ITC242 – Introduction to Data Communications Week 8 Topic 13 Wireless WANS Reading 2

1. ITC242 – Introduction to Data CommunicationsWeek 8Topic 13 Wireless WANSReading 2

2. Topic 12 – Circuit/Packet switching Learning Objectives • Define and describe the characteristics of: • Circuit switched network • Packet switched network • Describe the application of both circuit switching and packet switching networks • Compare Circuit/packet switched networks describing the advantages and disadvantages of each.

3. mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks” The Network Core

4. End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required Network Core: Circuit Switching

5. network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) Network Core: Circuit Switching • dividing link bandwidth into “pieces” • frequency division • time division

6. Example: 4 users FDM frequency time TDM frequency time Circuit Switching: FDM and TDM

7. Circuit Switching Applications • Public Telephone Network (PSTN) • Private Automatic Branch Exchanges (PABX / PBX) • Private Wide Area Networks (often used to interconnect PBXs in a single organization) • Data Switch

8. each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed Bandwidth division into “pieces” Dedicated allocation Resource reservation Network Core: Packet Switching resource contention: • aggregate resource demand can exceed amount available • congestion: packets queue, wait for link use • store and forward: packets move one hop at a time • Node receives complete packet before forwarding

9. store and forward: entire packet must arrive at router before it can be transmitted on next link Packet-switching: store-and-forward L R R R

10. packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Delay and loss in packet-switched networks A B

11. 1. nodal processing: check bit errors determine output link transmission A propagation B nodal processing queueing Four sources of packet delay • 2. queueing • time waiting at output link for transmission • depends on congestion level of router

12. 3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s transmission A propagation B nodal processing queueing Delay in packet-switched networks Note: s and R are very different quantities!

13. cars “propagate” at 100 km/hr toll booth takes 12 sec to service car (transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth? Time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec Time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr A: 62 minutes 100 km 100 km ten-car caravan toll booth toll booth Caravan analogy

14. Topic 13 – Wireless WANs Learning Objectives • Describe the properties and applications of the different types of satellite communications.

15. Satellite Communications • Two or more stations on or near the earth communicate via one or more satellites that serve as relay stations in space • The antenna systems on or near the earth are referred to as earth stations • Transmission from an earth station to the satellite is an uplink, from the satellite to the earth station is downlink • The transponder in the satellite takes an uplink signal and converts it to a downlink signal

16. Satellite Network

17. Geostationary Satellites • Circular orbit 35,838 km above the earth’s surface • Rotates in the equatorial plane of the earth at exactly the same angular speed as the earth • Remains above the same spot on the equator as the earth rotates

18. Advantages of Geostationary Orbits • Satellite is stationary relative to the earth, so no frequency changes due to the relative motion of the satellite and antennas on earth (Doppler effect). • Tracking of the satellite by its earth stations is simplified. • One satellite can communicate with roughly a fourth of the earth; three satellites separated by 120° cover most of the inhabited portions of the entire earth excluding only the areas near the north and south poles

19. Problems withGeostationary Orbits • Signal can weaken after traveling that distance • Polar regions and the far northern and southern hemispheres are poorly served • Even at speed of light, the delay in sending a signal 35,838 km each way to the satellite and back is substantial

20. LEO and MEO Orbits • Alternatives to geostationary orbits • LEO: Low earth orbiting • MEO: Medium earth orbiting

21. Satellite Orbits

22. LEO Advantages • Reduced propagation delay • Received LEO signal is much stronger than that of GEO signals for the same transmission power • LEO coverage can be better localized so that spectrum can be better conserved. • On the other hand, to provide broad coverage over 24 hours, many satellites are needed.

23. Satellite Network Applications • Television distribution • Long-distance telephone transmission • Private business networks • Military applications

26. Reading 2 – Wide Area and Large-Scale Networks Learning Objectives • Describe the basic concepts associated with wide area networks • Identify the uses, benefits, and drawbacks of WAN technologies such as ATM, FDDI, SONET, SMDS

27. WAN Transmission Technologies Some of the communication links employed to construct WANs include: • Packet-switching networks • Fibre-optic cable • Microwave transmitters • Satellite links • Cable television coax systems

28. WAN Transmission Technologies Three primary technologies are used to transmit communications between LANs across WAN links: • Analogue • Digital • Packet switching

29. Analogue Connectivity • PSTN – Public Switched Telephone Network • POTS – Plain Old Telephone System

30. Digital Connectivity • DDS – Digital Data Service: point-to-point, low data rates • E1 – high speed digital lines: 2.048Mbps = 30 x 64kbps voice channels + 2 x 64kbps signalling channels. • X.25: an interface between public packet switched networks and customers. • Frame Relay: point-to-point permanent virtual circuit technology.

31. Digital Connectivity ISDN – Integrated Services Digital Network: • BRI: Basic Rate Interface: consists of 2 B channels (64kbps each) – bearer channels for data, and one D channel (16Kbps) for setup and control. 2B+D • PRI: In Australia 30 B channels (64Kbps each) and 2 D channels (64Kbps each). 30B+2D

32. Advanced WAN Technologies • ATM: Asynchronous Transfer Mode: high speed, packet-switching. Uses fixed sized cells of 53 bytes. High levels of quality of service to allow for different data types. • SONET: Synchronous Optical Network: high speed Fibre optic WAN technology