Transcript Mainframes

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
Limited
Text Editing
Cluster
Controller
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
Limited
Text Editing
Cluster
Controller
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
A A
A
B
AAA B B
B
LongDistance
Line
A
Central
Site
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
LongDistance
Line
Central
Site
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; masterslave 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
Application
Human
User
LU6
Connection
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
PC with
TN3270E
Client
TCP/IP
Network
TN3270E Server