Open Systems and Data Link Protocols
November 7, 2002
Announcements• Short Test or Long Quiz: November 21
– Will cover Chapters 14, 15 and 16.
• Homework– Chapter 14: 3, 4, 6, 12, 22, 26 (hand in next
week)– Chapter 15: 4, 5, 6, 7, 16, 17, 26, 27, 40 (hand
in November 21)– Read Chapter 16 (pages 475-502) Internet-
ATM. It will not be discussed in class but the quiz bonus test will come from it.
Class Objectives
• Open Systems– Why do we need open standards?– Seven-layer OSI model
• Data Link Protocols– The past: BISYNC– The present: HDLC
Before Open Standards
• Getting two systems to communicate between them was a major effort.
• Once you purchased a system you were “stuck” with it for years for both hardware and software:– IBM– Digital– Unisys
With Open Systems
• System interoperability becomes much easier.
• Allows the combination of different hardware and software “solutions”.
• Results:– Lower costs due to increased competition– Faster implementation (alternate sources)– Easier system maintenance and troubleshooting
FIGURE 14-1 Seven-layer OSI model.
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OSI Layers
1. Physical: Concerned with the transmission of unstructured bit stream over physical medium; deals with the mechanical, electrical, functional, and procedural characteristics to access the physical medium.
2. Data Link: Provides for the reliable transfer of information across the physical link; sends blocks of data (frames) with the necessary synchronization, error control, and flow control.
OSI Layers (cont.)
3. Network: Provides upper layers with independence from the data transmission and switching technologies used to connect systems; responsible for establishing, maintaining, and terminating connections.
4.Transport: Provides reliable, transparent transfer of data between points; provides end-to-end error recovery and flow control.
OSI Layers
5. Session: Provides the control structure for communications between applications; establishes, manages, and terminates connections (sessions) between cooperating applications.
6. Presentation: Provides independence to the application processes from differences in data representation (syntax).
7. Application: Provides access to the OSI environment for users and also provides distributed information services.
FIGURE 14-2 (a) Computers connected point-to-point; (b) computer connected to several terminals in a multipoint (or multidrop) configuration.
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BISYNC
• Developed by IBM in 1964
• Byte or character oriented protocol
• Geared toward mainframe-terminal relationship
• Half-Duplex operation
• Use has been in decline due to better protocols
FIGURE 14-3 BISYNC message block format.
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FIGURE 14-4 BISYNC transparent text mode format.
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FIGURE 14-5 BISYNC point-to-point line control sequence.
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FIGURE 14-6 An error condition encountered causes the receiving station to request a retransmission.
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FIGURE 14-7 Acknowledgment, ACK, lost en route to the transmitting station.
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FIGURE 14-8 Host computer poll and selecting a device for transmitting data in a multipoint setup.
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FIGURE 14-10 Circuit switching network.
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FIGURE 14-11 Message switching network.
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FIGURE 14-12 Packet switching network.
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High-Level Data Link Control
• Bit-oriented protocol
• Used throughout the world
• Implemented in X.25 packet switching networks
• Full-duplex operation
• Flexible– Point-to-point and multi-point links– Primary-secondary and peer-to-peer interaction.– Works with long distance (e.g. satellite) as well
as short distance links.
FIGURE 14-13 HDLC frame format.
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FIGURE 14-14 HDLC framing pattern and bit order of transmission.
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FIGURE 14-15 Extending the HDLC ADDRESS BYTE field.
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FIGURE 14-16 Format of CONTROL BYTE for information transfer, supervisory, and unnumbered HDLC frames.
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FIGURE 14-18 HDLC supervisory frame structure.
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FIGURE 14-19 HDLC unnumbered frame structure.
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FIGURE 14-20 Zero-bit insertion range.
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FIGURE 14-21 HDLC zero-bit insertion, bit-stuffing technique.
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FIGURE 14-22 HDLC frame sequencing.
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