05-11-2016, 02:32 PM
[attachment=74652]
Introduction
In the 19th Century telegraph was most reliable way of long travel communication using wires
The bit rate was less than 10 per seconds
Samuel Morse moved data through same wire 100,000 times faster
Analog nature has challenges and limitation
Digital services is 20 times faster than analog system
Telecommunication Components
Media and Channels
Signals
analog
digital
Cables
Twisted-pair interference
Coaxial cable 20x more expensive
5.500 simultaneous phone calls
Fiber-optic
1 fiber 30.000 phone calls
Wireless
microwave
satellites
Radio, Infrared, Cellular Radio, Mobile computing
GPS global positioning system
BACKGROUND OF ANALOG SERVICES
Modem is the standard equipment used for these system
Earlier telephone system used analog signals
Telephone network handled millions of analog signals
Later circuitry changed to digital and only analog part of the system was the local loop
Allows frequency between 300-3000Hz to freely pass
Analog Services
Channels in Analog Services
Duplex: With modems, such a two-way exchange of information is called duplex communications. Duplex modems use two carriers to simultaneously transmit and receive data, each of which has half the bandwidth available
Half Duplex: In this case only one signal for transmission is used. It is slow data communications because often a modem must switch between sending and receiving modes
Echoplex: In this modem sends a character down the
phone line, and the distant modem returns the same character
Channels in Analog Services
Switching Modems: Switching modems are half duplex modems that reverse the direction of the signal at each end of the line in response to the need to send data.
You enter commands into a remote access system, and only after the commands are received does the remote system respond with the information .
Asymmetric Channel: asymmetrical modems cut the waiting by maintaining a semblance of two-way duplex communications while optimizing speed in one direction only.
Connection Enhancing Technologies
Line Compensation: The modem faces two challenges:
Not to interpret changes as data travel long distance.
To maintain the quality of the line to a high-enough standard to support its use for high-speed transmission.
Modems compensate for variations in telephone lines by equalizing these lines and use echo cancellation to eliminate the return of their own signals.
Echo Cancellation: The half-second delay imposed by the long distance the signals traveled from earth to satellite and back created annoying “echoes” on the line
Connection Enhancing Technologies
Fallback: Most modems use at most two carriers for duplex communications and these carriers are usually modulated to fill the available bandwidth capabilities.
When the top speed does not work, they attempt to communicate at lower speeds that are less critical of telephone line quality.
Multiple Carrier Modem: uses relatively simple
modulation on several simultaneous carrier signals.
generally cutting data speed in half, the multiple-carrier modems just stop using the carriers in the doubtful regions of the bandwidth.
Connection Enhancing Technologies
Data compression: Many of the bits that are sent through the telecommunications channel are meaningless or redundant, by eliminating those worthless bits, the information content of the data stream is more intense. The process of paring the bits is called data compression.
Error checking and error correction: Because all high-speed modems operate closer to the limits of the telephone channel, they are naturally more prone to data errors and have own built-in error-checking methods and correction methods.
Analog Standards
Modem Hardware
Computer Modem has five elements:
1.Interface circuitry for linking with the host computer
2.Circuits to prepare data for transmission by adding the proper start, stop, and parity bits
3.Modulator circuitry that makes the modem compatible with the telephone line
4. User interface that gives command
5. The package that gives the modem its physical embodiment
Computer Interface
At one time, all modems used a standard or enhanced serial port to link to the computer
All modems, whether installed outside your computer, in one of its expansion slots, or in a PCMCIA slot, make use of a serial or parallel communications port
In the case of serial modems, this interface converts the parallel data of your computer into a serial form suitable for transmission down a telephone line.
They need an input/output address and an interrupt to operate properly.
Data Preparation, Modulator
Pre-transmission preparation helps your modem deliver the highest possible data throughput while preventing errors from creeping in.
The incoming code stream may also be analyzed and compressed to strip out redundant information.
Microcontroller inside the modem performs these functions based on the communications standard.
Modulator circuitry actually converts the digital information from your computer into analog-compatible form.
User Interface & packing of Modem
The modem needs circuitry to connect with the telephone system. This line interface circuitry boosts the strength of the modem’s internal logic-level signals to a level matching that of normal telephone service.
Internal modems plug into an expansion slot in your computer. The connector in the slot provides all the electrical connections necessary to link to your computer
External modems are self-contained peripherals that accept signals from computer through a serial or parallel port and also plug into telephone line
Packaging of Modem
Pocket modems are compact external modems that are usually designed to plug directly into a port connector , eliminating one interface cable.
PC Card modems plug into that PCMCIA slots that are typically found in notebooks. They combine the advantage of cable-free simplicity of internal modems with the interchangeability of external modems.
Digital Services
Digital data transfer is 20 times faster than modem(Analog Services)
A pair of copper wire is used to carry the analog voice signal before 2000 in local loop
With change in time, local loop will be replaced with newer connection technology, such as coaxial cable or fiber optics to carry digital signals
Shifting of telecommunication services from analog to digital three technologies can provide you with a high-speed all-digital link;
(i) Telephone Services
(ii) Cable Services
(iii) Satellite Services
The most important limiting factor is availability of services
Telephone Services
The range of digital services supplied by telephone companies is wide and spans a range of data rates are listed below:
T1 Digital telephone services
The basic high-speed service provided by the telephone company is called T1, and started in the early 1960s
1st system developed by Bell Labs of 8KHz rate to sample analog signals and translate them into eight-bit digital values as a result of 64Kbps digital data stream.
Bells engineers combined 24 of these voice channels together to create a data frame 193 bits long, the extra bit length defining the beginning of the frame. The result was a data stream with a bit rate of 1.544Mbps.
T1 Digital telephone services
Its signal transmission method is called Alternate Mark Inversion (AMI), a formatting code for T1 transmissions over twisted-pair copper cable
AMI represents a zero (or space) by the absence of a voltage; a one (or mark) is represented by a positive or negative pulse, depending on whether the preceding one was negative or positive.
Outside of the United States, the equivalent of T1 services were called E1.
Most T1 lines installed today utilize High data rate Digital Subscriber Line (HDSL) which has bandwidth-reduction technique is splitting the signal across multiple phone lines
HDSL deliver high-speed data up to 1.544Mbps over two-Copper pair and 2.048Mbps over three copper pair
DSL Digital Services
Digital Subscriber Line(DSL) uses your ordinary telephone wires to carry high-speed digital signals.
The high frequencies of the digital signal are easily split from the low frequencies of the analog signal.
It is also called Asymmetrical Digital Subscriber Line or ADSL. It is asymmetrical because it offers a higher downstream data rate from the server compared to its upstream rates, from you back to the server.
Its speed is limited by distance, with longer distances imposing greater constraints
Another kind of DSL offers the same speed in both the upstream and downstream directions is termed as Symmetrical Digital Subscriber Line (SDSL)
VDSL Digital Services
Very-high-data-rate Digital Subscriber Line (VDSL) service is designed to initially operate asymmetrically at speeds higher than ADSL but for shorter distances, potentially as high as 51.84Mbps downstream for distances shorter than about 1000 feet, falling to one-quarter that at about four times the distance.
Proposed upstream rates range from 1.6Mbps to 2.3Mbps. In the long term, developers hope to make the service symmetrical.
ADSL, VDSL can share a pair of wires with an ordinary telephone connection or even ISDN service
SDS 56 Digital Services
Switched Data Services 56 is an archaic connection system that yielded a single digital channel capable of a 56Kbps data rate—the same as with a modem but with true digital signals
Traveled through conventional copper twisted-pair wiring
Installation costs, however, could be substantially higher
ISDN Digital Services
IDSN stands for “Integrated Services Digital Networks” came into existence in 1992
Two versions of ISDN are generally available
(i) Basic Rate Interface (BRI) which takes advantage of the copper twisted-pair wiring that’s already in place, linking homes /offices to telephone exchanges. ISDN line uses “2B1Q line coding” to carry three digital channels: two B (for Bearer) channels that can carry any kind of data (digitally encoded voice, fax, text, and numbers) at 64,000bps, and a D (or Delta) channel, operating at 16,000bps, that can carry control signals and serve as a third data channel.
The three channels can be independently routed to different destinations through the ISDN system.
The Max. distance is 18,000 feet. To accommodate longer run, distance can be increased by adding no of repeater.
ISDN Digital Services
(ii) Primary Rate Interface(PRI) This services delivers 23B channels(each operating 64,000bits/sec) and one D channel( 16,000 bits/sec.)
ISDN services is billed by time in use, not the amount of data received/transmitted.
Digital Cable Services
The coaxial cables used by most such services have bandwidths a hundred or more times wider than twisted pair.
Most cable television systems are meant to cover only a limited geographic range and do not interconnect.
Cable systems are designed for broadcast in a one-to-many fashion. Cable systems send the same signals to each of their subscribers.
The essence of the cable design is that all users share the same bandwidth
Cable Modem Service
Standards of Digital Cable Services
Several efforts at developing cable standards started in the mid–1990s.
The first major effort started at the IEEE, which formed a new working group called the Cable TV Media Access Control and Physical Protocol Working Group
The companies formed an independent limited partnership called Multimedia Cable Network System Partners, Ltd.
A revised version of the DOCSIS specification, version 1.1, was released in April 1999.
The new standard better defined signal parameters to guarantee the bandwidth of the signal and minimize delays
Standards of Digital Cable Services
The DOCSIS standard allows cable operators flexibility in choosing either 64- or 256- state quadrate amplitude modulation for the downstream data signals they provide.
Upstream, most cable operators have a choice of 16-state QAM or quadrature phase-shift keying, a more robust but slower modulation scheme most choose to ride over the higher noise levels prevalent at lower frequencies on cable.
In this asymmetrical system, cable operators often limit upstream bandwidth from individual users, often to as little as 320Kbps, although the standard allows upstream rates as high as 10Mbps
Standards of Digital Cable Services
Before the wide adoption of DOCSIS, some cable companies deployed Telco-return modems, which used the cable company’s high-speed coaxial cable for downstream data
With the move to DOCSIS, most cable modems now use only a cable connection because all signals ride across the same coaxial cable, all your neighbors have access to the packets of data you send and receive.
To maintain privacy, your cable modem automatically encrypts everything you send and receive using the Data Encryption Standard (DES) algorithm.
Digital Satellite Services
Satellite systems are inherently asymmetrical.
To send data, computer connects to the Web through a conventional dial-up modem. The satellite-based downlink operates at 400Kbps while your phone-based uplink struggles along at modem speed,14.4 to 56Kbps.
Satellites best fit a broadcast model and they move out their signals across wide areas for consumption by the multitudes rather than directly targeting individuals
The system pushes out selected Web and newsgroup
information, and your computer captures it as it is sent out, spooling the data to disk.
Digital Satellite Services
When we want to access one of the pushed Web sites or newsgroups, we read almost instantly from the cache
The system allows to choose which sites and groups to cache locally.
DirecPC uses a 21-inch elliptical antennae designed for roof mounting
The DirecPC antenna is a single-purpose device and can be used only for data, not satellite television reception
The system also requires a receiver (sometimes called a modem) that may be installed as an expansion board inside your computer or as a standalone external peripheral
Introduction
In the 19th Century telegraph was most reliable way of long travel communication using wires
The bit rate was less than 10 per seconds
Samuel Morse moved data through same wire 100,000 times faster
Analog nature has challenges and limitation
Digital services is 20 times faster than analog system
Telecommunication Components
Media and Channels
Signals
analog
digital
Cables
Twisted-pair interference
Coaxial cable 20x more expensive
5.500 simultaneous phone calls
Fiber-optic
1 fiber 30.000 phone calls
Wireless
microwave
satellites
Radio, Infrared, Cellular Radio, Mobile computing
GPS global positioning system
BACKGROUND OF ANALOG SERVICES
Modem is the standard equipment used for these system
Earlier telephone system used analog signals
Telephone network handled millions of analog signals
Later circuitry changed to digital and only analog part of the system was the local loop
Allows frequency between 300-3000Hz to freely pass
Analog Services
Channels in Analog Services
Duplex: With modems, such a two-way exchange of information is called duplex communications. Duplex modems use two carriers to simultaneously transmit and receive data, each of which has half the bandwidth available
Half Duplex: In this case only one signal for transmission is used. It is slow data communications because often a modem must switch between sending and receiving modes
Echoplex: In this modem sends a character down the
phone line, and the distant modem returns the same character
Channels in Analog Services
Switching Modems: Switching modems are half duplex modems that reverse the direction of the signal at each end of the line in response to the need to send data.
You enter commands into a remote access system, and only after the commands are received does the remote system respond with the information .
Asymmetric Channel: asymmetrical modems cut the waiting by maintaining a semblance of two-way duplex communications while optimizing speed in one direction only.
Connection Enhancing Technologies
Line Compensation: The modem faces two challenges:
Not to interpret changes as data travel long distance.
To maintain the quality of the line to a high-enough standard to support its use for high-speed transmission.
Modems compensate for variations in telephone lines by equalizing these lines and use echo cancellation to eliminate the return of their own signals.
Echo Cancellation: The half-second delay imposed by the long distance the signals traveled from earth to satellite and back created annoying “echoes” on the line
Connection Enhancing Technologies
Fallback: Most modems use at most two carriers for duplex communications and these carriers are usually modulated to fill the available bandwidth capabilities.
When the top speed does not work, they attempt to communicate at lower speeds that are less critical of telephone line quality.
Multiple Carrier Modem: uses relatively simple
modulation on several simultaneous carrier signals.
generally cutting data speed in half, the multiple-carrier modems just stop using the carriers in the doubtful regions of the bandwidth.
Connection Enhancing Technologies
Data compression: Many of the bits that are sent through the telecommunications channel are meaningless or redundant, by eliminating those worthless bits, the information content of the data stream is more intense. The process of paring the bits is called data compression.
Error checking and error correction: Because all high-speed modems operate closer to the limits of the telephone channel, they are naturally more prone to data errors and have own built-in error-checking methods and correction methods.
Analog Standards
Modem Hardware
Computer Modem has five elements:
1.Interface circuitry for linking with the host computer
2.Circuits to prepare data for transmission by adding the proper start, stop, and parity bits
3.Modulator circuitry that makes the modem compatible with the telephone line
4. User interface that gives command
5. The package that gives the modem its physical embodiment
Computer Interface
At one time, all modems used a standard or enhanced serial port to link to the computer
All modems, whether installed outside your computer, in one of its expansion slots, or in a PCMCIA slot, make use of a serial or parallel communications port
In the case of serial modems, this interface converts the parallel data of your computer into a serial form suitable for transmission down a telephone line.
They need an input/output address and an interrupt to operate properly.
Data Preparation, Modulator
Pre-transmission preparation helps your modem deliver the highest possible data throughput while preventing errors from creeping in.
The incoming code stream may also be analyzed and compressed to strip out redundant information.
Microcontroller inside the modem performs these functions based on the communications standard.
Modulator circuitry actually converts the digital information from your computer into analog-compatible form.
User Interface & packing of Modem
The modem needs circuitry to connect with the telephone system. This line interface circuitry boosts the strength of the modem’s internal logic-level signals to a level matching that of normal telephone service.
Internal modems plug into an expansion slot in your computer. The connector in the slot provides all the electrical connections necessary to link to your computer
External modems are self-contained peripherals that accept signals from computer through a serial or parallel port and also plug into telephone line
Packaging of Modem
Pocket modems are compact external modems that are usually designed to plug directly into a port connector , eliminating one interface cable.
PC Card modems plug into that PCMCIA slots that are typically found in notebooks. They combine the advantage of cable-free simplicity of internal modems with the interchangeability of external modems.
Digital Services
Digital data transfer is 20 times faster than modem(Analog Services)
A pair of copper wire is used to carry the analog voice signal before 2000 in local loop
With change in time, local loop will be replaced with newer connection technology, such as coaxial cable or fiber optics to carry digital signals
Shifting of telecommunication services from analog to digital three technologies can provide you with a high-speed all-digital link;
(i) Telephone Services
(ii) Cable Services
(iii) Satellite Services
The most important limiting factor is availability of services
Telephone Services
The range of digital services supplied by telephone companies is wide and spans a range of data rates are listed below:
T1 Digital telephone services
The basic high-speed service provided by the telephone company is called T1, and started in the early 1960s
1st system developed by Bell Labs of 8KHz rate to sample analog signals and translate them into eight-bit digital values as a result of 64Kbps digital data stream.
Bells engineers combined 24 of these voice channels together to create a data frame 193 bits long, the extra bit length defining the beginning of the frame. The result was a data stream with a bit rate of 1.544Mbps.
T1 Digital telephone services
Its signal transmission method is called Alternate Mark Inversion (AMI), a formatting code for T1 transmissions over twisted-pair copper cable
AMI represents a zero (or space) by the absence of a voltage; a one (or mark) is represented by a positive or negative pulse, depending on whether the preceding one was negative or positive.
Outside of the United States, the equivalent of T1 services were called E1.
Most T1 lines installed today utilize High data rate Digital Subscriber Line (HDSL) which has bandwidth-reduction technique is splitting the signal across multiple phone lines
HDSL deliver high-speed data up to 1.544Mbps over two-Copper pair and 2.048Mbps over three copper pair
DSL Digital Services
Digital Subscriber Line(DSL) uses your ordinary telephone wires to carry high-speed digital signals.
The high frequencies of the digital signal are easily split from the low frequencies of the analog signal.
It is also called Asymmetrical Digital Subscriber Line or ADSL. It is asymmetrical because it offers a higher downstream data rate from the server compared to its upstream rates, from you back to the server.
Its speed is limited by distance, with longer distances imposing greater constraints
Another kind of DSL offers the same speed in both the upstream and downstream directions is termed as Symmetrical Digital Subscriber Line (SDSL)
VDSL Digital Services
Very-high-data-rate Digital Subscriber Line (VDSL) service is designed to initially operate asymmetrically at speeds higher than ADSL but for shorter distances, potentially as high as 51.84Mbps downstream for distances shorter than about 1000 feet, falling to one-quarter that at about four times the distance.
Proposed upstream rates range from 1.6Mbps to 2.3Mbps. In the long term, developers hope to make the service symmetrical.
ADSL, VDSL can share a pair of wires with an ordinary telephone connection or even ISDN service
SDS 56 Digital Services
Switched Data Services 56 is an archaic connection system that yielded a single digital channel capable of a 56Kbps data rate—the same as with a modem but with true digital signals
Traveled through conventional copper twisted-pair wiring
Installation costs, however, could be substantially higher
ISDN Digital Services
IDSN stands for “Integrated Services Digital Networks” came into existence in 1992
Two versions of ISDN are generally available
(i) Basic Rate Interface (BRI) which takes advantage of the copper twisted-pair wiring that’s already in place, linking homes /offices to telephone exchanges. ISDN line uses “2B1Q line coding” to carry three digital channels: two B (for Bearer) channels that can carry any kind of data (digitally encoded voice, fax, text, and numbers) at 64,000bps, and a D (or Delta) channel, operating at 16,000bps, that can carry control signals and serve as a third data channel.
The three channels can be independently routed to different destinations through the ISDN system.
The Max. distance is 18,000 feet. To accommodate longer run, distance can be increased by adding no of repeater.
ISDN Digital Services
(ii) Primary Rate Interface(PRI) This services delivers 23B channels(each operating 64,000bits/sec) and one D channel( 16,000 bits/sec.)
ISDN services is billed by time in use, not the amount of data received/transmitted.
Digital Cable Services
The coaxial cables used by most such services have bandwidths a hundred or more times wider than twisted pair.
Most cable television systems are meant to cover only a limited geographic range and do not interconnect.
Cable systems are designed for broadcast in a one-to-many fashion. Cable systems send the same signals to each of their subscribers.
The essence of the cable design is that all users share the same bandwidth
Cable Modem Service
Standards of Digital Cable Services
Several efforts at developing cable standards started in the mid–1990s.
The first major effort started at the IEEE, which formed a new working group called the Cable TV Media Access Control and Physical Protocol Working Group
The companies formed an independent limited partnership called Multimedia Cable Network System Partners, Ltd.
A revised version of the DOCSIS specification, version 1.1, was released in April 1999.
The new standard better defined signal parameters to guarantee the bandwidth of the signal and minimize delays
Standards of Digital Cable Services
The DOCSIS standard allows cable operators flexibility in choosing either 64- or 256- state quadrate amplitude modulation for the downstream data signals they provide.
Upstream, most cable operators have a choice of 16-state QAM or quadrature phase-shift keying, a more robust but slower modulation scheme most choose to ride over the higher noise levels prevalent at lower frequencies on cable.
In this asymmetrical system, cable operators often limit upstream bandwidth from individual users, often to as little as 320Kbps, although the standard allows upstream rates as high as 10Mbps
Standards of Digital Cable Services
Before the wide adoption of DOCSIS, some cable companies deployed Telco-return modems, which used the cable company’s high-speed coaxial cable for downstream data
With the move to DOCSIS, most cable modems now use only a cable connection because all signals ride across the same coaxial cable, all your neighbors have access to the packets of data you send and receive.
To maintain privacy, your cable modem automatically encrypts everything you send and receive using the Data Encryption Standard (DES) algorithm.
Digital Satellite Services
Satellite systems are inherently asymmetrical.
To send data, computer connects to the Web through a conventional dial-up modem. The satellite-based downlink operates at 400Kbps while your phone-based uplink struggles along at modem speed,14.4 to 56Kbps.
Satellites best fit a broadcast model and they move out their signals across wide areas for consumption by the multitudes rather than directly targeting individuals
The system pushes out selected Web and newsgroup
information, and your computer captures it as it is sent out, spooling the data to disk.
Digital Satellite Services
When we want to access one of the pushed Web sites or newsgroups, we read almost instantly from the cache
The system allows to choose which sites and groups to cache locally.
DirecPC uses a 21-inch elliptical antennae designed for roof mounting
The DirecPC antenna is a single-purpose device and can be used only for data, not satellite television reception
The system also requires a receiver (sometimes called a modem) that may be installed as an expansion board inside your computer or as a standalone external peripheral