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-ondemand, 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 realtime 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-overIP 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 network
11.3.1 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 networks
11.4.1 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 25kHz1.1MHz 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)