DTJ Number 3 September 1987 - Digital Technical Journals

Terminal Servers on Ethernet Local Area Networkstypically 80 milliseconds . With many usersconnected, a host is interrupted much less oftenby gathering together all the characters typedby those users and sending them as a singlemessage.The lAT . protocol is divided into two distinctlayers, the virtual circuit layer and the slot layer.Virtual Circuit LayerThe virtual circuit layer establishes and maintainsan error-free communications path (avirtual circuit) between two nodes, typically aterminal server and a host, that wish to communicate.The connection is initiated by one end ofthe communications path and operates under thecontrol of the initiator. However, the circuit canbe terminated by either end. Typically, the virtUalcircuit connection is initiated when the firstterminal user requests a connection to a host systemto which no virtual circuit yet exists. Theinitiator of the virtual circuit is referred to as the"master node," the other end as the "slavenode." Thus the terminal server is normally themaster and the host the slave.The establishment of a virtual circuit connectionrequires a single message exchange. Informationsuch as protocol versions, message sizes,and node names are included in these messages.Simplified View of VirtualCircuit OperationWe start with a simplified explanation of the virtualcircuit operation. Once established, the dataexchange occurs as follows:• Every 80 milliseconds, the master sends to theslave a message containing any data that mustbe sent.• On receiving this message , the slave processesany data in that message and sends back areply containing any data waiting to be sent inthat direction.have ignored errors that may occur in messagedelivery, and we have assumed message deliveryeven when there is no data to send. We willexamine the implications of these cases shortly.The protocol as defined is, in effect, a requestresponseone. Such a protocol has the characteristicthat only one data link buffer need be allocatedat each end of the virtual circuit. This factcan be important for hosts that need to supportlarge numbers of virtual circuits without dedicatinglarge quantities of buffer space to that task.The termination of a virtual circuit can occurfrom either end; under normal conditions, however,the master usually initiates the closing.The IAT protocol defines three messages atthe virtual circuit layer: the start, run, and stopmessages. Thus for a typical virtual circuit, wemight see the exchange of messages depicted inFigure 2 (again, making the stated simplifyingassumptions) .Knowing the built-in limits on maximum messagesize and the rate at which IAT messages areexchanged, we determined that the maximumamount of data that can be transferred across anyvirtual circuit is just under 150,000 bits per secondin each direction. (In fact, the IAT protocoldefines a method for increasing the availablebandwidth for a virtual circuit by using multipledata link messages. To date, there has been noTIME(MILLISECONDS)MASTER0 START510 RUN1590 RUN95170 RUNSLAVESTARTRUNRUN• On receiving this reply, the master processesany data that was in the message.175RUN• Eighty milliseconds after one message wassent, the next message is seiu from the master.The message round-trip time is typically lessthan 10 milliseconds. This operation is timerdriven on the master, the terminal server, andevent driven (by message receipt) on the slave,the host. The operation is simplified because wenRUNn+5RUNn+10 STOPFigure 2 Exchange of Messages78Digital Tecbnical]ournalNo. 3 September 1986