CMPE 80N Winter 2004 Lecture 5

1. CMPE 80NWinter 2004Lecture 5 Introduction to Networks and the Internet

2. Announcements • First quiz on Friday, 01.16. • Covers material up to and including 01.14. • Closed books, notes, etc.

3. Data Transmission • Analog and digital transmission. • Historically, communication infrastructure for analog transmission. • Digital data needed to be converted: modems (modulator-demodulator).

4. Modems • MODEM = Modulator/Demodulator • Converts digital to analog before transmitting over analog channel (e.g., telephone networks). • To transmit data: DAC (digital-to-analog converter) • To receive data: ADC (analog-to-digital converter) • 2-way communication: needs two modems. • Each modem contains circuitry to encode outgoing data and decode incoming data.

5. Types of Communication • Half-duplex communication: • Only one party can talk at a time. • E.g., walkie-talkie. • Full-duplex communication: • Both parties can talk at the same time. • E.g., telephone • Modems use full-duplex communication.

6. Modems (cont’d) • Modems contain complex circuitry to: • Modulate/demodulate the analog signal. • Allows for the transmission of moderately high bit-rate over the telephone line. • Compress the data. • Reduces the amount of bits to be transmitted • Detect bit errors due to transmission. • Achievable bit rates. • E.g., 14.4 - 56 Kb/s

7. Input/Output Connections

8. I/O Connections • How to connect peripheral devices to a computer. • Serial and parallel connections. • Serial: bits are sent one at a time. • Sequence of bits sent in parallel using parallel wires.

9. I/O Connections: Standards • Standard: defines the details of a particular technology. • RS-232 is a standard for serial communication between digital devices. • It’s full duplex. • 20-30 Kb/s. • Can only connect one device at a time. • So if you want to connect a PC to many devices, you need as many cables coming out of your PC.

10. USB and FireWire • USB (Universal Serial Bus). • Can connect many devices through a USB hub. • Bitrates: 12 Mb/s (USB 1.1) to 480 Mb/s (USB 2.0). • Provides power to small devices (e.g., mouse). • Firewire (IEEE 1394). • Can connect many devices through a FireWire hub. • Bitrates: up to 400 Mb/s. • Very popular for video cameras and storage systems, also to connect two devices (without a PC). • Can provide power to small devices (e.g., video cameras).

11. Some Considerations • RS-232, USB, Firewire, all have constraints on the maximum length of the wire. • We already know a solution: modem. • Uses the telephone network. • However, modems provide insufficient bitrate. • Also, when using the modem, you cannot use the telephone for voice communication!

12. Solution: Broadband Coonections

13. Broadband Connection Types • Integrated Services Digital Network (ISDN). • Asymmetric Digital Subscriber Line (ADSL). • Cable Modem. • Wireless. • Satellite Links.

14. ISDN • ISDN provides for communication of digitized voice and data to subscribers over the conventional “local loop” (i.e., using the same wiring as for analog telephone). • In the Basic Rate Interface (BRI), ISDN offers three separate digital channels (2B+D). • All on the same wire! (Multiplexing) • Primary Rate Interface (23B+D). • Requires higher capacity lines than local loop!

15. ISDN (cont’d) • The two B channels are intended to carry digital voice, data, or digital streams • Bitrate of each B-channel: 64 kb/s (overall, 128 kb/s) • The D channel is used as a control channel • E.g., to request services which are then supplied over the B channels, to carry caller ID information, etc. • Bitrate of D channel: 16 kb/s.

16. ISDN (cont’d) • To connect computer to ISDN, user needs a special network termination device (NT1). • NT1 device a.k.a. ISDN modem. • A modem converts a digital signal to an analog signal; ISDN is inherently digital, so no such conversion is necessary. • Need to dial a number to start a connection

17. ISDN (cont’d) • ISDN was initiated in 1984, and was available in the USA in the early 90s. • It was an attempt to replace the analog phone system with a digital voice+data system. • It never really succeeded… • Currently, ISDN is obsolete, because it offers limited bitrate at a fairly high price. • Still a possibility for Internet connection where other forms of broadband are not available.

18. ADSL • ADSL allows transmission of high bit-rates over local loop. • It does not require any changes in the wiring. • In addition, it does not preempt the local loop. • A user can use the telephone for analog voice communication and at the same time transmit data or stream video. • It requires a splitter and a ADSL modem • The splitter separates voice/fax signals from data stream. • PC to ADSL modem: typically USB.

19. ADSL scheme

20. ADSL (cont’d) • To achieve high bitrate transmission, ADSL must use sophisticated technology • It is “adaptive”: ADSL modems at the two ends probe the line between them to find its characteristics, and then agree to communicate using techniques that are optimal for that line. • Depending on the characteristics of the wiring, different bit-rates can be achieved • If a house is too far form the “End office” (switching center), ADSL is not available.

21. ADSL (cont’d) • ADSL is asymmetric: it provides a higher bit-rate downstream than upstream. • Downstream: 32 kb/s to 6.4 Mb/s (more typically, 1.5 Mb/s) • Upstream: 32 to 640 kb/s (more typically, 256 kb/s) • Asymmetry is OK when high bitrate data is transmitted to the user. • E.g.: Video-On-Demand, Internet radio… • In some cases, symmetric communication is preferable. • E.g.: Videoconferences

22. Cable Modems • CATV (Community Antenna TV, or cable TV) uses coax cable (less susceptible to interference) • 1-Km coax cable can accommodate bitrates of 1-2 Gb/s! • Only one cable is used for a neighborhood • Different TV channels are multiplexed on it. • Cable systems are designed to carry many more television signals than currently available. • There is unused capacity that can be used for data communication! • >80% of US homes are already reached by CATV

23. Cable Modems (cont’d) • User can connect using a cable modem • A splitter separates the TV and the data signals. • Problem: all users in the neighborhood share the same available capacity in the same cable! • If all users in the neighborhood transmit data at the same time, the available bitrate is reduced. • E.g., if there are 50 Mb/s available, and 100 users in the neighborhood use it simultaneously, each user has only 0.5 Mb/s

24. Cable Modems (cont’d) • Coax cables from several neighborhoods connect to a concentrator • The concentrator uses high capacity fiber optics cables to connect to the head end, which is connected to the Internet. • Communication is asymmetric • Originally, CATV was designed only for downstream communication! • Available bitrates: • Downstream: 1.5 to 2 Mb/s • Upstream: 128 kb/s

25. Satellite Systems • Digital communication satellites were deployed by telecommunication companies as an alternative to terrestrial lines. • They can now be used as “local loop” technology (e.g., DirectPC). • Advantages: • Can reach arbitrary geographic locations. • Does not require wiring. • Has high bandwidth. • Perfect for broadcasting (can reach many users at once).

26. Satellite Systems (cont’d) • Disadvantages: • It’s a shared medium (the bitrate depends on the number of simultaneous users). • Delay (latency) can be relatively high (<1s) • Not ideal for playing interactive app’s (e.g., games). • You have to put a dish on your roof! • Initially, uplink was not provided. • Needed to use a separate phone line to uplink information. • Nowadays it is a two-way system.

27. Physical Layer: Summary • Different types of signal: • Analog and digital. • Analog communication infrastructure: • Need to convert digital to analog before transmitting: ADC. • DAC before entering computer. • Digitization: • Sampling. • Sampling period and frequency (samples/sec or Hertz). • Sample representation (quantization). • Bit rate.

28. Physical Layer: Summary (Cont’d) • Modems. • Input/output connections. • RS 232. • USB. • Firewire. • Broadband. • ISDN. • ADSL. • Cable modem. • Satellite.

29. Other Wireless Networks

30. Cellular Networks • Cellular phones:voice. • Cellular networks: shift from voice to data. • New wireless devices: pagers, PDAs. • New services: Web access, e-mail, instant messaging, etc.

31. Cellular Concept: Motivation • Early mobile radio systems: • Large coverage with single, high-powered transmitter. • But, no frequency re-use due to interference. • Since finite spectrum allocation, need: high capacity (number of users) with limited spectrum and wide coverage.

32. Some Cellular Terminology • Mobile. • Base station. • Mobile Switching Center (MSC). • Handoff. • Cell.

33. Cellular Architecture mobile BS BS cell cell

34. Cellular Fundamentals • System-level idea, no major technological changes. • Many low-power transmitters instead of single, high power on (large cell). • Service area divided into small cells covered by each low power transmitter. • Each transmitter (or base station) allocated a portion of the spectrum. • Nearby BSs assigned different channel group to minimize interference. • Scalability: as more users subscribe, more BSs can be added using lower transmission power).

35. Frequency Reuse E B G C A G F D E F

36. Handoff/Handover • Mobile hosts can change cells while communicating. • Hand-off occurs when a mobile host starts communicating via a new base station. • Handoff decision made based on signal strength.

37. Cellular Networks: Evolution • Evidence of the wireless success! • Since 1996, number of new mobile phone subscribers exceeded number of new fixed phone subscribers! • 1st. Generation (1G): analog technology. • FDMA. • Analog FM.

38. Second Generation (2G) • Most of today’s cellular networks use 2G standards. • Early 90s. • Digital technology. • Lighter, smaller devices with longer battery life. • Better reception and channel utilization.

39. 3G Wireless Networks • Multi-megabit Internet access, VoIP, ubiquitous “always-on” access. • Single mobile device for everything (integrated service approach). • New, world-wide standard. • International Mobile Telephone 2000 (IMT 2000)

40. Wireless Local Area Networks • Local area network connectivity using wireless communication. • IEEE 802.11 WLAN standard. • Example: WaveLan, Aironet • Wireless LAN may be used for • Last hop to a wireless host. • Wireless connectivity between hosts on the LAN.

41. Other WLAN Standards • HomeRF • Proponents of 802.11 frequency hoping-spread spectrum (FH-SS). • HomeRF 2.0 • 10 Mbps FH-SS. • HIPERLAN • Europe, mid 1990s. • Similar capability to IEEE 802.11b.

42. MANETs • Mobile, (wireless), multi-hop ad-hoc networks. • Formed by wireless hosts which may be mobile. • Without (necessarily) using a pre-existing infrastructure. • Routes between nodes may potentially contain multiple hops. • Mobilitty cause routes to change.

43. Multi-hop • May need to traverse multiple hops to reach destination.

44. Why MANETs ? • Ease of deployment. • Speed of deployment. • Decreased dependence on infrastructure.

45. Many Applications • Personal area networking. • Cell phone, laptop, ear phone, wrist watch. • Military environments. • Soldiers, tanks, planes. • Civilian environments. • “Smart” environments. • Emergency operations • Search-and-rescue • Policing and fire fighting • Monitoring and surveillance.