Unicast, Multicast, and Broadcast Frames

A station can address its frames for transmission using one of three methods:

• Broadcast addressing— The station sends the frame to all stations in the broadcast domain.
• Group or multicast addressing— The station addresses its frames to a subset of all stations in the broadcast domain that belong a predefined group.
• Unicast addressing— The station addresses its frames to a specific station.

Figure 1-5 depicts these addressing types. Ethernet networks use all three methods. No one method is a panacea. Each method has pros and cons for its use.

802.3u Fast Ethernet

As Ethernet became more accepted as a standard for data networking, users began demanding more bandwidth. To calm the screaming masses, the IEEE announced 802.3u, the standard for 100 Mbps Ethernet in 1995. Although there were a number of 100 Mbps solutions for Ethernet, two have become the most common options: 100BASE-TX and 100BASE-FX (both are collectively referred to as 100BASE-X). 100BASE-X technology is based on the non-IEEE standard FDDI (ANSI X3T9.5). FDDI was the de facto 100 Mbps standard before Fast Ethernet and had a number of advantages to shared Ethernet.

100BASE-TX applies the 100BASE-X specification to Category 5 twisted-pair cabling. 100BASE-TX is similar to 10BASE-T in many ways, but unlike 10BASE-T, 100BASE-TX requires Category 5 cabling. 100BASE-TX performs a great deal of high-frequency signaling that requires a higher grade of cable than the Category 3 required for 10BASE-T. 100BASETX also has the same distance limitation of roughly 100 m that 10BASE-T has, meaning the same cabling infrastructure can be leveraged (assuming it is Category 5 or better).

The network diameter and Ethernet slot time for Fast Ethernet networks change from Ethernet to 100BASE-X networks. The Ethernet slot time defines the maximum network diameter by stipulating that the diameter should not exceed the distance a 512-bit frame can travel before the transmitting station is done sending that frame. Fast Ethernet systems maintain the use of the 512-bit frame size to maintain backward compatibility with legacy Ethernet systems.

For Ethernet networks, the maximum diameter is 2800 m. With 100BASE-TX, the transmit operations occur 10 times faster than the transmit operations of Ethernet stations. Accordingly, for a sending station to detect a collision after sending the 512-bit frame, the frame can only travel one-tenth the distance. This limit reduces the maximum network diameter from 2800 m to roughly 200 m. The loss of distance does not pose a real issue because most Fast Ethernet deployments use 100BASE-TX, which has a maximum distance of 100 m anyway.

100BASE-FX is a variant of 100BASE-X that uses multimode fiber as the medium to transmit data. The network interface card (NIC) converts electric signals into pulses of light that are sent over the fiber medium to the receiving NIC. The receiving NIC then translates the light pulses back into electrical signals that the receiving station can process.

100BASE-FX uses the same encoding mechanism as 100BASE-TX, but that is where the similarities end between 100BASE-TX and 100BASE-FX. Because 100BASE-FX uses light to carry data through the medium, there is no electromagnetic interference to be concerned with. This setup allows for a more efficient signaling scheme. The maximum network diameter for 100BASE-FX is roughly 400 m in half-duplex mode. 100BASE-FX can also operate in full-duplex mode. (Duplex modes are discussed next.) Full-duplex operation essentially eliminates the issues surrounding collisions, so 100BASE-FX can safely extend to distances beyond 400 m. In fact, using standard 62.5/125 micron multimode fiber, 100BASEFX can extend to 2 km while in full-duplex mode. If connectivity requirements dictate distances beyond 2 km, single-mode transceivers are available that allow 100BASE-FX to operate over single-mode fiber to distances up to 40 km. The cost of single-mode transceivers and single-mode fiber is an order of magnitude more expensive than its multimode brethren, but the solution exists if needed.


Full-Duplex Operation

CSMA/CD is the methodology that half-duplex Ethernet and Fast Ethernet is based on. As described earlier, CSMA/CD is like a telephone conference call. Each participant must wait until the medium is available before he can speak. In 1995, the IEEE ratified 802.3x, which specifies a new methodology for transmission in Ethernet networks known as full-duplex operation. Full-duplex operation allows a station to send and receive frames simultaneously, allowing greater use of the medium and higher overall throughput (see Figure 1-8). Fullduplex operation significantly changes the requirements placed on end stations, however.


Full-duplex operation works only in a point-to-point environment. There can be only one other device in the collision domain. Stations connected to hubs, repeaters, and the like are unable to operate in full-duplex mode. Stations connected back-to-back or connected to Layer 2 switches (that support full-duplex mode) are able to use full-duplex mode.

The capability to transmit and receive at the same rate allows stations to better utilize the network medium. The bandwidth available to the station is theoretically doubled because the station has full access to the medium in the send direction and the receive direction. In the case of 100BASE-X, this access gives each station up to 200 Mbps of maximum bandwidth. For end stations, such as PCs, the truth is that few stations transmit and receive at the same time. Stations such as servers and networking infrastructure such as routers and switches can take advantage of full-duplex mode in a manner that end stations cannot. The devices aggregate sessions and connections from the edge of the network to the core and back. They send and receive traffic distributed in both the send and receive directions, so these links are able to really take advantage of the extra bandwidth that full-duplex operation provides.

Full-duplex operation allows Ethernet topologies to break free from the distance limitations that half-duplex operations impose on them. Ironically, only fiber-based interfaces can take advantage of additional distances (as 100BASE-FX does) because twisted-pair deployments are distance-limited by the physical medium itself and not the network diameter imposed by Ethernet or Fast Ethernet time slots.