DC 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

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

DC Architectures l Strategy for connecting information technology equipment into functional systems l Communications functions kept modular: – stratified – isolated – segregated from each other l Allows changes in functions without affecting other aspects of computing l E.g., shape of datacomm connectors can be modified without affecting application programs

DC OSI Model l Open Systems Interconnection Model l Intl Std Organization (ISO) l Universal architecture for datacomm l Allow interoperability of equipment from all manufacturers l Defines 7 layers for specific functions l Number of layers is arbitrary

DC OSI Model The 7 layers l Application (7) l Presentation (6) l Session (5) l Transport (4) l Network (3) l Data Link (2) l Physical (1) UPPER LAYERS LOWER LAYERS } }

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

DC OSI Model Data Link Layer (2) l Error-free data transmission – examines bits for errors – error recovery – flow control l E.g., bisync, SDLC, HDLC IEEE 802.3, (see later)

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

DC OSI Model Transport Layer (4) l Data transfer l Takes messages from upper layers and breaks into pieces for lower layers l Blocking l End-to-end control l Multiplexing l Mapping l E.g., TCP, UDP

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

DC OSI Model Presentation Layer (6) l Data interpretation l Format & code transformations l Appearance of output l E.g., ASCII, EBCDIC, TELNET, printer drivers, formats

DC OSI Model Application Layer (7) l User application programs/processes l Management functions l E.g., terminal emulators, Mosaic, Netscape, ARPA, NS/3000, SMTP, X.400, rlogin, XWINDOWS

DC OSI Applications OUTPUT INPUT

DC SNA l Systems Network Architecture l IBM proprietary architecture l De facto standard but declining in use l Complex terminology l Read section on SNA in text (pp ) l Omit review questions 7-5 through 7-10 on page 156 l Not discussed further in this course

DC Protocols l Polling – Host: Ready to Send? – Terminal: Yes / No l Selecting – Host: Ready to Receive (ENQ)? – Terminal: Yes, Acknowledge (ACK) – No, Negative Acknowledge (NAK) l Automatic Repeat Request (ARQ) – Check each block for correctness – Stop-and-wait ARQ – Go-back-N continuous ARQ

DC Common Link-Level Protocols l Let devices share communications link l Define queries/responses for polling & selecting l Byte-oriented protocols (character-oriented) – control sequences in whole bytes – e.g., BSC (Binary Synchronous Communications) l Bit-oriented protocols – single bits for control information – e.g., SDLC (Synchronous Data Link Control)

DC Common Link-Level Protocols BSC l IBM proprietary and de facto standard – half-duplex – stop-and-wait ARQ – point-to-point and multipoint datacomm l Designed for ASCII, EBCDIC (and rarely used Transcode) l Not suitable for satellite or other long-delay transmission modes l Data transmitted in frames

DC Common Link-Level Protocols BSC frames l Data frames – user data – headers allow specific addressing l Control information frames – terminal and host communicate without user involvement – terminal buffers transmissions until receipt of acknowledgement

DC Common Link-Level Protocols SDLC l IBM proprietary – but de facto standard – used in SNA l Similar protocols: – ISO HDLC (High-Level Data Link Control) – CCITT Link Access Procedure-Balanced (LAP-B)

DC Common Link-Level Protocols SDLC l Full duplex (usually) l Go-back-N continuous ARQ l Suitable for channels even with long delays l SDLC Frames – Information frames (I-Frames) data – Supervisory frames (S-Frames) control – Unnumbered frames (U-Frames) specialized applications

DC Common Link-Level Protocols Protocol Converters l Adapt data flow to link systems using dissimilar link-level protocols l Code converters translate ASCII and EBCDIC l E.g., asynchronous terminal linked to SDLC host

DC TCP/IP Transmission Control Protocol / Internet Protocol l Originally designed for ARPANET l Eventually became part of the definition for being part of the Internet l TCP – works at layer 4 (transport) of OSI stack – ensures reliable delivery

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

DC TCP/IP l SMTP (Simple Mail Transfer Protocol) – universal format for exchange l Telnet – allows access to remote host – simulates terminal l FTP (File Transfer Protocol) – remote viewing of directories – file transfers l SNMP (Simple Network Mgmt Protocol) – see later (Chapter / Section 9)

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