Local Area Network Overview

1. Local Area Network Overview ENTC 345 Dr. Ana Goulart Assistant Professor

2. Reading • Stallings, Chapter 15 and 16

3. Review • M/M/1 modeling in Opnet Run Simulation View and Analyze Results Create Network Models Choose Statistics

4. Review • Example of transmission over a shared bus in which we calculated transmission and propagation delays, and minimum frame requirements to avoid collisions

5. Local Area Network Overview • LAN Applications • Personal computer LANs • Backend networks • interconnecting large systems (mainframes and large storage devices) • Storage area Networks (SANs) • shared storage facility (eg. hard disks, tape libraries, CD arrays) • accessed using a high-speed network (eg. Fibre Channel)

6. Local Area Network Overview • LAN Applications (cont.) • High speed office networks • Backbone LANs • interconnect low speed local LANs

7. LAN Architecture • Topologies • Transmission Medium • Layout • Medium Access Control

8. LAN Topologies Bus Ring Tree Star

9. Bus and Tree Topologies • used with multipoint medium • transmission propagates throughout medium • heard by all stations • need to regulate transmission • to avoid collisions and hogging • terminator absorbs frames at end of medium • tree a generalization of bus • headend connected to branching cables

10. Frame Transmission on a Bus LAN

11. Ring Topology • a closed loop of repeaters joined by point to point links • receive data on one link & retransmit on another • links unidirectional • stations attach to repeaters • data in frames • circulate past all stations • destination recognizes address and copies frame • frame circulates back to source where it is removed • media access control determines when a station can insert frame

12. Star Topology • each station connects to central node • usually via two point to point links • either central node can broadcast • physical star, logical bus • only one station can transmit at a time • or central node can act as frame switch

13. Choice of Topology • reliability • expandability • performance • needs considering in context of: • medium • wiring layout • access control

14. Bus LAN Transmission Medium • twisted pair • early LANs used voice grade cable • didn’t scale for fast LANs • not used in bus LANs now • baseband coaxial cable • uses digital signalling • original Ethernet • less convenient compared to star topology twisted pair

15. Ring and Star Usage • ring • very high speed links over long distances • single link or repeater failure disables network • star • uses natural layout of wiring in building • best for short distances • high data rates for small number of devices

16. Choice of Topology • reliability • expandability • performance • needs considering in context of: • medium • wiring layout • access control

17. Example • Problem 15.4 • Baseband bus, 10 Mbps, length 1 Km • Mean time to send a frame of 1000 bits? (assume mean distance of 375m)

18. Example B • Problem 15.4 b) If 2 stations begin to transmit at exactly the same time, their packets will interfere with each other. If each transmitting station monitors the bus during transmission, how long before it notices an interference, in seconds? In bit times? B

19. Example B A to First bit of the frame B A to + tprop …. Llink B A to+ tx …. Last bit of the frame

20. Example And finally, all bits are transmitted and received B A to + tprop + tx last bit of the frame

21. LAN Protocol Architecture

22. IEEE 802 Layers (1) • Physical • encoding/decoding of signals • preamble generation/removal • bit transmission/reception • transmission medium and topology

23. IEEE 802 Layers (2) • Logical Link Control • interface to higher levels • flow and error control • Media Access Control • on transmit assemble data into frame • on receive disassemble frame • govern access to transmission medium • for same LLC, may have several MAC options

24. IEEE 802 Committee Architecture 802.10 Security 802 Overview & Arch. 802.1 Management 802.1 Bridging Data Link Layer 802.2 Logical Link Layer 802.3 Medium Access 802.3 Physical 802.4 Medium Access 802.4 Physical 802.5 Medium Access 802.5 Physical 802.9 Medium Access 802.9 Physical 802.11 Medium Access 802.11 Physical 802.12 Medium Access 802.12 Physical Physical Layer

25. LAN Protocols in Context

26. Media Access Control • where • central • greater control, single point of failure • distributed • more complex, but more redundant • how • synchronous • capacity dedicated to connection, not optimal • asynchronous • in response to demand

27. Asynchronous Systems • round robin • each station given turn to transmit data • reservation • divide medium into slots • good for stream traffic • contention • all stations contend for time • good for bursty traffic • simple to implement • tends to collapse under heavy load

28. MAC Frame Handling • MAC layer receives data from LLC layer • fields • MAC control • destination MAC address • source MAC address • LLC • CRC • MAC layer detects errors and discards frames • LLC optionally retransmits unsuccessful frames

29. Connection of Two LANs

30. Bridge Function

31. Bridge Function • connects similar LANs • identical physical / link layer protocols • minimal processing • can map between MAC formats • reasons for use • reliability • performance • security • geography