DTJ Number 3 September 1987 - Digital Technical Journals

The Extended Local Area Network Architecture and LANBridge 100MICRO-PROCESSORlPROGRAMMEMORYIETHERNETADDRESSMEMORYPACKETETHERNETETHERNETME M ORY INTERFACE INTERFACEETHERNETLANETHERNETLANFigure 5High-level Block Diagrammore , it must have additional time available toperform other functi(;)ns, such as updating timersand running the spanning-tree algorithm to correctpossible faulty network configurations. Atpower-up or system reset, the microprocessormust verify via diagnostic code that the entiresystem is operating correctly.The design of the real-time code paths wasfairly straightforward. It was written in a detailedoutline form for a generic processor. That. allowed us to understand the requirements andselect a processor with enough power. Fromthis design , it was quite clear that a highperformancemicroprocessor was required. The10-MHz MC68000 chip from Motorola, Inc., waschosen based on its available power and attractiveprice.In this design, the microprocessor has a privatememory. Thus instruction and local-data accesswill not conflict with packet data flowing to andfrom the two Ethernet interfaces. Some of thismemory is ROM, · which contains all the codeneeded by the bridge to be fully functional onpower-up. The bridge also contains RAM, used asa writeable data area. There is a small amount ofnonvolatile RAM (NVRAM) , which stores systemspecificparameters that must survive power failuresand be available on the next system start-up.Ethernet InterfaceThe Ethernet interface is a complex function thatis implemented most economically in VLSI. Theinterface can be implemented at the board level ,but only at considerably greater expense. Since aVLSI implementation was clearly the most attractiveoption, we explored a number of alternativesources for it.Data integrity is one of the more importantconsiderations in designing a bridge . In particular,the bridge should not cause undetectabledata errors in a packet delivered to a destinationstation. This injunction implies that either thepacket memory in the bridge must have a verylow probability of error or the original CRC generatedby the source station must be forwarded ·with the packet. If the original CRC travelsthrough a bridge with the packet, then anypacket memory errors will be detected as transmissionerrors at the destination station.The only available chip set that allowed packetsto be transmitted without a recalculated andappended CRC was one made by Advanced MicroDevices Corporation. This chip set was calledLANCE (Local Area Network Controller for Ethernet). Although other considerations were important,this very important capability was thedeciding factor in our selection process.Network Address Look- upThe network address look-up mechanism is oneof the most interesting aspects of the LAN­Bridge 100 design. Upon receiving a packet, thebridge must locate the information associatedwith its destination address so that a forwardingdecision can be made. In addition, the sourceaddress must be added to the database unless it68Digital TecbnicaljournalNo . 3 September 1986