Data and Computer Communications

1. Data and Computer Communications Chapter 15 – Local Area Network Overview Ninth Edition by William Stallings

2. Local Area Network Overview The whole of this operation is described in minute detail in the official British Naval History, and should be studied with its excellent charts by those who are interested in its technical aspect. So complicated is the full story that the lay reader cannot see the wood for the trees. I have endeavored to render intelligible the broad effects. —The World Crisis, Winston Churchill

3. Local Area Networks (LANs) • usually owned by the organization that is using the network to interconnect equipment • key elements: • topology • transmission medium • wiring layout • medium access control

4. LAN Topologies

5. Bus and Tree

6. Frame Transmissionon Bus LAN

7. 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 transmitted in frames • circulate past all stations • destination recognizes address and copies frame • frame circulates back to source where it is removed • medium access control determines when a station can insert frame

8. Frame TransmissionRing LAN

9. Star Topology • each station connects to common central node • usually via two point-to-point link • one for transmission and one for reception

10. Choice of Topology • medium • wiring layout • access control

11. Bus LAN Transmission Media cont…

12. Bus LAN Transmission Media • only baseband coaxial cable has achieved widespread use

13. Ring and Star Topologies

14. Choice of Medium • constrained by LAN topology • capacity • to support the expected network traffic • reliability • to meet requirements for availability • types of data supported • tailored to the application • environmental scope • provide service over the range of environments

15. Media Available

16. LAN Protocol Architecture

17. IEEE 802 Layers • Physical Layer • Encoding / decoding of signals • preamble generation / removal • bit transmission / reception • transmission medium and topology

18. IEEE 802 Layers • Logical Link Control Layer (LLC) • provide interface to higher levels • perform flow and error control • Media Access Control • on transmit assemble data into frame • on reception disassemble frame, perform address recognition and error detection • govern access to transmission medium • for same LLC, may have several MAC options

19. LAN Protocols in Context

20. Logical Link Control • transmission of link level PDUs between stations • must support multi-access, shared medium • relieved of some details of link access by the MAC layer • addressing involves specifying source and destination LLC users • referred to as service access points (SAPs)

21. LLC Services

22. LLC Service Alternatives

23. LLC Protocol • modeled after HDLC • asynchronous balanced mode • connection mode (type 2) LLC service • unacknowledged connectionless service • using unnumbered information PDUs (type 1) • acknowledged connectionless service • using 2 new unnumbered PDUs (type 3) • permits multiplexing using LSAPs

24. MAC Frame Format

25. Medium Access Control (MAC) Protocol • controls access to the transmission medium • key parameters: • where • greater control, single point of failure • more complex, but more redundant • how • synchronous • capacity dedicated to connection, not optimal • asynchronous • response to demand • round robin, reservation, contention

26. Asynchronous Systems

27. MAC Frame Handling • MAC layer receives data from LLC layer • PDU is referred to as a MAC frame • fields: • MAC control • destination MAC address • source MAC address • LLC • CRC • MAC layer detects errors and discards frames • LLC optionally retransmits unsuccessful frames

28. Bridges • connects similar LANs with identical physical and link layer protocols • minimal processing • can map between MAC formats • reasons for use: • reliability • performance • security • geography

29. Bridge Function

30. Bridge Design Aspects • no modification to frame content or format • no encapsulation • exact bitwise copy of frame • buffering to meet peak demand • contains routing and address intelligence • may connect more than two LANs • bridging is transparent to stations

31. Bridge Protocol Architecture • IEEE 802.1D defines architecture • MAC level designates endpoint • bridge does not need LLC layer

32. Connection of Two LANs

33. Bridges andLANs withAlternativeRoutes

34. Fixed Routing • simplest and most common • suitable for Internets that are stable • a fixed route is selected for each pair of LANs • usually least hop route • only changed when topology changes • widely used but limited flexibility

35. Spanning Tree • bridge automatically develops routing table • automatically updates routing table in response to changing topology

36. Frame Forwarding • maintain forwarding database for each port • for a frame arriving on port X:

37. Address Learning • can preload forwarding database • when frame arrives at port X, it has come from the LAN attached to port X • use source address to update forwarding database for port X to include that address • have a timer on each entry in database • if timer expires, entry is removed • each time frame arrives, source address checked against forwarding database • if present timer is reset and direction recorded • if not present entry is created and timer set

38. Spanning Tree Algorithm • address learning works for tree layout if there are no alternate routes in the network • alternate route means there is a closed loop • for any connected graph there is a spanning tree maintaining connectivity with no closed loops • algorithm must be dynamic

39. Loop of Bridges

40. Interconnecting LANs - Hubs • active central element of star layout • each station connected to hub by two UTP lines • hub acts as a repeater • limited to about 100m by UTP properties • optical fiber may be used out to 500m • physically star, logically bus • transmission from a station seen by all others • if two stations transmit at the same time have a collision

41. Two Level Hub Topology

42. Buses, Hubs and Switches • can improve performance using a layer 2 switch • can switch multiple frames between separate ports • multiplying capacity of LAN

43. Shared Medium Bus and Hub

44. Layer 2 Switch Benefits • no changeto attached devices to convert bus LAN or hub LAN to switched LAN • e.g. Ethernet LANs use Ethernet MAC protocol • have dedicated capacity equal to original LAN • assuming switch has sufficient capacity to keep up with all devices • scales easily • additional devices attached to switch by increasing capacity of layer 2

45. Types of Layer 2 Switches • store-and-forward switch • accepts frame on input line, buffers briefly, routes to destination port • see delay between sender and receiver • boosts overall integrity • cut-through switch • use destination address at beginning of frame • switch begins repeating frame onto output line as soon asdestination address is recognized • highest possible throughput • risk of propagating bad frames

46. Layer 2 Switch vs. Bridge • differences between switches & bridges: • layer 2 switch can be viewed as full-duplex hub • incorporates logic to function as multiport bridge • new installations typically include layer 2 switches with bridge functionality rather than bridges

47. A Partitioned LAN Configuration

48. Virtual LANs (VLANs) • subgroup within a LAN • created by software • combines user stations and network devices into a single broadcast domain • functions at the MAC layer • router required to link VLANs • physically dispersed but maintains group identity

49. A VLAN Configuration

50. Defining VLANs • broadcast domain consisting of a group of end stations not limited by physical location and communicate as if they were on a common LAN • membership by: • port group • MAC address • protocol information