Telephone data networks

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Chapter 9
Using Telephone
and Cable Networks
for Data Transmission
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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9-2 DIAL-UP MODEMS
Traditional telephone lines can carry frequencies
between 300 and 3300 Hz, giving them a bandwidth of
3000 Hz. All this range is used for transmitting voice,
where a great deal of interference and distortion can
be accepted without loss of intelligibility.
Modem Standards
Topics discussed in this section:

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Figure 9.6 Telephone line bandwidth
•The signal bandwidth must be smaller than the cable bandwidth.

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Modem
stands for modulator/demodulator.
Note
• A modulator creates a bandpass analog signal from binary data.
• A Demodulator recovers the binary data from the modulated
signal.

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Figure 9.7 Modulation/demodulation

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Modem standard
V.32
 Uses a combined modulation and encoding technique called trellis-
coded modulation.
 Data stream is divided into 4-bit stream.
 A pentabit (5-bit pattern) is transmitted
 The extra bit is used for error detection.
 V32 calls with a baud rate of 2400 = 4 X 2400 = 9600 bps.
V.32bis
 Uses 7 bits/baud 1 bit for error control.
 V.32 calls with a baud rate of 2400 = 6 X 2400 = 14400 bps>
 V.32bis enables the modem to adjust its speed upward or downward
depending on the quality of the line or signal (fall-back and fall-
forward).

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Figure 9.8 The V.32 and V.32bis constellation and bandwidth

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Modem standard
V.34bis
 Bit rate of 28,800 bps with 960-point constellation to 1664-point
constellation for a bit rate of 33,600 bps.
V-90
 Traditionally modems have a limitation on data rate (max. 33.6
Kbps)
 V-90 modems (56k modems) can be used (up to 56Kbps) if using
digital signaling.
 For example, Through an Internet Service Provider (ISP).
 V-90 are asymmetric
 Downloading rate (from ISP to PC) has a 56 Kbps limitation.
 Uploading rate (from PC to IST) has a 33.6 Kbps limitation.

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Figure 9.9 Uploading and downloading in 56K modems

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Modem standard
V.92
 Modems can adjust their speed, and if the noise allows, they can
upload data at the rate of 48 Kbps. The downloading rate is still 56
Kbps.
 Modem can interrupt the Internet connection when there is an
incoming call if the line has call-waiting service.

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Using Telephone and Cable Networks for Data Transmission
 Telephone networks were created to provide voice
communication.
 Need to communicate digital data resulted in invention of
the dial-up modem.
 With the appearance of the internet and the need for
high-speed downloading and uploading (modem too
slow), the telephone companies added a new technology
(DSL: digital subscriber line)

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9-3 DIGITAL SUBSCRIBER LINE
• After traditional modems reached their peak data rate, telephone companies
developed another technology, DSL,
•to provide higher-speed access to the Internet.
• Digital subscriber line (DSL) technology is one of the most promising for supporting
high-speed digital communication over the existing local loops.
DSL technology is a set of technology
ADSL
VDSL
HDSL
SDSL
The set is often referd to as xDSL, where x can be replaced by A,V,H, or S.

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ADSL
Asymmetric DSL (ADSL)
 Is like a 56k modem provides higher speed (bit rate) in the downstream direction than in the
upstream direction. ( Why called Asymmetric)
 Designers of ADSL divided available bandwidth of the local loop unevenly for the
residential customer.
• It is not suitable for businesses who need a large bandwidth in both direction.
Using Existing Local Loops
 How with ADSL we reach a data rate that was never achieved with modems?
 The existing local loops (twisted-pair) can handle bandwidths up to 1.1 MHz.
 Filter installed at the end office of the telephone company where each local
loop terminates limits the bandwidth to 4 KHz (voice communication).
 If filter is removed, the entire 1.1 MHz is available for voice and data
communication.

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ADSL
Adaptive Technology
 1.1 MHz is theoretical bandwidth of the local loop.
 Numbers of factors affect the bandwidth:
 Distance between the residence and the switching office
 The size of the cable,
 The signaling used.
 Designers of ADSL were aware of this problem and used an adaptive
technology.
 The system uses a data rate based on the condition of the local loop line.
 The data rate of ADSL is not fixed; it changes based on the condition and
type of the local loop cable.

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Discrete Multitone Technique (DMT)
 The modulation technique that has become standard for ADSL is called the
discrete multitone technique which combines QAM and FDM.
 The DMT divides a 1.104 MHz bandwidth into 256 channels about 4.312
kHz each.
Figure 9.10 Discrete multitone technique (DMT)

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Figure 9.11 Bandwidth division in ADSL

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Discrete Multitone Technique (DMT)
 Voice. Channel 0 is reserved for voice communication.
 Idle. 1 to 5 are not used and provide a gap between voice and data
communication.
Upstream data and control:
 Channels 6-30 (25 channels) are used for upstream data transfer
and control:
 1 channel for control.
 24 channels are for data transfer.
 If there 24 channels, each using 4 kHz with QAM modulation:
 Bandwidth = 24 X 4000 X 15 = 1.44 Mbps.
 However, the data rate is normally below 500 kbps because some of
the carriers (channels) are deleted at frequencies where the noise
level is large (some of channels may be unused).

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Discrete Multitone Technique (DMT)
Downstream data and control
 Channels 31-255 (225 channels) are used for downstream data and
control:
 1 channel for control.
 224 channels for data transfer.
 If there are 224 channels, we can achieve up to 13.4 Mbps:
 Bandwidth = 224 X 4000 X 15 = 13.4 Mbps.
 However, normally the data rate is below 8 Mbps because some of
carriers are deleted at frequency where the noise level is large
(some of channels may be unused).

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 ADSL modem installed at a customer’s site.
 The local loop connects to a splitter which separates voice and data
communications.
 ADSL modem modulates and demodulates the data, using DMT, and
creates downstream and upstream channels.
Figure 9.12 ADSL modem

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Figure 9.13 Digital Subscriber Line Access Multiplexer (DSLAM)
Digital Subscriber Line Access Multiplexer (DSLAM)
 At the telephone company site, instead of an ADSL modem,
 A device called a DSLAM is installed that functions similarly.
 It packetizes the data to be sent to the Internet (ISP server)

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ADSL Lite
 ADSL can be expensive and impractical enough to discourage most
subscribers:
 The installation of splitters at the border premises
 New wiring for the data line.
 ADSL Lite is new technology (universal ADSL or splitterless ADSL ):
 Plug ASDL lite modem directly into a telephone jack and connected to the
computer.
 Splitting is done at the telephone company.
 ADSL lite uses 256 DMT carriers
 with 8-bit modulation (instead of 15-bit)
 However, some of the carriers may not be available.
 Maximum downstream data rate 1.5 Mbps.
 Upstream data rate of 512 kbps.

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HDSL
HDSL (high-bit-rate digital subscriber line)
 Alternative for T-1 line (1.544 Mbps)
 The T-1 uses alternate mark inversion (AMI) encoding which is very
susceptible to attenuation at high frequencies.
 This limits the length of T-1 to 1 km (3200 ft) (repeater is needed for
longer distance)
 Increase the cost.
 HDSL uses 2B1Q encoding which is less susceptible to attenuation.
 A data rate of 1.544 Mbps (up to 2 Mbps) without repeater up to a
distance of 3.86 km (12000 ft).
 HDSL uses two twisted pairs (One pair for each direction) to achieve
full-duplex transmission.

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SDSL
The symmetric digital subscriber line (SDSL) is a one twisted-pair
version of HDSL.
 It provides full-duplex symmetric communication up to 768 kbps in each
direction.
 Could be considered an alternative to ADSL because !!!!
 Although this feature meets the need of most residential subscribers, is not
suitable for business
 because send and receive data in large volumes in both direction.

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VDSL
The very high-bit-rate digital subscriber line (VDSL)
 Similar to ADSL, uses coaxial, fiber-optic, or twisted-pair
cable for short distances.
 The modulating technique is DMT:
 25 to 55 Mbps for upstream at distance of 3000 to 10,000
 3.2 Mbps for downstream

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Table 9.2 Summary of DSL technologies

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9-4 CABLE TV NETWORKS
The cable TV network
•started as a video service provider,
•but it has moved to the business of Internet access.
Traditional Cable Networks
Hybrid Fiber-Coaxial (HFC) Network
Topics discussed in this section:

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Figure 9.14 Traditional cable TV network
• Cable TV started to distribute video signals to locations with poor or no reception
in the late 1940s.
• Called community antenna TV (CATV) because”
• Antenna on the top of a tall hill or building received the signal
• Distributed via coaxial cable to the community

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Traditional cable TV network
 The cable TV office called the head end
 Receives video signals from broadcasting stations
 Feeds the signals into coaxial cables
 Signals became weaker with distance, so amplifiers were installed through
the network.
 There could be up 35 amplifiers between the head end and the subscriber
premises.
 At the end splitter split the cable, and taps and drop cables make the
connection to the subscriber premises.
 The traditional cable TV system used coaxial cable end to end:
 Because of:
 Attenuation of the signals
 The use of large number of amplifiers,
• Communication in the traditional cable TV network is unidirectional.

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Figure 9.15 Hybrid fiber-coaxial (HFC) network
• The network use a combination of fiber-optic and coaxial cable.
• From the cable TV office to a box, called the fiber node, is optical fiber.
• From the fiber node through the neighborhood and into the house is coaxial
cable

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Hybrid fiber-coaxial (HFC) network
 The regional cable head (RCH) serves up to 400,000 subscribers.
 The RCH feed the distribution hubs each serves up to 40,000
subscribes.
 The distribution hub does
 the modulation and distribution of signals.
 Feed the signals to the fiber nodes through the fiber-optic cables.
 The fiber node splits the analog signals, so the same signal is sent
to each coaxial cable.
 Each coaxial cable serves up to 1000 subscribers.
 The fiber-optic cables reduce the need for amplifiers down to eight
or less.
 The reason to move fro traditional to hybrid infrastructure is to make
the cable network bidirectional.

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9-5 CABLE TV FOR DATA TRANSFER
• Cable companies are now competing with telephone companies for the
residential customer who wants high-speed data transfer.
• DSL uses unshielded twisted-pair cable, which is very susceptible to
interference.
•Which impose an upper limit on the data rate.
•Another solution is the use of the cable TV network.
Bandwidth
Sharing
CM and CMTS
Data Transmission Schemes: DOCSIS
Topics discussed in this section:

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Figure 9.16 Division of coaxial cable band by CATV
• Coaxial cable in HFC system has a bandwidth that ranges 5-750 MHz.
• Cable company has divided this bandwidth:
Each TV channel
occupies 6 MHZ,
how many
channels we can
accommodate?
80
. From the
Internet to the
subscriber
premises.
. Each
channels is 6
MHz.
Each
channel
is divided
into 6
MHz

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Downstream data are modulated using
the 64-QAM modulation technique.
Note

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The theoretical downstream data rate
is 30 Mbps.
Note
• 5 bits/Hz X 6 MHz
• Because the cable modem is connected to the computer through a
1oBase-T cable, this limits the data rate to 10 Mbps.

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Upstream data are modulated using the QPSK modulation technique.
Note
• Uses lower frequencies that are more susceptible to noise and
interference, thus the QAM technique is not suitable.
• The better solution is QPSK.

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The theoretical upstream data rate is 12 Mbps.
Note
• in QPSK, 2 bits/baud is used.
• (2 bits/Hz X 6 MHz)

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Sharing
Both upstream and downstream bands are shared by the subscribers.
Upstream Sharing
 Bandwidth is 37 MHz, there is 6 MHz channels.
 How can six channels shared in an area with 1000, 2000 or 100,000
subscribes.
 The solution is time sharing by dividing the band into channels using FDM.
 One channel is allocated for upstream for one subscriber.
Downstream sharing
 Has 33 channels of 6 MHz.
 Each channel must be shared between a group of subscribers.
 Here we have a multicasting situation.
 Subscriber with the matched address receives the data and the other subscribers
discard the data.

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Figure 9.17 Cable modem (CM)
CM and CMTS
To use a cable network for data transmission, we need two key devices:
1. A cable modem
2. A cable modem transmission system (CMTS)

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Figure 9.18 Cable modem transmission system (CMTS)

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Data Transmission Schemes DOCSIS
DOCSIS defines all the protocols necessary to transport data from a CMTS to a CM.
Upstream Communication
1. The CM checks the downstream channels for a specific packet periodically sent by
the CMTS. The packet asks any new CM to announce itself on a specific upstream
channel.
2. The CMTS sends a packet to the CM, defining its allocated downstream and
upstream channels.
3. The CM then starts a process, called ranging, which determines the distance
between the CM and CMTS. This process is required for synchronization between
all CMs and CMTSs for minislots used for timesharing of the upstream channels.
4. The CM sends a packet to the ISP, asking for the Internet address.
5. The CM and CMTS the exchange some packets to establish security parameters.
6. The CM sends its unique identifier to the CMTS.
7. Upstream communication can start in the allocated upstream channel: the CM can
contend for minislots to send data.
Downstream Communication
The CMTS sends the packet with the address of the receiving CM, using the allocated
downstream channel.