DC 7 - 1

DATACOMM

John Abbott College JPC

Architectures & Protocols

M. E. Kabay, PhD, CISSP

Director of Education, ICSA

President, JINBU Corp

Copyright © 1998 JINBU Corp.

All rights reserved

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Architectures & Protocols

Architectures OSI Model OSI Applications SNA Protocols Common Link-Level Protocols TCP/IP

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Architectures

Strategy for connecting information technology equipment into functional systems

Communications functions kept modular:– stratified– isolated– segregated from each other

Allows changes in functions without affecting other aspects of computing

E.g., shape of datacomm connectors can be modified without affecting application programs

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OSI Model

Open Systems Interconnection Model Intl Std Organization (ISO) Universal architecture for datacomm Allow interoperability of equipment from all

manufacturers Defines 7 layers for specific functions Number of layers is arbitrary

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OSI Model

The 7 layers Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data Link (2) Physical (1)

UPPERLAYERS

LOWERLAYERS

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OSI Model

Physical Layer (1) Transmits bits I/F to outside world Always hardware Connectors, voltages, amplitudes…. E.g., RS-232-C

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OSI Model

Data Link Layer (2) Error-free data transmission

– examines bits for errors– error recovery– flow control

E.g., bisync, SDLC, HDLCIEEE 802.3, 802.5 (see later)

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OSI Model

Network Layer (3) Routing messages through networks Switching messages among nodes Segmenting data into packets Blocking data Error recovery Flow control E.g., X.25, ISDN (see later)

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OSI Model

Transport Layer (4) Data transfer Takes messages from upper layers and

breaks into pieces for lower layers Blocking End-to-end control Multiplexing Mapping E.g., TCP, UDP

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OSI Model

Session Layer (5) Administration & control of sessions Logon/logoff (login/logout) I/F to host and remote file systems E.g., Berkeley Sockets, NET IPC

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OSI Model

Presentation Layer (6) Data interpretation Format & code transformations Appearance of output E.g., ASCII, EBCDIC,

TELNET, printer drivers,e-mail formats

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OSI Model

Application Layer (7) User application programs/processes Management functions E.g., terminal emulators,

Mosaic, Netscape, ARPA, NS/3000, SMTP, X.400, rlogin,XWINDOWS

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OSI Applications

OUTPUT INPUT

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SNA

Systems Network Architecture IBM proprietary architecture De facto standard — but declining in use Complex terminology Read section on SNA in text (pp. 136-143) Omit review questions 7-5 through 7-10 on

page 156 Not discussed further in this course

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Protocols

Polling – Host: Ready to Send?– Terminal: Yes / No

Selecting– Host: Ready to Receive (ENQ)?– Terminal: Yes, Acknowledge (ACK)– No, Negative Acknowledge (NAK)

Automatic Repeat Request (ARQ)– Check each block for correctness– Stop-and-wait ARQ– Go-back-N continuous ARQ

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Common Link-Level Protocols Let devices share communications link Define queries/responses for polling &

selecting Byte-oriented protocols (character-oriented)

– control sequences in whole bytes– e.g., BSC (Binary Synchronous

Communications) Bit-oriented protocols

– single bits for control information– e.g., SDLC (Synchronous Data Link

Control)

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Common Link-Level ProtocolsBSC IBM proprietary and de facto standard

– half-duplex– stop-and-wait ARQ– point-to-point and multipoint datacomm

Designed for ASCII, EBCDIC (and rarely used Transcode)

Not suitable for satellite or other long-delay transmission modes

Data transmitted in frames

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Common Link-Level ProtocolsBSC frames Data frames

– user data– headers allow specific addressing

Control information frames– terminal and host communicate without

user involvement– terminal buffers transmissions until receipt

of acknowledgement

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Common Link-Level ProtocolsSDLC IBM proprietary

– but de facto standard– used in SNA

Similar protocols:– ISO HDLC (High-Level Data Link Control)– CCITT Link Access Procedure-Balanced

(LAP-B)

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Common Link-Level ProtocolsSDLC Full duplex (usually) Go-back-N continuous ARQ Suitable for channels even with long delays SDLC Frames

– Information frames (I-Frames)

• data– Supervisory frames (S-Frames)

• control– Unnumbered frames (U-Frames)

• specialized applications

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Common Link-Level ProtocolsProtocol Converters Adapt data flow to link systems using

dissimilar link-level protocols Code converters translate ASCII and EBCDIC E.g., asynchronous terminal linked to SDLC

host

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TCP/IP

Transmission Control Protocol / Internet Protocol

Originally designed for ARPANET Eventually became part of the definition for

being part of the Internet TCP

– works at layer 4 (transport) of OSI stack– ensures reliable delivery

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TCP/IP

IP Works at layer 3 (network) Best-effort delivery only Packets called datagrams Connectionless because every packet routed

individually

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TCP/IP

SMTP (Simple Mail Transfer Protocol)– universal format for e-mail exchange

Telnet– allows access to remote host– simulates terminal

FTP (File Transfer Protocol)– remote viewing of directories– file transfers

SNMP (Simple Network Mgmt Protocol)– see later (Chapter / Section 9)

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Homework

Read Chapter 7 of your textbook in detail, adding to your workbook notes as appropriate.

Review and be prepared to define or expand all the terms listed at the end of Chapter 7 of your textbook except for those relating to SNA (no hand-in required)

Answer exercises 7-1 through 7-4 and 7-11 through 7-17 on pages 156 of the textbook using a computer word-processing program or absolutely legible handwriting (hand in after quiz tomorrow morning).