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Although a regular bridge or router typically will delay a full-size packet by about 1,200 microseconds, a cut-through Ethernet switch will measure transit delay in tens of microseconds. This difference in transit delay will force a quantum difference in the resulting end-to-end throughput. This type of switching, however, may propagate corrupted packets or ones containing errors.
Exhibit 3 lists a few factors that can effect end-to-end throughput. As can be seen, the store-and-forward architecture avoids many of the problems associated with a cut-through architecture. This includes short or fragmented frames as well as the smooth handling of multicast packets. In addition, this architecture provides for a much easier migration to high-speed LAN technologies such as Fast Ethernet or ATM.
Nebula 2000 Port | Port Type | Full Duplex |
---|---|---|
Port | Console | N/A |
Port | Trace Port | (LAN analyzer) |
Port | WAN Port | Supports IP (RIP) SLIP routing |
Port 1 | 10Base-T/RJ-45 | No |
Port 2 | 10Base-T/RJ-45 | No |
Port 3 | 10Base-T/RJ-45 | No |
Port 4 | 10Base-T/RJ-45 | No |
Port 5 | 10Base-T/RJ-45 | No |
Port 6 | 10Base-T/RJ-45 | No |
Port 7 | 10Base-T/RJ-45 | Yes |
Port 8 | 10Base-T/RJ-45 | Yes |
Ethernet switch configurations vary with their application design. For example, desktop switches are designed to provide a high-speed link between desk terminals using bandwidth-intensive applications. Ports are available to link several terminals as well as a server.
Workgroup switches support Ethernet links or segments, with separate ports for each link. Workgroup switches are connected to hubs to provide segment connectivity. There may be several high-speed links to connect servers. A WAN port may be included to link to other remote LANs.
Hub or backbone switches are used to connect several corporate resources across a collapsed backbone. These switches also provide ports for Ethernet links as well as high-speed WAN links that are used to link remote LANs by the public network (e.g., frame relay, T1, and ATM).
Exhibit 4 shows the basic configuration for a typical midrange workgroup Ethernet switch, the Nebula 2000. This is a typical base line configuration for a stackable type switch. The summary for the Nebula 2000 Ethernet Switch states that any 10Base-T port can easily be connected to either a 10Base-T transceiver or 10Base-T device without requiring special crossover cables. Two of the 10Base-T ports are configurable to an F-DX operation. An F-DX link connecting two switches will enable the switches to transmit and receive simultaneously between them.
The full-duplex operation is a modification of a normal half-duplex operation. This technology enables a high-performance, low-latency connection between multiple servers, or other Nebula switches. In Exhibit 5, the high-speed SPARC station can be connected to the other high-speed SPARC station, and the workstation can be connected to the server.
Exhibit 5. Dual Nebula 2000 Switches Connected with a Full-Duplex Link.
An F-DX (20M bps) link is established between each of these devices. Because there is no media access delay nor collisions for any packet forwarded to a full-duplex link, network latency is greatly enhanced. In the full-duplex architecture of the Nebula 2000 Switch in Exhibit 5, all switching takes place within the Ethernet switch because the Ethernet switch serves as F-EP for the segments distributed behind the switch.
Not every Ethernet LAN is a candidate for switching. An ideal application requires the presence of multiple hubs residing in multiple departments, floors, or buildings. There should be the presence of high-performance workstations with applications that demand high bandwidth (e.g., CAD/CAM, imaging, and multimedia). These conditions would be characteristic of large campus environments typically associated with academic or industrial research activities.
Typically, in these environments there will be clusters of high-performance workstations (e.g., Sun SPARC and DEC Alpha) used for scientific, imaging, and multimedia applications. These applications generate large amounts of traffic with high demands for bandwidth. In this environment, Ethernet switches serve as a natural extension of the network segments. These switches would be capable of supporting parallel internetworking of subsegments thereby optimizing the capability of the network without sacrificing the investment in the infrastructure.
Exhibit 6 shows an overview of the University of California Supercomputer Center network that supports the academic and research requirements for University of California San Diego as a large user constituency scattered around the world. In this network, the planners have deployed an inexpensive Ethernet switch (Nebula 2000) to link a number of external and internal networks to their diverse computing resources.
This design concept makes use of a collapsed backbone network supported by a Nebula 2000 switch concentrating four different networks within their Supercomputer Center in La Jolla. The four-tier filtering capability of the Nebula 2000 makes it possible to extend the vast resources of the Supercomputer center to outside researchers, keeping all users within a well-defined set of boundaries.
A router connects the Nebula 2000 to an FDDI network within the center, which in turn provides links to the Cray C-90, Intel Paragon XPS 30, and various other computing resources.
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