The Physical Connection

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Appendix A: Technology Overview

Local Loop

The Physical Connection

Local Loop

The physical connection (typically, twisted pair of copper wire) connecting the end user (subscriber) to the central office.

Central Office

Typically where the local loop is terminated for connection to the public switched telephone network (PSTN).

Customer Premises

Typically the building owned by the end user who uses services provided from the central office to enter the public switched telephone network (PSTN).

Access Device

A physical device that terminates the local loop, such as a CSU/DSU for T1.

Analog vs. Digital


  • Older technology

  • Designed for voice

  • Inefficient for data

  • Noisy and error prone

  • Lower speed

  • High overhead


  • Newer technology

  • Designed for data and voice

  • More test options

  • Discrete-level information

  • Higher speeds

  • Low overhead

Analog data implies continuity, continuously varying level information signals, as contrasted to digital data that is concerned with discrete states, 1s and 0s. The information content of an analog signal is conveyed by continuously varying some characteristic such as amplitude, frequency or phase of a voltage or other characteristic of the signal. Computers, motors, lights and other electrical devices generate electrical noise (unwanted electrical signals), which produce undesirable variations on the information content of an analog signal, thus making it error prone.

The information content of the digital signal is conveyed through discrete changes in the state of the signal such as the presence or absence of voltage (1 or 0) or discrete changes in voltage (+3, 0, -3) thereby making the information content of a digital signal less susceptible to electrical noise. Since a digital signal consists of 1s and 0s, it is easy to identify the signal sent (1 or 0) and re-create the signal and transmit it further.

This is the primary reason digital communications improve the quality, reliability and speed of communications. Any noise or distortion introduced along the circuit is eliminated when the signal is regenerated.

Digital Service

Dedicated vs. Switched

Dedicated Service

  • Point-to-point service

  • Customer has exclusive use 24x7

  • Connection is always available

  • Customer must pay for mileage

Switched Service

  • Physical path may vary

  • Shared network

  • Customer connects on demand

  • Customer pays for usage

  • Service provider can oversubscribe capacity

Dedicated service is a circuit between fixed endpoints. The circuit is owned by an end user or leased from a common carrier. The service is available 24 hours a day, 7 days a week, 52 weeks a year, for exclusive use by that user. This service may also be referred to as a leased line or nailed-up circuit.

Switched service is a circuit for which the endpoints may vary with each usage. The circuit is provided by a common carrier, which is routed through a switched network, providing circuit switching between public end users. There are two types of switched technology: circuit switched and packet switched. In a switched circuit, a call is established only for as long as needed and then the session is disconnected.

Access Technologies

Dedicated Service

Switched Service

  • DDS

  • Switched 56 (SW56)

  • T1

  • Basic Rate ISDN (BRI)

  • E1

  • Primary Rate ISDN (PRI)

  • T3

  • Frame Relay

  • xDSL

Technology Overview - Dedicated Services

Dedicated services are a type of digital communications that is non-switched, providing the customer with the ability to have a constant transmission path from point-to-point. A dedicated line may be leased (leased-line) or owned.

  • DDS Technology

  • T1 Technology

  • T1 Carrier Technology

  • E1 Technology

  • E1 Carrier Technology

  • T3 Technology

  • T3 Carrier Technology

  • xDSL Technology

  • HDSL2 Technology

DDS Technology

DDS is a 4-wire, dedicated service with available data rates from 2.4 to 64 kbps. In LAN applications, DDS is deployed as a dedicated point-to-point service. In this type of configuration the two DDS endpoints are always connected. It best serves applications where a continuous connection is required between two or more LANs. DDS is also used to deploy Frame Relay in applications requiring access rates of 64k or less.

DDS was originally an AT&T nationwide non-switched special service network for synchronous data at speeds up to 56 kbps.

T1 Technology

T1 digital communications were introduced in the early 1960s to reduce the amount of copper cable needed to carry the same number of telephone conversations as analog communications.

The term T1 circuit is commonly used to identify a multiplexed 24 channel, 1.544 Mbps digital data circuit providing communications between two facilities or from a local service provider. T1 refers to the transport of a DS-1 formatted signal onto a copper, fiber or wireless medium for deploying voice, data or video-conferencing services. The T1 is part of an extensive digital hierarchy that starts with 24 DS0s at 64 kbps. These individual DS0s are used to provide voice or digital data to support point to point or network applications. By combining multiple DS0s, a high-speed interface can be provided to support a synchronous interface to a LAN router or voice PBX. For distances longer than one mile, a repeater is placed every mile to regenerate the signal.

E1 Technology

E1 is the European equivalent to the American T1. Although both E1 and T1 use 64 kbps channels, they differ in many aspects. E1 is a point-to-point, dedicated, 2.048 Mbps communications circuit that carries 32 channels contrasted with T1's 24 channels. Of these 32 channels, 30 channels transmit voice and data. Unlike T1, E1 always provides clear channel 64 kbps channels.

Of the two remaining channels, one uses time slot 16 and is used for signaling and carrying line supervision (such as whether the telephones are on-hook or off-hook). The other remaining channel uses time slot 0, and is used for synchronization, channel control, and framing control.

There are two options for the physical media:

  • 120 ohm twisted pair cabling, typically foil shielded. This is called a balanced interface and uses a DB-15 or 8-pin modular connector.

  • 75 ohm coaxial cable. This is called an unbalanced interface because the two conductors do not have an equal impedance to ground, and uses a BNC connector.

T3 Technology

T3 is a physical transmission medium, which varies from DS3, which is the actual data, video, and voice that is transmitted over the medium. DS3 can be transmitted over microwave radio, fiber optics, 75 ohm coaxial cable (in-house wiring with distance limitations of 450 feet). The line coding for a T3 line is Bipolar Three Zero Substitution (B3ZS). The framing formats available are M12/M23, M13 or C-Bit Parity. DS3 can be delivered as channelized or non-channelized. Channelized DS3 is delivered as 28 individual DS1s and 672 individual DS0s.

Each DS1 may come from a remote location. The telephone company's central office will do the subdivision of the DS3 to each site. Non-channelized DS3 involves no DS2 or DS1 multiplexing. This service is delivered as a T3 pipe with the bandwidth being 44.2 Mbps. It is generally used in point-to-point applications (one customer sending data to one remote site). Any subdivision of bandwidth is performed at each customer site rather than the central office.

M13 and C-Bit parity formats supports both of the applications above.

xDSL Technology

xDSL is the generic name used to represent a wide variety of digital subscriber line technologies including HDSL, ADSL and IDSL.

High Bit Rate Digital Subscriber Line (HDSL) is the most widely available and used xDSL service in North America today. HDSL technology has been developed to allow the transport of a standard DS1 signal over the outside plant wiring. With HDSL electronics at both the central office and the customer's premises, it's possible to extend a full-duplex 1.544 Mbps signal of voice, data, and video applications over two pairs of copper wire across private or leased copper facilities to distances of 9,000 feet (26 AWG) or 12,000 feet (24 AWG) without redesigning the copper loop and without expensive repeaters. The specific rates achievable with DSL depend on factors such as the DSL technology used, the distance between endpoints and the wire size.

High-bit-rate Digital Subscriber Line 2 (HDSL2) is a technology that transmits a T1 on 2 wire (one pair) 24 AWG copper wire without repeaters for 12,000 feet. HDSL2 enjoys all the benefits of HDSL (the first version) while using half of the copper. Please see the HDSL2 section for more information.

ISDN Digital Subscriber Line (IDSL) is also commonly used for applications that require ISDN BRI signaling in a dedicated mode. IDSL can be extended up to 18,000 feet and can transmit digital data at rates up to 144 Kbps.

HDSL technology also provides T1 capability for private campus networks by utilizing existing copper loop plant originally designed to carry much slower signals.

HDSL2 Technology

High-bit-rate Digital Subscriber Line 2 (HDSL2) is a technology that transmits a T1 on 2 wire (one pair) 24 AWG copper wire without repeaters for 12,000 feet . In comparison, HDSL (the first version) uses 4 wires to do the same job HDSL2 does with 2 wires.

In 1961, the Bell System deployed the first digital T-1 circuit. The T-Carrier System is a 2-way transmission path, one cable pair for each direction of transmission. This T-1 replaced analog carriers that were being deployed. The digital circuit brought many advantages to the telecommunications arena. They were more economical and improved the quality of transmission.

It is also important to note that in this traditional method of deployment, repeaters are required at about every 6000 feet for the intermediate repeater sections. The first repeater from the Central office is usually no more than 3000 feet away. The repeater closest to the customer location is usually no more that 3000 feet away.

In the early 1990s, HDSL circuits were developed. These circuits helped the carriers to deploy T1 circuits faster with less engineering. HDSL is called repeaterless T1 because it can reach 12,000 feet on 24 gauge wire. This requires a transceiver unit (HTU-C) at the Telephone company central office and a transceiver unit (HTU-R) at the endpoint. The transmit pair and receive pair do not need to be binder group separated as in traditional T1. If more distance is needed one or two HREs (HDSL Range Extenders) can be deployed for 24,000 or 36,000 feet of loop length. The HTU-C powers the HTU-R and HREs. HDSL is also useful in a campus or dry wire application.

In the late 1990s, there is another significant development, HDSL2.

HDSL2 uses the Trellis Coded PAM (Pulse Amplitude Modulation) line code. This yields 3 bits per baud. Trellis Coded PAM along with POET (Partially Overlapped Echo Canceled Transmission) allows HDSL2 to deliver all 24 channels using 2 wires , while maintaining a loop distance of 12,000 feet on 24 AWG. In comparison, the first version of HDSL uses the 2B1Q line code which yields 2 bits per baud.

HDSL2 and HDSL can both tolerate bridged taps, but neither can tolerate load coils. A single bridged tap can be no more than 2000 feet and total length of all bridged taps cannot exceed 2500 feet. As mentioned earlier HDSL2 and HDSL can reach a distance of 12,000 feet on 24-AWG cable non-loaded, non-bridged tapped cable. This distance is 9,000 feet if the cable is 26-AWG

Switched Services

Switched services are a type of digital communications that is active only when the customer initiates a connection. There are two types of switched technology: circuit switched and packet switched. Each allows the customer to be billed only when the line is active. This is often referred to as a dial-up service. A customer could use this service to dial another customer's site and only pay for the service while the connection is active.

Circuit Switching

  • This is a specific type of bandwidth that is owned by the customer. It is active any time the customer initiates a connection. It is a direct connection to another site, which is simply used to transmit data.

  • A network that provides temporary constant paths of bandwidth while active. The connection is established when the user places a call to another party. The user has exclusive use of the path.

Switched 56 Technology

Switched 56 is a 56 kbps digital data service that is purchased from some local exchange carriers or interexchange carriers. It may be deployed on either 2 or 4 wires depending on the local carrier's capabilities. In both 4-wire and 2-wire Switched 56 applications, 56 kbps is the standard network operating rate accessed, or access is required on an "as demanded" basis and 24-hour-a-day connectivity is not required.

The 4-wire circuit has been developed and used by AT&T and is available in AT&T 4ESS or 5ESS central office switches. The maximum travel distance is 18,000 feet on a 26-gauge wire without a repeater.

The 2-wire type circuit with out-of-band signaling was developed and used by Northern Telecom in its Datapath equipment. This also supports switched 64 kbps clear channel service. Both 2-wire and 4-wire Switched 56 services are quickly being replaced by Basic Rate ISDN (BRI).

ISDN Services

ntegrated Services Digital Network (ISDN) was first established by the CCITT (Consultative Committee on International Telegraph and Telephone) in 1980 to integrate an all-digital public switched telephone network. This is accomplished through ISDN.

ISDN offers a full range of enhanced services supporting voice, data and video applications through standard interfaces over a single twisted pair of copper. ISDN provides a means of integrating these services and modernizing communications networks to provide information movement and management efficiency.

Two types of ISDN service are Basic Rate ISDN and Primary Rate ISDN. BRI can transmit data up to 128 kbps. PRI (which is transmitted over a T1 line) can transmit data up to 1.536 Mbps.

An LDN (Local Directory Number) is a customer's seven digit ISDN phone number. A SPID (Service Profile Identifier) is a unique identifier that is used to represent the service and feature identifiers of a particular ISDN line or service provider. This number generally is 10+ digits long and includes the LDN.

BRI Technology

Basic Rate ISDN service divides a standard telephone line into three digital channels capable of simultaneous voice and data transmission. The three channels are comprised of two Bearer (B) channels at 64 kpbs each and a data (D) channel at 16 kbps, also known as 2B+D.

The B channels are used to carry voice, video, and data to the customer's site.

The D channel is used to carry all signaling information associated with connection control as-well-as supplementary services.

Multiple B channels can be used at the same time. The D channel can also be used to carry packetized data. BRI uses 2B1Q line coding. The 2B means that the coding method contains two binary information elements in a single quaternary 1Q.

All of the voice and data carried over ISDN are represented in a single stream of 0s and 1s, which are paired up and expressed as one of the four (quaternary) voltage levels.

BRI Reference Model

The above diagram is based on the ITU-T's standard for ISDN interfaces.


U-interface is a 2-wire digital telephone line that runs from the telephone company's central office to an NT1 device.

NT1 (Network Termination Type 1)

NT1 is a Basic Rate ISDN-only device that converts a service provider's U-interface to a customer's S/T-interface. It can be stand-alone or integrated into a terminal adapter.


S/T-interface is a common way of referring to either an S- or T-interface. This can be used to connect directly to an ISDN 2B+D NT1 or an NT2 device with a terminal adapter. This type of interface is often found on Terminal Equipment Type 1.


TE1 (Terminal Equipment Type 1) is ISDN-ready equipment that can directly connect to the ISDN line (often using an S/ T-interface). Examples are ISDN phones, ISDN routers, ISDN computers, etc.

TA (terminal adapter)

TA is a device that allows non-ISDN-ready equipment to connect to an ISDN line. This device can have an integrated NT1.


R-interface is a non-ISDN interface such as an EIA-232 or a V.35 interface. This type of interface is often found on TE2.

TE2 (Terminal Equipment Type 2)

TE2 is equipment that cannot directly connect to an ISDN line. A common example of this device is a PC, or a non-ISDN-ready router. A TA must be used to connect to the ISDN line.

Primary Rate ISDN (PRI)

Primary Rate Interface (PRI) ISDN is a user-to-network interface consisting of twenty-three 64 kbps bearer (B) channels and one 64 kbps signaling (D) channel carried over a 1.544 Mbps DS-1 circuit. The B channels carry data, voice or video traffic. The D channel is used to set up calls on the B channels.

Packet Switched Service

A packet switched network is a network where data is carried in the form of packets. This data would be given an ID on a per packet basis and sent across the network in the most efficient way.

Frame Relay Technology

Frame Relay is a digital packet network that provides all the features and benefits of a dedicated DDS or T1 network, but without the expense of multiple dedicated circuits. Frame Relay is deployed over the same services used to deploy DDS and T1. In a Frame Relay network, circuits are connected to a packet switch within the network that ensures packets are routed to the correct location. Frame Relay is an ideal, cost-effective solution for networks with bursty traffic that require connections to multiple locations and where a certain degree of delay is acceptable.

Frame Relay is optimized for use over higher-speed and very low error-rate data circuits. To reduce latency in Frame Relay networks the switches do not perform error correction (other than discarding corrupted frames) or flow control (other than setting Forward Explicit Congestion Notification and Backward Explicit Congestion Notification bits in the frame header). If the user equipment does not react to those notifications, then the network discards bits when it gets congested. All other functions of error congestion and flow control are left to the customer premises equipment.

One of the main advantages contributing to recent growth in the Frame Relay industry is the cost effectiveness of this type of service. Frame Relay service is a cost-effective solution for networks with bursty traffic requiring connections to multiple locations and where a certain degree of delay is acceptable. It also allows a voice circuit to share the same virtual connection as a data circuit, again, saving money.

Frame Relay Terms

Frame Relay Access Device – FRAD

A generic name for a device that multiplexes and formats traffic for entering a Frame Relay network.

Access Line

A communications line interconnecting a Frame Relay-compatible device to a Frame Relay switch.


Sporadic use of bandwidth that does not use the total bandwidth of a circuit 100% of the time.


The process of placing protocol-specific frames inside Frame Relay frames.

CIR (Committed Information Rate)

The committed rate (usually less than the access rate) which the carrier guarantees to be available to transfer information to its destination under normal circumstances for a particular PVC.

DE (Discard Eligibility)

A user-set bit indicating that a frame may be discarded in preference to other frames if congestion occurs, to maintain the committed quality of service within the network.

DLCI (Data Link Connection Identifier)

A unique number identifying a particular PVC endpoint within a user's access channel in a Frame Relay network and has local significance only to that channel.

BECN (Backward Explicit Congestion Notification)

A bit set by a Frame Relay network to notify an interface device (DTE) that congestion avoidance procedures should be initiated by the sending device.

FECN (Forward Explicit Congestion Notification)

A bit set by a Frame Relay network to notify an interface device (DTE) that congestion avoidance procedures should be initiated by the receiving device.

SNA (Systems Network Architecture)

An IBM network structure that use SDLC data link protocol in a polled environment.

SDLC (Synchronous Data Link Control)

SNA uses SDLC exclusively as the transport for an SNA network.

HDLC (High Level Data Link Control)

A generic protocol used to transmit code-transparent, serial information over a link connection. Unlike SDLC, control information is always placed in the same location.

UNI (User Network Interface)

Describes the connection between the user and the public network service provider.

NNI (Network to Network Interface)

Describes the connection between two public service network providers.

Frame Relay Reference Model


Permanent Virtual Circuit


Physical connection to Frame Relay equipment


Data Link Connection Identifier


Committed Information Rate


Techspeak for the old "time-sharing"


Local Area Network


Channel Service Unit


User-to-Network Interface


Network-to-Network Interface


Quality Of Service


Data Terminal Equipment


Discard Eligible


Frame Relay Access Device


Data Service Unit

The above diagram shows the possible interfaces associated with a Frame Relay network.

DTE (Data Terminal Equipment)

User terminal equipment which creates information for transmission; for example, a user's PC or a router.

FRAD (Frame Relay Access Device)

A generic name for one of a family of devices, usually located at a customer site, which multiplexes and formats traffic for entering a Frame Relay network.


A customer owned, physical layer device that connects DTE, such as a router, to an access line, such as a T1, from the network service provider. Traditionally, DSUs were network equipment used in conjunction with customer-owned CSUs to terminate access lines. Because of regulatory changes, there is no need for physical separation of CSU and DSU any longer. Most so-called DSUs now marketed are really combination CSU/DSUs.


The physical medium, such as a twisted pair of wire, connecting the subscriber to the local exchange company's central office.

ATM Technology

Asynchronous Transfer Mode (ATM) is defined as a form of packet switching which allows high-speed transmission of data through the use of small, fixed-length packets (cells) rather than frames as used in Frame Relay.

ATM was originally developed to be an integral part of what made up Broadband ISDN (B-ISDN).ATM was designed as a truly integrated service that could carry multiple channels of voice, data, and video on the same physical connection.

Unlike TDM, where every packet is given a time slot, ATM utilizes a first-come/first-serve method of transferring data. When the cell is generated, it is given whatever data is available for transfer without regard for data order. ATM switches use a type of store and forward switching to allow for the quick data transfer.

Another advantage of ATM/TDM applications is the ability to carry voice, data, and video over the same pipe of bandwidth. With TDM, you may need to groom channels from a service for specific applications (i.e.: voice, data, video). This can lead to an inefficient use of bandwidth which ATM avoids.

ATM was designed to run over Fiber, but can also be implemented over coaxial cable and copper twisted pair.

The cells are a fixed size of 53 bytes, with 48 bytes being used as the payload and 5 bytes used as the header information.

ATM Connectivity

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