51-20-97 DATA COMMUNICATIONS MANAGEMENT INTRODUCTION TO 100BASE-T: FAST (AND FASTER) ETHERNET Colin Mick INSIDE How It Works: An Iso View, 100BASE-TX, 100BASE-T4, 100BASE-T2, Gigabit Ethernet, Impacts on Network Design INTRODUCTION 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, multiport 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 PAYOFF IDEA Fast Ethernet (100BASE-T), an extension to the IEEE802.3 Ethernet standard to support service at 100M bps, 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. Network managers can use 100BASE-T to improve bandwidth and still maximize investments in equipment, management tools, applications, and network support personnel. 12/97 Auerbach Publications 1998 CRC Press LLC
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 pairs of Category 5 unshielded twisted pair (UTP) or shielded twisted pair (STP) cables. 100BASE-T4 supports operation over four pairs of Category 3, Category 4, or Category 5 UTP or STP cables. 100BASE-T2 supports operation over two pairs 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 (anticipated 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 VIEW Exhibit 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 100Mbps 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. 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 inter-
EXHIBIT 1 100M bps Standards Model face (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 2): 100BASE-TX, 100BASE-T4, 100BASE-T2, and 100BASE-FX. 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 3). TP/PMD uses continuous signaling, unlike the dis-
EXHIBIT 2 100BASE-T Physical Layers crete signaling used with 10BASE-T. In 10BASE-T, when a station is finished sending a frame, it sends a few idle signals and then goes quiet, except for a link pulse, which is sent every 16 ms to indicate that the link is still good. EXHIBIT 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 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 62.5-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 EXHIBIT 4 10BASE-T-FX Uses 2-strand, 62.5/125 micron fiber Connector: MIC, ST, SC (converters available) Uses FDDI TP/PMD specification Continuous signaling scheme 4B5B coding scheme Transmits over 1-fiber and receives over 1-fiber 100M bps data rate Full and half duplex Detects and signals far end faults
EXHIBIT 5 100BASE-TX 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. 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.) 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
EXHIBIT 6 100BASE-T4 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 7 100BASE-T4 Signaling
EXHIBIT 8 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. EXHIBIT 9 Media Independent Interface (MII)
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 halfduplex 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 bidirectional 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 2 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 twospeed (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. Repeaters and Repeater Connections Repeaters provide for shared media operation in 10BASE-T and 100BASE-T via the CSMA/CD protocol. 10BASE-T networks have a collision domain diameter of 1000 meters. This permits building large, singlecollision domain networks using hierarchical, cascaded repeating hubs to increase port density. 100BASE-T does not permit hierarchical cascading of hubs because the maximum collision domain for UTP is slightly more than 200 meters (see Exhibit 10). Two techniques can be used to build large, single-collision domain networks (i.e., increase port density). One technique is to use modular hubs, where ports can be added by inserting additional multiport cards into the hub chassis. A second is to use stackable hubs stand-alone repeaters that can be connected via high-bandwidth stacking ports that do not impact the collision domain. EXHIBIT 10 Repeater Connection Styles
EXHIBIT 11 100BASE-T Topologies Topology Rules Topology rules for half-duplex 100BASE-T networks are shown in Exhibit 11. Copper links are limited to 100 meters by the U.S. cabling standard EIA/TIA-568-A. A collision domain containing two copper links can contain one class I repeater and two 100-meter copper links; or two class II repeaters, two 100-meter copper links, and a 5-meter copper interrepeater link. A collision domain containing a class I repeater with two fiber links can support two fiber links of 136 meters, for a collision domain diameter of 272 meters. A collision domain containing a class I repeater can also support one copper link of 100 meters and a single fiber link of 160 meters. A fiber DTE-DTE half-duplex collision domain (e.g., a switch-toswitch or switch-to-server) can support a 412-meter fiber link. Links of up to 2 kilometers can be supported over fiber by operating in full-duplex mode, which turns off the CSMA/CD portion of the protocol and requires a dedicated link (see Exhibit 12). Gigabit Ethernet Work to extend the Ethernet family to 1000M-bps (gigabit) operation is well underway. The first products using the new technology were demonstrated at Networld + Interop Las Vegas in May 1997, and the first products start shipping during the summer of 1997. Initial products will support operation over 62.5-micron multimode fiber (1000BASE-SX), 50- micron single-mode fiber (1000BASE-LX), or short lengths (to 25 meters)
EXHIBIT 12 Full Duplex of coaxial cable (1000BASE-CX). The operation of these products is being defined in a supplement to the IEEE 802.3 standard entitled 802.3z It is scheduled for completion in early 1998. A second supplement, entitled 802.3ab, will define gigabit Ethernet over 100-meter, four-pair Category 5 copper links (1000BASE-T). It is scheduled for completion in late 1998. IMPACTS ON NETWORK DESIGN Fast Ethernet is a family of 100M-bps signaling systems for use with the standard Ethernet MAC layer. The family consists of four signaling systems (100BASE-TX, 100BASE-T4, 100BASE-T2, and 100BASE-FX) and technologies that support automatic start-up (Auto-Negotiation), shared media operation (Repeaters), full-duplex operation, and flow-control to manage traffic flow. Fast Ethernet devices work seamlessly with legacy Ethernet systems: they have the same MAC layer, the same frame format, and the same CS- MA/CD protocol for shared media operation. Auto-Negotiation ensures that all 100BASE-T devices operating over copper links automatically configure themselves to operate with link partners. This makes 100BASE- T a very economical technology for adding high-bandwidth links to legacy systems. Higher-speed operation reduces the diameter of 100BASE-T shared media collision domains to approximately 200 meters for copper. Collision domains can be extended through the use of fiber and connected via switches to build large, complex networks. Full-duplex operation improves bandwidth for bidirectional links and increases the maximum
length of fiber links to kilometers. Port density within a single collision domain is expanded through the use of modular and stackable hubs. 100M bps is not the endpoint for Ethernet. 1000M-bps (gigabit) devices were demonstrated in the spring of 1997 and began shipping in the summer of 1997. Targets for gigabit Ethernet operation are 700 meters for full-duplex single mode fiber links, 25 meters for short-haul copper coax links, and 100 meters for Category 5 copper links. Colin Mick is a consultant in Palo Alto, CA.