Mainframes • Contain about 70% of corporate data from operations (accounting, payroll, billing, etc.) • Often the “database server” in web-enabled database applications
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
UserSite
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
ClusterController
LimitedText 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
ClusterController
LimitedText 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
CentralSite
CentralSite
Long-Distance
Line
A A A A AA A
BB
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
CentralSite
CentralSite
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
CommunicationsController
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
LU6Connection
Application
HumanUser
Mainframes• Physical Units
– Do not deal directly with end users
– Mainframe, communications controller, cluster controller
– Path control network connects cluster controller and communication controller
PathControlNetwork
PathControlNetwork
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/IPNetwork
TCP/IPNetwork
PC withTN3270E
Client
TN3270E Server