Numbering Plan
A numbering plan is a system that identifies communication points within a communications network through the structured use of numbers. The structure of the numbers is divided to indicate specific regions or groups of users. It is important that all users connected to a telephone network agree on a specific numbering plan to be able to identify and route calls from one point to another.
Telephone numbering plans throughout the world and systems vary dramatically. In some countries, it is possible to dial using 5 digits and others require 10 digits. To uniquely identify every device that is connected to public telephone networks, the Comite Consultatif Internationale de Telegraphique et Telehonique (CCITT) devised a world numbering plan that provides codes for telephone access to each country. These are called country codes. Coupled with the national telephone number assigned to each subscriber in a country, the country code telephone makes that subscribers number unique worldwide. The International Telecommunications Union (ITU) administers the World Numbering Plan standard E.164 publishes any new standards or modifications to existing standards on the Internet.
Each country defines its public telephone network numbering plans. The United States and Canada adopted the North American Numbering Plan (NANP) that allows the two countries to appear as one when dialing internally. Each country has a country code prescribed by the World
Numbering Plan so they are accessed internationally as separate entities. The NANP is based on 10 digit numbering (NXX-NXX-XXXX). The number consists of a 3-digit area code, a 3-digit central office code, and a 4-digit line number. The first three digits (NXX) are the Numbering Plan Area (NPA) or area code. It is this 3-digit code that designates one of the numbering plan areas in the North American Numbering Plan for direct distance dialing. Originally, the format was N0/1X, where N is any digit 2 through 9 and X is any digit. From 1995 on, the acceptable format is NXX.
With the massive requirement for telephone numbers generated by Internet access, fax machines, and cellular telephones, new area codes are being placed in service at an all time high rate. This is causing the telecommunications industry and standards bodies in North America to consider the implementation of “number portability”. When this occurs each subscriber will be assigned telephone numbers permanently (e.g., all subscribers in North America will dial ten digits to make a local call and take their number with them when they move.).
Friday, March 21, 2008
Tuesday, March 18, 2008
PSTN : Switching Systems
Switching Systems
Switching systems are assemblies of equipment that setup, maintain, and disconnect connections between multiple communication lines. Switching systems are often classified by the type of network they are part of (e.g., packet or circuit switched) and the methods that are used to control the switches. The term “switch” is sometimes used as a short name for switching system. Public telephone switching systems have many switches within their network. A typical switch can handle up to 10,000 communication lines each.
Early switches used mechanical levers (crossbars) to interconnect lines. Modern switches use computer systems to dynamically setup, maintain, and disconnect communication paths through one or more switches. True computer-based switching came about through the introduction of the electronic switching systems (ESS’s). ESS EOs did not require a physical connection between incoming and outgoing circuits. Paths between the circuits consisted of temporary memory locations that allowed for the temporary storage of traffic. For an ESS system, a computer controls the assignment, storage, and retrieval of memory locations so that a portion of an incoming line (time slot) could be stored in temporary memory and retrieved for insertion to an outgoing line. This is called a time slot interchange (TSI) memory matrix. The switch control system maps specific time slots on an incoming communication line (e.g., DS3) to specific time slots on an outgoing communication line.
The public telephone network switching system architecture uses a distributed switching system that has a hierarchy of switching levels. Distributed switching systems connect calls through the nearest switching system. With distributed network architecture, the call processing requirements are distributed to multiple points. Using a multilevel hierarchy structure for switching systems allows switching to occur at lower levels of switching unless the telephone call must pass between multiple switches. At that point, the call is passed up to a higher-level switch for transfer to more distant locations.
In conjunction with distributed network architecture, the ability to perform “dynamic routing” furthers the network’s resiliency to faults. Sometimes called “adaptive routing”, dynamic routing automatically re-routes communication paths or circuits as the network traffic levels (e.g., levels of congestion) change or as paths go in or out of service.
A key part of public telephone networks is system reliability. As a result, in the event of equipment failure in such a network, backup (redundant) equipment must provide for continued service. Although this increases the reliability of switching systems, it also increases the system cost (for additional backup equipment) and complexity (recovery management systems).
Public telephone switching systems use EO telephone switches to connect the telephone network to end customers. These switches serve as an end node switch that and provides local dial and access to local and long distance services. Switches that are used to interconnect switches to each other are called tandem switches.
Some systems use mini-switches called remote digital terminals that are located near the EO switch. These mini-switches act as concentrator lines of voice channels between the end customers and the EO switching system. Concentrators grouping multiple communication lines into more efficient trunked (multi-channel) lines.
Switching systems are assemblies of equipment that setup, maintain, and disconnect connections between multiple communication lines. Switching systems are often classified by the type of network they are part of (e.g., packet or circuit switched) and the methods that are used to control the switches. The term “switch” is sometimes used as a short name for switching system. Public telephone switching systems have many switches within their network. A typical switch can handle up to 10,000 communication lines each.
Early switches used mechanical levers (crossbars) to interconnect lines. Modern switches use computer systems to dynamically setup, maintain, and disconnect communication paths through one or more switches. True computer-based switching came about through the introduction of the electronic switching systems (ESS’s). ESS EOs did not require a physical connection between incoming and outgoing circuits. Paths between the circuits consisted of temporary memory locations that allowed for the temporary storage of traffic. For an ESS system, a computer controls the assignment, storage, and retrieval of memory locations so that a portion of an incoming line (time slot) could be stored in temporary memory and retrieved for insertion to an outgoing line. This is called a time slot interchange (TSI) memory matrix. The switch control system maps specific time slots on an incoming communication line (e.g., DS3) to specific time slots on an outgoing communication line.
The public telephone network switching system architecture uses a distributed switching system that has a hierarchy of switching levels. Distributed switching systems connect calls through the nearest switching system. With distributed network architecture, the call processing requirements are distributed to multiple points. Using a multilevel hierarchy structure for switching systems allows switching to occur at lower levels of switching unless the telephone call must pass between multiple switches. At that point, the call is passed up to a higher-level switch for transfer to more distant locations.
In conjunction with distributed network architecture, the ability to perform “dynamic routing” furthers the network’s resiliency to faults. Sometimes called “adaptive routing”, dynamic routing automatically re-routes communication paths or circuits as the network traffic levels (e.g., levels of congestion) change or as paths go in or out of service.
A key part of public telephone networks is system reliability. As a result, in the event of equipment failure in such a network, backup (redundant) equipment must provide for continued service. Although this increases the reliability of switching systems, it also increases the system cost (for additional backup equipment) and complexity (recovery management systems).
Public telephone switching systems use EO telephone switches to connect the telephone network to end customers. These switches serve as an end node switch that and provides local dial and access to local and long distance services. Switches that are used to interconnect switches to each other are called tandem switches.
Some systems use mini-switches called remote digital terminals that are located near the EO switch. These mini-switches act as concentrator lines of voice channels between the end customers and the EO switching system. Concentrators grouping multiple communication lines into more efficient trunked (multi-channel) lines.
Wednesday, March 12, 2008
PSTN : Local Loop
Local Loop
The local loop is the connection (wireless or wired) between a customer’s telephone or data equipment and a LEC or other telephone service provider. Traditionally, the local loop (also called “outside plant”) has been composed of copper wires that extend from the EO switch. The EO is the last switching office in the telephone network that connects customers to the telephone network.
The EO switch cables meet the copper (or other types of lines) at the main distribution frame (MDF). The MDF is a wiring rack that allows technicians to splice the local loop lines with the lines from the switching system. Local loop lines leave the MDF in bundles (possibly thousands of wires in each bundle) and arrive in other junction points such as local distribution frames (LDF). The LDF allows the connection of the final connection (the “drop”) to the business or residence. At the entry to the customer’s location, there is often a network termination (NT) device that isolates the telephone network from the wiring inside the customers building.
Figure 1 depicts a traditional local loop distribution system. This diagram shows a central office (CO) building that contains an EO switch. The EO switch is connected to the MDF splice box. The MDF connects the switch to bundles of cables in the “outside plant” distribution network. These bundles of cables periodically are connected to local distribution frames (LDFs). The LDFs allow connection of the final cable (called the “drop”) that connects to the house or building. A NT block isolates the inside wiring from the telephone system. Twisted pair wiring is usually looped through the home or building to provide several telephone connection points, or jacks, so telephones can connect to the telephone system
Figure 2: Local Loop
The local loop is the connection (wireless or wired) between a customer’s telephone or data equipment and a LEC or other telephone service provider. Traditionally, the local loop (also called “outside plant”) has been composed of copper wires that extend from the EO switch. The EO is the last switching office in the telephone network that connects customers to the telephone network.
The EO switch cables meet the copper (or other types of lines) at the main distribution frame (MDF). The MDF is a wiring rack that allows technicians to splice the local loop lines with the lines from the switching system. Local loop lines leave the MDF in bundles (possibly thousands of wires in each bundle) and arrive in other junction points such as local distribution frames (LDF). The LDF allows the connection of the final connection (the “drop”) to the business or residence. At the entry to the customer’s location, there is often a network termination (NT) device that isolates the telephone network from the wiring inside the customers building.
Figure 1 depicts a traditional local loop distribution system. This diagram shows a central office (CO) building that contains an EO switch. The EO switch is connected to the MDF splice box. The MDF connects the switch to bundles of cables in the “outside plant” distribution network. These bundles of cables periodically are connected to local distribution frames (LDFs). The LDFs allow connection of the final cable (called the “drop”) that connects to the house or building. A NT block isolates the inside wiring from the telephone system. Twisted pair wiring is usually looped through the home or building to provide several telephone connection points, or jacks, so telephones can connect to the telephone system
Tuesday, March 11, 2008
Public Switched Telephone Networks (PSTNs)
Public telephone networks are unrestricted dialing telephone networks that are available for public use to interconnect communication devices. Public telephone networks within countries and regions are a standard integrated system of transmission and switching facilities, signaling processors, and associated operations support systems that allow communication devices to communicate with each other when they operate.
Figure 1 shows a basic overview of the Public Switched Telephone Network (PSTN) as deployed in a typical metropolitan area. PSTN customers connect to the end-office (EO) for telecommunications services. The EO processes the customer service request locally or passes it off to the appropriate end or tandem office. As Different levels of switches interconnect the parts of the PSTN system, lower-level switches are used to connect end-users (telephones) directly to other end-users in a specific geographic area. Higher-level switches are used to interconnect lower level switches.
Figure 1: Public Switched Telephone Network (PSTN)
Switches send control messages to each other through a separate control-signaling network called signaling system number 7 (SS7). The SS7 network is composed of signaling transfer points (STPs) and service control point (SCP) databases. A STP is used to route packets of control
messages through the network. SCP’s are databases that are used by the network to process or reroute calls through the network (such as 800 number toll free call routing). SS7 also provides for the newer features such as incoming call identification and automatic call rerouting used by some service companies that provide 24/7, worldwide dial-in support.
Overview
Public telephone networks include local loops (access lines), switching systems, numbering plans, and are coordinated by network management systems. Post, telephone, and telegraph (PTT) and local exchange carriers (LEC’s) are the established telephone network operators or companies that provide local telecommunications services. For most countries, PTTs are government operated telephone systems. In the United States, LEC’s are granted franchises to provide telephone services to certain geographical areas as mandated by the Federal Communication Commission (FCC). Recently, deregulation and privatization of telecommunication systems worldwide have allowed the creation of a new competing local exchange carriers (CLECs). CLEC’s provide similar services as LEC’s and PTTs. In some cases, CLECs provide services by leasing existing lines from incumbent local exchange carriers (ILECs) and reselling services on these lines. In other cases, CLECs install new communication lines or provide connection by wireless service.
Figure 1 shows a basic overview of the Public Switched Telephone Network (PSTN) as deployed in a typical metropolitan area. PSTN customers connect to the end-office (EO) for telecommunications services. The EO processes the customer service request locally or passes it off to the appropriate end or tandem office. As Different levels of switches interconnect the parts of the PSTN system, lower-level switches are used to connect end-users (telephones) directly to other end-users in a specific geographic area. Higher-level switches are used to interconnect lower level switches.
Switches send control messages to each other through a separate control-signaling network called signaling system number 7 (SS7). The SS7 network is composed of signaling transfer points (STPs) and service control point (SCP) databases. A STP is used to route packets of control
messages through the network. SCP’s are databases that are used by the network to process or reroute calls through the network (such as 800 number toll free call routing). SS7 also provides for the newer features such as incoming call identification and automatic call rerouting used by some service companies that provide 24/7, worldwide dial-in support.
Overview
Public telephone networks include local loops (access lines), switching systems, numbering plans, and are coordinated by network management systems. Post, telephone, and telegraph (PTT) and local exchange carriers (LEC’s) are the established telephone network operators or companies that provide local telecommunications services. For most countries, PTTs are government operated telephone systems. In the United States, LEC’s are granted franchises to provide telephone services to certain geographical areas as mandated by the Federal Communication Commission (FCC). Recently, deregulation and privatization of telecommunication systems worldwide have allowed the creation of a new competing local exchange carriers (CLECs). CLEC’s provide similar services as LEC’s and PTTs. In some cases, CLECs provide services by leasing existing lines from incumbent local exchange carriers (ILECs) and reselling services on these lines. In other cases, CLECs install new communication lines or provide connection by wireless service.
Monday, March 10, 2008
Telecom : Repeaters
Repeaters are devices or circuits that are located between transmitting and receiving devices to improve the quality the signal that is delivered between them. A repeater obtains some or all of the signal from the transmitter, amplifies and may adjust (change a frequency) or filter the signal, and retransmits the signal to the receiver(s).
Repeaters can be analog or digital. Analog repeaters amplify the received signal for retransmission. Analog repeaters amplify both the desired signal and any noise that is added to the communication lines. This limits the maximum number of analog repeaters that can be used and this limits the maximum distance for analog communication lines. Digital repeaters can receive and recreate digital signals. Digital repeaters are also called regenerative repeaters. The regenerative process allows digital signals to be transferred at great distances with minimal errors at the receiving end.
Repeaters can be analog or digital. Analog repeaters amplify the received signal for retransmission. Analog repeaters amplify both the desired signal and any noise that is added to the communication lines. This limits the maximum number of analog repeaters that can be used and this limits the maximum distance for analog communication lines. Digital repeaters can receive and recreate digital signals. Digital repeaters are also called regenerative repeaters. The regenerative process allows digital signals to be transferred at great distances with minimal errors at the receiving end.
Sunday, March 9, 2008
Channel Multiplexing
Channel multiplexing is a process that divides a single transmission path into several parts that can transfer multiple communication (voice and/or data) channels. Multiplexing may be frequency division (dividing into frequency bands), time division (dividing into time slots), code division (dividing into coded data that randomly overlap), or statistical multiplexing (dynamically assigning portions of channels when activity exists).
When several communications channels are connected over a common channel, a device called a multiplexer is used. The multiplexer combines multiple incoming (input) signals onto one common communications channel through the process of time, frequency, or code sharing. At the other end of the communication line, a demultiplexer device is used to separate the channels (output) at the receiving end.
When a digital channel is divided into multiple digital sub channels, the separate channels are called logical channels. Each logical channel is assigned a portion of the bits from the digital communications channel.
Frequency Division Multiplexing
A device that converts the information signal into a format that is suitable for transmission is called a transmitter. The device that receives and decodes the transmitted signal is called a receiver. When a transmitter and receiver are combined into one device, it is called a transceiver.
Frequency Division Multiplexing (FDM)
Frequency division multiplexing is a process of allowing multiple channels to share a frequency band by dividing up a frequency band into smaller frequency bandwidth channels. Each of these smaller channels provides for a separate communications channel.
Time Division Multiplexing
Figure below shows how a frequency band can be divided into several communication channels. When a device is communicating on a FDM system using a frequency carrier signal, it’s carrier channel is completely occupied by the transmission of the device. For some FDM systems, after it has stopped transmitting, other transceivers may be assigned to that carrier channel frequency. When this process of assigning channels is organized, it is called frequency division multiple access (FDMA). Transceivers in an FDM system typically have the ability to tune to several different carrier channel frequencies.
Carrier signals can co-exist with each other on an FDM system without interference if they are operating at different frequencies. Because the modulating signal slightly changes the carrier signal, this produces small changes in frequency. This results in a single radio signal that occupies a frequency range, depending on the type and amount of information that is changing the electromagnetic wave. The maximum amount of frequency change is typically called the channel bandwidth. Hence, a carrier signal should not typically operate in areas that other radio carrier signals may occupy.
As a carrier signal is modulated (amplitude, frequency, or phase), several other small energy signals at different frequencies are created. Some of the signals produced by the modulation process fall outside the designated frequency bandwidth. Although the amount of energy that falls outside the designated bandwidth is usually small, they may cause interference with other devices that are communicating on other nearby channels.
To help protect from unwanted interference, when multiple carrier signals are operating in an FDM system, a guard band is usually used to protect adjacent carriers from interference. Guard bands are a portion of a resource (frequency or time) that is dedicated to the protection of a communication channel from interference due to radio signal energy or time overlap of signals. While guard bands protect a desired communication channel from interference, the guard band also uses part of the valuable resource (frequency bandwidth or time period) for this protection.
Time Division Multiplexing (TDM)
Time division multiplexing (TDM) is a process of sharing a single carrier channel by dividing the channel into time slots that are shared between simultaneous users of the carrier channel. When a transceiver communicates on a TDM system, it is assigned a specific time position on the carrier channel. By allowing several users to use different time positions (time slots) on a single carrier channel, TDM systems increase their ability to serve multiple users with a limited number of channels by dividing a frequency band into time slots. Time slots are grouped into repetitive frames. Each communication channel is assigned to one (or several) time slot(s) within a frame.
To allow TDM systems to provide continuous voice communication to a transceiver that can only transmit for brief periods, TDM systems use digital signal processing to characterize and compress digital signals into short time-slices. Figure below shows how a single carrier channel is time-sliced into three communication channels. Transceiver number 1 is communicating on time slot number 1 and mobile radio number 2 is communicating on time slot number 3. Each frame on this communication system has three time slots.
Code Division Multiplexing (CDM)
Code division multiplexing uses a method of spreading an information signal using different codes on a wide bandwidth communication channel (typically digital signals). For CDM channels, the frequency bandwidth of the carrier channel is much larger than the bandwidth of the original information signal. Because the channel bandwidth is very large, information from other channels operating in the same frequency band is relatively small. This allows multiple communications channels to operate in the same frequency bandwidth at the same time. There are various forms of CDM. The most popular forms of spread spectrum include frequency hopping and direct spread spectrum.
Frequency hopping is a multiplexing technology where transceivers may share a frequency band by transmitting for brief periods of time on an individual carrier channels and then hopping to other carrier channels to continue transmission. Each transceiver is assigned to a particular hopping pattern and collisions that occur are random. These errors only cause a loss of small amounts of data that may be fixed through error detection and correction methods.
Direct spread spectrum is relatively new commercialized (verses militarized) modulation technique that is used primarily in cellular and satellite systems. Direct sequence spread spectrum systems mix a relatively long digital code with a small amount of communication data (information signal) to produce a combined signal that is spread over a relatively wide frequency band. To receive the signal, the long code is used to extract the original signal.
Because the energy is spread over a wide bandwidth, multiple spread spectrum channels with different codes can co-exist with minimal interference. Figure 3.8 shows how a single direct sequence spread spectrum communication channel can have several channels. In this example, there are 3 different code patterns that are used for communication channels. When a receiver uses the reference code, a direct sequence spread spectrum system can build a mask as shown in Figure below for each conversation allowing only that information which falls within the mask to be transmitted or received.
Code Division Multiplexing
Digital Speech Interpolation (DSI)
In addition to multiplexing through channel division, statistical multiplexing can also be used by distributing transmission of a communications channel over idle portions of multiplexed channels. An example of statistical multiplexing is digital speech interpolation (DSI). DSI is a technique that dynamically allocates time slots for voice or data transmission to a user only when the have voice or data activity. This increases the system capacity as transmission for other users can occur when others are silent.
Digital speech interpolation (DSI) is a digital form of a process known as time assigned speech interpolation (TASI). The DSI technique that dynamically allocates channels (usually time slots) for voice or data transmission to a user only when the have voice or data activity. This increases the system capacity as transmission for other users can occur when others are silent.
A system that has DSI capability assigns information transmission based in speech activity. The DSI system senses activities of speech signals and availability of communication channels in a system and dynamically transmits information signals on available communications channels. Because speech conversation is composed of pauses and alternating directions of communications (usually one person speaks at a time), the use of TASI increases the efficiency of a communications system of approximately 2:1. For example, a 96 channel communications circuit that uses TASI can provide service approximately 192 calls.
Figure below shows the process of multiplexing using DSI. This diagram shows a communication circuit that has 96 independent communication channels (one communication link that has 96 time slots). The DSI system monitors the activity of each voice conversation (a voice channel) using a voice activity detector (VAD). The VAD is an electronic circuit that senses the activity (or absence) of voice signals. This is used to inhibit a transmission signal during periods of voice inactivity.
Digital Speech Interpolation (DSI)
When the VAD detects that speech is active, the DSI system assigns the information to specific time slots on the communications channel. The DSI transmitter system identifies the voice channel at the beginning of the transmission so the DSI receiver can assign it to an output voice channel. When the voice activity detector senses a pause in communication, the DSI transmitter sends an ending message on the channel allowing the channel to be placed back into a pool of available communication channels. The next time the speech activity detector senses the voice channel is again active, the DSI transmitter will select a channel from the pool of available communication channels and the process begins again. Each time, the DSI receiver will assign the information to the correct output voice channel.
When several communications channels are connected over a common channel, a device called a multiplexer is used. The multiplexer combines multiple incoming (input) signals onto one common communications channel through the process of time, frequency, or code sharing. At the other end of the communication line, a demultiplexer device is used to separate the channels (output) at the receiving end.
When a digital channel is divided into multiple digital sub channels, the separate channels are called logical channels. Each logical channel is assigned a portion of the bits from the digital communications channel.
A device that converts the information signal into a format that is suitable for transmission is called a transmitter. The device that receives and decodes the transmitted signal is called a receiver. When a transmitter and receiver are combined into one device, it is called a transceiver.
Frequency Division Multiplexing (FDM)
Frequency division multiplexing is a process of allowing multiple channels to share a frequency band by dividing up a frequency band into smaller frequency bandwidth channels. Each of these smaller channels provides for a separate communications channel.
Figure below shows how a frequency band can be divided into several communication channels. When a device is communicating on a FDM system using a frequency carrier signal, it’s carrier channel is completely occupied by the transmission of the device. For some FDM systems, after it has stopped transmitting, other transceivers may be assigned to that carrier channel frequency. When this process of assigning channels is organized, it is called frequency division multiple access (FDMA). Transceivers in an FDM system typically have the ability to tune to several different carrier channel frequencies.
Carrier signals can co-exist with each other on an FDM system without interference if they are operating at different frequencies. Because the modulating signal slightly changes the carrier signal, this produces small changes in frequency. This results in a single radio signal that occupies a frequency range, depending on the type and amount of information that is changing the electromagnetic wave. The maximum amount of frequency change is typically called the channel bandwidth. Hence, a carrier signal should not typically operate in areas that other radio carrier signals may occupy.
As a carrier signal is modulated (amplitude, frequency, or phase), several other small energy signals at different frequencies are created. Some of the signals produced by the modulation process fall outside the designated frequency bandwidth. Although the amount of energy that falls outside the designated bandwidth is usually small, they may cause interference with other devices that are communicating on other nearby channels.
To help protect from unwanted interference, when multiple carrier signals are operating in an FDM system, a guard band is usually used to protect adjacent carriers from interference. Guard bands are a portion of a resource (frequency or time) that is dedicated to the protection of a communication channel from interference due to radio signal energy or time overlap of signals. While guard bands protect a desired communication channel from interference, the guard band also uses part of the valuable resource (frequency bandwidth or time period) for this protection.
Time Division Multiplexing (TDM)
Time division multiplexing (TDM) is a process of sharing a single carrier channel by dividing the channel into time slots that are shared between simultaneous users of the carrier channel. When a transceiver communicates on a TDM system, it is assigned a specific time position on the carrier channel. By allowing several users to use different time positions (time slots) on a single carrier channel, TDM systems increase their ability to serve multiple users with a limited number of channels by dividing a frequency band into time slots. Time slots are grouped into repetitive frames. Each communication channel is assigned to one (or several) time slot(s) within a frame.
To allow TDM systems to provide continuous voice communication to a transceiver that can only transmit for brief periods, TDM systems use digital signal processing to characterize and compress digital signals into short time-slices. Figure below shows how a single carrier channel is time-sliced into three communication channels. Transceiver number 1 is communicating on time slot number 1 and mobile radio number 2 is communicating on time slot number 3. Each frame on this communication system has three time slots.
Code Division Multiplexing (CDM)
Code division multiplexing uses a method of spreading an information signal using different codes on a wide bandwidth communication channel (typically digital signals). For CDM channels, the frequency bandwidth of the carrier channel is much larger than the bandwidth of the original information signal. Because the channel bandwidth is very large, information from other channels operating in the same frequency band is relatively small. This allows multiple communications channels to operate in the same frequency bandwidth at the same time. There are various forms of CDM. The most popular forms of spread spectrum include frequency hopping and direct spread spectrum.
Frequency hopping is a multiplexing technology where transceivers may share a frequency band by transmitting for brief periods of time on an individual carrier channels and then hopping to other carrier channels to continue transmission. Each transceiver is assigned to a particular hopping pattern and collisions that occur are random. These errors only cause a loss of small amounts of data that may be fixed through error detection and correction methods.
Direct spread spectrum is relatively new commercialized (verses militarized) modulation technique that is used primarily in cellular and satellite systems. Direct sequence spread spectrum systems mix a relatively long digital code with a small amount of communication data (information signal) to produce a combined signal that is spread over a relatively wide frequency band. To receive the signal, the long code is used to extract the original signal.
Because the energy is spread over a wide bandwidth, multiple spread spectrum channels with different codes can co-exist with minimal interference. Figure 3.8 shows how a single direct sequence spread spectrum communication channel can have several channels. In this example, there are 3 different code patterns that are used for communication channels. When a receiver uses the reference code, a direct sequence spread spectrum system can build a mask as shown in Figure below for each conversation allowing only that information which falls within the mask to be transmitted or received.
Digital Speech Interpolation (DSI)
In addition to multiplexing through channel division, statistical multiplexing can also be used by distributing transmission of a communications channel over idle portions of multiplexed channels. An example of statistical multiplexing is digital speech interpolation (DSI). DSI is a technique that dynamically allocates time slots for voice or data transmission to a user only when the have voice or data activity. This increases the system capacity as transmission for other users can occur when others are silent.
Digital speech interpolation (DSI) is a digital form of a process known as time assigned speech interpolation (TASI). The DSI technique that dynamically allocates channels (usually time slots) for voice or data transmission to a user only when the have voice or data activity. This increases the system capacity as transmission for other users can occur when others are silent.
A system that has DSI capability assigns information transmission based in speech activity. The DSI system senses activities of speech signals and availability of communication channels in a system and dynamically transmits information signals on available communications channels. Because speech conversation is composed of pauses and alternating directions of communications (usually one person speaks at a time), the use of TASI increases the efficiency of a communications system of approximately 2:1. For example, a 96 channel communications circuit that uses TASI can provide service approximately 192 calls.
Figure below shows the process of multiplexing using DSI. This diagram shows a communication circuit that has 96 independent communication channels (one communication link that has 96 time slots). The DSI system monitors the activity of each voice conversation (a voice channel) using a voice activity detector (VAD). The VAD is an electronic circuit that senses the activity (or absence) of voice signals. This is used to inhibit a transmission signal during periods of voice inactivity.
When the VAD detects that speech is active, the DSI system assigns the information to specific time slots on the communications channel. The DSI transmitter system identifies the voice channel at the beginning of the transmission so the DSI receiver can assign it to an output voice channel. When the voice activity detector senses a pause in communication, the DSI transmitter sends an ending message on the channel allowing the channel to be placed back into a pool of available communication channels. The next time the speech activity detector senses the voice channel is again active, the DSI transmitter will select a channel from the pool of available communication channels and the process begins again. Each time, the DSI receiver will assign the information to the correct output voice channel.
Friday, March 7, 2008
Signaling : In-Band Signaling,
Signaling is the process of transferring control information such as connection addresses, call supervision codes, or other connection information between communication switching equipment and other communications equipment or systems. The basic functions of signaling include initiate a call or line connection (call setup), maintain a communication link, and to end a call or connection (call teardown). Signaling comes in two basic forms: in-band signaling and out-of-band signaling.
In-Band Signaling
In-band signaling sends control messages in the same communication channel that is used for voice or data communication. During the period of in-band signaling, the voice or data communication is temporarily inhibited (muted) to allow the transfer of control messages. The types of in-band address signaling include dial pulse (DP), dual-tone multi-frequency (DTMF), MF (Multi-Frequency), audio signaling, and line control.
Dial pulse (DP) signaling senses and counts the changes in current flow, such as from a rotary dial telephone, to allow the user to send address information (dialed digits) to the telephone system.
DTMF signaling is a means of transferring information from a user to the telephone network through the use of in-band audio tones. Each digit of information is assigned a simultaneous combination of one of a lower group of frequencies and one of a higher group of frequencies to represent each digit or character.
Multifrequency (MF) signaling is a type of in-band address signaling method that represents decimal digits and auxiliary signals by pairs of frequencies from the following group: 700, 900, 1100, 1300, 1500, and 1700 Hz. These audio frequencies are used to indicate telephone address digits, precedence, control signals, such as line-busy or trunk-busy signals, and other required signals.
On modern telephone systems, most in-band signaling only occurs between the end-user and his serving central office telephone switch.
These signals travel over the same audio line as the voice or data call. Examples of other these in-band signaling messages include:
- Dial tone (the circuit is working)
- Busy tone (the circuit is unavailable)
- Fast busy tone (the system is busy)
- DTMF or pulse digits (send dialed digit information)
- Special functions such as # and * (activate other services)
- Telephone systems also can sense line condition as a signaling method. When the central office senses a grounding of the line (ground start) or a reduction in voltage (off-hook loop-start), it produces a dialtone signal (audio signaling) to inform the user service is available. Wink start is another line activation signal that is used by the telephone switch to indicate to end-user telephone systems of a change in status. Wink signals are brief 140 msec interruptions of communication.
Out-of-Band Signaling
Out-of-band signaling is signaling that travels over a separate path from voice and data calls but carries control information about the calls such as call setup, call routing, caller-id, call tear-down, etc. For out-of-band signaling the telecommunication industry uses a standard called Common Channel Signaling and Control (CCS). The current version of CCS is known as Signaling System 7 (SS7). Through standardization all telephone companies implement SS7 and thus can interact smoothly with very few errors.
Using SS7, switches can more effectively route calls and even query centralized databases for additional control information. The advent of SS7 has brought with it many new features such as caller-id. It has also been instrumental allowing for the phenomenal growth the industry has seen.
In-Band Signaling
In-band signaling sends control messages in the same communication channel that is used for voice or data communication. During the period of in-band signaling, the voice or data communication is temporarily inhibited (muted) to allow the transfer of control messages. The types of in-band address signaling include dial pulse (DP), dual-tone multi-frequency (DTMF), MF (Multi-Frequency), audio signaling, and line control.
Dial pulse (DP) signaling senses and counts the changes in current flow, such as from a rotary dial telephone, to allow the user to send address information (dialed digits) to the telephone system.
DTMF signaling is a means of transferring information from a user to the telephone network through the use of in-band audio tones. Each digit of information is assigned a simultaneous combination of one of a lower group of frequencies and one of a higher group of frequencies to represent each digit or character.
Multifrequency (MF) signaling is a type of in-band address signaling method that represents decimal digits and auxiliary signals by pairs of frequencies from the following group: 700, 900, 1100, 1300, 1500, and 1700 Hz. These audio frequencies are used to indicate telephone address digits, precedence, control signals, such as line-busy or trunk-busy signals, and other required signals.
On modern telephone systems, most in-band signaling only occurs between the end-user and his serving central office telephone switch.
These signals travel over the same audio line as the voice or data call. Examples of other these in-band signaling messages include:
- Dial tone (the circuit is working)
- Busy tone (the circuit is unavailable)
- Fast busy tone (the system is busy)
- DTMF or pulse digits (send dialed digit information)
- Special functions such as # and * (activate other services)
- Telephone systems also can sense line condition as a signaling method. When the central office senses a grounding of the line (ground start) or a reduction in voltage (off-hook loop-start), it produces a dialtone signal (audio signaling) to inform the user service is available. Wink start is another line activation signal that is used by the telephone switch to indicate to end-user telephone systems of a change in status. Wink signals are brief 140 msec interruptions of communication.
Out-of-Band Signaling
Out-of-band signaling is signaling that travels over a separate path from voice and data calls but carries control information about the calls such as call setup, call routing, caller-id, call tear-down, etc. For out-of-band signaling the telecommunication industry uses a standard called Common Channel Signaling and Control (CCS). The current version of CCS is known as Signaling System 7 (SS7). Through standardization all telephone companies implement SS7 and thus can interact smoothly with very few errors.
Using SS7, switches can more effectively route calls and even query centralized databases for additional control information. The advent of SS7 has brought with it many new features such as caller-id. It has also been instrumental allowing for the phenomenal growth the industry has seen.
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