High Speed LANs

The LAN world has been dominated by 10Mbps Ethernet. What to do when the Ethernet becomes a bottleneck? The inevitable tension arises between preserving investment and maintaining backward compatibility and providing improved performance.

Switched Ethernet

If you leave the NICs alone and don't change the wiring, then one means of improving performance is to replace the hub and eliminate the bottleneck of the single shared bus. The bus can be replaced with a switch which allows for multiple, simultaneous transmissions between nodes.

The switched hub consists of a single very high speed backplane connecting multiple cards, each of which has some number of host NICs connected to them. Communication between hosts within the same card doesn't require the backplane. The cards can be simple and run a standard Ethernet collision domain, or they may be buffered so that multiple hosts on the same card can be sending and receiving at the same time.

The details of the backplane are proprietary to the vendor of the hub, but this doesn't matter as what the host NICs see is a standard 802.3 LAN.

The throughput of the LAN as a whole can be much faster, which helps individual hosts, but note that the actual data rate to any single host is still limited to the max of 10Mbps.

Fast Ethernet

How to improve on the max rate of 10Mbps?

Good reference for this topic is  Description Page: Quick Reference Guides to 100 Mbps Fast Ethernet .

Two ideas competed to become the "upgrade" to make Ethernet faster. They both use the same frame format, but one changed the MAC protocol and the other didn't. Compatibility versus technical superiority.

The first used the CSMA/CD protocol, but made the speed higher, and hence the cable shorter. This has been standardized as part of 802.3 and is known as 100Base-T Fast Ethernet.

The second approach got rid of CSMA/CD for a "demand priority" approach where access is coordinated by the central hub. This too is now an IEEE standard (802.12) but seems to have lost in the market place. This scheme also can transport token ring frames, and hence is known as 100VG-AnyLAN.

The 100Base-T Fast Ethernet can be run on either twisted pair (2 pair or 4), or fiber. The 4-pair option doesn't require the same quality of UTP as the 2-pair option. The fiber option extends the distance from NIC to hub up to 2km. The signalling standards were copied from the ANSI standard for FDDI.

The maximum collision domain diameter of a 100Mbps Ethernet on copper is 200m. This is roughly a factor of 10 smaller than the max 2.5km found in 10Mbps Ethernet, as you would expect.

A means of auto-negotiating speeds with hubs has been created so that NICs of both 10 and 100 Mbps can live on the same hub. Interesting to note that this wouldn't work with a mixed LAN and co-ax wiring. With UTP and a hub each link is point-to-point, so can be negotiated independently.

Neither of the new fast Ethernet standards supports any form of co-axial cable medium.

FDDI

ANSI standard token ring at 100Mbps with a maximum length of 200 km. The max cable length is 2km between repeaters, and no more than 1000 nodes on the ring.

Often used as backbone to connect LANs together, hasn't become widely used in the LAN itself.

Uses multimode fibers and LEDs

Error spec is 2.5x10^10 but better is achievable

Topology

Two rings, transmitting in oposite directions for redundancy, wire centers can reconfigure into a single long ring. Stations can connect to both rings, or just a single ring depending on need for fault tolerance, cost.

Synchronous communication requires tight clock tolerance, can send frames of 4500 ? after long preamble for synch.

Data frame format is similar to 802.5 (TR) standard, allows for multiple simultaneous messages on the TR by piggybacking

FDDI 2 allows for synchronous circuit switched traffic for voice and ISDN. Special synchronous frames launched by a special monitor station support this. Slots in these frames are reserved and are timed to come every 125us for 1/8000 voice sampling rate.