Because they're more efficient, more convenient, and increasingly less expensive, Ethernet switching hubs are replacing router-repeater setups in LANs
Tadesse W. Giorgis
Ten years ago, LAN wire speeds didn't come close to approaching bottleneck status. But these days, bandwidth is a jealously guarded resource, and the actual throughput on a crowded 10-Mbps Ethernet network or a 16-Mbps token-ring network is often slower than what you get from even a 28.8-Kbps modem. Anxiety-ridden network administrators are calling for more bandwidth at reasonable prices--and they're buying Ethernet switching hubs.
Without a switching hub, one fast workstation can choke a 10-Mbps Ethernet bandwidth in no time and take the network down with it. Ten-Mbp
s Ethernet switches don't sacrifice installed bases of network hardware and software. Until the newer, faster standards fall into place (which could take years), they present a solid interim solution.
Ten-Mbps Ethernet switching hubs alleviate traffic jams by making virtual connections between transmitting and receiving nodes and sending data only to each packet's Ethernet destination address (i.e., a type of private connection) rather than broadcasting data to everyone. This improves every node's network performance, and it offers a security benefit as well. Many LANs rely on routers and repeaters to distribute network data, but repeaters aren't capable of port-specific transmission, and routers don't conserve bandwidth. Although the cost per port of switching hubs is currently greater than that of router/repeater combinations, prices are falling, due largely to the increased use of ASICs.
Most of the switching logic and management capability of the 29 hubs we tested is hard-coded into ASICs th
at manage specific ports. Developing the ASICs is expensive. But once vendors have the design, ASICs cost much less than the commonly used general-purpose RISC-based Intel 960 processors. Most 960 designs in switches use dual processors, with one CPU for switching and one for management. Port-specific flow requires two CPUs at each port and is expensive.
Switching hubs come in both Ethernet and token-ring varieties. We tested only the Ethernet type. Store-and-forward switches receive each packet into a memory buffer and examine them for errors and undesirable fragments before transmission. Cut-through switches examine only the header segment of a frame to obtain its destination address (and source address for virtual LAN support) before they begin transmitting partially received packets. As a result, cut-through switches exhibit shorter latency (i.e., forwarding delays) than do store-and-forward switches. But store-and-forward switches provide protocol-based filtering and more sophisticated virtual LAN
grouping based on membership rules.
In contrast to cut-through switches, store-and-forward switches can switch packets between standard Ethernet and Fast Ethernet or between standard Ethernet and FDDI (Fiber Distributed Data Interface) networks, when configured with both the standard and fast switch types. Cut-through switches cannot handle speed conversions unless they include some form of frame buffering. A fast network called vBNS (very high speed Backbone Network Service) was recently announced by MCI and the National Science Foundation. It combines ATM (asynchronous transfer mode) and SONET (Synchronous Optical Network) technologies and should achieve speeds of 600 Mbps by 1996. Advances such as this will make speed-switching devices essential.
We chose standard 10-Mbps Ethernet switches because they represent a large installed base. For in-house workgroup needs, 10 Mbps is usually sufficient.
HOW TO USE THIS GUIDE
We combine our low- and high-level performance test results with usability and features ratings to choose winners in categories by technology (i.e., cut-through and store-and-forward) and then by configuration or expandability (i.e., stackable and rack-mountable). Some are hybrids. You can configure Kalpana's cut-through EtherSwitch EPS-2115M to perform a few store-and-forward functions.
Using charts, we summarize test details about the winners and runners-up in each of these categories.
Overall Evaluation Score:
A weighted average of each switch's low-level and applications performance score (75 percent), the features score (15 percent), and the usability score (10 percent), based on a 10-point scale. Higher numbers are better.
Features:
Reflects the quality of documentation, ease of setup, and ease of operation.
Usability:
Categorized by suitability to small or large networks, rate management tools, virtual LAN support, scala
bility, compatibility, and fault tolerance.
Price Per Port Test Configuration:
List prices for test configurations. They vary because of the number of ports, speed, features, and bundled software.
Inside a Switching Hub
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RISC PROCESSOR
A dedicated RISC processor (like the 960 shown here) handles FDDI (Fiber Distributed Data Interface) translation and SNMP.
POWER
Power supplies and fans.
FLASH-MEMORY CHIPS
For remote software downloads. Upgraded via the serial port.
PORTS
Look for flexi
bility in port density and media type, including 16 or 8 10Base-T or 12 10Base-FL.
ASICS
Enable optimized performance with full wire speed filtering and forwarding rates.
Store-and-forward switching enables multiple LAN types and speeds (10-/100-Mbps Ethernet, FDDI).
Cams in the ASIC store the forwarding table of Ethernet addresses.
INTERFACES
Modular, optional high-speed interfaces mix-and-match FDDI, 100Base-T, 100Base-VG, Any LAN, and ISL Interswitch Link.
NETWORK INTERFACE CONTROLLERS
Convert Ethernet serial data to parallel data.
29 Switching Hubs Save The Bandwidth
