4.10 Suggested References

Isochronous data is data that is sent at fixed intervals. A common example is digital voice data, where voice samples are taken at fixed intervals and delivered to the network. All ATM traffic is connection-oriented and uses small fixed-size switched cells 53 bytes long, 5 of which are headers and 48 are data. Cells have very limited addressing information and can only be passed along heavyweight pre-established connections.
ATM's long-distance communication bias is evident in its cost and design tradeoffs. Despite its ability to be configured into large high-bandwidth networks, it is rarely used where cost-effective local communication is needed. In a SAN or LAN environment it is almost always more effective to over-provision with low-cost bandwidth (the Fast Ethernet approach) than to use expensive controlled bandwidth. In the WAN environment, where ATM should be an obvious choice, you are faced with the problems of converting from datagrams to a connection-oriented system.
A second issue limiting its acceptability in computer communication is its connection model. ATM networks are designed to be used only with virtual point-to-point connections, much like establishing a phone call. Every communication must be preceded by a costly "dial" phase. To minimize this overhead with typical IP traffic, the system is typically run in "IP over ATM" mode, where the ATM network provides only virtual point-to-point links. This ignores the expensive, complex parts of the switch, utilizing only the switching fabric.
5.2.2 FDDI
FDDI, Fiber Distributed Data interface, is a 100Mbps packet network originally designed for fiber-optic links. It uses a token ring access protocol that allows slightly higher link bandwidth utilization, but results in a typically higher packet latency. Besides a larger packet size of 4500 bytes, it has few advantages over the much less expensive Fast Ethernet. FDDI gained popular use as a campus backbone network in the early '90s because it was the only technology that delivered 100 Mbps of bandwidth at the time. FDDI is sometimes used in Beowulf clusters because it can be tuned for high bandwidth or low delay around the token ring. Fast Ethernet sustained bandwidth now exceeds that of FDDI, so unless you require the low latency, it is probably a poor interconnect choice. It used to be that you would chose a token ring network when you wanted to prevent potential network starvation situations present in broadcast media, where a workstation might never gain access to the network. With high performance Fast Ethernet switches, starvation is no longer a problem.

 



How to Build a Beowulf
How to Build a Beowulf: A Guide to the Implementation and Application of PC Clusters (Scientific and Engineering Computation)
ISBN: 026269218X
EAN: 2147483647
Year: 1999
Pages: 134

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