Home' RTCA Documents for Review : DO-230H FRAC Contents 255
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More application load on less server hardware requires a higher-performance network, and also requires a
very low latency, lossless architecture that lends itself to a two-tier approach. Storage traffic cannot tolerate
the buffering and latency of extra switch hops through a three-tier architecture that includes a layer of
aggregation switching. Alternatives to the Spanning Tree standard to facilitate this are being pursued in the
IEEE’s Data Center Bridging/Converged Enhanced Ethernet work, and ANSI T11’s Fiber Channel-over-
VLANs (Virtual LANs) are logical groupings of network users and resources connected to defined ports
on a switch. For an ISSA, VLANs enable identified ISSA data to be isolated from other network users.
Other potential benefits include:
Simplified administration when moving or changing groups of computers.
Better control over broadcasts.
Tightened network security through traffic filtering on each port.
Increased server security through unique VLAN segmenting.
Many switches support VLANs based on physical port, IEEE 802.16 tag, or combinations of port and tag
assignments. In practice, most VLANs use a combination of both methods. The 802.16 standard becomes
extremely useful for multiple VLANs and the need to span a server or another resource computer across
certain VLAN ports.
VLANs permit the system designer to group devices logically into a single broadcast domain which means
that broadcast traffic for a particular workgroup can be confined to just those devices that need to see it,
thus reducing traffic to the rest of the network in addition to securing the data from unauthorized users of
the network. This also results in increased connection speed due to the elimination of latency from router
connections. The system designer can use this opportunity to create a network that is independent of
physical location and group users into logical workgroups, e.g., if a department has users in three different
locations, the users can now access servers and printers as if they were all in the same building.
VLANs require a switched network, and the switches must be VLAN ready. Some older switches are not
able to use this feature for lack of configuration support, particularly LANs configured around hubs. If new
equipment is to be purchased for the VLAN, the switches should be specified to have Layer 3 capability.
Managing switched VLANs can be challenging, and the management functions should be addressed during
system design. A system administrator should be provided with a GUI interface for segmenting or sub-
netting the network, for monitoring VLAN performance and effectiveness in isolating data streams, and for
facilitating VLAN re-configuration during the moves/adds/changes which frequently occur at airports.
While consumed or delivered bandwidth may be much less than the interface bandwidth, Ethernet
connections--especially client-facing connections--on average operates at a fraction of the available
bandwidth anyway, thanks to the bursting nature of the data traffic. With the exception of WAN
connectivity of large data centers, historical evidence for service provider leased lines and data services
also points to LAN and WAN connection utilization far less than 100%.
TCP/IP typically requires about 20 bytes in overhead. Approximately 34 bytes are required for the Media
Access Control policy, and a delay of 192 μsec is incurred in accessing the 802.11b highway (20 μsec is
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