Chapter 11: Entertainment networks and high-speed modems

1. Chapter 11: Entertainment networks and high-speed modems

2. Outline • Introduction • cable TV networks • Satellite television networks • Terrestrial television networks • High-speed PSTN access technologies

3. Introduction • Entertainment applications include movie/ video-on-demand, broadcast television and interactive television • Figure 11.1 • Typical rates are • VCR-quality video with sound 1.5Mbps(MPEG-1) • Broadcast-quality video with sound 4/6/8Mbps (MPEG2,Main) • Studio-quality television with sound 9/15/18Mbps (MPEG2,Main) • High-definition television with sound 60/80Mbps (MPEG2,High)

5. 11.2 cable TV networks • Community antenna television(CATV) networks: These were designed to distribute broadcast television programs to customer premises geographically distributed around an area • Frequency division multiplexing (fdm): In order to transmit multiple TV programs concurrently, modulated transmission is used with each program allocated a fixed bandwidth

6. 11.2 cable TV networks • Each channel is allocated an fixed amount of bandwidth-6MHz in North America and 8MHz in Europe-which ensures that signals relating to the two channels are cleanly separated • The bandwidth used is limited to 88/110 at the low end through to 300, 450, or 550MHz • Figure 11.2

7. distribution cable bandwidth utilization.

8. 11.2.1 HFC networks • Hybrid fiber coax (HFC) network: optical fiber is used in the main trunk network and coaxial cable is limited to the local distribution network • Each distribution cable provides cable services to between 125 and 500 homes distributed over an area of 3 miles • The use of fiber means that the signal attenuation is much reduced so removing the necessity of having a large number of amplifiers

9. 11.2.1 HFC networks • The guard band(GB) at the lower end is 8/10MHz and 50MHz at the upper end • GB is necessary to ensure that the new services are cleanly separated • RF modems converts the source digital bitstream into an analog signal for transmission • Upstream/return direction is required from the subscriber to the cable headend from 5MHz through to 30/42/48/65MHz • Figure 11.3

11. 11.2.1 HFC networks • Cable modem termination system(CMTs) is used to control the transmissions to all the CMs to relay packets • In the downstream direction, each packet is simply broadcast and is received by all the attached CMs • In the upstream direction, the attached CMs must compete for the use of the upstream channel • Access to the upstream channel is controlled by the CMTS • Figure 11.4

12. Figure 11.4 Cable modem termination system schematic.

13. 11.2.1 HFC networks • Once CM obtains permission to start to relay frames, the CMTS responds by assigning a unique service identifier(SID) and one or more service flow identifiers(SFID) to the CM • When a CM wishes to transmit a frame, the cable MAC in the CM first sends a Request(REQ) to the cable MAC in the CMTS which indicates the amount of bandwidth • The CMTS responds by returning a Grant message which indicates the needed bandwidth allocated

14. 11.2.1 HFC networks • All CMs must be in time synchronism with the master clock in the CMTS • Ranging : Each CM must know the round-trip propagation delay time between itself and the CMTS • Both time synchronization and ranging must be carried out during the initialization phase of the CM

15. 11.2.1 HFC networks • The CMTS broadcasts a SYNC message that contains the current state of this counter on the downstream channel at periodic (10ms) intervals • Once a CM is in time synchronism, it invokes the ranging procedure to determine its round-trip correction(RTC) time • With time synchronism, it searches each MAP from the downstream channel for an initial maintenance (IM) region • When it finds one, it immediately transmits a Ranging Request(RNG-REQ) message

16. 11.2.1 HFC networks • On receipt of the RNG-REQ message, the CMTS can estimate the round-trip propagation delay from the CM to the CMTS • The CMTS responds by returning a Ranging Response(RNG-RSP) message that contains the computed offset to be used by the CM • On receipt of this, the CM first proceeds to locate another IM region in the next MAP it receives • Figure 11.7

18. 11.2.1 HFC networks • In the header of each bandwidth allocation MAP is a pair of fields, one called the data backoff start(DBS) and the other the data backoff end (DBE) • The range is known as the backoff window • The value of DBE is the maximum backoff window • Providing the Grant messaged is received, the Request message was received successfully by the CMTS and no collision occurred

19. 11.2.1 HFC networks • If a Grant is not received, the CM must try again • It first doubles its current backoff window, the CM proceeds to compute a new random number within the limits of the new window • A maximum retry limit of 16 is used, if the maximum backoff window is reached, the CM discards the frame • Figure 11.8

21. 11.2.1 HFC networks • During periods of heavy traffic on the upstream channel, a grant message may specify a smaller number of minislots to transfer the user data frame • The MAC layer within the CM automatically splits the user data frame into a number of smaller fragments for transmission over the cable • Piggyback request: when the cable MAC prepares the first fragment, it computes the number of minislots required to send the remainder of the frame including the 16bytes of overhead

22. 11.2.1 HFC networks • Service flow:The CMTS schedules transmissions so that the agreed QoS parameters with each service class are met • Each service flow is characterized by a separate service flow identifier • Unsolicited grant:these are intended for use for service flows involving packets containing real-time information

23. 11.2.1 HFC networks • Real-time polling:the CMTS periodically polls each CM with an active service flow to make a bandwidth request at intervals of about 1ms • Unsolicited grant with activity detection:the packetization process associated with voice-over-IP exploits the silence periods between talk spurts by ceasing to send packets during these periods • Non-real-time polling:this service is intended for use with non-real-time applications that involve the transfer of large volumes of data

24. 11.2.1 HFC networks • Each packet is made up of a 4-bytes header and a 184-byte payload • The first byte of the header is for synchronization purposes ; A second 13-bit field is used to identify the type contents in the payload • The 184-byte payload field is used to transfer the cable MAC frames • For each CM to determine if a pinter bye is present, a single bit in the 4-byte packet header called the payload unit start indicator(PUSI) bit is set to 1 • Figure 11.11

26. 11.2.1 HFC networks • Multichannel multipoint system (MMDS) • Local multipoint distribution system (LMDS) • Both use ommidirectional transmitters and provide a similar range of services to those provided with a coaxial cable distribution network • The difference is the area of coverage of the transmissions and the number of channels suppoted • Figure 11.14

28. 11.3 Satellite television network • It broadcasts a set of TV programs to the set-top boxes of a large number of subscribers • Geostationary earth orbit(GEO):at 36000km height, from a point on the earth’s surface, the satellite appears stationary • The signal of each TV program is frequency modulated onto a separate carrier which results in a basic channel bandwidth of 36 MHz

29. 11.3 Satellite television network11.3.1 Broadcast television principles • Each channel is allocated a separate carrier signal in both the uplink and downlink directions with the same fixed spacing between channels • Figure 11.16

31. 11.3.1 Broadcast television principles • Satellite intermediate frequency(SAT-IF):The frequency of the uplink signal exceeds the bandwidth of a coaxial cable, hence the received signal is amplified in the LNB/C and downconverted • Prime focus antenna:the disadvantage of this design is the LNB/C inhibits the direct waves in the center of the dish from being collected • Offset focus antenna: This has the effect of increasing significantly the antenna’s efficiency • Figure 11.17

33. 11.3.1 Broadcast television principles • In the uplink direction a narrow beam width is used to ensure the maximum amount of energy in the signal transmitted • A wide beam width is used in the downlink direction to ensure the signal is received by all the antennas within the satellite’s footprint • The frequency range of the uplink channel is from 5.925 to 6.425 GHz and that of downlink channels from 3.7 to 4.2GHz • Typical channel bandwidths of 40MHz are used

34. 11.3.2 Digital television • The frequency band used in digital TV is called the Ku band which covers the frequency range from 10.7 through to 14.5 GHz • The band from 10.7 to 11.7 GHz is mainly used for newer analog TV transmissions • For digital TV, the band from 12.2 to 12.7 GHz for the North American digital broadcast satellites (DBS) and 11.7 to 12.5 GHz for the European digital video broadcasting-satellites (DVB-S)

35. 11.3.2 Digital television • The bitstreams of multiple TV programs are multiplexed together into a single bitstream made up of 188-byte packets • Interleaving: very long error bursts in a block can be broken down into smaller bursts • Convolutional encoder: It minimize the effect that satellite transmissions are susceptible to randomly distributed single bit errors • Figure11.18

37. 11.3.3 Interactive services • Local interaction:each MPEG2 program multiplexer supports an optional data channel for various purposes such as pay-per-view • Anonymous response to broadcasts: a low bit rate interaction channel is involved which is a PSTN • Purchase requests:typically this is in response to a product or service that is being offered via a TV broadcast

38. 13.4 Terrestrial television networks11.4.1 Broadcast television principles • Multiple-frequency network(MFN):Each transmitter operates in a different frequency band from its neighbors and the network • Single-frequency network(SFN): all transmitters operate using the same frequency band • Figure 11.19

40. 11.4.2 Digital television • These waves, such as microwaves reflected from buildings and various atmospheric conditions, take longer time to reach the antenna than the direct wave and lead to an effect─multipath • Intersymbol interference(ISI):multipath dispersion causes the signals relating to a previous bit to interfere with the signals relating to the next bit • Coded orthogonal frequency division multiplexing (COFDM): is used for the transmission of the high bit rates associated with digital television

41. 11.4.2 Digital television • Guard interval: ensure that all delayed versions of the direct-path signals that make up the symbol have been received • Inverse discrete Fourier transform (IDFT): the generation of each symbol is carried out digitally using a mathematical technique • Figure 11.20

43. 11.5 High-speed PSTN access technologies • Digital subscriber line(DSL) • ISDN DSL(IDSL) uses a single pair • High-speed DSL(HDSL) uses two pairs • Single-pair DSL(SDSL) is a simpler version of HDSL which operates over a single pair • Asymmetric DSL (ADSL) • Very-high-speed DSL (VDSL)

44. 11.5.1 ADSL • It provides a downstream bit rate of up to 1.5M bps and an upstream bit rate of up to 384 kbps • As with ADSL, the actual bit rates achievable depend on the length and quality of the line • POTS splitter is to separate out the POTS and ADSL signals • Low-pass filter from 0-4kHz that passes only the POTS signal and a high-pass filter from 25kHz-1.1MHz that passes only the ADSL signals

45. 11.5.1 ADSL • ADSL-Lite: the advantage is the much simplified NT as it avoids the use of a POTS splitter and filters • The disadvantage is that some interference can be experienced with the basic telephony service when the high bit rate service is being used • Figure 11.23

47. 11.5.1 ADSL • The modulation method used with ADSL modems is called discrete multitone (DMT) • Frequency division duplex(FDD): the bitstream in each direction is transmitted concurrently using a different portion of the allocated bandwidth and set of carriers • Figure 11.24

48. Figure 11.24 Example DMT frequency usage: (a) bits per carrier allocation, (b) duplex frequency usage.Bits per carrier

49. 11.5.2 VDSL • Bit rates can be up to 20M bps in each direction when used in a symmetric configuration or up to 52 Mbps in a asymmetric configuration with a return path of up to 1.5 Mbps • The duplexing method is likely to be based on time-division duplexing (TDD)