1. Chapter 10 - X.25 and Network Management
This is about the Wide Area Network using
X.25 packet switching.
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2. Objectives The X.25 network The network management in various networks
Modem diagnosis
Network design
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3. X.25 Defines three layers only not the whole seven layers including:
Physical layer
Data link layer
Network layer
Does not define internal subnet protocols.
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4. Example of X.25 Network Sunday, April 21, 2019Sunday, April 21, 2019
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5. X.25 Level 1 Equivalent to ISO/OSI Physical layer X.21 bis
Interface to analog network
Equivalent to RS-232C - V.24.
X.21
Interface to digital network (may also support X.21 bis/RS232C to those which does not support).
Limited Support in Hong Kong
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6. X.25 Level 2
This is consistent with OSI layer 2. The function of this level is to ensure reliable communication between DTE and DCE.
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7. X.25 Level 2 protocols LAP (SARM) (Set Asynchronous Response Mode)
LAP-B (SABM) (Set Asynchronous Balance Mode)
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8. comparison between the OSI and X.25 protocol structure.
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9. X.25 Level 3
This is similar to OSI layer 3 and is called the X.25 Packet Level Protocol (PLP). It provides:
Virtual circuits. (With permanent virtual circuits and switched virtual circuits analog to leased and switched lines in PSTN.)
Datagram (Sending individual connectionless oriented packets through different communication paths)
PAD support
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10. Call Establishment Establish virtual circuit
Call request by sending a Call Request packet
Call accepted by responding a Call Accepted packet
Data exchange once the connection has been established.
To terminate the connection is
To send a Clear Request packet
Wait for the confirmed by receiving a Clear Confirmation packet
Time-out is used to establish limits on how long it takes to get connections.
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11. Procedures of exchanging data packets
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12. Packet assembler-disassembler
Terminal to X.25 Network
X.28 - this defines the interface between the terminal and the PAD.
X.29 - this defines the host (DTE)-PAD interface.
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13. X.25 packet header
The relationship between a frame and a packet with appropriate labelling for virtual channels and paths.
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14. The services offered by X.25 include:
Switched virtual circuits (SVC) (Analog to public switched telephone service)
Permanent virtual circuits (PVC) (The connection is automatically established once the device connecting to X.25 network is booted up.)
Datagrams(Equivalent to connectionless service. Not explicit virtual path is set up prior to sending packets.)
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15. Network Management
Once a network has been established, it is a good idea if one can
control it;
monitor its performance;
diagnose faults;
reconfigure in the event of failure.
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16. THREE Factors for Network Management
People, There are not many on the market and requires on-the-job training. Usually,, the problem for the systems designer (network engineer) is to estimate the likely incidence of faults for each component in the data communications systems.
Equipment, This includes: testing equipment for diagnosing faults; reconfiguring equipment such as patching and switching facilities.
Software, Among other things,, the software can provide performance statistics to help sort out the bottleneck.
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17. Measures of System Effectiveness
Three factors, namely, availability, reliability and cost-effectiveness are used to measure how well the system is.
Availability, It means that all necessary components are operable and accessible. For a connection to CityU through the CityU Link Plus, it includes your PC, Windows 95, Browser, modem, telephone exchange, remote server etc.
Accessibility means the user can make use of the component when needed.
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18. Performance Factor
Mean Time Between failures (MTBF), and Mean time to Repair (MTTR).
A modem with MTBF of 1000 hours that operates an average of three days a week, 20 days a month, would be expected to fail once every 1000/(3 x 20 ) = 16 months.
MTTR is the average amount of time required to restore the failure into normal service such as replacing faculty modem using 30 minutes.
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19. Reliability
It is about the probability that the system will continue to function over a given operating period.
If a transaction requires 1 second for a response to be received, then the reliability of the system is the probability that the system will not fail during that second.
Reliability includes the error characteristics of component such as modem, PC, Windows 2000 and the transmission telephone line.
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20. Performance Statistics
Statistical information can be collected in the following IBM devices:
IBM Host (IBM Product Code 3090)
Front End Processor (3725)
Intelligent Terminal Control Unit (3174)
External Equipment (3278/3279)
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21. Performance Statistics Factor
Character in/out, This indicates the loading on the system and the traffic fluctuation during the day. It is helpful in keeping track of traffic growth.
Polls sent, Is particular useful on networks using statistical multiplexers
Retries, This indicates the number of messages that are re-transmitted. This statistic can be a useful diagnostic tool and can foreshadow a potential network component failure.
Time-outs, Indicates how many polls or messages sent to terminals did not get a response.
Response Times, This is both a planning tool and a diagnostic aid.
Line utilization, This gives an indication of the loading on a line.
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22. Goals of Datacomm Testing
The goals of datacommunication testing are to:
Increase system availability by
Minimizing the network down time
Expediting repairs by identifying precisely the faulty network components
Eliminating finger pointing by randomly repairing functioning network components
Debug network applications
Develop good relationships with data communications vendors
Isolate the failing component within the network
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23. Testing domain Analog testing Transmission impairment and continuity
Measuring sets include voltmeter
Digital
Bit Error Rate Tester. It is used to measure the ratio between the error bits against the totally received bits including the error bit.
Protocol
Serial Data Analyzer (It captures the status of every V24 interface lead for every bit time on the interface. Data can be stored on tape or diskette for later analysis.)
LAN Protocol Analyzer (To measure the network loading, performance and traffic pattern.)
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24. Breakout box
The use of break-put box to diagnose the network connection between a modem and terminal.
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25. Protocol Analyzer
The use of protocol analyzer to monitor the contents of data movement between a computer and a modem.
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26. Characteristics of protocol analyzer
Simulate and Monitor Modes
Simulate mode is to configure the protocol analyzer to act as a DTE or DCE in the network usually used to perform software verification.
Monitor mode is to configure the protocol analyzer to act as a monitor to keep check the data movement between a DTE and DCE.
Softkey menu operation instead of using hard-code keys in the small console to configure the protocol analyzer
Use BASIC language to program the analyzer to emulate DTE/DCE and capture special data stream.
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27. Modem diagnostics
By looping the analog side, the messages sent by DTE will be returned and processed by DTE to ensure that the analog and digital side of modem, and data link between two modems are working properly.
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28. End-to-end Test
The text messages will be sent by one of the computers and be captured by another to verify the communication is in proper condition.
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29. Remote Digital Loopback test
It uses a machine to perform loop-back text by looping the remote digital side. This can detect the send/return path.
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30. To design a network
Must be within the capabilities of the protocols supported by terminal and host computer
Must be fast enough to carry all the traffic at the required times
Must be sufficiently reliable to meet the needs of users of the service.
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31. Redundancy Standby modem Standby line Standby Equipment
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32. Standby modem
A single standby modem is installed to replace the defective modem once a hardware problem is found
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33. Standby telecommunication lines
A duplicated telecommunication line is leased from Hong Kong Telecom using different route to replace the defective line once it is identified.
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34. Standby Front end processor
A Front-end Processor is standby to replace the defective machine in case it is not working properly.
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