1. The Physical Layer Chapter 2 CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Theoretical Basis for Data Communications Guided Transmission Media Wireless Transmission Communication Satellites Digital Modulation and Multiplexing Public Switched Telephone Network Mobile Telephone System Cable Television Revised: August 2011

2. The Physical Layer CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Foundation on which other layers build Properties of wires, fiber, wireless limit what the network can do Key problem is to send (digital) bits using only (analog) signals This is called modulation Physical Link Network Transport Application

3. The Public Switched Telephone Network CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Structure of the telephone system » Politics of telephones » Local loop: modems, ADSL, and FTTH » Trunks and multiplexing » Switching »

4. TUNALI Computer Networks 1 4 Structure of the Telephone System (1) (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

5. Structure of the Telephone System (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 A hierarchical system for carrying voice calls made of: Local loops, mostly analog twisted pairs to houses Trunks, digital fiber optic links that carry calls Switching offices, that move calls among trunks

6. TUNALI Computer Networks 1 6 Major Components of the Telephone System Local loops  Analog twisted pairs going to houses and businesses Trunks  Digital fiber optics connecting the switching offices Switching offices  Where calls are moved from one trunk to another

7. The Politics of Telephones In the U.S., there is a distinction for competition between serving a local area (LECs) and connecting to a local area (at a POP) to switch calls across areas (IXCs) Customers of a LEC can dial via any IXC they choose CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

8. Local loop (1): telephone modems CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Telephone modems send digital data over an 3.3 KHz analog voice channel interface to the POTS Rates <56 kbps; early way to connect to the Internet

9. TUNALI Computer Networks 1 9 Digital Subscriber Lines Modems are designed to carry human voice and they are slow for digital communications ADSL design issues Works on local loops Does not affect the telephone or fax machine Faster than 56 Kbps Always on with fixed monthly charge

10. TUNALI Computer Networks 1 10 DMT Discrete Multitone This is the underlying technology of ADSL Divides the 1.1 MHz spectrum of the local loop into 256 independent channels of 4312.5 Hz each Channel 0 is for plain telephone (POTS) Channel 1-5 are not used to prevent interference The rest of the channels are used for user data Typical speed is 64 Kbps upstream 1 Mbps downstream

11. Local loop (2): Digital Subscriber Lines CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 DSL broadband sends data over the local loop to the local office using frequencies that are not used for POTS Telephone/computers attach to the same old phone line Rates vary with line −ADSL2 up to 12 Mbps OFDM is used up to 1.1 MHz for ADSL2 −Most bandwidth down

12. TUNALI Computer Networks 1 12 Digital Subscriber Lines (2) Bandwidth versus distance over category 3 UTP for DSL.

13. Local loop (3): Fiber To The Home CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 FTTH broadband relies on deployment of fiber optic cables to provide high data rates customers One wavelength can be shared among many houses Fiber is passive (no amplifiers, etc.)

14. TUNALI Computer Networks 1 14 Time Division Multiplexing Using Pulse Code Modulation, a codec digitizes analog signals (voice) by sampling at 8000 samples per second (Nyquist Rate for 4 KHz band) corresponding to 125  sec/sample. Each sample is quantized to an 8-bit digital value leading to an 64 Kbps data rate. Each 64 Kbps voice channel is then multiplexed by using T1 carrier standards

15. TUNALI Computer Networks 1 15 Time Division Multiplexing (3) The quantization process can be carried out in various ways Differential PCM is one popular way The difference between the current and previous value of the voltage is coded Delta modulation −The difference above can at most be 1

16. TUNALI Computer Networks 1 16 Time Division Multiplexing (4) Delta modulation.

17. Trunks and Multiplexing (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Calls are carried digitally on PSTN trunks using TDM A call is an 8-bit PCM sample each 125 μs (64 kbps) Traditional T1 carrier has 24 call channels each 125 μs (1.544 Mbps) with symbols based on AMI

18. TUNALI Computer Networks 1 18 Time Division Multiplexing (5) Multiplexing T1 streams into higher carriers.

19. Trunks and Multiplexing (2) SONET (Synchronous Optical NETwork) is the worldwide standard for carrying digital signals on optical trunks Keeps 125 μs frame; base frame is 810 bytes (52Mbps) Payload “floats” within framing for flexibility CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

20. Trunks and Multiplexing (3) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Hierarchy at 3:1 per level is used for higher rates Each level also adds a small amount of framing Rates from 50 Mbps (STS-1) to 40 Gbps (STS-768) SONET/SDH rate hierarchy

21. Trunks and Multiplexing (4) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 WDM (Wavelength Division Multiplexing), another name for FDM, is used to carry many signals on one fiber:

22. TUNALI Computer Networks 1 22 Switching Circuit Switching A physical path is established all the way from sender to receiver Message Switching No physical path from sender to receiver is established Data is transferred in a store and forward fashion among switching nodes one hop at a time Packet switching Similar to message switching however block size is limited and message is segmented into packets

23. PSTN uses circuit switching; Internet uses packet switching Switching (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 PSTN: Internet:

24. Switching (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Circuit switching requires call setup (connection) before data flows smoothly Also teardown at end (not shown) Packet switching treats messages independently No setup, but variable queuing delay at routers Circuits Packets

25. Switching (3) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Comparison of circuit- and packet-switched networks

26. Mobile Telephone System CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Generations of mobile telephone systems » Cellular mobile telephone systems » GSM, a 2G system » UMTS, a 3G system »

27. Generations of mobile telephone systems CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 1G, analog voice −AMPS (Advanced Mobile Phone System) is example, deployed from 1980s. Modulation based on FM (as in radio). 2G, analog voice and digital data −GSM (Global System for Mobile communications) is example, deployed from 1990s. Modulation based on QPSK. 3G, digital voice and data −UMTS (Universal Mobile Telecommunications System) is example, deployed from 2000s. Modulation based on CDMA 4G, digital data including voice −LTE (Long Term Evolution) is example, deployed from 2010s. Modulation based on OFDM

28. TUNALI Computer Networks 1 28 Advanced Mobile Phone System (2) In 1982, Bell Labs developed Advanced Mobile Phone System (AMPS) The geographic region is divided up into cells typically 1-20 km across each using some set of frequencies By using relatively small cells, the transmission frequencies can be reused in a nearby but not adjacent cells Smaller cell size also leads to less power for transmission and smaller hand-held telephones

29. TUNALI Computer Networks 1 29 AMPS (3) At the center of each cell is a base station to which all the telephones in the cell transmit Base stations are connected to MTSO (Mobile Telephone Switching Office) that is connected to at least one telephone system When mobile telephone leaves the cell, the base station asks to neighboring base stations how much power they receive from the phone The ownership is transferred to the base station that receives the strongest signal from the phone The telephone is informed about its new boss If a call is in progress, the telephone changes channel since the old one is not used in the new cell

30. Cellular mobile phone systems CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 All based on notion of spatial regions called cells −Each mobile uses a frequency in a cell; moves cause handoff −Frequencies are reused across non-adjacent cells −To support more mobiles, smaller cells can be used Cellular reuse pattern Smaller cells for dense mobiles

31. TUNALI Computer Networks 1 31 Channels in AMPS 832 full-duplex channels 832 simplex channels in 824-849 MHz band 832 simplex channels in 869-894 MHz band Each simplex channel is 30 KHz wide FDM is used Four channel categories Control: To manage the system Paging: To alert the mobile users Access: For call setup and channel assignment Data: Voice, fax or data (only 45 for a cell!)

32. TUNALI Computer Networks 1 32 Call Management in AMPS When a phone is switched on, it scans a preprogrammed list of 21 control channels to find the most powerful signal From the control channel, it learns the numbers of the paging and access channels The phone then broadcasts its 32-bit serial number and 34-bit telephone number in a packet with error correcting code When the base station hears the announcement, it tells the MTSO, which records the existence of its new customer Mobile phone reregisters once every 15 minutes

33. TUNALI Computer Networks 1 33 Making a Call The phone sends the number to be called and its own identity on the access channel The base station forwards the call to MTSO The MTSO finds an idle channel for the call and sends it back on the control channel The mobile phone automatically switches to the selected voice channel and waits for the called party to pick up the phone

34. TUNALI Computer Networks 1 34 Receiving a Call Idle phones listen to the paging channel When a call is placed by another phone, a packet is sent to the callee’s home MTSO to find out where it is. Then a packet is sent to the base station in its current cell which in turn broadcasts on the paging channel The callee answers the broadcast and the base station tells the callee the channel number which in turn automatically switches to the assigned channel and starts making ringing sounds

35. TUNALI Computer Networks 1 35 Security Issues Any receiver that can tune in can hear everything A thief can pick up a 34-bit serial number and 34-bit telephone number and use it in their calls

36. GSM – Global System for Mobile Communications (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Mobile is divided into handset and SIM card (Subscriber Identity Module) with credentials Mobiles tell their HLR (Home Location Register) their current whereabouts for incoming calls Cells keep track of visiting mobiles (in the Visitor LR)

37. GSM – Global System for Mobile Communications (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Air interface is based on FDM channels of 200 KHz divided in an eight-slot TDM frame every 4.615 ms Mobile is assigned up- and down-stream slots to use Each slot is 148 bits long, gives rate of 27.4 kbps

38. TUNALI Computer Networks 1 38 GSM (2) A portion of the GSM framing structure.

39. TUNALI Computer Networks 1 39 Third-Generation Mobile Phones: Digital Voice and Data Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (replace e-mail, fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.)

40. UMTS – Universal Mobile Telecommunications System (1) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Architecture is an evolution of GSM; terminology differs Packets goes to/from the Internet via SGSN/GGSN Internet

41. UMTS – Universal Mobile Telecommunications System (2) CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Air interface based on CDMA over 5 MHz channels Rates over users <14.4 Mbps (HSPDA) per 5 MHz CDMA allows frequency reuse over all cells CDMA permits soft handoff (connected to both cells) Soft handoff

42. Cable Television CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Internet over cable » Spectrum allocation » Cable modems » ADSL vs. cable »

43. TUNALI Computer Networks 1 43 Community Antenna Television An early cable television system.

44. Internet over Cable CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Internet over cable reuses the cable television plant Data is sent on the shared cable tree from the head- end, not on a dedicated line per subscriber (DSL) ISP (Internet)

45. TUNALI Computer Networks 1 45 Internet over Cable (2) The fixed telephone system.

46. Spectrum Allocation CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Upstream and downstream data are allocated to frequency channels not used for TV channels:

47. Cable Modems CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cable modems at customer premises implement the physical layer of the DOCSIS standard QPSK/QAM is used in timeslots on frequencies that are assigned for upstream/downstream data

48. Cable vs. ADSL CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011 Cable: +Uses coaxial cable to customers (good bandwidth) −Data is broadcast to all customers (less secure) −Bandwidth is shared over customers so may vary ADSL: +Bandwidth is dedicated for each customer +Point-to-point link does not broadcast data −Uses twisted pair to customers (lower bandwidth)

49. TUNALI Computer Networks 1 49 Required Reading Tanenbaum Chapter 2 Sections 2.6 2.7 2.8

50. TUNALI Computer Networks 1 50 Homework Tanenbaum Chapter 2 Problems 18 24 29 35 (37) 36 (38)

51. End Chapter 2 CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011