Transcript Telephone Network - Dr. Rajiv Srivastava

Lecture 10, 11 & 12
Telephone Network
Telephone Network
TELEPHONE NETWORK
The telephone network had its beginnings in the
late 1800s. It was originally designed for voice
communication only. The entire network was
originally an analog system using analog signals
to transmit voice. With the advent of the
computer era, the network, in the 1980s, began
to carry data in addition to voice. During the last
decade, the telephone network has undergone
many technical changes. The network is now
digital as well as analog.
Major Components
The telephone network, as shown in next
Figure, is made of three major components:
1. local loops,
2. trunks, and
3. switching offices.
The telephone network has several levels of
switching offices such as end offices, tandem
offices, and regional offices.
14.4
Figure : A telephone system
Local loops
• Local loop is a twisted pair cable that is used for connection for
connecting a subscribers telephone to the nearest end office.
• The local loop has a bandwidth of 4 kHz if it is used for voice
communication.
• In an 8- digit local telephone number the first four digits define
the office and the next four digits define the local loop number.
Trunks
• Trunks are the transmission media that handle the
communication between offices.
• The signals originating from many end offices are multiplexed to
form a common signal which is then transmitted over the
trunks.
• The trunks thus handle hundreds or thousands of
connections simultaneously. The transmission media used
as trunks is usually optical fiber cables or satellites links.
Switching offices
• Connecting every subscriber permanently with all the
others subscriber using wires will be impossible things.
• Hence the telephone companies use switching located at
the switching offices to establish connection as and when
required, between the calling and the called subscriber.
• Earlier the electromechanical rotary switches but being
used but now in electronic exchanges, the electronic
switches are used.
Local Access Transport Areas(LATAs)
The LATA is an American concept. The United States
was divided into more than 200 local-access transport
areas (LATAs). The number of LATAs has increased
since then.
A LATA can be a small or large metropolitan area. A
small state may have a single LATA; a large state may
have several LATAs. A LATA boundary may overlap
the boundary of a state; part of a LATA can be in one
state, part in another state.
14.8
Figure : Switching offices in a LATA
14.9
• Types of LATA
1. Intra LATA
2. Inter LATA
Intra-LATA services
These services are provided by local exchange carriers. Since
1996, there are two types of LECs:
1.
incumbent local exchange carriers (ILEC)
2.
competitive local exchange carriers (CLEC)
Inter LATA Services
The services between LATAs are Handled by interchange
carriers (IXCs). These carriers are also called as long
distances companies that provide communication services
between two customers in deferent LATAs.
communications in circuit switching
networks
• Communication via circuit switching takes place
over three phases of operation as follows:
• Circuit establishment
• Data transfer
• Circuit disconnect
1. Circuit establishment
In a circuit switching network, before any signal is transmitted, it is
necessary to establish as end-to-end link.
The node to node links are usually multiplexed. they either use FDM
or TDM
2. Data transfer
The data can be analog or digital depending on the nature of the
network
Generally all the internal connection are duplex.
3. Circuit Disconnect
After some time the connection between two user transmitted usually
by the action of one or two stations.
Circuit switching is insufficient in most of the application.
•The entire channel capacity id dedicated for the duration
of connection, even if the data is not being transferred.
•Once the circuit is established, the network is effectively
transparent to the user with no delays involved.
PSTN
PSTN (public switched telephone network) is the
world's collection of interconnected voice-oriented
public telephone networks, both commercial and
government-owned. It's also referred to as the Plain
Old Telephone Service (POTS). A public
telecommunication network can be described using
four generic architectural components.
1. Subscribers
2. Subscribers line
3. Exchanges
4. Trunks
FIG OF PSTN
Subscribers
• These are the services that attach to the
network. Example is telephone, PCs etc
Subscriber line
• The line between the subscriber and the network
is called as subscriber line.
• It is also called as subscriber loop or local loop.
• All the local loop connection are made are using
the twisted wires.
• The length of local loop cab be in the range of
few kilometers or few hundred kilometers.
Exchanges
• The switching centers in the networks are called
as exchanges.
• End office is a switching center which directly
supports the subscriber.
• Typically as end office can support a few
thousand subscribers.
Trunks
• Trunks are the branches between the exchanges.
• The signal travelling on them are either FDM or
TDM.
ROUTING IN CIRCUIT SWITCHING
• The modern telephone network is developed in order to provide the basic
telephone services.
• The basic telephone service provides the two way real time transmission of voice
signals.
• The bandwidth of each voice signal is 4 kHz.
• The telephone network operates on the principle of circuit switching. The block
diagram of the basic telephone network .
• Following are the three phase of connection oriented communication.
• When the user pickup his telephone, it activate a switch in the local telephone
office of call request.
• A dial tone is given to the calling party and the dialed number of the called
subscriber is recognized.
• The source switch uses the telephone signaling network to find the rout and
allocate resource to the network of destination office.
• The destination office then alerts the called party about the incoming call by
ringing its phone.
• This ring is sent back to the calling party . the conversation can begin when the
called party lifts the telephone.
Making A Connection
The subscriber telephones are connected
through the local loops, to end office or central
office.
• The switching station at the end office is
accessed through dialing.
• Two types of dialing are practically used.
1. Rotary or pulse dialing.
2. Touch tone dialing (DTMF dialing)
Pulsed Dialing:
• As shown in fig (A) the telephone instrument has a routing
dial. Corresponding to each digit dialed a group of pulses is
generated and set to the exchange. E.g. if digit “3” is dialed
then a group of 3 pulses is produced.
• Two pulses are produced for digit 2 and so on. Ten pulse
are produced when a “0” is dialed .
• The total of each pulse i.e. P remains always constant. The
make to break ratio M:B is 33% 66% as shown in fig.
• An inter digit pulse is automatically generated in order to
give sufficient time to the exchange switching circuits.
• The time taken to send a digit will not be constant. “1” will
take shortest time (only 1 pulse) whereas a “0” will take
longest time (10 pulse).
DTMF dialing [Dual Tone Multi-Frequency Dialing]
• A rotary dial is very slow in dialing e.g. it takes about 12 seconds to
dial a 7 digit numbers.
• To increase the speed of dialing the so called “touch tone” push
button key pad is used in the modern telephone instruments.
• In this scheme a key pad of 12 push buttons arranged in four rows
with three keys per row is used.
• As we touch a push button on the key pad a voice frequency
“tone” is produced.
• This tone is unique for each push button. It is generated by the
combination of two frequencies. One from the lower band and the
other from the upper band.
• Therefore the dialing is also known as the dual tone multi
frequency dialing. (DTMF). This is as shown in fig.
• If the digit “8” is pressed the tone that is produced and
sent to the exchange is a combination of 852 Hz from the
lower band and 1336 Hz from the upper band.
• Special frequency selective filters are used in the telephone
exchange to pass only the frequencies for DTMF.
• DTMF is much faster than the pulsed dialing. The time
required to recognize any digit tone is only 50 m sec.
• The inter digit interval is of 50 m sec. which makes the total
tome required to send any digit to be 100 m sec. this time
does not depend on which digit is dialed. It is same for all
digits.
Comparison of Pulse and DTMF Dialing
S.
Pulse Dialing
no
1 A train of pulse is produced to represent
the dialled digit. The number of pulses
represent the dialed digit
2 Low speed dialing
DTMF Dialing
A unique tone is produced by combination
of two frequencies. The tones for the
various digits are different each other.
It is a high speed dialing
3
Mechanically rotating dial is used.
Key pad is used
4
Time taken to send different digits is
different from each other
Same tome is required for sending any
digit.
Types of services Given By
Telephone Networks
• Analog services
– Analog switched services
– Analog leased services
• Digital Services
Analog switched services
This is the normal telephone service. The signal on
a local loop is analog and it has the bandwidth
between 0 and 4 kHz.
• The call between any two subscribers is
established via a series of switches or exchange.
• Some of the commonly provided services are as
follows:
1.Local call services:
• It is the service normally provided for a flat
monthly rate
Analog Switched Service
Analog Leased Service
2. Toll call services:
• A toll call can be intra-local office or inter local
office.
• Depending on the geographical area, the call may
have to be routed via a toll office.
• The subscriber is supposed to pay the fee for call.
3. 800 services :
• This service is used for providing free calls. The call
is free for the caller but it is paid by the called party.
• But the rates are less expensive than the normal
long distance call
4. WATs:
• WAT is a wide area telephone service. it is opposite to
the 800 services.
• For this service the outgoing calls are paid by the
subscriber. The service is less expensive and the charges.
Are based on number of calls.
• This service is applicable to calls inside a state or even
outside the state.
5. 900 services:
• The 900 services are like 800 services. Here the call is
paid by the caller and it is more expensive than the
normal long distance call.
Dial-Up Service
Traditional telephone lines can carry frequencies
between 300 and 3300 Hz, giving them a bandwidth
of 3000 Hz.
A dial-up service uses a modem to send data through
telephone lines.
The term modem is a composite word that refers to
the two functional entities that make up the device: a
signal modulator and a signal demodulator.
14.34
Figure : Telephone line bandwidth
Figure : Modulation/demodulation
Figure : Dial-up network to provide Internet access
Analog Hierarchy
Digital services:
• Digital services are being offered in the recent
times.
• These are less sensitive to noise than the
analog services. So less interference is
observed.
• Two commonly used digital services are:
1. Switched/56 services
2. Digital data services (DDS).
3. DS
Switched/56 services
• It allows data rates to go upto 56kbps.
• A subscriber using normal telephone line cannot connect to
a telephone or computer using switched/56 even if he is
using a modem.
• The line in switched/56 services is already digital so that
user doesn’t have to use modems.
• But they need to use another device called Digital Services
Unit (DSU)
• The DSU changes the rate of the data created by the
subscriber to 56 kbps.
• This service supports bandwidth on demand.
• This service can support the video conferencing,
multimedia, fast facsimile and fast data transfer.
Switched/56 Service
Digital Data Service (DDS)
• DDS is the digital counterpart of an analog leased
line.
• It is a digital leased line with data rates of 64
kbps.
DDS
3. DS Hierarchy
T-1 Line
Signaling
The telephone network, at its beginning, used a
circuit-switched network with dedicated links to
transfer voice communication. The operator
connected the two parties by using a wire with two
plugs inserted into the corresponding two jacks. Later,
the signaling system became automatic. Rotary
telephones were invented that sent a digital signal
defining each digit in a multi-digit telephone number.
As telephone networks evolved into a complex
network, the functionality of the signaling system
increased. The signaling system was required to
perform other tasks.
Control Signaling
• A circuit switching network uses the control signals
to manage the following function.
1. To establish calls.
2. To maintain calls.
3. To terminate calls.
• For large public telecommunication network a
relatively complex control signaling scheme is
required to be used.
signaling function:
Some of most important signaling function are as follows :
• To provide dial tone, ring tone and busy tone and busy
tone to the subscribers as and when required.
• To transmit the number dialed by the subscriber to the
switching offices which attempt to complete the
connection.
• To transmit information between switches to indicate that
a call cannot be completed.
• To transmit information between switches to indicate that
the call has ended.
• To send a signal to make a telephone ring (at the called
subscriber)
Control Signal
• The network management control signals are used for
controlling the overall routing selection process such as
changing the previously planned routes, or for modifying
the operating characteristics of the network in the event of
overloading or failure condition.
• Signaling can also be classified functionally as follows.
Supervisory function
• These functions generally have a binary character (true /
false or on/ off).
• These example of supervisory functions are request for
service, answer alerting and return to idle.
• These signals are used to check if the desired resources is
available or not and also to communicate the status of the
requested resources.
Address Signals
• These are used for identifying a subscriber.
• An address signal is generated by a calling party when he
dials a telephone number.
• This address propagates through the network to help the
routing function and to locate the called subscriber.
Call information
• it includes those signals which provide
information about the status of the call to the
subscriber.
• These are audible tones.
Network management signals
• These are used for maintenance, trouble
shooting and overall operation of the
network.
• These signals can be in the form of message.
Common channel signaling
• Traditionally the control signaling in a circuit
switching network is the in channel signaling.
• The same channel which carries the voice
information (call) is used to carry the control
signals as well.
• The advantage of in channel signaling is that
no additional transmission facilities are
needed for signaling.
Types of inchannel signaling
• There are two types of inchannel signaling as
follows.
1. Inband signaling
2. Out of band signaling
Inband signaling
• Inband signaling not only use the same physical
path as the voice but it also uses the same
frequency band as the voice signal.
• This type of signaling has certain advantages.
One of them is that due to some electromagnetic
properties as those of the voice, the control
signals can go everywhere the voice signal can
go.
• Another advantage is that if the path is faulty
then it is not possible to establish a call because
the control signals which set up the call
themselves cannot travel on the faulty path.
Out of band Signaling
• The voice signal occupies the frequency band from 300Hz to
3400 Hz as shown in fig. but a standard band of 0 to 4000 Hz
has been allotted to the voice signal.
• Hence the band from 3400 to 4000 Hz remains un-utilized.
• The out of band signaling uses this unutilized narrow band of
frequencies for accommodating the control signals as shown in
fig.
• The advantage of out of band signaling is that control signals
can be sent with or without the presence of voice signals on the
line.
• This allows a continuous supervision and control of a call.
• But the disadvantage of out of band signaling is that additional
circuitry is required to handle the signaling band plus the
signaling rate are slower because of the narrow bandwidth used
for the signaling purpose.
Figure : Data transfer and signaling network
• To transmit information used for the billing
purpose.
• To transmit information giving the status of
equipment or trunks in the network. This
information can then used for the purpose routing
and maintenance.
• To transmit information which is useful for the
diagnosis of faults and for the isolation in case of
system failure?
• To control special equipments such as the satellite
channel equipment.
Digital Subscriber Line (DSL)
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 telephone. DSL
technology is a set of technologies, each differing in
the first letter (ADSL, VDSL, HDSL, and SDSL).
14.59
Digital Subscriber Lines (DSL)
• Earlier we have seen the use of MODEM to produce
digital signals, so as to facilitate the internal access
through telephone.
• But the traditional MODEMS cannot provide very
high data rates.
• This disadvantage can be overcome with the help of
following technologies.
1. The digital subscriber line (DSL)technology
2. Cable modem
3. SONET
DSL Technology
• DSL stands for digital subscriber line and it has
been developed for providing a high speed
access to the internet.
• It support the high speed digital communication
over the existing local loops.
• The DSL technology is classified as follow:
• Thus DSL technology is actually a set of
technologies which is referred to as x DSL
where x can be A,V,H or S.
DSL
ADSL
VDSL
HDSL
SDSL
ADSL
• Short for Asymmetric Digital Subscriber Line.
• ADSL is a type of DSL Broadband communications technology used
for connecting to the Internet.
• ADSL allows more data to be sent over existing copper telephone
lines (POTS), when compared to traditional modem lines.
• A special filter, called a microfilter, is installed on a subscriber's
telephone line to allow both ADSL and regular voice (telephone)
services to be used at the same time.
• ADSL requires a special ADSL modem and subscribers must be in
close geographical locations to the provider's central office to
receive ADSL service.
• Typically this distance is within a radius of 2 to 2.5 miles.
• ADSL supports data rates of from 1.5 to 9 Mbps when receiving data
(known as the downstream rate) and from 16 to 640 Kbps when
sending data (known as the upstream rate).
ADSL
• Similar to 56k modem, it provides higher bit rate in
the downstream direction (from internet to the user)
as compared to the bit rate of in the upstream
direction(from user to internet) this is why it is called
as asymmetrical.
• ADSL divides the bandwidth of a twisted pair cable of
1 MHz into 3 bands as shown in figure.
• The first band is between 0 and 25 Khz. It is used for
regular telephone(plain old telephone sets-POTS)
• This service uses only 4 kHz of this band the rest is used as
guard band to separate the voice channel from the data
channels.
• The second band is from 50kHz to 200 kHz.it is used for
upstream communication.
• The third band is from 250Khz to 1 MHz It is used for
downstream communication.
• ADSL technology is designed for the residential users. it is
not suitable for business applications.
Bands for ADSL
0
1000
Downstream
Upstream
POT
S
25
50
200
250
Frequenc
y (kHz)
Use of Existing Local Loop
• The ADSL uses the existing local loop and still it can
support very high data rates. Let us see how.
• Actually the twisted pair local telephone loop can
support the bandwidth upto 1.1 MHz. The
bandwidth reduces to 4 kHz due to the filters
installed at the end of line.
• If this filter is removed then the bandwidth can
again become 1.1 MHz.
• Hence the existing local loop with this modification
can have a bandwidth of 1.1 MHz and ADSL can
make use of the existing loop.
Adaptive Technology
• The bandwidth of 1.1 MHz is not always available
practically. It will depend on various factors such as
distance ,cable size,type of signaling etc.
• So ADSL has been designed to operate as an
adaptive technology. That means depending on the
condition and actually available bandwidth, the
data rate of ADSL is adjusted.
• Thus ADSL is an adaptive technology. The data rate
of the system is adjusted based on the conditions of
the local loop.
Modulation techniques:
• Two modulation techniques can be used for
ADSL:
1. Carrierless Amplitude/Phase(CAP)
2. Discrete Multitone Technique(DMT)
• CAP is modulation technique that is similar to
QAM but with one important difference i.e. the
carrier signal is eliminated.
Discrete Multitone Techniques (DMT)
• DMT is the standard modulation technique for ADSL.; it combines
QAM and FDM.
• Typically the available bandwidth of 1.1MHz is divided into 256
channels. However this is not a rule. The division may change
system to system.
• Figure shows the block schematic of the DMT system.
• Channel 0 is reserved for the voice communication.
• Channel 1 to 5 are called as idle channels since they are not used.
These channels create a gap between the voice and data
channels.
• Channels 6 to 30 (25 channels) are used for the upstream data
transfer and control. These 24 channels are for data transfer and
one channel for control.
• The downstream bit rate is 224 channels * 4 kHz * 15=13.4Mbps.
Figure
Bandwidth division for DMT
• For 24 channels 31 to 255(255 channels) are
allotted for downstream data transfer and control.
Out of 255 channels, one is reserved for control and
remaining 224 are reserved for data.
Actual Bit Rate:
• The actual bit rates are normally as follows:
• Upstream: 64kbps to 1 mbps
• Downstream: 500 kbps to 8 mbps
ADSL MODEM
• The ADSL modem installed at the customer’s site is
shown in figure.
• The local loop is connected to the filter which
separates voice and data from each other.
• The voice goes to the telephone instrument.
• The data goes to ADSL modem which modulates it using
DMT and creates the upstream and downstream channels.
Figure: ADSL modem
Figure : ASDL point-to-point network
(DSL Access
Multiplexer)
xDSL(other DSL technologies)
• ADSL is one of a number of recent schemes for
providing the high speed digital transmission
of the subscribe line.
• Some of the most important new schemes in
the field of digital transmission are collectively
called as xDSL
• The are as follows:
1. HDSL
2. SDSL
3. VDSL
HDSL
• High-bit-rate digital subscriber line (HDSL) is a
telecommunications protocol standardized in 1994.
• It was the first digital subscriber line (DSL) technology to use
a higher frequency spectrum of copper, twisted pair cables.
• HDSL (High bit-rate Digital Subscriber Line), one of the earliest
forms of DSL, is used for wideband digital transmission within a
corporate site and between the telephone company and a
customer.
• The main characteristic of HDSL is that it is symmetrical: an
equal amount of bandwidth is available in both directions.
HDSL can carry as much on a single wire of twisted-pair cable
as can be carried on a T1 line (up to 1.544 Mbps) in North
America or an E1 line (up to 2.048 Mbps) in Europe over a
somewhat longer range and is considered an alternative to a
HDSL
• High-bit-rate digital subscriber line (HDSL) is a
telecommunications protocol standardized in 1994. It was
the first digital subscriber line (DSL) technology to use a
higher frequency spectrum of copper, twisted pair cables.
• It is an alternative to the PCM TDM system i.e. T-1
line(1.544Mbps). The T-1 line uses alternate mark
inversion(AMI) encoding which is affected by high frequency
attenuation.
• This limits the length of T-1 line to 1-km. The HDSL uses 2BIQ encoding
method which is less susceptible to attenuation. A data rate of 2 Mbps can
be achieved upto a length of 3.6 km.
• The HDSL is a form of DSL providing T-1/E-1 connections over two or three
twisted pair copper lines.
• HDSL is not a typical consumer service, it is mostly used to replace
traditional T-1/E-1 technology.
• It allows DS-1 signals to be transported over
distance of upto 12,000 feet (Approx 3600 mtrs)
on copper twisted pair without repeaters.
• It uses full duplex transmission, using echo
cancellation with each pair carrying 784 kb/s the
lower bit rate allows for a lower frequency range
of operation that reduces channel loss and nearend crosstalk.
• HDSL-based hardware can be installed in central
office unit as shown in figure. it is a standalone
transport system located near DS-1 cross
connected frame, in the outside plant on at the
customer premises.
• By using the existing copper in the local loop,
HDSL technology helps telephone companies
for quick and easy installation, provide an
intelligent network interface, which reduces
maintenance, and deliver revenue-generating
high capacity service over copper wires as
they fiberize the local loop and as they
convert to SONET architecture.
Comparison of T1 and HDSL technology:
SNo
T-1 technology
HDSL technology
1
Line must be pre-conditioned
2
Requires T-1 line repeaters.
3
Simplex transmission.
Uses AMI line code
Affected by high frequency
attenuation.
Sensitive to loop swapping
and polarity reversal.
Allows bridge taps and
cable gauge changes.
No repeaters for cable
length less than 12,000
feet.
Full duplex transmission.
Uses 2BIQ line code.
Less susceptible to high
frequency attenuation
Transparent to loop
swapping and polarity
reversal.
Requires less bandwidth.
4
5
6
7
Requires more bandwidth
SDSL
• Short for symmetric digital subscriber line, a technology
that allows more data to be sent over existing copper
telephone lines (POTS).
• SDSL supports data rates up to 3 Mbps. SDSL works by
sending digital pulses in the high-frequency area of
telephone wires and can not operate simultaneously with
voice connections over the same wires.
• SDSL requires a special SDSL modem. SDSL is
called symmetric because it supports the same data rates
for upstream and downstream traffic. A similar technology
that supports different data rates for upstream and
downstream data is called Asymmetric digital subscriber
line (ADSL). ADSL is more popular in North America,
whereas SDSL is being developed primarily in Europe.
SDSL
• It is same as HDSL but uses a single twisted pair cable. In this
method echo cancellation technique is used to achieve the
full duplex mode of transmission.
• This technology provides the same bandwidth in both
directions, i.e. upsteam and downstream. The same high
quality performance is present in the upstream and
downstream.
• SDSL provides transmission speeds within a T-1/E-1 range of
up to 1.5Mbps at maximum range of 12,000 to 18,000 feet
from a central telephone office.
• SDSL is generally used by small and medium sized businesses
that have an equal need to download and upload data over
the internet. It is also well suited for large file transfers
towards and away from users for web site hosting services.
VDSL
• Very-high-bit-rate digital subscriber
line (VDSL or VHDSL) is a digital subscriber line (DSL)
technology providing data transmission faster
than asymmetric digital subscriber line (ADSL) over a single
flat untwisted or twisted pair of copper wires (up to
52 Mbit/s downstream and 16 Mbit/s upstream), and
on coaxial cable (up to 85 Mbit/s down- and
upstream)using the frequency band from 25 kHz to
12 MHz. These rates mean that VDSL is capable of
supporting applications such as high-definition television,
as well as telephone services (voice over IP) and
general Internet access, over a single connection. VDSL is
deployed over existing wiring used for analog telephone
service and lower-speed DSL connections. This standard
was approved by ITU in November 2001.
VDSL
• It is similar to ADSL but uses co-axial, fiber optic or
twisted pair cable for short distance of 300 to 1800
meters.
• It uses the discrete multitone modulation technique
with a bit rate of 50 to 55 Mbps downstream and 7.5
and 2.5 Mbps upstream.
• The basic features of a VDSL system are:
1. Flexible scalability for symmetric and asymmetric
transmission.
2. Single duplex transmission
3. Field programmable bit rates depending on noise
environment and cable length
4.
Parallel operation of plain old telephone service (POTS) and / or
ISDN.
• To reach a large number of customers
(subscribers) with the existing copper wire twisted
pair distribution network the VDSL system should
use only one twisted pair and coexist with POTS
and ISDN.
• To take care of the coexistence with POTS and
ISDNN the VDSL spectrum should meet the
frequency plan as shown in fig 12.10.2.
• Besides this, depending on the means for
separating downstream and upstream
transmission, any possible configuration of
symmetric and asymmetric system should not
interface with one another.
Comparison of xDSL Schemes
SNo
Parameter
ADSL
HDSL
1
Mode
Asymmetric
symmetric
2
Signaling
Frequency
Analog
1 to 5 MHz
digital
196kHz
digital
196kHz
5
Copper pairs
Bits/second
1
1.5 to 9 Mbps
downstream
16 to 640 kbps
upstream
2
1.544
2.048
Mbps
1
1
1.544 or 13 to
2.048
52Mbps
Mbps
down
stream 1.5
to 2.3Mbps
upstream.
6
Bits/cycle
3
4
variable
4
SDSL
or
symmetric
4
VDSL
asymmetric
analog
10MHz
variable
Comparing DSL Types
There are several variations on DSL technology. In fact, there are so many
that you will often see the term xDSL, where x is a variable, when the
discussion is about DSL in general.
Asymmetric DSL (ADSL) - It is called "asymmetric" because the download
speed is greater than the upload speed. ADSL works this way because
most Internet users look at, or download, much more information than they
send, or upload.
High bit-rate DSL (HDSL) - Providing transfer rates comparable to a T1
line (about 1.5 Mbps), HDSL receives and sends data at the same speed,
but it requires two lines that are separate from your normal phone line.
ISDN DSL (ISDL) - Geared primarily toward existing users ofIntegrated
Services Digital Network (ISDN), ISDL is slower than most other forms of
DSL, operating at fixed rate of 144 Kbps in both directions. The advantage
for ISDN customers is that they can use their existing equipment, but the
actual speed gain is typically only 16 Kbps (ISDN runs at 128 Kbps).
Multirate Symmetric DSL (MSDSL) - This is Symmetric DSL that is
capable of more than one transfer rate. The transfer rate is set by the
service provider, typically based on the service (price) level.
Rate Adaptive DSL (RADSL) - This is a popular variation of
ADSL that allows the modem to adjust the speed of the
connection depending on the length and quality of the line.
Symmetric DSL (SDSL) - Like HDSL, this version receives and
sends data at the same speed. While SDSL also requires a
separate line from your phone, it uses only a single line instead
of the two used by HDSL.
Very high bit-rate DSL (VDSL) - An extremely fast connection,
VDSL is asymmetric, but only works over a short distance using
standard copper phone wiring.
Voice-over DSL (VoDSL) - A type of IP telephony, VoDSL allows
multiple phone lines to be combined into a single phone line that
also includes data-transmission capabilities.
The chart below provides a comparison of the various DSL
technologies:
As you can see, VDSL provides a significant performance boost
over any other version. But for VDSL to become widely
available, it must be standardized.
Cable Modem
• Now a days, the cable companies have to come forward
to provide high speed access to the internet
• Due to DSL technology, it has become possible to
provide the high data rate connection for the residential
user on the local loop
• But DSL technology uses the existing unshielded twisted
pair cable which does not perform well in the presence
of external interference or noise.
• So the data rate on such cables cannot be increased
beyond a certain limit.
• A solution to this problem is to use cable TV network.
CABLE NETWORK
The cable networks became popular with
people who just wanted a better signal. In
addition, cable networks enabled access to
remote broadcasting stations via microwave
connections. Cable TV also found a good
market in Internet access provision, using
some of
the channels originally designed for video.
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Traditional Cable Networks
Cable TV started to distribute broadcast video signals
to locations with poor or no reception in the late
1940s. It was called community antenna television
(CATV) because an antenna at the top of a tall hill or
building received the signals from the TV
stations and distributed them, via coaxial cables, to
the community. Figure shows a schematic diagram of
a traditional cable TV network.
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Figure Traditional cable TV network
• The receiving antenna is generally set up on the top of a tall
building.
• It receives the video signals from the broadcasting stations
and supplies them to the cable TV office called the head end.
• The head end processes these signals and feeds them into
coaxial cable.
• Amplifiers are inserted at regular intervals in order to amplify
the signal passing from the head end to the user’s house.
• There could be unto 35 amplifiers between the head end the
user’s premises.
• The splitters are used to split the signal from one to many
paths. Then the taps and the drop cables make the
connection to the users residences.
Disadvantages of traditional cable system:
• A large number of amplifiers are required
• The bandwidth of the coaxial cable is limited.
• Due to the use of amplifiers the system
becomes unidirectional. Signal can travel only
from head end to the user’s residence.
• The biggest disadvantage of the traditional
cable TV network is that this network is
unidirectional.
HFC network
• The improve version of cable network is called as hybrid
fiber-coaxial (HFC) network.
• This is the second generation of the cable network and it
uses a combination of fiber optic and coaxial cable for the
transmission of signal.
• Figure shows the block schematic of the HFC network.
• The antenna placed on a tall building receives the
broadcast signals and passes them on to a block known as
distribution hub.
• The transmission medium from the antenna to the box
called fiber node is the optical cable as shown in figure
• However from the fiber node to user residence the coaxial cables are
being used.
• the regional cable head (RCH) is generally capable of serving upto
400,00users
• the RCH feeds into the distribution hubs each one of which can serve
upto 40,000 users.
• The distribution hub carries out two important operations, namely
modulation and distribution of signals.
• These signal are then fed to the fiber nodes through
the optical fiber cables as shown in figure.
• The fiber node splits the analog signals so as to
send the same signal to each coaxial cable.
• Hence the number of amplifiers are required to be
used is reduced as compared to the conventional
cable network.
• Typically unto eight amplifiers are required to be
used, as compared to 35 in the conventional cable
network
• The HFC cable TV network is a bidirectional
network. This is its biggest advantage.
Bandwidth of HFC network:
• Since the last part of HFC network uses coaxial cable,the
bandwidth of this network is seceded by the bandwidth of
the coaxial cable.
• So the bandwidth range approximately from 5MHz to
750MHz as shown in figure.
• This bandwidth is divided into three bands as shown in
table given.
• Table shows HFC network bands
Band name
Frequency range
Upstream data
5MHz-42MHz
Video band
54MHz-550MHz
Downstream data
550mhZ-750MHz
Figure HFC network
Figure Coaxial cable bands
Video band
• This band extends from 54MHZ to 550 MHz.it
basically carries all the TV channels.
• If each TV channel needs a bandwidth of 6
MHz then the video band can accommodate
about 83 TV channels.
Downstream Data Band
• Downstream data travels from the internet to
the subscriber as shown in figure, the dawn
stream data band extends from 550MHZ to
750 MHz
• This band is also divided into 6MHz channels.
Modulation technique
• The modulation technique used for downstream
data band is either 64-QAM or 256QAM.
Data Rate
• With 64-qam,we can receive the downstream data
at the rate upto 30Mbps.but since the cable
modem is connected to the computer through a
10 base T cable the data rate is limited only to 10
Mbps.
• So theoretical downstream data rate is 30 Mbps
but practically it is restricted to 10Mbps.
UPSTREAM DATA BAND:
• The upstream data travel from the subscriber
to the internet. As shown in figure, the
upstream band extends from 5MHz to 42
MHz.
• This band is also divided into 6MHz channels
• Modulation technique used is QPSK
• Theoretically the upstream data rate is 12
Mbps but practically this rate cannot be
achieved.
Cable network for data transmission
• in order to utilize the cable network for data
transmission we have to use two device namely
CM(cable modem) and CMTS(cable modem
transmission system).
Cable Modem(CM)
• Given figure shows the block schematic of cable
modem it is similar to ADSL modem and it is installed
at the user premises.
• The filter separates out video and data in the
incoming signal from the cable.
• The video signal is applied to TV whereas the data
signal is applied to the computer via the Cable
Modem.
Figure Cable modem
Cable Modem Transmission System(CMTS)
• The cable companies install the CMTS inside the
distribution hub as shown in figure.
• the data from internet is passed through the CMTS
to the combines the video signal from the haed and
also is applied to the combiner.
• The combiner output contains video as well as data
from the internet. this signal is put on the optical
fiber for routing it to the subscribers.
• The data from the subscribers coming via the
optical fiber4 is applied to the CMTS, and the CMTS
passes it to the internet.
Figure CMTS
Extra Information
BRIEF OF CIRCUIT SWITCHING
Echo in Transmission
• In computer telecommunications, echo is the display or
return of sent data at or to the sending end of a transmission.
Echo can be either local echo, where the sending device itself
displays the sent data, or remote echo, where the receiving
device returns the sent data that it receives to the sender
(which is of course simply no local echo from the point of
view of the sending device itself). That latter, when used as a
form of error detection to determine that data received at the
remote end of a communications line are the same as data
sent, is also known as echoplex, echo check, or loop check.
When two modems are communicating in echoplex mode,
for example, the remote modem echoes whatever it receives
from the local modem
SONET
• Synchronous Optical Networking (SONET) and
Synchronous Digital Hierarchy (SDH) are
standardized protocols that transfer multiple digital
bit streams synchronously over optical fiber using
lasers or highly coherent light from light-emitting
diodes (LEDs). At low transmission rates data can
also be transferred via an electrical interface. The
method was developed to replace the
Plesiochronous Digital Hierarchy (PDH) system for
transporting large amounts of telephone calls and
data traffic over the same fiber without
synchronization problems.