Chapter 11: Entertainment networks and high-speed modems

Outline Introduction cable TV networks Satellite television networks Terrestrial television networks High-speed PSTN access

technologies

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)

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

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

distribution cable bandwidth utilization.

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

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

11.2.1 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

Figure 11.4 Cable modem termination system schematic.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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)

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

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

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

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

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)