DTJ Number 3 September 1987 - Digital Technical Journals

1 New Products,. •..1 _., - ·-. -- ::_Figure 8 DECserver 200Another feature of the DECserver 200 takesadvantage of the new DECconnect cablingscheme, allowing coQnections to be made usingDEC4 23 wiring. This feature allows communicationsat up to 19.2 Kbau.d over cable that is neithertwisted pair nor shielded, for relatively longdistances of up to 1000 feet. Figure 8 shows theDECserver 200 hardware.SummaryUnlike other existing packet-oriented transportlayer architectures, the LAT transport layerimplements asymmetric connection management,asymmetric data flows , and timer-basedmessage exchanges.The most unusual innovation of the IAT architectureis the use of multicasting as a presentationlevel naming service . On Ethernet, packetsare normally addressed to the adapter of a .specific system. However, the Ethernet specificationdescribes a form of logical addressing calledmulticast addressing. In this scheme a packetaddressed to a multicast address is receivednearly simultaneously by many independent systems.LAT uses these messages to completelyconfigure the topology automatically. Thisaction means that installing a terminal server is assimple as plugging it into the Ethernet and waitingfor services to be advertised.·Asymmetric connection management considerablysimplifies the complexity of the protocol inwhich terminal servers initiate connections tohost systems. If a host system wants to connect toa terminal server, that connection must be solicitedfrom the terminal server. This protocolsolves the problem of having many host systemscompeting independently for the same resource .The first "solicitation" is serviced by a connection,and subsequent requests are queued on afirst-in, first-out basis. IOn a particular terminal srver, all devices thatare logically connected to the same host systemshare messages both to and from that host .Within each message , each user's data is containedwithin slots. This ultiplexing, in conjunctionwith the delay timer, reduces furtherthe number of messages e:?cchanged. For example,as more users log in to a host system, thenumber of messages exchanged remains constantat approximately 12 per second in eachdirection, even as the lengths of the messagesincrease.The DECserver 100 and · DECserver 200 arelow-cost implementations of the IAT architecture,allowing terminals and other asynchronousdevices to be configured in a flexible and costeffectivemanner in a LAN ..Acknowledgments ,Over the years, a large number of people havecontributed to the architecture and productsdescribed in this paper. The authors acknowledge, with gratitude, all this work. In addition, anumber of people have taken the time to reviewthis paper and have made!many helpful suggestions;to these. people we a.lso extend our thanks.References1 . J. Morency et al ., ''The: DECnetjSNA Gateway.IProduct - A Case St;udy in Cross VendorNetworking,'' Digitl Te chnical journal(September 1986, this issue) : 35-53.Digital Technical journalNo. 3 September 198687