Previous Table of Contents Next


In TP/PMD, a continuous stream of idle symbols is sent when data is not being transmitted. To ease the transition between data and idle signals, a JK symbol sequence is added to the front of a data frame and a TR symbol sequence is added to the end of the frame before transmission of idle symbols begins. The JK, TR, and idle transmission patterns must be added to Ethernet frames when they are transmitted via the TP/PMD specification.

Exhibit 4.10Base-T-FX

• Uses 2-strand, 62.5/125 micron fiber
• Connector: MIC,ST,SC (converters available)
— Continuous signaling scheme
— 4B5B coding scheme
• Transmit over 1-fiber and receives over 1-fiber
• 100M bps data rate
• Full and half duplex
• Detects and signals far end faults

Both 100BASE-TX and 100BASE-FX use 4B5B coding. This means it takes 5 baud (signal transitions on the wire) to transmit 4 bits of information. This is vastly more efficient than the Manchester coding used for 10BASE-T, which requires 2 baud to send each bit across the wire.

Exhibit 4 summarizes the attributes of 100BASE-FX. It uses two strands of 63.5-micron fiber. All standard connectors are listed in the specification — different manufacturers support different types of connectors. 100BASE-FX uses the FDDI TP/PMD specification with continuous signaling and 4B5B coding. The data clock runs at 125 MHz, providing a signaling rate of 100M bps with the 80% efficiency of 4B5B coding. One fiber is used for transmitting data, the other for receiving data. It can support both half-duplex and full-duplex operation and has automatic link detection.

100BASE-TX

Exhibit 5 summarizes the attributes of 100BASE-TX. It operates over two pairs of Category 5 UTP or STP, and uses Category 5 certified RJ-45 connectors. It uses the 125-MHz 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.

100BASE-T4

100BASE-T4 (see Exhibit 6) is a more complex signaling system because it must support a 100M-bps data rate over cable certified for operation at 16 MHz. 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 bidirectional) while listening for collisions on the remaining pair. It uses a much more sophisticated coding system called 8B6T.


Exhibit 5.  100BASE-TX.


Exhibit 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 8 bits of data into 6 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 25 MHz (8 bits transmitted as 6 ternary symbols over three wire pairs at 25 MHz.)


Exhibit 7.  100BASE-T4 Signaling.

This process is diagrammed in Exhibit 7: 1 byte (8 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.

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 bidirectional transmission of 50M-bps data streams over each of the two wire pairs (see Exhibit 8).

Because of its robust encoding, 100BASE-T2 emits less noise during use and is less susceptible to noise from external sources. When used with 4-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.


Exhibit 8.  Media-Independent Interface (MII).


Exhibit 9.  100BASE-T Auto Negotiation (2).

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 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.

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.


Previous Table of Contents Next

Copyright © CRC Press LLC