Mainframes

honorableclunkSoftware and s/w Development

Oct 30, 2013 (4 years and 10 days ago)

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Mainframes


Contain about 70% of corporate data from
operations (accounting, payroll, billing,
etc.)


Often the “database server” in web
-
enabled
database applications

Mainframes


Dominated by IBM


Mainframe competitors build clones


Called
plug
-
compatible machines

Terminal
-
Host Communication


Traditionally, Just a Terminal, Host, and
Transmission Line (Chapter 1)


Poor response time


Poor user interface: sending graphics over a
distance is expensive (and lines usually are
slow)


Inadequate for
production workers

who use
their terminals hours per day

Mainframe Communication


User site has multiple terminal users


3270 Terminals


High speeds, some color, some graphics

User

Site

3270 Terminal

Mainframe Communication


Cluster Controller at User Site


Supports a cluster of terminals and printers


Provides limited on
-
screen
text editing power

to terminals


This elimination of text editing work allows the
mainframe to focus on high
-
value database
chores

Cluster

Controller

Limited

Text Editing

Mainframe Communication


Cluster Controller at User Site


Supports a cluster of terminals and printers


Provides limited on
-
screen text editing power
to terminals


This also
reduces response time

because editing
is done locally

Cluster

Controller

Limited

Text Editing

Mainframe Communication


Cluster Controller at User Site


Multiplexes transmissions of multiple terminals
and printers to the central site


This
reduces transmission costs
, which are
expensive for higher
-
speed long
-
distance links

Central

Site

Long
-

Distance

Line

A

A

A

A

A

A

A

B

B

B

B

Mainframe Communication


Transmission Line


Long
-
distance lines are expensive per bit sent


But 3270 terminals need high speeds


Multiplexes terminal communication onto 56
kbps, 1.544 Mbps or faster line to give high
speed but keep cost reasonable

Central

Site

Long
-

Distance

Line

Mainframe Communication


Central Site


Communications Controller


Handles multiplexing to
reduce transmission
cost


Handles detailed interactions with cluster
controllers,
freeing mainframe to deal with
database processing

Communications

Controller

Mainframe Communication


Mainframe


Handles high
-
value database work


Must be freed of low
-
value communications
processing work to be economically efficient

Mainframe

Mainframe Communication


Reducing Response Time


Text editing work is done locally


Still delay for heavy database work on mainframe


Reducing Transmission Costs


Multiplexing, and


Cluster controller provides limited local screen
editing, so fewer bits need to be transmitted to and
from the mainframe

Mainframe Communication


Reducing Work the Mainframe Needs to Do, so
that it can Focus on High
-
Value Database
Processing


Cluster controller handles most text
-
editing chores
freeing mainframe from having to support this work,
and


Communications controller handles details of
communication with cluster controllers, freeing
mainframe from having to support this work

Application Servers


NOT Part of Mainframe Communications


Can act as terminals or cluster controllers


Transparent to mainframe: no need to do
anything differently on mainframe system

App Server

App Server

Mainframes


Use SNA Standards Architecture Instead
of TCP/IP


Not peer
-
to
-
peer control; Master
-
slave
control under a
System Services Control
Point

program on a mainframe


SSCP program governs all
sessions among
devices

SSCP

Mainframes


SNA Standards Architecture


Like TCP/IP and OSI, uses layering


Uses OSI standards at the physical and data link
layers

Mainframes


SNA Standards Architecture


SNA
Path Control

layer is like TCP/IP internet
layer and OSI network layer


SNA
Transmission Control

layer is like the OSI
and TCP/IP transport layer


However, not peer to peer operation;
master
-
slave operation

under the control of the SSCP

Mainframes


SNA Standards Architecture


Highest layers are like OSI layers


Network Addressable Unit (NAU) Services

layer is like
OSI session layer


Data Flow Control

layer is like OSI presentation layer


However, applications are not standardized within SNA


There is
no SNA application layer

Mainframes


SNA NAUs


Communication takes place between
network
addressable units (NAUs)


Unit is a general name for a communicating
entity


In networks, communicating entities must have
addresses; So they are network addressable
units

Mainframes


Logical Units


Deal directly with end users


Terminals

are logical units!


Connection points on mainframe (not
mainframe itself) deal with application
programs,which are considered to be end
users

LU6

Connection

Application

Human

User

Mainframes


Physical Units


Do not deal directly with end users


Mainframe, communications controller, cluster
controller


Path control network

connects cluster
controller and communication controller

Path

Control

Network

Mainframes


SSCP


Third type of NAU


Program located on the mainframe


In classic SNA, two other NAUs can only be
connected under the control of the SSCP

Mainframes


Versions of SNA


Classic SNA


All communication under the control of SSCP


Advanced Peer
-
to
-
Peer Networking (APPN)


Newer; NAUs can connect directly


High
-
Performance Routing


Classic SNA and APPN are difficult to route


Newer still; HPR improves routing

Mainframes


SNA and Router Networks


To link cluster controllers to communications
controller over routed networks


Data Link Switching (DLSw)

standard supports
SNA transmission through routers


High
-
Performance Routing (HPR)

is better

Mainframes


Mainframes and TCP/IP Networks


TN3270E


TN3270E servers

communicate with
mainframe


Users have PCs with
TN3270E client
software

that
emulates 3270 terminals

TCP/IP

Network

PC with

TN3270E

Client

TN3270E Server