1. 09/20/2007EETS 73041 Chapter 2/2 (Physical Layer) Public Switched Telephone System (2) The Mobile Telephone System Cable Television

2. 09/20/2007EETS 73042 Public Switched Telephone System (2) Trunks and Multiplexing Switching The Mobile Telephone System PLMN Cable Television

3. 09/20/2007EETS 73043 Frequency Division Multiplexing Group hierarchy 1 group = 12 voice channels (48 kHz) 1 supergroup = 5 groups = 60 voice channels (240 kHz) 1 mastergroup = 5 supergoups = 300 voice channels (1200 kHz) Multiplexed channel Original bandwidths The bandwidths raised in frequency

4. 09/20/2007EETS 73044 Wavelength Division Multiplexing Each channel carries 2.5 Gbps and they multiplex 40 channels going to 200 Channels. Because of slow electric to optical conversion bandwidth is less than 5 GHz or 10 Gbps. Since fiber bandwidth is 25000 GHz there is theoretical room for 5000 channels.

5. 09/20/2007EETS 73045 Time Division Multiplexing (CCITT) T1 carrier (1.544 Mbps) Voice: 7 bits for data 1 for signaling. Data: 23 data channels and 24-th channel for signaling. E1 carrier 32*64 kbps = 2.048 Mbps 30 used for voice/data 31 and 32 used for signaling 8-th bit every 6-th frame.

6. 09/20/2007EETS 73046 Time Division Multiplexing hierarchy Multiplexing T1 streams into higher carriers.

7. 09/20/2007EETS 73047 Sonet time Division Multiplexing First two bytes contain frame synchronization bit pattern. SONET frame: 810 bytes/125 mks = 51.84 Mbps. SPE (Synchronous Payload Envelope – user data) First row in line overhead points to the first payload byte.

8. 09/20/2007EETS 73048 SONET and SDH * multiplex rates * SDH – Synchronous Digital Hierarchy

9. 09/20/2007EETS 73049 Circuit Switching (a) Circuit switching. (b) Packet switching.

10. 09/20/2007EETS 730410 Message Switching (a) Circuit switching (b) Message switching (c) Packet switching

11. 09/20/2007EETS 730411 Circuit switched vs. packet-switched networks

12. 09/20/2007EETS 730412 The Mobile Telephone System First-Generation Mobile Phones: Analog Voice AMPS 800 MHz band Second-Generation Mobile Phones: Digital Voice 1900 MHz IS-136, IS-54, IS-95 2.5 generation introduces data over voice network: GSM -> GPRS. Third-Generation Mobile Phones: Digital Voice and Data: UMTS/W-CDMA, CDMA- 2000. Fourth-Generation Mobile Phones: WiFi and WiMax

13. 09/20/2007EETS 730413 Advanced Mobile Phone System (a)Frequencies are not reused in adjacent cells: 832 full duplex channels, 21 for control hardwired into phone. Typical 45 channels/cell. (b) To add more users, smaller cells can be used.

14. 09/20/2007EETS 730414 TDM Channel Categories 800 MHz + 1900 MHz band 30 KHz channels are divided into four categories: Control (base to mobile) to manage the system. 21 channels hardwired into phone ROM. Paging (base to mobile) to alert users for calls for them Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fax, or data. Voice is compressed to 8 kbps for 3 slots per frame or 4 kbps for 6 slots per frame.

15. 09/20/2007EETS 730415 D-AMPS (Digital Advanced Mobile Phone System)||TDMA (a) D-AMPS channel with three users. (b) D-AMPS channel with six users.

16. 09/20/2007EETS 730416 GSM: Global System for Mobile Communications GSM uses 124 frequency channels 200 kHz each of which uses an eight-slot (992 slots) TDM system.

17. 09/20/2007EETS 730417 GSM framing structure

18. 09/20/2007EETS 730418 CDMA concept 1 1 -1 -1 1 1 -1 -1 d 1 o =1 d 1 1 =-1 Data bits Chips Senders 1 1 1 d 2 o =1d 2 1 =1 1 1 1 Data bits Chips Z i, 1 m = d i 1 c m 1 Z i, 2 m = d i 2 c m 2 2 2 2 -2 1 1 -1 -1 1 1 Channel Z i, * m d 1 o =1 d 1 1 =-1 d 1 i = (  m Z i, * m c 1 m )/M -1 -1 1 1 1 1 1 1 d 2 i = (  m Z i, * m c 2 m )/M d 2 o =1 d 2 1 =1 Chip rate Spreading factor = chip_rate/data_rate. dB = 10 log( spreading rate/data rate ) has the same effect as dB (signal/noise).

19. 09/20/2007EETS 730419 Third-Generation Mobile Phones: Digital Voice and Data Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (e-mail, fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.)

20. 09/20/2007EETS 730420 Cable Television Community Antenna Television Internet over Cable Spectrum Allocation Cable Modems ADSL versus Cable

21. 09/20/2007EETS 730421 An early cable television system

22. 09/20/2007EETS 730422 Phone and Internet over Cable

23. 09/20/2007EETS 730423 TV over Subscriber Loop The fixed telephone system.

24. 09/20/2007EETS 730424 TV Spectrum Allocation for Internet access Upstream; QPSK is used due to large noise. Downstream: QAM-64 and QAM-256 is used. For 6 MHz bandwidth it is 36 Mbps.

25. 09/20/2007EETS 730425 Cable Modems Upstream channels are divided into minislots. Headend assigns upstream minislot and downstream channels (184 byte payload length) to the powered-up modem. Typical minislot is 8 bytes. Collision is Slotted Aloha with random backoff doubled each successive collision. IP address is dynamically allocated to modem using DHCP (Dynamic Host Configuration Protocol).

26. 09/20/2007EETS 730426 The Data Link Layer Chapter 3

27. 09/20/2007EETS 730427 Functions of the Data Link Layer Error free service provided to the Network Layer Framing Error Control: dealing with transmission errors Flow Control: regulating data flow (through acknowledgments) so that slow receivers are not swamped by fast senders.

28. 09/20/2007EETS 730428 Relationship between packets and frames

29. 09/20/2007EETS 730429 Services Provided to Network Layer (a) Virtual communication. (b) Actual communication.

30. 09/20/2007EETS 730430 DLL within a router

31. 09/20/2007EETS 730431 Framing with character count A character stream: (a) Without errors: first byte contains the message length (in bytes). (b) With one error: receiver looses synchronization and cannot recover.

32. 09/20/2007EETS 730432 Framing with ‘esc’ character: byte stuffing (PPP) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

33. 09/20/2007EETS 730433 Framing with zero insertion (HDLC) (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing. Flag bit pattern: 0 1 1 1 1 1 1 0 Zero insertion eliminates flag bit pattern to appear in the output text

34. 09/20/2007EETS 730434 Error Detection and Correction (using additional bits) Error-Correcting Codes (used in voice communication) Error-Detecting Codes (used in data communication) Sender receives a positive acknowledgment if data received correctly and negative ack to ask for retransmission. What to do if sent frame is completely lost? Solution is to use a timer slightly larger than round-trip delay. If sender doesn’t receive ack it retransmits. There is a possibility that receiver receives the same frame twice. To avoid that the frames are numbered in increasing order and each frame carries this (sequence) number.

35. 09/20/2007EETS 730435 Hamming Distance Hamming distance d: the number of bits in which two words differ. To mistaken one word with the other at least d bits must be in error. Hamming distance of the code: the minimum Hamming distance between any two code words. For error detection with d bit error the code must have distance d+1. S1S1 S2S2 d d + 1 d For error correction with d bit error the code must have distance 2d+1. R S 1 S 2 are two valid code words. R is the word received. Since not within the set of valid words it is an error. We have no way to say whether R is S 1 or S 2. Hence error detection.

36. 09/20/2007EETS 730436 Error Correction S1S1 S2S2 d 2d + 1 d Code with hamming distance 2d + 1 is capable of error correction within d or less erroneous bits. If R is received we say that the sending code word is S 2. R

37. 09/20/2007EETS 730437 Hamming code corrects single bit errors 000 101 011 110 100 010 001 111 m - word bits r - check bits n = m + r code bits Example: m = 1 n = 3 r = 2 For single bit error correction: each valid word has n bit pattern dedicated to it (obtained by one bit change in a valid word). Therefore (n+1)*2 m <= 2 n. (m + r + 1)*2 m <= 2 m+r. m+r+1 <= 2 r m r 12 complete 233 43 complete 7 4 1000 10

38. 09/20/2007EETS 730438 Hamming (11,7) Code p1p1 p2p2 d1d1 p3p3 d2d2 d3d3 d4d4 p4p4 d5d5 d6d6 d7d7 Data word (without parity) 1001000 p1p1 010100 p2p2 010100 p3p3 1001 p4p4 0000 Data word (with even parity) 00110010000 p1p1 p2p2 d1d1 p3p3 d2d2 d3d3 d4d4 p4p4 d5d5 d6d6 d7d7 Parity checkParity bit Received word00110010001 p1p1 010101fail1 p2p2 010101 1 p3p3 1001pass0 p4p4 0001fail1 Coding Decoding: the position of error bit = 8*p 4 + 4*p 3 + 2*p 2 + p 1 -> 8 + 2 + 1 = 11.

39. 09/20/2007EETS 730439 Use of a Hamming code to correct burst errors Words are sent leftmost column first then the next column etc. If the burst error (burst <= 12) occurs it can be recovered by Hamming correction code.

40. 09/20/2007EETS 730440 Cyclic Redundancy Check Error Detecting Codes Only words (represented as a single number) that are divisible by given divisor are valid. In modulo arithmetic there is finite number of these words. They are evenly spaced (equidistant from each other). Say all number up to 100 divisible by 7 are legal. Coding: transform message M word into code word T using generator polynomial G. Assume we want to send M = 50. 1.Multiply M*10 -> 500 (10 is the order of G) 2.Divide M*10 : G -> 500 : 7 = 71 + 3/7 -> 500 = 71*7 + 3 (remainder) 3.Subtract remainder from M*10 -> T = 500 – 3 = 497 (certainly divisible by 7). Decoding: T/G -> 497/7 = 71 (good word divisible by 7)

41. 09/20/2007EETS 730441 CRC codes G(x) is generating polynomial of degree r. M(x) is a message word of degree m > r. Coding 1.Append r 0’s to the M(x) -> x r M(x). 2.Divide modulo 2: x r M(x) : G(x) = Q(x) + R(x)/G(x) 3.T(x) = x r M(x) - R(x). (Certainly divisible by G(x)) On the next slide G(x) = x 4 + x + 1 = 10011 -> r = 4. M(x) = x 9 + x 8 + x 6 + x 4 + x 3 + x + 1 = 1101011011 -> m = 9. 1.x 4 M(x) = 11010110110000 2.R(x) = x 4 M(x) : G(x) = 1110 = x 3 + x 2 + x < G(x) 3.T(x) = x 4 M(x) + R(x) = 11010110111110 (certainly divisible by G(x)).

42. 09/20/2007EETS 730442 Calculation of the polynomial code checksum.

43. 09/20/2007EETS 730443 Analysis of CRC coding: choice of G(x) Decoding: Receiver calculates the remainder: (T(x) + E(x))/G(x) = E(x)/G(x). 1.Single bit error: E(x) = x i. If G(x) contains 2 or more terms x i will never be divisible by G(x) => single error is always detected. 2.Double error: E(x) = x i + x j = x j (x i-j + 1) -> x k + 1 (0 < k < frame length) 3.G(x) = x 15 + x 14 + 1 detects double errors up to 32768 frame length. …. 4.Polynomial coding with r check bits will detect all burst errors with the burst error length <= r. CRC calculations are performed on a fly by shift registers with feedback that is determined by G(x).