Data Communications

1. Data Communications Local Area Network Technology

2. LAN Applications (1) • Personal computer LANs • Low cost • Limited data rate • Back end networks and storage area networks • Interconnecting large systems (mainframes and large storage devices) • High data rate • High speed interface • Distributed access • Limited distance • Limited number of devices

3. LAN Applications (2) • High speed office networks • Applications include desktop image processing, and • High capacity local storage • Backbone LANs • Used to interconnect low speed local LANs • Disadvantages include: • Reliability – backbone failure can be catastrophic • Capacity – has to support a large capacity • Cost – can be high

4. LAN Architecture • Four basic components when describing a particular LAN: • Protocol architecture • Topologies • Media access control • Logical Link Control • Let’s examine each of these components in more detail

5. Protocol Architecture • LANs involve the lower layers of OSI model • IEEE 802 reference model is standard • Physical layer • Media access control (MAC) sublayer • Logical link control (LLC) sublayer

6. IEEE 802 v OSI

7. 802 Layers - The Physical Layer • What functions are provided by the physical layer? • Encoding/decoding of signals • Preamble generation/removal • Bit transmission/reception • Transmission medium and topology

8. 802 Layers -Media Access Control Sublayer • Assembly of data into frame with address and error detection fields • Disassembly of frame • Address recognition • Error detection • Govern access to transmission medium • Not found in traditional layer 2 data link control • For the same LLC, several MAC options may be available

9. MAC Frame Format • MAC layer receives data from LLC layer • MAC layer adds control field, destination MAC address, source MAC address, and CRC to LLC data unit • MAC layer detects errors and discards frames • (LLC optionally retransmits unsuccessful frames)

10. Typical Frame Format

11. Logical Link Control Sublayer • LLC provides the interface to higher levels • It provides transmission of link level PDUs between two stations • Unlike other link control protocols, LLC must support multi-access, shared medium • And it is relieved of some link access details by MAC layer • Addressing involves specifying source and destination LLC users • Referred to as service access points (SAP) • Typically the user is a higher level protocol

12. LLC Services • Based on HDLC • Unacknowledged connectionless service – a datagram service - leave the error and flow control to a higher layer, such as TCP • Connection mode service – a logical connection is set up between 2 users and error and flow control are provided • Acknowledged connectionless service – a cross between the first two – datagrams are acked but no prior logical connection is set up

13. LAN Protocols in Context

14. Five Basic LAN Topologies • Bus • Original, but no new installations • Tree • Special case of bus – multiple branches • Ring • Pretty much dead on LANs, more on MANs • Star-wired bus • Wireless

15. LAN Topologies

16. Star-wired Bus LANs • Use unshielded twisted pair wire • Minimal installation cost • Attach to a central active hub • Two links • Transmit and receive • Hub repeats incoming signal on all outgoing lines • Link lengths limited to about 100m • Fiber optic - up to 500m • Logical bus - with collisions

17. Hubs and Switches • Shared medium hub • Central hub • Hub retransmits incoming signal to all outgoing lines • Only one station can transmit at a time • With a 10Mbps LAN, total capacity is 10Mbps • Switch • Hub acts as switch • Incoming frame switches to appropriate outgoing line • Unused lines can also be used to switch other traffic • With two pairs of lines in use, overall capacity is now 20Mbps

18. Switch • No change to software or hardware of workstations / devices • Each device has dedicated capacity! • Scales well • Store and forward switch • Accept input, buffer it briefly, then output • Cut through switch • Take advantage of the destination address being at the start of the frame • Begin repeating incoming frame onto output line as soon as address recognized • May propagate some bad frames

19. Hubs and Switches

20. Wireless LANs • Mobility • Flexibility • Hard to wire areas • Reduced cost of wireless systems • Improved performance of wireless systems

21. Wireless LAN Applications • LAN Extension - large buildings, hard to connect locations • Cross building interconnection • Nomadic access • Ad hoc networks

22. Example Applications • Factory floor workers can access part and process specs • College students can connect to campus net from almost any location • Medical professionals can access patient data bedside or on location • Office workers can move laptops from cubicle to meeting room to … • Retail sales handhelds, warehouse transactions, stock market uses, and many, many more

23. Basic Components • Backbone wired LAN that wireless workstation is going to connect to • Control module (CM) - the interface device between a wireless workstation and the wired LAN; contains either bridge or router functionality plus access logic such as CSMA, polling or token-passing; aka access point • User module (UM) - wireless workstation or device

24. Single Cell Wireless LAN

25. Multi Cell Wireless LAN

26. Cross Building Interconnection • Point to point wireless link between buildings • Typically connecting bridges or routers • Used where cable connection not possible • e.g. across a street

27. Nomadic Access • Mobile data terminal • e.g. laptop • Transfer of data from laptop to server • Campus or cluster of buildings

28. Ad Hoc Networking • Peer to peer • Temporary • e.g. conference

29. Wireless LAN Configurations

30. Wireless LAN Requirements • Throughput - should be reasonably high • Number of nodes - may need to support 100s of nodes throughout the LAN • Connection to backbone - most require a CM type of connection to backbone • Service area - should typically be 100 to 300 m diameter • Battery power consumption - should be low, so MAC sublayer cannot be in constant communication with access point

31. Wireless LAN Requirements • Transmission robustness and security - have to avoid interference and eavesdropping • Collocated network operation - might have to support two or more wireless LANs in the same area • License free operation • Handoff/roaming • Dynamic configuration of workstations

32. Wireless LAN Technology • All current wireless LAN products fall into one of the following categories: • Infrared (IR) LANs - limited to a single room • Spread spectrum LANs - no FCC licensing required • Narrow band microwave

33. Protocols You Should Know • IEEE 802.11 • IEEE 802.11a • IEEE 802.11b • IEEE 802.11g • HiperLAN

34. IEEE 802.11 LANs • Basic service set (BSS, or cell) • Set of stations using same MAC protocol • Competing to access shared medium • May be isolated or may connect to backbone via access point (bridge) • Extended service set • Two or more BSSs connected by distributed system • Appears as single logic LAN to LLC level

35. Types of 802.11 Stations • No transition • Stationary or moves within direct communication range of single BSS • BSS transition • Moves between two BSSs within a single ESS • ESS transition • From a BSS in one ESS to a BSS in another ESS • Disruption of service likely

36. 802.11 Physical Types • Infrared • 1Mbps and 2Mbps • Wavelength 850-950nm • Direct sequence spread spectrum • 2.4GHz ISM band • Up to 7 channels • Each 1Mbps or 2Mbps • Frequency hopping spread spectrum • 2.4GHz ISM band • 1Mbps or 2Mbps

37. 802.11 MAC Layer • What kind of access protocol should a wireless network use? • CSMA-type makes sense for ad-hoc networks where there is no central authority • A centralized access protocol makes sense for systems employing a base station / access point; especially useful for high priority or time sensitive data

38. 802.11 MAC Layer • Protocol created : Distributed wireless foundation MAC (DWFMAC) • A distributed access control protocol with an optional centralized control built on top of that • The MAC layer is divided into two sublayers: DCF and PCF • DCF uses contention-based access • PCF uses a centralized MAC algorithm

39. 802.11 MAC Layer • Distributed coordination function (DCF) • The lower sublayer • CSMA • No collision detection (cell may be too wide and each workstation may not hear all other workstations) • Also includes a set of delays which essentially provides a set of priority levels

40. DCF Priority Scheme • If medium is idle, station waits to see if medium remains idle for a time equal to IFS (interframe space). If still idle, transmit • If medium is busy (either initially found busy or becomes busy during IFS), station continues to listen • When medium becomes idle, station delays another IFS. If still idle after IFS, station chooses a random backoff factor. When backoff counter reaches zero, transmit packet

41. DCF Priority Scheme • Where is priority scheme? • Short IFS (SIFS) - used for all immediate response actions, eg ACKs, CTSs, poll responses • Midlength IFS (PIFS) - used by the centralized controller in the PCF scheme when issuing polls • Long IFS (DIFS) - used as a minimum delay for ordinary async frames contending for access

42. 802.11 MAC Layer • Point coordination function (PCF) • Optional and implemented on top of DCF • Polling performed by central master • Polling performed round robin fashion • Polled station may respond with SIFS

43. IEEE 802.11b • First modification to the 802.11 standard • Uses the 2.4 GHz band • Transmits data up to 11 Mbps (theoretically, in practice – more like 6 Mbps)

44. IEEE 802.11a • Higher speed protocol • Transmissions in the 5 GHz band • Uses a modulation technique called orthogonal frequency division multiplexing • Can run at several data rates, up to 54 Mbps • First products shipped in 2002.

45. IEEE 802.11g • Modification on 802.11b • Extends the 2.4 GHz technology to 54 Mbps • Products now appearing but no final standard yet exists (as of 5/19/03) • Since same frequency range as 802.11b, can use same layout – just need to replace NICs and access points

46. HiperLAN/2 • Drafted by the European Telecommunications Standards Institute • Like 802.11a, promises up to 54 Mbps data rates in the 5 GHz band • Some consider HiperLAN/2 to be technically superior to the IEEE standard

47. Interesting Facts • Higher power requirements to transmit the 5 GHz signals may make it difficult for laptops • 802.11b interface cards dropping from $150 to $75 • 802.11a cards may start at around $200 and drop to $150 ?? • 802.11b access points $500 - $600? • 802.11a access points may start around $1000 and drop to $650 - $750?

48. Interesting Facts • The range of a 5.4 GHz radio is less than that of a 2.4 GHz radio • Rough rule - 802.11b has a range of approx 250 - 300 feet; 802.11a will have a range of approx 90 feet • Shorter wavelength of 5.4 GHz has trouble going through walls, floors, furniture, etc. • You will need roughly 4 times as many 5 GHz access points as 2.4 GHz access points

49. Bridges • Ability to expand beyond single LAN • Provide interconnection to other LANs/WANs • Use Bridge (switch) or router • Bridge is simpler • Connects similar LANs • Identical protocols for physical and link layers • Minimal processing • Router more general purpose • Interconnect various LANs and WANs

50. Why Bridge? • Reliability • Performance • Security • Geography