More on TerminalHost Communication

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More on Terminal-Host Communication

• Module H

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Terminal-Host Systems

• Components
• Terminal
• Host (locus of processing)
• Transmission line

Host
Terminal
Transmission Line
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Terminal Emulation

• People Already Have PCs
• Host operating systems only work with terminals
• Do not want to buy a terminal as well
• PCs can emulate (act like) terminals

Host
PC
Transmission Line
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Locus of Processing

• Processing is Done on the Host
• Usually, poor response times
• Usually poor user interface (mostly text, simple
  graphics if any)

Host
Terminal
Transmission Line
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Legacy Systems

• Moving applications from hosts to other
  platforms, especially client-server
• Saves money
• Improves response times

Host
Terminal
Transmission Line
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Legacy Systems

• However, moving applications is slow
• Many applications with large investments
• So in many firms, host legacy systems still run a
  large share of all applications

Host
Terminal
Transmission Line
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Matching Terminals and Hosts

• Designed as a Complete System
• Optimized for specific uses
• Cannot mix and match any terminal with any host
• Most hosts only work with a few terminals

Host
Terminal
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VT100 Systems

• Use VT100 terminals
• Inexpensive
• Low-performance
• For light users
• Lowest common denominator supported by many hosts

VT100 Terminal
Supportive Host
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IBM Mainframe Systems

• Designed to support hundreds of simultaneous
  terminal users dispersed around the globe
• Designed for heavy users, such as clerks, who
  will use the system many hours per day and so
  need fast response times and reliability
• High performance, high cost
• Use SNA protocols instead of TCP/IP

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IBM AS/400 Systems

• AS/400s are small business computers
• Minicomputers optimized for business transactions
• Widely used in industry

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Windows Terminals

• Run Windows Programs
• Execution is done on a terminal server
• Results are shown on the WT screen
• Can run Windows applications

Windows Terminal Server
Windows Terminal
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Network Computers (NCs)

• Often replace terminal-host systems
• But processing is done on network computer, so it
  is not a true terminal
• Low cost devices
• Programs downloaded from servers
• Limited to Java language programs

NC
Java
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VT100 Terminal Systems
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Basic Operation

• Each keystroke is sent to the host for processing
• Echoed back before shown on screen
• Dumb terminal

Host
VT100 Terminal
Keystroke
Echo
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Basic Operation

• Each keystroke is sent to the host for processing
• Long delays if host is overloaded
• Places a heavy load on the host and network

Host
VT100 Terminal
Keystroke
Echo
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VT100 Limitations

• Designed in the 1970s
• Simplicity allows it to be supported easily, so
  widely supported
• Poor user interface
• Text only
• Monospaced font
• Monochrome (text against contrasting background)

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VT100 Limitations

• Slow Speed
• Uses serial ports
• 9-pin and 25-pin versions both work fine
• These ports were designed to work at less than 20
  kbps
• Although modern serial ports are faster, VT100s
  still run at 9,600 bps or less

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VT100 Limitations

• Limited to ASCII codes
• Designed for printed text in English
• 7 bits per byte
• Last bit in byte is for error detection

Symbol
ASCII
H
1001000
h
1101000
space
0100000
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VT100 Limitations

• 8-Bit ASCII
• Used on PCs
• No eighth error checking bit
• All 8 bits represent data

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Asynchronous Transmission

• Each character is placed in a frame by itself
• Each frame is sent separately
• There may be time wasted between frames

H
e
l
l
o
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Asynchronous Transmission

• Each frame has ten bits
• For 8-bit ASCII start bit, stop bit, 8 data bits

Stop Bit (1)
Start Bit (0)
8 Data Bits
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Asynchronous Transmission

• Idle line state is one (1111111111)
• Start bit (always 0) changes the line state to
  signal the start of the frame
• Stop bit (always 1) gives one time period of idle
  so that next start bit can be read unambiguously

Start Bit (0)
Stop Bit (1)
8 Data Bits
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Asynchronous Transmission

• Data bits sent in reverse order
• If 11110000,
• Sent as 00001111
• So if parity is even, send 0000011111
• Also reversed in 7-bit ASCII

Start Bit (0)
Stop Bit (1)
8 Data Bits (00001111)
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Asynchronous Transmission

• For 7-Bit ASCII, still 10 bits per frame
• Start bit
• 7 data bits (remember that order of bits is
  reversed)
• parity bit
• Stop bit

Start Bit (0)
Stop Bit (1)
7 Data Bits
Parity Bit (1 or 0)
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Parity

• For 7-bit ASCII Only (No Parity 8-bit ASCII)
• Transmit an 8th bit per character
• Even parity sum of data and parity bits is even
• To send 1110000 (odd), send 00001111
• Note that order of data bits is reversed
• To send 1111000 (even), send 00011110
• Odd parity sum is odd
• If error is detected, the character is simply
  discarded

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Asynchronous Transmission

• Another Look at 7-Bit ASCII Frames (direction
  reversed)

4. Parity Bit
1. No Data State (1)
3. 7 Data Bits
7. Next Character
6. No Data State (1)
2. Start Bit (Always 0)
5 Stop Bit (Always 1)
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Asynch Settings are Not Standardized

• Number of data bits/use of parity
• Number of stop bits
• Usually one but can be more for longer rests
  between frames
• Full versus half duplex
• Almost always full duplex today
• Speed (2,400 bps, 9600 bps, etc.)
• Setting up a VT terminal requires you to know
  what the host requires for these parameter

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VT100 Terminal Emulation

• Turn a 2,000 PC into a 200 VT100 Terminal
• Needed to be able to talk to some hosts
• Serial Ports are all the hardware you need
• Implement asynchronous ASCII transmission

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VT100 Terminal Emulation

• Only Need Software
• Communications program
• Dials the host and logs you in
• Transfers files between PC and host
• Executes scripts (often-repeated series of
  instructions)

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Telnet

• Reach Host Through the Internet
• Need telnet program on PC and Host
• Log in as VT100 terminal user

Telnet Program
Telnet Program
Host
Internet
PC
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Telnet

• Reach Host Through the Internet
• No setup needed (parity, etc.) this is all
  standardized
• Limited to the Internet

Telnet Program
Telnet Program
Host
Internet
PC
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IBM Mainframe Communication
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IBM Mainframe Communication

• Designed to Support ...
• Thousands of Terminals
• Scattered Across the World
• With high-performance computing
• With high reliability

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Hardware Elements

• Mainframe Computer
• Very-high-speed machine for even the largest
  applications
• Optimized for business processing (disk and file
  handling)

Mainframe
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Hardware Elements

• IBM Mainframe Operating Systems
• MVS/CICS and MVS/IMS for transaction processing
• VM/CMS for decision support applications

Mainframe
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Hardware Elements

• Terminal
• High-speed interface up to 2 Mbps
• Graphics (limited) and color
• Usually called 3270 terminals after early model
  numbers

3270 Terminal
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Hardware Elements

• 3270 Terminal Emulation
• Does not use asynchronous communication
• Uses EBCDIC instead of ASCII
• Requires kit consisting of expansion board and
  software

PC Emulating 3270 Terminal
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Support from PC Networks

• Synchronous Communication Gateway Server
• Acts as cluster controller
• Provides terminal emulation without hardware on
  client PCs
• Converts between TCP/IP or IPX/SPX and SNA

Communications Controller
Gateway
LAN
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Hardware Elements

• Cluster Controller
• Manages several 3270 terminals and printers in an
  office or remote site
• Provides processing power for text editing
• Good response time for simple text editing tasks
• Reduces the load on the host and the network

Cluster Controller
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Hardware Elements

• Communications Controller
• Manages cluster controllers
• Reduces the load on the mainframe
• Implements multiplexing with cluster controller
  for multiple terminal-host connections to reduce
  transmission costs

Multiplexing
Cluster Controller
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IBM Mainframe Systems

• Response Time
• Fast terminals
• Fast mainframe
• Fast transmission lines
• Cluster controller editing to reduce some
  response times
• Rich User Interface
• Graphics
• Color

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IBM Mainframe Computers

• Reduce the Load on the (Expensive) Mainframe
• Communications controller offloads communications
  work
• Cluster controllers local editing capability
  offloads simple editing work
• Mainframe can focus on the application(s)

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IBM Mainframe Computers

• Reduce the Load on the Network
• Long-distance communication is expensive
• Multiplexing of lines between the cluster
  controller and the mainframe reduces costs
• Local editing via the cluster controller
  eliminates communication associated with simple
  text editing

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Systems Network Architecture (SNA)

• IBM mainframes do not use TCP/IP
• They use Systems Network Architecture (SNA)
• Architecture designed for IBM mainframes
• Used by other mainframe vendors as well
• SNA traffic can dominate long-distance
  communication costs

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SNA Software

• On the mainframe, SSCP
• Systems Services Control Point
• To manage all connections between network
  addressable units (later)
• On the mainframe, VTAM
• Virtual Telecommunications Access Method
• On the communications controller, NCP
• Network Control Program

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SNA NAUs

• Network Addressable Units (NAUs)
• Resources that can communicate with one another
• This requires an address
• User NAUs
• Human users
• Application programs
• Only application programs have addresses, but
  users are still included in the picture

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SNA NAUs

• Logical Unit (LU) NAUs
• LUs deal with users
• Terminals (LU 2) deal with users
• Mainframe connections to application programs (LU
  6)
• Note logical units can be hardware!

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SNA NAUs

• Physical Unit (PU) NAUs
• Units that do not deal with users
• Cluster controllers (PU2)
• Communications controllers (PU4)
• VTAM (PU5)
• Note physical units can be software (VTAM)!

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Role of SSCP

• System Services Control Point program
• Before two NAUs can communicate, SSCP must allow
  it.
• Central point of control very different from
  TCP/IP, OSI
• SSCP sets up a session
• Manages the communication during the session
• Breaks down the session

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Layering

• Very different from OSI, TCP/IP and IPX/SPX
• Designed for terminal-host communication
• Master-slave focus, not peer-peer
• Real time interaction versus sending messages and
  later getting responses

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Layering

• End User
• Governs application program interactions
• Not really in SNA
• NAU Services
• Data formatting
• Program interfaces
• Data Flow Control
• Flow control within a session

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Layering

• Transmission Control
• Controls sequence numbers for frames
• Path Control
• Similar to the OSI network layer
• Data Link Control
• Similar to the OSI data link layer
• Physical Layer
• Similar to the OSI physical layer