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1-2
Fast (and Faster) Ethernet

COLIN MICK

Fast Ethernet (100BASE-T) is an extension to the IEEE802.3 Ethernet standard to support service at 100M bps. It is virtually identical to 10BASE-T, in that it uses the same media access control (MAC) layer, frame format, and carrier sense multiple access with collision detection (CSMA/CD) protocol. This means that network managers can use 100BASE-T to improve bandwidth and still make maximum use of investments in equipment, management tools, applications, and network support personnel.

100BASE-T is designed to work transparently with 10BASE-T systems. Switches (high-speed, multi-port bridges) are used to connect existing 10BASE-T networks to 100BASE-T technology. By building networks with 100BASE-T and 10BASE-T linked with switches and repeating hubs, network designers can build networks that provide four levels of service:

  Shared 10M-bps service.
  Dedicated (switched) 10M-bps service.
  Shared 100M-bps service.
  Dedicated 100M-bps service.

Operating at higher speeds with the same frame size and the CSMA/CD protocol requires that 100BASE-T collision domain diameters be smaller — typically about 200 meters. In 100BASE-T, larger networks are built by combining collision domains by way of switches. Fiber (100BASE-FX) links are used to support long (i.e., 412 meters in half duplex, 2 kilometers in full duplex) cable runs. Within a single collision domain, port density is increased by using modular or stacking hubs.

The 100BASE-T standard (IEEE802.3u, 1995) currently defines four physical layer signaling systems:

  100BASE-TX supports operation over two pair of Category 5 unshielded twisted pair (UTP) or shielded twisted pair (STP) cables.
  100BASE-T4 supports operation over four pair of Category 3, Category 4 or Category 5 UTP or STP cables.
  100BASE-T2 supports operation over two pair of Category 3, Category 4, or Category 5 UTP or STP cables.
  100BASE-FX supports operation over two 62.5 micron multimode fibers.

Products for 100BASE-TX and 100BASE-FX are available from a wide range of manufacturers. Products for 100BASE-T4 are supported by a smaller group of manufacturers and no products have yet been offered for 100BASE-T2 (this should happen in summer 1997).

In addition, the Fast Ethernet standard has recently added support for full-duplex operation and flow control. Full duplex operation is broadly available in current 10BASE-T and 100BASE-T products. Support for flow control (which can be used to manage traffic flows between intermediate devices to avoid dropping frames) should begin appearing in products by late 1997. (Many manufacturers already offer proprietary forms of flow control in their products.)

HOW IT WORKS: AN ISO VIEW0

Exhibit 1-2-1 depicts an ISO seven-layer diagram comparing 10BASE-T and 100BASE-T. Both 10BASE-T and 100BASE-T defined operations at the lower half of the data link layer (known as the Media Access or MAC layer) and the physical layer. Extension of the Ethernet standard to 100M bps operation required one small change to the MAC layer operation specified in the IEEE802.3 standard. Originally, timing was defined in absolute terms (i.e., an external reference clock). As a result, timing specifications were defined in milliseconds, nanoseconds, and picoseconds. To support 100M bps operation, timing was respecified relative to the internal clock of the MAC. This meant that specifications were defined in bit times.


Exhibit 1-2-1.  100M bps Standards Model

Several changes were made at the physical layer. In 10BASE-T, coding (i.e., conversion of data bits to symbols) is done in the PLS layer, directly below the MAC. A mechanical interface called the attachment unit interface (AUI) is situated directly below the PLS. Below the AUI is the PMA layer, which converts the digital symbols into analog symbols that can be sent across the wire and a media dependent interface (MDI)—a socket for connecting the cable.

100BASE-T puts the coding, called the physical coding sublayer (PCS), below the mechanical interface. This was done to make it possible to offer a variety of coding systems that could be packaged in a transceiver along with the analog/digital circuitry for connection via the mechanical interface. The mechanical interface used for 100BASE-T is called the Media Independent Interface (MII). It is similar to the AUI, but offers a larger data path and the ability to move management information between the PHY and the MAC. A simple mapping function, called the Reconciliation Sublayer, handles linking the MII to the MAC. As noted previously, 100BASE-T currently supports four signaling systems (see Exhibit 1-2-2): 100BASE-TX, 100BASE-T4, 100BASE-T2, and 100BASE-FX.


Exhibit 1-2-2.  100Base-T Physical Layers

Two 100BASE-T signaling systems—100BASE-TX and 100BASE-FX—are based on the transport protocol/physical medium dependent (TP/PMD) specification developed by the ANSI X3T12 committee to support sending fiber distributed data interface (FDDI) signals over copper wire (see Exhibit 1-2-3). TP/PMD uses continuous signaling, unlike the discrete signaling used with 10BASE-T. In 10BASE-T, when a station is done sending a frame, it sends a few idle signals and then goes quiet, except for a link pulse, which is sent every 16ms to indicate that the link is still good.


Exhibit 1-2-3.  100Base-T (TX and FX) Frames

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.

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

Exhibit 1-2-4 summarizes the attributes of 100BASE-FX. It uses two strands of 623.5 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 a 125Mhz, 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.


Exhibit 1-2-4.  10Base-T-FX


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