DATACOMM John Abbott College JPC Architectures Protocols M

Architectures & Protocols l l l l DC 7 - 2 Architectures OSI Model OSI Applications SNA Protocols Common Link-Level Protocols TCP/IP

Architectures l l DC 7 - 3 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

OSI Model l l l DC 7 - 4 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

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) DC 7 - 5 } } UPPER LAYERS LOWER LAYERS

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 7 - 6

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, 802. 5 (see later) DC 7 - 7

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 7 - 8

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 7 - 9

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 7 - 10

OSI Model Presentation Layer (6) l Data interpretation l Format & code transformations l Appearance of output l E. g. , ASCII, EBCDIC, TELNET, printer drivers, e-mail formats DC 7 - 11

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 7 - 12

OSI Applications OUTPUT DC 7 - 13 INPUT

SNA l l l l DC 7 - 14 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

Protocols l l l DC 7 - 15 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

Common Link-Level Protocols l l DC 7 - 16 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)

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 7 - 17

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 7 - 18

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 7 - 19

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 7 - 20

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 7 - 21

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 7 - 22

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 7 - 23

TCP/IP l l DC 7 - 24 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)

Homework l l l DC 7 - 25 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).