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7. LAN NETWORK TECHNOLOGIES

7.1. Ethernet at 10 Mbps

There is a vast installed base (about 40 million Ethernet nodes) of 10 Mbps Ethernet and 4 or 16 Mbps Token Ring Local Area Networks (LANs) using coaxial cable or twisted copper wire. Most new LANs used twisted copper wire cable. Ethernet uses a contention method to enable workstations attached to the same cable to share the data bandwidth on the cable. Nodes transmit to the network on demand, but continually monitor the network to see if another node is transmitting at the same time. If this occurs both nodes cease transmission, and try again later at random intervals. The throughput with such a system is limited to about half the available bandwidth or 5 to 6 Mbps. Multimedia file servers can be attached to such LANs. Both Novell and Microsoft have developed software to support video on these traditional LANs.

Video in an Intel DVI or MPEG form requires 1 to 2 Mbps per user, so it is clear that only a handful of users can run video applications simultaneously. Audio requires lower bandwidth but is sensitive to unpredictable delays. For instance an intensive file transfer or video stream could prevent an audio application from transmitting onto the LAN for several tens of milliseconds, which is sufficient to reduce the intelligibility of voice. Improved performance is possible if LAN segments are divided up into segments with only a few attached users. Switched Ethernet takes this approach to the limit by enabling only one user to access a single segment which is then connected to a higher capacity network at a central hub by a switch. The price per port of Ethernet switches ranges from £400 to £1000. The use of a switch permits the filtering of packets based on address. Switching times need to be fast enough to deliver packets at the basic 10 Mbps Ethernet line rate. An Ethernet switch would be equipped with a higher speed ATM or FDDI interface to other networks. However even with one user per Ethernet Switch port or segment contention between different applications on the same machine or incoming and outgoing traffic can occur.

7.2. Fast Ethernet

The IEEE has set out the aims of the 100 Mbps Ethernet LAN standard [Rame93] as follows:-

Most of the 10 Mbps parameters including the CSMA/CD media access protocol will remain unchanged and the standard is being named 100Base-T. Another 100 Mbps standard called 100VG-AnyLAN [LAN95] has also been proposed and changes the media access protocol to a demand priority system. Fast Ethernet products are only just starting to appear. Use of both standards will require a change out of workstation and hub cards. The cost of 100 Mbps Ethernet cards will be targeted to be comparable with high performance 10Base-T cards.

7.3. FDDI

The Fibre Distributed Data Interface (FDDI) [Minoli93] was the only standards based technology operating at 100 Mbps for some time. FDDI has experienced slow market penetration due to the high cost of cards, still around £600. The first version of FDDI was developed as a campus trunk network for data.

Logically FDDI consists of a dual ring, but it may be implemented as a physical star. Key feature of FDDI include :-

An upgraded FDDI standard called FDDI II has been designed. In addition to the data, packet switched mode in FDDI an isochronous circuit switched service is made available by imposing a 125 micro second frame structure. The 100 Mbps bandwidth can be split between packet data and up to fifteen isochronous channel operating at 6.144 Mbps each.

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