Fast Ethernet (100Base-T) is an extension of IEEE802.3 CSMA/CD. Its LAN has a star topology connection for up to 210 meters in diameter using either UTP or fiber cable. Fast Ethernet use the same MAC layer as 10Base-T Ethernet, so Ethernet users can easily migrate to Fast Ethernet. Applications and higher level protocols developed on 10Base-T Ethernet will run on 100Base-T Fast Ethernet without modification, and 100Base-T adaptors are available to switch between the wide used 10 Mbps ethernet and the 100 Mbps standard. In addition to having the advantages of Ethernet, Fast Ethernet has a 100 Mbps throughput make it competitive with FDDI networks. The limit for a Ethernet is 2500m and 1024 hosts with no more than 4 repeaters between each host pair. The maximum segment length for Fast Ethernet is only 100m, up to 2000m with the fiber optic link.
FDDI (Fiber Distributed Data Interface) is a 100 Mega-bit technology using a timed token over a dual ring of trees. An FDDI network consists two independent rings that transmit data in the opposite direction, so that it is able to tolerate a single break in the cable. Each host implements a small elasticity buffer that temporarily holds the bits of a frame as they pass through the host, so that frames are able to be sent over the ring without all the hosts having to be synchronized. FDDI also allows SAS (single attachment stations) in the configuration of the ring. These improvements for FDDI give it a 100 Mbps throughput. The limit for a FDDI is 100km and 1000 hosts. FDDI can transmit a frame up to 4500 bytes, which about 3 times as much as Ethernet can do.
Are FDDI networks in danger of dying away?
Let’s compare the two technologies in terms of throughput, latency, deterministic, configuration, maintenance, compatibility, reliability, cost, user community, and interconnectivity.
- throughput – On shared media LANs such as Fast Ethernet and FDDI, line speed decreases in proportion to the number of nodes contending for a portion of the total available bandwidth. Now assume that same amount of hosts on the LAN, even thought Fast Ethernet and FDDI have the same 100 Mbps throughput, FDDI will ran faster than Fast Ethernet because there collision detection could reduce the bandwidth by 30-50%.
- latency – FDDI has much higher latency since data frames has to be passed by many hosts in between. For Ethernet, the latency is relative lower, although it is nondeterministic based on random wait after detected collision.
- deterministic – FDDI is deterministic. But Ethernet is not, because of the wait for a random time after detecting a collision.
- configuration and maintenance – It is less complicated with Ethernet LAN, because of its simple structure. Adding one more host on Ethernet is much simpler than do it on FDDI. Ethernet protocol is simple and hosts can be installed on the fly without taking the whole network down.
- distance – 100m for twisted pair and 2000m for fiber optics on Fast Ethernet segment, FDDI is clearly the better choice with a distance up to 100km.
- host load – Both Fast Ethernet and FDDI can accept up to about 1000 hosts. For FDDI, more hosts means potential longer latency. For Fast Ethernet, more hosts mean more collision and more congestion. Even worse for higher than 60% loading, the overall throughput of Ethernet could be stalled, because hosts will be busy with detecting collision/waiting, and thus are not able to transmit.
- compatibility – Ethernet users can easily migrate to Fast Ethernet. Applications and higher level protocols developed on 10Base-T Ethernet will run on 100Base-T Fast Ethernet without modification, and 100Base-T adaptors are available to switch between the wide used 10 Mbps ethernet and the 100 Mbps standard.
- reliability – Ethernet is simple, has less things to break, and thus is pretty reliable. For FDDI, the whole LAN could be brought down if one or more hosts in the ring break. The dual ring is only able to tolerate a single break in the cable.
- cost – Fast Ethernet although does not have the technical edge over FDDI in terms of speed, it is much more easier to configurate and maintain than FDDI. In addition to that, Fast Ethernet products cost a fraction to what would be for its FDDI counterparts.
- users – Ethernet has more users. One of the most important obstacles to the installation of high-performance networks are users failure to accept the new technology. In the case of Ethernet a large community of users were convinced to install a commonly agreed type of high performance network because manufacturers are able to provide standard add on features, which as they are produced in bulk, have been offer cheaper.
- interconnectivity – when FDDI and Fast Ethernet are bridged, only 50 Mbps can be handled before exhibit heavy packets loss in their Client-Server experiment. In contrast, Ethernet switches deliver dedicated bandwidth.
FDDI is more efficient with its high bandwidth at 100 Mbps level, but Fast Ethernet is the better for the latency sensitive application. Fast Ethernet is more cost effective and already a larger user community. So we believe that Fast Ethernet is the wining technology over FDDI.