Virtual Circuits

A virtual circuit or virtual channel (VC) is a logical connection between two communication ports in one or more communication networks. There are two types of VC’s: permanent virtual circuits (PVC’s) and switched virtual circuits (SVC’s).

A SVC is an automatically and temporarily created virtual connection that is used for a communication session. A PVC is a virtual circuit is manually created for a continuous communication connection. To create a PVC, routing tables in switches are manually configured one time to provide a continuous connection of end points through a network. The ability to dynamically or manually create virtual connections through a network has created a new type of network referred to as value-added networks (VAN’s). Rather than purchase leased circuits between corporate locations some companies chose to contract with VAN’s to transport their data between their sites. With leased data circuits from each corporate site to the closest VAN point-of-presence (POP), usually in the same metropolitan area, a company could establish PVC’s between its sites through the VAN’s network. These functioned similar to leased circuits and often provided a monthly cost saving to the corporation, yet delivered practically the same service as leased data circuits.

Carrier System : Optical Carrier (OCx), ISDN Digital Subscriber Line (IDSL), Integrated Digital Loop Carrier (IDLC),

Optical carrier (OCx) transmission is a hierarchy of optical communication channels and lines that range from 51 Mbps to more than 39 Gbps. Lower level OC structures are combined to produce higher-speed communication lines. There are different structures of OC. The North American optical transmission standard is called synchronous optical network (SONET) and the European (world standard) is synchronous digital hierarchy (SDH). OCx has been used to represent the digital transmission standards where the “x” denotes the multiple of 51.84 Mbps service. Optical carrier standards continue up through OC768 but some definition at the higher levels is still lacking. The following is an abbreviated list of the optical carrier systems:

Optical Carrier 1 (OC1) – Operates at 51.84 Mbps;

Optical Carrier 3 (OC3) – Operates at 155.52 Mbps (3 X OC1)

Optical Carrier 9 (OC9) – Operates at 466.56 Mbps (9 X OC1);

Optical Carrier 12 (OC12) – Operates at 622.08 Mbps (12 X OC1).

Optical Carrier 192 (OC192) – Operates at 9.95 Gbps

Optical Carrier 256 (OC256) – Operates at 13.27 Gbps.

Optical Carrier 768 (OC768) – Operates at 39.81 Gbps

Synchronous Digital Hierarchy (SDH) is an international digital transmission format used in optical (fiber) networks standardized that is similar (but not identical) to the title Synchronous Optical Network (SONET) used in the United States. SDH uses standardized synchronous transmission according to CCITT standards G.707, G.708 and G.709. These standards define data transfer rates, defined optical interfaces and signal structure formats.

ISDN Digital Subscriber Line (IDSL)
ISDN Digital Subscriber Line (IDSL) is a hybrid of ISDN and DSL technologies. It uses the same data formatting as ISDN devices on the copper wire pair and delivers up to 144 kilobits per second bandwidth through two 64 kbps channels and one 16 kbps channel. The key difference for IDSL systems is that the IDSL system only uses the 64 kbps DS0 channels and the ISDN control channel (D channel) is ignored. The IDSL system effectively multiplies the number of channels on a single copper pair by 2x. The ability to avoid using ISDN signaling is very important as software upgrades for switching systems, to allow ISDN operation can cost more than $500,000 per switch.

Integrated Digital Loop Carrier (IDLC)
Integrated digital loop carrier (DLC) is a digital transmission technology that is used between the central office and groups of customers. The IDLC system is composed of two primary parts: an integrated digital terminal (IDT) and a remote digital terminal (RDT). The IDT concentrates up to 96 lines on to a single 24 channel T1 line. It does this by assigning central office channels to time slots on the IDLC line (between the IDT and RDT) as needed. The RDT reverses the process by assigning a time slot to an access line. The RDT also changes the format of the time slot to the access technology of choice (e.g., ISDN or analog).

The key advantages to the DLC carrier system is that some of the switching function is moved closer to the customer (in the RDT) and increased cost effective transmission through the increased sharing of local loop copper lines. Because the RDT in the DLC system acts as a repeater, this also extends the range of access lines from the central office to the end customer.

Unfortunately, DLC systems are not transparent to DSL systems. Although it is possible to install digital subscriber line network equipment (co-locate) along with RDT equipment, the RDT equipment housings and power supplies were not originally designed to hold additional equipment.

An RDT is divided into three major parts; digital transmission facility interface, common system interface, and line interface. The digital transmission interface terminates the high-speed line and coordinates the signaling. The common system interface performs the multiplexing/de-multiplexing, signaling insertion and extraction. The line interface contains digital to analog conversions (if the access line is analog) or digital formatting (if the line is digital).

Figure 4.23 shows an integrated digital loop carrier system. The DLC carrier system is composed of two basic parts: the Integrated Digital Terminal (IDT) and the Remote RDT. The IDT dynamically connects access lines (actually digital time slots) in the switching system to time slots on the communications line between the IDT and RDT. The RDT reverses the process and converts the high speed DLC channel into independent POTS channels (DS0s) that are connected via local loop lines to homes or businesses.


Figure 1: Digital Loop Carrier (DLC) System

The most common multiplexer is the SLC (subscriber line carrier)-96. This system, routinely referred to as a “Slick96”, can deliver 96 voice circuits (the equivalent 4 T-1’s) to the customer site. This methodology overcomes distance problems associated with providing voice services to remote customers as well as free additional twisted pair for future use.

Carrier System : Digital Subscriber Line (DSL)

Digital subscriber line is the transmission of digital information, usually on a copper wire pair. Although the transmitted information is in digital form, the transmission medium is usually an analog carrier signal (or the combination of many analog carrier signals) that is modulated by the digital information signal.

Digital subscriber line (DSL) was first used in the 1960s to describe the T-1 circuits that were extended to the customer premises. Later the same term was used to describe ISDN basic rate interface (BRI) (2B+D, 144 Kbps) and primary rates interface (PRI) (23B+D, 1.544 Mbps). There are several different digital subscriber line technologies. Each of these DSL technologies usually has a prefix to indicate the specific variant of DSL technology. Hence, the “x” in xDSL indicates that there are many forms of xDSL technology.

DSL transmission allows high-speed data transmission over existing twisted pair telephone wires. This has the potential providing high-speed data services without the burden of installing new transmission lines (e.g., for Internet access).

DSL service dramatically evolved in the mid 1990s due to the availability of new modulation technology and low cost electronic circuits that can do advanced signal processing (e.g., echo canceling and multiple channel demodulation). This has increased the data transmission capability of twisted pair copper wire to over 50 Mbps.

The data transmission capability of a DSL system varies based on the distance of the cable, type of cable used, and modulation technology. There are several different DSL technologies. Each of the DSL technologies mixes different types of transmission technologies to satisfy a specific business need. Some DSL systems allow simultaneous digital and analog transmission and are compatible with analog POTS systems.

Figure 1 shows a basic DSL system. This diagram shows that the key to DSL technologies is a more efficient use of the 1 MHz of bandwidth available on a single pair of copper telephone lines. A DSL system consists of compatible modems on each end of the local loop. For some systems, the DSL system allows for multiple types of transmission on a single copper pair. This includes analog or ISDN telephone (e.g., POTS) and digital communications (ADSL or VDSL). This diagram shows that there are basic trade offs for DSL systems. Generally, the longer the distance of the copper line, the lower the data rate. Distances of less than 1,000 feet can achieve data rates of over 50 Mbps.


Figure 1: Basic Digital Subscriber Line (DSL) System

The first digital subscriber lines (DSLs) were developed due to the need for cost effective quality communication over copper wire. The first digital transmission system was the T1 line. This system had a maximum distance of approximately 6,000 feet prior to needing repeaters.

The T1 digital transmission system used a very complex form of digital transmission. A new high-speed digital subscriber line technology was developed to replace T1 transmission technology. HDSL systems increased the distance that high-speed digital signals could be transmitted without the user of a repeater/amplifier. The HDSL system did require 2 (or 3) pairs of wires to allow simultaneous (send and receive) up to 2 Mbps of data transmission. To conserve the number of copper pairs for data transmission, symmetrical digital subscriber line (SDSL) technology was developed. Although SDSL systems offered lower data rates than HDSL, only 2 wire pairs were required. Since SDSL was developed, the HDSL system has evolved to a 2nd generation (HDSL2) that allows the use of 2 wire pair for duplex transmission with reduced emissions (lower egress). New efficient modulation technology used by ADSL systems dramatically increased the data transmission rates from the central office to the customer to over 6 Mbps (some ADSL systems to 8 Mbps). To take advantage of integrated services digital network (ISDN) equipment and efficiency, an offshoot of ISDN technology that was adapted for the local loop developed called ISDN digital subscriber line (IDSL). Asymmetric digital subscriber line (ADSL) systems evolved to rate adaptive digital subscriber line (RADSL) allow the data rate to be automatically or manually changed by the service provider. To simplify the installation of consumer based DSL equipment, and low data transmission offshoot of ADSL developed that is called ADSL-Lite. Using similar technology as the ADSL system, very high-speed digital subscriber line (VDSL) was created to provide up to 52 Mbps data transfer rates over very short distances.

Figure 2 shows the evolution of DSL systems. This diagram shows that high-speed digital subscriber line technology has been readily available since the 1970s. In the late 1990’s, the addition of advanced signal processing technology allowed DSL technology to rapidly increase transmission speed to over 50 Mbps in short distances.


Figure 2: Evolution of DSL
Digital Subscriber Line (DSL)

Carrier System : Integrated Services Digital Network (ISDN)

Integrated services digital network (ISDN) is a structured all digital telephone network system that was designed to replace (upgrade) existing analog telephone networks. The ISDN network supports for advanced telecommunications services and defined universal standard interfaces that are used in wireless and wired communications systems. There are two key user interfaces defined for ISDN networks: basic rate interface (BRI) and primary rate interface (PRI).

The basic rate interface (BRI) is the smallest transmission system (or interface) available through ISDN. BRI provides for two 64 kbps bearer channels (B channels) and a 16 kbps signaling (data) channel (D channel). This configuration is also is also referred to as 2B+D.

The primary rate interface (PRI) is a standard high-speed data communications interface that is used in the ISDN system. This interface provides a standard data rates for T1 1.544 Mbps and E1 2.048 Mbps. The interface can be divided into combinations of 384 kbps (H) channels, 64 kbps (B) channels and includes at least one 64 kbps (D) control channel.

Integrated Services Digital Network (ISDN)

Carrier System : Digital Signal Level (DSx)

Digital signal level (DSx) transmission is a hierarchy of digital communication channels and lines that range from 64 kbps to 565 Mbps. Lower level DS structures are combined to produce higher-speed communication lines. There are different structures of DS levels used throughout the world with significant variations between North American and European systems. DSx has been used to represent the digital transmission standards where the “x” denotes which service is under discussion.

Trunk carrier (T Carrier) is often used to describe the DSx level. Trunk carrier uses (Tx) to represent the digital transmission standards where the “x” denotes the multiplexed level of trunk service. Tx is the actual transmission structure where DSx is the digital signal levels. Outside the United States, E Carrier (Ex) is used to represent the transmission carrier.

The following represent the North American DS structure:

Digital Signal 0 (DS0) - is the smallest digital channel operating at 64 kbps. It represents a single digitized analog voice channel;

Digital Signal 1 (DS1) – normally referred as a T-1, it is composed of twenty four voice channels packed into a 193 bit frame and transmitted at 1.544 Mbps. The unframed version, or payload, is 192 bits at a rate of 1.536 Mbps;

Digital Signal 2 (DS2) – normally referred to as a T-2, it is composed of four T-1 frames packed into a higher level frame transmitted at 6.312 Mbps;

Digital Signal 3 (DS3) – normally referred to as a T-3, it is composed of twenty-eight T-1 frames packed into a level frame transmitted at 44.736 Mbps.

Fractional T1 (FracT) - A data transmission rate that is a portion of the total capacity of a T-1 communications line (1.544 Mbps) but greater than a DS0 (64Kbps).

European standards are different from the North American noted above. The European hierarchy is as follows:

Digital Signal 0 (DS0) - is the smallest digital channel operating at 64 kbps. It represents a single digitized analog voice channel;

Digital Signal 1 (DS1) – normally referred as an E-1, it is composed of thirty voice channels and two controls channels. It uses a 256-bit frame and operates at 2.048 Mbps.

Digital Signal 2 (DS2) – normally referred to as a E-2, it is composed of four E-1 frames packed into a higher level frame transmitted at 8.448 Mbps;

Digital Signal 3 (DS3) – normally referred to as an E-3, it is composed of sixteen E-1 frames packed into a level frame transmitted at 34.368 Mbps.

Digital Signal 4 (DS4) – normally referred to as a E-4, it is composed of sixty-four E-1 frames packed into a higher level frame transmitted at 139.268 Mbps;

Digital Signal 5 (DS5) – normally referred to as an E-5, it is composed of two hundred fifty-six E-1 frames packed into a level frame transmitted at 565.148 Mbps.

Digital Signal Level (DSx)

Carrier Systems : Plain Old Telephone Service (POTS)

Carrier systems are a combination of a transmission medium, specification of signal types and levels along with specific protocol controls (communication rules). The types of carrier systems range from simple audio telephone POTS carriers to high-speed optical carrier (OC-x) transmission systems.

Plain Old Telephone Service (POTS)
Plain old telephone service (POTS) is a transmission system that is used to provide basic telephone service. It is the common term used for residential telephone service.

Between the late 1800’s through the 1990’s, telephone transmission had remained basically the same. Acoustic energy from the customer was converted to electrical signal by a microphone in a handset. This electrical energy was applied through a hybrid electrical device to the telephone line through the speaker in the handset. A telephone hybrid device (often called a “magic” device by telephone personnel) transferred energy from the microphone to into the telephone 2-wire line while extracting most the remote microphone energy and applying it to the speaker. At the same time at the other end of the connection, the same process was occurring.

Figure 1 shows a hybrid telephone transmission system. This diagram shows that microphone energy from user #1 is added to the 2-wire transmission circuit via the hybrid assembly and the microphone energy from user #2 is subtracted from the circuit by the speaker #1. At the same time, the microphone energy from user #2 is added to the 2-wire transmission circuit and speaker #2 subtracts the microphone energy from user #1. This hybrid process allows 2 wires to contain a composite of both microphone signals.


Figure 1: Hybrid Telephone Transmission

Transmission Medium Limitations

Some of the limitations of transmission lines that reduce their ability to transfer analog and digital information include limited frequency response of the transmission lines, crosstalk, noise from external sources that cause distortion, non-terminated tap lines, and signal attenuation that results from line splices and line resistance.

Frequency Response
The twisting of copper wire pairs provides good frequency response for low frequency audio signals. Unfortunately, twisted copper wire pairs are not specifically designed for high frequency transmission. Analog signals have a frequency range of up to 3.4 kHz and most of the DSL technologies use frequencies up to 1.1 MHz. As the frequency applied to the copper wire pair increases, the attenuation of the line increases and signal energy leaks (emits) from the wire pair.

Figure 1 shows the typical frequency response of a twisted pair of copper wires. The frequency response depends on a variety of factors including the dimension of the copper wire (gauge), insulation type and installation environment (twisting or stapling of the wire).


Figure 1: Frequency Response of Copper and Coax Wire

Crosstalk (Signal Leakage)
Crosstalk is the undesired coupling of a signal from one communications channel to another. Crosstalk occurs when some of the transmission signal energy leaks from the cable. This leakage is called signal egress (emission from the line).

Crosstalk on communication systems can be divided into two categories: near end crosstalk (NEXT) and far end crosstalk (FEXT). Figure 2 shows two types of crosstalk. NEXT results when some of the energy that is transmitted in the desired direction seeps into one (or more) adjacent communication lines from the originating source. FEXT occurs when some of the digital signal energy leaks from one twisted pair and is coupled back to a communications line that is transferring a signal in the opposite direction. Generally, NEXT is more serious than FEXT as the signal interference levels from NEXT are higher.


Figure 2: FEXT and NEXT Crosstalk

Signal Ingress
Signal ingress occurs when electrical signals from other sources (such as radio or lightning spikes) enter into the transmission line. Figure 3 shows a source of signal ingress from a nearby radio tower that may occur in a transmission system. This diagram shows that a high power AM radio transmission tower that is located near a telephone line couples some of its energy onto the telephone line. This interference signal (radio ingress) usually reduces the data transmission capacity of a digital subscriber line (DSL).


Figure 3: Radio Signal Ingress

Bridge Tap Reflections
A bridge tap is an extension to a communication line that is used to attach two (or more) end points (user access lines) to a central office. Bridge taps provide connection options to the telephone company on connecting different communication lines to a central office without having to install new pairs of wires each time a customer requests a new telephone line.

The connection of one (or more) bridge taps on a communication line that is used for plain old telephone service (POTS) does not usually cause signal distortion. However, unterminated bridge taps that are installed on communication lines that transfer high frequency DSL signals can result in signal distortion. The signal distortion comes from the reflections of signal energy reflections off the bridge taps.

When an electrical signal is applied to the end of a copper wire, electrical energy begins to travel down the copper wire. Ideally, when the energy reaches the end of the copper wire, the signal is absorbed at the other end (called a matched line). If the end of the wire is not connected, some (or all) of the energy is reflected back to the beginning of the line.

Figure 4 shows how reflections from bridge line tap can cause distortion. This signal shows that some of the energy from the bridge tap is reflected back to the communications line. This reflected signal is a delayed representation of the original signal. Typically, bridge taps must be removed from communications lines that use DSL technology.


Figure 4: Bridge Tap Reflections

Loading Coils
Loading coils are sometimes used to adjust the frequency response of a communication line to better transfer audio signals. While these loading coils work well for specific types of signals (e.g., audio signals), they can disable the ability of the line to be used for other types of signals (e.g., high frequency DSL signals.)

Figure 5 shows that there may be several installed audio loading coils on a single local loop line. Although these loading coils improve the audio frequency response, they must be removed to allow for high-frequency transmission for systems such as DSL.


Figure 5: Audio Loading Coils

Line Splice Attenuation
Telephone cables usually come in 500 feet roles. Because most telephone lines are several thousand feet from the central office, several cable splices are required. Each line splice attenuates the signal and the amount of signal attenuation varies depending on the type of splice (solder, twist, or pegs) and the amount of corrosion inside the splice.

Because the average distance for local access lines is over 10,000 feet, there are more than 20 splices in the average local access loop. Each of these splices offers the potential for corrosion and increased resistance.

One method that is used to decrease the effects of corrosion (and reduce the attenuation) is to continuously run electric current through the copper wire pair. This “sealing current” is a small amount of direct current that is passed through a copper wire to reduce the corrosion effects of the splice points. The sealing current effectively maintains conductivity of mechanical splices that are not soldered. The direct current effectively punches holes in the corrosive oxide film that forms on the mechanical splices.

Line Resistance Attenuation
The copper cable also has resistance (impedance) that is dependent on the size (diameter) of the cable. The resistance of the copper wire increases as the diameter decreases (gauge number increases). The higher the line resistance, the more of the signal energy is dissipated by the line and less energy is transferred to the receiving device.

Figure 6 shows how line resistance attenuation and the wire size decreases. This diagram shows that cables with larger diameter copper wires are typically used to in the distribution system. As the distribution system nears its destination, the size of the wire often decreases.


Figure 6: Line Resistance Attenuation

Group Delay (Dispersion)
Group delay is the amount of delay a particular group of frequencies experience as they travel through a transmission medium. Because transmitted signals are composed of multiple frequency parts (e.g., high frequency components for rapid signal changes), the delay of some of the parts results in distortion of the transmitted signal.

Figure 7 shows how group delay can cause pulsed signals, such as in digital transmission system, can cause signal distortion. This diagram shows that a digital pulse signal is actually composed of many low, medium, and high frequency components. As the pulse is transmitted through the transmission line, some of the frequency components are delayed more than others. This results in a distorted pulse at the receiving end of the transmission line.


Figure 7: Group Delay (Dispersion)