1 ITC242 – Introduction to Data Communications Week 8 Topic 13 Wireless WANS Reading 2.
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1 ITC242 – Introduction to Data Communications Week 8 Topic 13 Wireless WANS Reading 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 The Network Core 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”
4 Network Core: Circuit Switching End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required
5 Network Core: Circuit Switching network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces” –frequency division –time division
6 Circuit Switching: FDM and TDM FDM frequency time TDM frequency time 4 users Example:
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 Network Core: Packet Switching 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 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 Bandwidth division into “pieces” Dedicated allocation Resource reservation
9 Packet-switching: store-and- forward store and forward: entire packet must arrive at router before it can be transmitted on next link R R R L
10 Delay and loss in packet- switched networks packets queue in router buffers packet arrival rate to link exceeds output link capacity packets queue, wait for turn A B packet being transmitted (delay) packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers
11 Four sources of packet delay 1. nodal processing: –check bit errors –determine output link A B propagation transmission nodal processing queueing 2. queueing –time waiting at output link for transmission –depends on congestion level of router
12 Delay in packet-switched networks 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 (~2x10 8 m/sec) propagation delay = d/s A B propagation transmission nodal processing queueing Note: s and R are very different quantities!
13 Caravan analogy 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 toll booth toll booth ten-car caravan 100 km
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
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 with Geostationary 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
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