Distribuci´ on muestral de la raz´ on de varianzas

GAS provides support for the gateway access pro­ tocols (GAP) . GAS i s basically a message switcher that receives messages from the IBM SNA network session and sends them along the correct DECnet logical links. GAS also takes mes­ sages received from DECnet logical links and sends them to the correct IBM SNA sessions. A DECnet logical link and an IBM SNA session are associated with one another at connection time. Connections into the IBM applications are always initiated from the DECnet side of the gateway.

GAS concerns itself only with the SNA bind message because it determines the buffer size that the gateway will receive from and transmit to the IBM SNA network. These sizes are allo-

Digital TecbnicalJournal No. 3 September 1986

cated from a common buffer pool, and each side of the connection has a maximum allocation limit. The buffers are allocated until the allocation exceeds the maximum allowed. This method provides the best allocation of buffer memory, but it does not guarantee a fixed number of sessions since a single buffer can be allocated that exceeds the allocation limit for a session. Therefore, the 32 sessions that the gateway documentation discusses only occur if the allocation limits are not exceeded by the sessions. The server must per­ form protocol work only at the start and end of the session.

Remote job Server (R]SRV)

RJSRV is by far the most complicated program in the gateway. RJSRV supports multiple SNA remote job entry workstations. Each workstation contains a DECnet control link, multiple IBM sessions, and multiple network files. This handling of many different linkages has almost transformed the server from a simple message switcher to a " micro-operating" system, since it performs these activities in real time. This micro-operating system provides a scheduler for events, common termina­ tion routines, and common buffer allocation methods.

As with GAS, a DECnet host program initiates

the connection with RJSRV, thus establishing the workstation connection. The number of workstations and the sessions per workstation are limited only by the available memory in the gateway. Because of this l i mitation , each workstation is treated as an RSX pro­ gram logical address space (PLAS) region. Ses­ sions can then be allocated from the PLAS region.

The messages from the DECnet control link are parsed, and the actions taken vary depending on the current workstation state. At the same time, IBM SNA sessions may be active, receiving printer or punch records, or transmitting reader records. The server provides all the SNA proto­

cols for transmission control (TC) , data flow control (DFC) , and function management head­ ers (FMH) . In addition, RJSRV provides support for SNA character strings (SCS) and LUI com­ pression. These facilities and the permutations of different states make R)SRV rich in functional­ ity and fairly complex in terms of its internal structure. Digital TecbnicalJournal No. 3 September 1986 I

I

I

I

Host Command Facillty Server

(HCFSRV)

!

HCFSRV is a program lying

1

₁ midway in complex-_. ity between GAS and RJSR:V. HCFSRV performs some SNA protocols fo

�

the sess ions that have been established. It d,iffers, however, from the other servers in that the IBM application initiates the connection.

Af

₁ ter the IBM applica- tion session has been eStablished, HCFSRV receives the VMS host name and establishes a _I DECnet logical link with that node. HCFSRV then continues to provide SNA protocol support _I after the session to the VMS host has been estab- lished. This server can ha

d

dle multiple sessions

. I

from the IBM network, but the number of ses- sions is limited by the a

rri

ount of buffer space

available.

r

i I

The Gateway Hardw4re

The DECnetjSNA Gateway

[

software runs on two hardware configurations': a PDP- 1 1 j24 with RX02 disks, DMR l l s for

(

the DECnet connec­ tion, and DUPl l s for the 1sNA connection; and the Digital Ethernet Communications Server (DECSA) . DECSA is the ne

rw

ork equivalent of a communications controller, such as the DZ 1 1 ,

I •

DMF32, or DUPl l . A serv(lr is a shared resource for the hosts in an Ethernet andjor wide area

I

networks connected to an1 Ethernet. The se�er performs specific communications functions for , I

these hosts. The hardware components are packaged in a freestandin

g

, table-top unit with self-contained power and cooling; it can operate in an office environment o

i-

_I in a computer room. At start-up, the unit performs a brief self-test. Then the appropriate se

nt

er software is down­ line loaded from a Phase _• W I DECnet host on the same Ethernet, and the unit begins operations as a DECnetjSNA Gateway.

i

Summary

I

We ·have enumerated the

!

many diverse issues that need to be addressed as part of a network _I interconnection process. This process is, to say _I the least, a complex one.

I

An effective network interconnection scheme can result only from an effective architectural

!

and implementation

process. 1 ·

Numerous aspects of cross-network intercon­ nect must be considered i'f the final result is to meet the end-user's ne,eds . The following aspects should be considered.

5 1

The DECnetjSNA Gateway Product

• One must clearly understand the properties of

all architectures to be interconnected to deter­ mine the most effective level of interconnec­ tion between them from a base services stand­ point.

• In implementing the interconnect, one must

take consistent approaches that take into account both the turnkey functions to be implemented as well as end-user requirements concerning those functions. (For example, is it effective to split functions across multiple sys­ tems, and if so, what are the benefits?) A modular approach that uses effectively both hardware and software "building blocks" is also important for reliability, maintainability, and reusability considerations. Thus it is as important to provide a modular implementation consisting of known, proven software segments as it is to use a framework that allows "mixing and match­ ing" pieces to facilitate the development of new functions for various base systems. Once a struc­ ture has been defined, the turnkey functions themselves must be of a bidirectional nature, allowing users in one environment equal access to the resources 'of the other (provided, of course, they are authorized to do so) .

Coincident with all this functionality is the need to manage it effectively, either from a cen­ tralized point in the network or at the distributed points closest to the actual work being done. An interconnect structure must be chosen that allows the continuing use of existing mecha­ nisms with convenient "hooks" to access other environments, if needs so dictate. Finally, the approach chosen must be flexible enough to allow existing structures to migrate conveniently to more cost-effective technologies as they become available, all without disrupting the user interface. The preservation of existing user investment must always be a key concern.

All these goals were met in the existing DECnetjSNA Gateway product set. Our approach is the.result of a carefully considered structure, not of an ad-hoc collection of functionality. That structure facilitates the rapid development of new functionality today and preserves existing application investments for the increasingly dis­ tributed processing world of tomorrow. We expect these products to be key components of the network that eventually becomes the system. ·

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In document PARA ESTUDIANTES DE CIENCIAS (página 127-130)