The initial implementations of the LAT protocol were on the terminal servers described above and on VAX/VMS host systems . The servers i m p l e m e n t e d o n l y t h e m a s t e r e n d of t h e LAT protocol , whereas the hosts implemented the s l ave end . Fo l l ow-on i m p l e m entations have added s i m i l a r s u p po r t for a d d i t i o n a l host systems: t h e MicroVMS , RSX- 1 1 M - PLUS , MicroRSX, ULTRIX-3 2 , ULTRIX-3 2m, TOPS- 1 0 , and TOPS-2 0 systems.
Each system implementation offers access to the command interpreter as the service access point. Figure 5 illustrates this support.
U L T R I X-32 TOPS-20
SYSTEM SYSTEM MicroVMS MicroRSX TOPS- 1 0
SYSTEM SYSTEM DECserver 1 00 . . . T E R M I NALS SYSTEM ETH E R N ET T E R M I N A L S E R V E R T E R M I NALS DIAL-IN MODEMS
Figure 5 Additional LA T Host Support
84 Digital Tecbnicaljournal
VAX/VMS HOST DECserver 1 00 TERMINALS NON-LAT HOSTS ETHERNET TERMINAL SERVER
TERMINALS DIAL-IN PERSONAL MODEMS COMPUTERS
Figure 6 Ethernet Configured as a Service Node
Version 2_0 of the Ethernet Terminal Server, released in August 1985, added the reverse-IAT implementation, permitting a server to offer additional services to which terminal users can connect_ This imple mentation permits sessions to be created within the box as well as across the network, thus forming a switch style of operation in a single server. The types of services that may be offered by the terminal server can be grouped into the following three categories.
The first category is connections to non-IAT hosts. In this mode, the server acts as the Ether net connection for systems (typically not made by Digital) that cannot themselves offer IAT ser vices on the Ethernet. Asynchronous ASCII ports on these systems are connected to a terminal server. Terminal users on the same or different terminal servers can connect to the service offered. They can then communicate with the non-IAT host as though it were connected to the Ethernet.
The second category is service for dial-out modems. Terminal users can connect to a port in a pool of dial-out modems. The users can then use the appropriate ASCII protocol to create a dialed connection and then access the remote system via its own dial-in port.
Digital Techntcaljournal No. 3 September 1986
The third category is serv
i
ce for personal com- puters (PC). They can be cb
I nnected to terminal servers and run in either of the terminal emula- tion modes. Each PC thus acts as though it were a dumb terminal. A PC can 'also run in file trans fer mode when connected to another PC via .the same, or another, terminal server. Figure 6 illus trates the terminal server as a service node.Subsequent versions of the Ethernet Ter minal Server, the DECserver 1 0 0, and the VMS
LTDRIVER software all permit asynchronous printers to connect to terlninat servers. These versions also allow print qu¢ues to be directed to the printers from hosts. The IAT protocol has been enhanced so that th
�
connection mecha nism remains under the cti
j. ntrol of the terminal server (for the reasons of efficiency mentioned previously). That enhancement allows a host to "solicit" a connection from a port on a terminal server. Once the connection has been made, data transfer can occur as in the normal interactive terminal case, except that the printer output is under the direction of a VMS print symbiont. It is possible, with these implementations, to direct the queues from multiple systems to a single printer or bank of printer
s being offered as a common service. When a connection request is made while the printer is�
I eing used by anotherI
85
Terminal Servers on Ethernet Local Area Networks
system, the connection request can be queued. This queuing provides a basic mechanism for sharing printers among multiple systems.
Some of Digital 's personal computers now implement the master end of the IAT protocol and can operate as simple single-session terminal servers. These servers are implemented as part of
the DECnet-DOS and ProjDECnet releases and allow the PC to emulate a terminal connected to a terminal server. Combining this feature with the servers that offer services, a PC user can con nect to any PC that is connected to a terminal server for file transfer applications, to a dial-out modem, or to a non-LAT host system . Data integrity is provided "end-to-end" in PC-based implementations due to the lack of twisted pair, or simihir, wiring. Figure 7 shows the connec tions to asynchronous printers and IAT from per sonal computers.
Within the IAT environment, the service name offered by a host system does not always have to represent the command interpreter on a given system, though this is by far the most common use today. Instead, a service name could repre sent an application program, which might be run automatically when a connection request is made. Alternatively, using the solicited-connec tion mechanism currently employed for printers,
DECnet-DOS
VAX/VMS HOST
D ECserver 1 00
applications programs could initiate connections to terminals (or other asynchronous devices) located within the LAN.
DECserver 200
The DECserver 1 00 interconnects terminals in an office environment at a very low price. Soon after it was announced, it became clear that modem-controlled lines and connections to non rAT host systems should also be priced just
as low.
Thus the DECserver 200 project was initi ated to produce a new server based on the DEC server 1 00 design, but with modem control capa bilities. Moreover, this product had to meet the original cost goals of the DECserver 1 00. This project involved a redesign of the printed circuit board, yet retained the same system architecture. A faster version ( 1 0 MHz) of the same MC68000 microprocessor was used, and memory was increased from 1 28KB to 384KB of RAM and
from 5 1 2 bytes to 2KB of
NVRAM
. This increaseallowed room for the implementation of modem control software and support for non-IAT hosts (i.e. , reverse-rAT capabilities) . The increase also allowed a larger service directory database to be stored and an enhanced on-line help capability to be added. PRO/DECnet NON-LA T HOSTS ETHERNET TERMINAL SERVER ETHERNET TERMINAL SERVER .
TERMINALS ASYNCHRONOUS TERMINALS DIAL-IN ASYNCHRONOUS
PRINTERS MODEMS PRINTERS
Figure 7 Asynchronous Printers and LA T on PCs
86 Digital Tecbnicaljournal
No. 3 September 1986
1 New Products
1_.,
::_
Figure 8 DECserver 200
Another feature of the DECserver 200 takes advantage of the new DECconnect cabling scheme, allowing coQnections to be made using DEC423 wiring. This feature allows communica tions at up to 1 9.2 Kbau.d over cable that is nei ther twisted pair nor shielded, for relatively long distances of up to 1 000 feet. Figure 8 shows the DECserver 200 hardware.
Summary
Unlike other existing packet-oriented transport layer architectures, the LAT transport layer implements asymmetric connection manage ment, asymmetric data flows, and timer-based message exchanges.
The most unusual innovation of the IAT archi tecture is the use of multicasting as a presenta tion level naming service . On Ethernet, packets are normally addressed to the adapter of a . specific system. However, the Ethernet specifica tion describes a form of logical addressing called multicast addressing. In this scheme a packet addressed to a multicast address is received nearly simultaneously by many independent sys tems. LAT uses these messages to completely configure the topology automatically. This action means that installing a terminal server is as simple as plugging it into the Ethernet and wait-
ing for services to be advertised. ·
Asymmetric connection management consider ably simplifies the complexity of the protocol in which terminal servers initiate connections to host systems. If a host system wants to connect to a terminal server, that connection must be solic ited from the terminal server. This protocol solves the problem of having many host systems
Digital Technical journal
No. 3 September 1986
competing independently for the same resource. The first "solicitation" is serviced by a connec tion, and subsequent requests are queued on a first-in, first-out basis.
I
On a particular terminal s�rver, all devices that are logically connected to the same host system share messages both to and from that host . Within each message, each user's data is con tained within slots. This �ultiplexing, in con junction with the delay ti
m
er, reduces further the number of messages e:?cchanged. For exam ple, as more users log in to a host system, the number of messages exchanged remains con stant at approximately 1 2 per second in each direction, even as the lengths of the messages increase.The DECserver 1 00 and · DECserver 200 are low-cost implementations of the IAT architec ture, allowing terminals and other asynchronous devices to be configured in a flexible and cost effective manner in a LAN ..
Acknowledgments
,
Over the years, a large number of people have contributed to the architecture and products described in this paper. The authors acknowl edge, with gratitude, all this work. In addition, a number of people have taken the time to review this paper and have made
!
many helpful sugges tions; to these. people we a.lso extend our thanks.References
1 . J. Morency et al., ''The: DECnetjSNA Gateway . I
Product - A Case St;udy in Cross Vendor
Networking,'' Digit
�
l Technical journal(September 1986, this issue) : 35-53. 87
Paul R. Beck james A. Krycka