP and uses Category 5 certified RJ-45 connectors. It uses the 125Mhz data clock, continuous signaling, and 4B5B coding of 100BASE-FX, but adds signal scrambling and MLT-3 conditioning to deal with noise problems associated with sending high-frequency signals over copper. 100BASE-TX uses exactly the same connector pinouts as 10BASE-T. It transmits over one pair and receives over the other. It supports half-duplex and full-duplex operation.


Exhibit 1-2-5.  100Base-TX

100BASE-T4

100BASE-T4 (see Exhibit 1-2-6) is a more complex signaling system because it must support a 100M bps data rate over cable certified for operation at 16Mhz. This is accomplished by increasing the number of cable pairs used for data transmission and using a more sophisticated coding system. 100BASE-T4 starts with the two pairs used for 10BASE-T—one for transmit and one for receive—and adds two additional pairs that are used bidirectionally. This means that when transmitting, 100BASE-T4 always transmits over three pairs (one dedicated and two bi-directional) while listening for collisions on the remaining pair. It uses a much more sophisticated coding system called 8B6T.


Exhibit 1-2-6.  100Base-T4

Unlike other coding systems that use binary (0, 1) codes, 100BASE-T4 uses ternary (+1, 0, -1) codes, which enable it to pack eight bits of data into six ternary symbols. By using 8B6T coding and three wire pairs for transmission, 100BASE-T4 provides a 100M-bps data transmission rate with a clock speed of only 25MHz (8bits transmitted as 6 ternary symbols over three wire pairs at 25mhz.)

This process is diagrammed in Exhibit 1-2-7: one byte (eight bits) of data is encoded into 6 ternary symbols which are transmitted sequentially across three wire pairs. Unlike 100BASE-TX and 100BASE-FX, 100BASE-T4 does not support full-duplex operation.


Exhibit 1-2-7.  100Base-T4 Signaling

100BASE-T2

100BASE-T2 provides a more robust and noise-resistant signaling system capable of operating over two pairs of Category 3, Category 4, or Category 5 UTP or over STP links and supporting both half-duplex and full duplex operation. It uses an extremely sophisticated coding system called PAM5X5, which employs quinary (five-level—+2, +1, 0, -1, -2) signaling. In addition, it uses hybrid circuitry to enable simultaneous bi-directional transmission of 50M-bps data streams over each of the two wire pairs (see Exhibit 1-2-8).


Exhibit 1-2-8.  Media Independent Interface (MII)

Because of its robust encoding, 100BASE-T2 emits less noise during use and is less susceptible to noise from external sources. When used with four-pair Category 5 cable bundles, it can coexist with other signaling systems. A single four-pair bundle can carry two 100Base-T2 links, one 100Base-T2 link, and one 10BASE-T link, or one 100BASE-T link and one voice (telephone) link.

Media Independent Interface (MII)

The Media Independent Interface is a mechanical interface to the Ethernet MAC, similar to the AUI, which is used to connect transceivers (see Exhibit 1-2-9). The MII supports a nibble-wide data path, a station management interface, and command and status registers. It uses a 40-pin connector, similar in appearance to mini-small computer systems interface (mini-SCSI) connectors.


Exhibit 1-2-9.  100Base-T Auto-Negotiation (2)

Auto-Negotiation

Auto-Negotiation provides automatic link testing and configuration for UTP signaling systems. All 100BASE-T systems using UTP or STP go through Auto-Negotiation prior to establishing a link. During this start-up process, 100BASE-T systems on each side of a link:

  Check the link.
  Exchange coded information defining the abilities of each link partner (e.g., 10BASE-T half duplex operation, 10BASE-T full-duplex operation, 100BASE-TX half-duplex operation, 100BASE-TX full-duplex operation, 100BASE-T2 half-duplex operation, 1000BASE-T2 full-duplex operation or 100BASE-T4 operation).
  Go to an internal lookup table to determine the highest common operation mode.
  Configure themselves as per the table.
  Turn off Auto-Negotiation.
  Open the link.

If one end of the link is a 10BASE-T system that does not support Auto-Negotiation, the partner is automatically configured for 10BASE-T half-duplex operation (default mode). When confronted with another networking technology that uses the RJ-45 connector (e.g., Token Ring) Auto-negotiation will automatically fail the link.

Auto-Negotiation is based on the link pulse used in 10BASE-T. For Auto-Negotiation, the link pulse is divided into 33 fast link pulses that are used to carry pages of coded information between link partners.

Full Duplex Operation

Full-duplex operation supports simultaneous signaling in both directions over dedicated links by turning off the CSMA/CD collision detection circuitry. It provides some increase in bandwidth over links that have a high proportion of bi-directional traffic such as switch-switch and switch-server links. In addition, full-duplex operation increases the maximum length of fiber links. Whereas a half-duplex link is limited to 412 meters by the need to detect collisions, full-duplex operation supports links of up to two kilometers, because no collision detection is required. This increased link length is only useful for fiber links, signal attenuation limits, and copper link length to 100 meters for both half- and full-duplex operation.

Flow Control

Flow control provides a method for controlling traffic flows between intermediate devices (primarily switches and routers) and between intermediate devices and servers to avoid dropping packets. Currently two-speed (10/100 or 100/1000) operation requires large buffers to reduce the probability of dropping packets when a continuous stream of packets is sent from a high-speed to a low-speed device (e.g., 100M bps to 10M bps or 1000M bps to 100M bps). In such a scenario, when the buffers fill, the intermediate device drops the unbuffered packets.

Flow control provides a management alternative to having large buffers. When a buffer approaches full, the receiving device can send a flow control packet back to the sending device to stop the incoming packet stream. When the buffers of the receiving device empty, packet transmission starts again. This eliminates dropped packets and allows manufacturers to build switches with smaller buffers, which reduces costs.


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