Services : Switched Data Service, Leased Lines and Digital Subscriber Line (DSL)

Switched Data Service
Switched data transmission services is the providing of data transmission service between points that are setup by or for the customer. A specific form of switched data transmission service offered in public telephone networks is called switched multi-megabit data service (SMDS). SMDS provides high-speed data connections in metropolitan areas that are often used for LAN-to-LAN connections where there are several interconnection points (ports). Interfaces to the SMDS networks normally range from T-1 (1.544 Mbps) to T-3 (44.736 Mbps) although 56 Kbps and fractional T-1 are offered in some locations.

Figure 1 shows an example of the cost structure of SMDS data transmission services. This diagram shows that the user pays an installation fee, a port fee for each access port to the data transmission network, and a monthly usage fee based on the data transmission rate used by the customer.


Figure 1: Cost of SMDS Data Transmission Service

Leased Lines
Leased lines are telecommunications circuits (either two-wire or four-wire) rented/leased from a telephone company to connect two or more locations on a permanent basis. Leased lines are normally associated with data services or voice PBX tie line services. Leased lines are ordered as either analog or digital circuits. Analog circuits provide a single full duplex (two-way) path between locations. They terminate in either telephone switches/instruments or in modems. Digital leased lines, on the other hand, terminate in customer service units (CSU’s) rather than modems. The cost of leased lines depends on the region of service, specific carrier pricing plan, and on distance (line length). As a result, leased lines often connect the end user to another carrier that interconnects another leased line to allow connection to its destination. As a result, leased line prices are often quoted from the customer’s location to an EO or POP of a carrier.

Figure 2 shows the typical costs involved in pricing of point-to-point leased lines in the United States. This table shows that average leased line costs for 56 kbps lines is approximately $240 per month. For a T1 line, the average cost is approximately $900 per month and the monthly cost for a DS3 (45 Mbps) connection is approximately $4800.


Figure 2: Typical Cost of Leased Line Service in United States

Digital Subscriber Line (DSL)
Digital subscriber line (DSL) service is a data service that offers varying data transmission rates to customer. DSL service usually connects users directly to an Internet service provider (ISP). DSL service is generally lower in cost than leased line cost. The difference between DSL service and leased line service is that DSL service does not usually guarantee a data transmission rate.

Figure 3 shows an example of cost of DSL service for an ADSL line. This table shows that a customer pays an initial DSL connection fee, purchases or leases a data interface (e.g., router), and pays a monthly subscription of approximately $50 per month.


Figure 3: Cost of Digital Subscriber Line Service

Services : Voice and Centrex

Services
The key services provided in public switched telephone networks include voice (audio bandpass), Centrex, switched data communications service, leased line, and digital subscriber line.

Voice
Voice service is the providing of audio communication circuits that can pass analog frequencies below 3.3 kHz. Voice service is commonly called plain old telephone service (POTS). Voice service remains the core of telephone service as in 2000, the amount of voice traffic transferred per month was more than 53,000 terabytes per month [5].

The newer CO switches have enhanced voice services to allow residential customers to have practically all the features normally associated with PBX’s that serve businesses such as: call waiting, distinctive ringing, voice mail (with signaling or stutter dial tone), feature telephones, and incoming WATS. Some of the newer features are packaged (bundled) together so their actual cost is not readily known.

Figure 1 shows the cost of local telephone service in the United States and that the costs are based on a recurring charge with unlimited usage. The customer may also pay additional recurring fees for advanced services. The cost elements are reasonably standard but the costs vary among LEC’s/CLEC’s. At times the variations between LEC’s in geographies are substantial.


Figure 1: Cost of Local Telephone Service in the United States. Source: Federal Communications Commission (FCC)

Outside the United States, the cost structure for local telephone service is often based on actual usage with a per minute rate ranging from 2 to 6 cents per minute.

Centrex
Centrex is a service offered by a local telephone service provider (primarily to businesses) that allows the customer to have features that are typically associated with a PBX. These features include 3 or 4 digit dialing, intercom features, distinctive line ringing for inside and outside lines, voice mail, call waiting indication, and others.

Centrex services have had many names over the years, but, whatever the name, the purpose of this offering was always the same: an alternative to customer premises PBX’s. Centrex services flourished and still have a place for many large, dispersed entities such as large universities and major medical centers.

One of the major selling points for centrex is the lack of capital expenditure up front. That coupled with the reliability associated with centrex due to its location in the telephone company CO have kept centrex as the primary telephone system in many of the businesses referenced above. PBX’s, however, have cut into what was once a quite lucrative market for the telephone companies and are now the rule rather than the exception for business telephone service. This has come about because of inventive ways of funding the initial capital outlay and the significantly lower operating cost of a PBX versus a comparable centrex offering.

System : Passive Optical Network (PON)

Passive Optical Network (PON)
A passive optical network (PON) combines, routes, and separates optical signals through the use of passive optical filters that separate and combine channels of different optical wavelengths (different colors). The PON distributes and routes signals without the need to convert them to electrical signals for routing through switches.

PON networks are constructed of optical line termination (OLT), optical splitters and optical network units (ONUs). OLTs interface the telephone network to allow multiple channels to be combined to different optical wavelengths for distribution through the PON. Optical splitters are passive devices that redirect optical signals to different locations. ONU’s terminate or sample optical signals so they can be converted to electrical signals in a format suitable for distribution to a customer’s equipment. When used for residential use, a single ONU can server 128 to 500 dwellings. In 2001, most PON’s use ATM cell architecture for their transport between the provider EO or point of presence (POP) and the ONU (in some case even to the user workstation). When ATM protocol is combined with PON system, it is called ATM passive optical network (APON).

Figure 1 shows an APON that locates ONUs near residential and business locations. This network uses ATM protocol to coordinate the PON. ONU interfaces are connected via fiber to an OLT located at the provider’s EO or POP. Each ONU multiplexes user channels (between 12 and 40) into an optical frequency spectrum allocated to that ONU. Up 32 ONU’s can share access to a single PON using the features of dense wave division multiplexing (DWDM). Some newer PON’s use high density wave division multiplexing (HDWDM). Use of HDWDM increases the number of ONU’s per PON from 32 to 64. This diagrams shows that a PON that uses HDWDM can support approximately 2500 residential customers.


Figure 1: Passive Optical Network (PON)

System : Digital Loop Carrier (DLC)

Digital Loop Carrier (DLC)
Digital loop carrier (DLC) is a high efficiency digital transmission system that uses existing distribution cabling systems to transfer digital information between the telephone system (central office) and a telephone or other communication device. There are two types of DLC: universal digital loop carrier (UDLC) and integrated digital loop carrier (IDLC).

The UDLC is a system that consists of RDTs and central office terminals (COTs). Optical systems such as synchronous optical network (SONET) can transfer signals transparently through the COT to the RDT. The RDT provides an interface between the digital transmission line (e.g., DS1) and the customer’s access line. The RDT can dynamically assign time slots from the communication line to customer access lines.

Integrated digital loop carrier (IDLC) is a digital line interface that has been re-engineered to integrate within a switch (usually as card) and shares the internal bus structure of the switch. This function (or card) is called an integrated digital terminal (IDT). Using the IDT, the switch can directly communicate with a remote digital terminal (RDT) that is closer to the end customer using an efficient multi-channel communication line. The RDT provides an interface between the high-speed digital transmission line (e.g., DS1) and the customer’s access line. The RDT can dynamically assign time slots from the communication line to customer access lines. Because customer access lines are not used at the same time, an RDT that interfaces to a DS1 line (24 channels) usually provides service to 96 customer access lines.

The key advantages to DLC carrier systems are the cost effective transmission and the ability to rapidly add, delete, or change customer services without having to dispatch an installation technician. The DLC system offers improved efficiency through the use of existing distribution cabling systems. DLC systems also offer the ability to extend the range of access lines from the central office to the end customer as the RDT effectively operates as a repeater.

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).

DLC initially allowed 40 analog telephone connections to be extended to the remote neighborhoods using a device called an SLC-40. Later an SLC-96 (known as a “slick 96”) was put into service that allowed 96 voice grade analog circuits to be extended from the CO on just ten (10) pairs thus reclaiming 86 pairs per installation. Still in use the SLC-96 has allowed the LEC’s to conserve much of their installed outside copper infrastructure.

Unfortunately, DLC systems are not transparent to other systems such as 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.

Figure 1 shows the deployment of an integrated digital loop carrier (IDLC) application in a local telephone distribution network. This diagram shows that a switching system has been upgraded to include an IDT and an RDT has been located close to a residential neighborhood. 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 can connect to up to 96 residential telephone lines. When a call is to be originated, the RDT connects (locally switches) the residential line to one of the available channels on the DS1 interconnection line. The IDT communicates with the RDT using the GR-303 standard.


Figure 1: Integrated Digital Loop Carrier (IDLC)

System : Digital Subscriber Line (DSL)

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.

A DSL network is composed of several key parts; this includes a local access line provider, DSL access provider, backbone network aggregator, ISP provider, and other media providers. DSL services can be provided by a single service provider or may result from the combination of processes from different service providers. The communication network can be divided into several parts; local access lines (copper), voice communications network (PSTN), high-speed digital subscriber line (DSL), aggregator (interconnection), Internet service provider (ISP) and content provider (media source). These network parts and the service providers who operate them, must interact to provide most DSL services.

The physical parts of a DSL network include a subscriber access device, network access lines and digital subscriber line access module (DSLAM). There are many configuration options for a DSL network. They vary from a simple end-user’s modem bridge that connects a single end-user’s computer to the DSL network to complex multi-channel, asynchronous transfer mode (ATM) systems that connect routers and set-top boxes.

Figure 1 shows the functional parts of DSL network. This diagram shows that end user equipment adapts, or converts analog and digital signals to a high-speed DSL transmission signal via a DSL modem (an ATU-R for an ADSL system). The copper wire carries this complex DSL signal to a DSL modem at that connects to the central office (an ATU-C for an ADSL system) where it is converted back to its analog and digital components. The analog POTS portion of the signal (if any) is routed to the central office switching system. The high-speed digital portion is routed to a digital subscriber line access module (DSLAM). The DSLAM combines (concentrates) the signals from several ATU-Cs and converts and routes the signals to the appropriate service provider network.


Figure 1: DSL Network Diagram

System : Integrated Digital Services Network (ISDN)

Integrated Digital Services Network (ISDN)
A structured all digital telephone network system that was developed 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.

ISDN provides several communication channels to customers via local loop lines through a standardized digital transmission line. ISDN is provided in two interface formats: a basic rate (primarily for consumers) and high-speed rate (primarily for businesses). The basic rate interface (BRI) is 144 kbps and is divided into three digital channels called 2B + D. The primary rate interface (PRI) is 1.54 Mbps and is divided into 23B + D. The digital channels for the BRI are carried over a single, unshielded, twisted pair, copper wire and the PRI is normally carried on (2) twisted pairs of copper wire.

The “B” channels operate at 64kb per second digital synchronous rate and the “D” channel is a control channel. The D channel is used to coordinate (signal) the communication with the telephone network. When used on the BRI line, the D channel is 16kbps and when provided on the PRI channel, the D channel is 64 kbps. Because the amount of telephone system control signaling is relatively small, the D channel can also be used for low speed packet data messaging. The 64 kbps “B” channels can be used for voice and data. On the BRI system, the two B channels can be combined for 128 kbps data connection.

ISDN telephone lines exclusively use digital transmission. This requires a customer to replace their analog telephones with ISDN digital telephone equipment if they upgrade to ISDN service. ISDN service is typically provided using modular plugs. These plugs include a RJ45 interface (8 pin) for data equipment (called a BRI-S/T) and the other physical connection type is a two-wire, RJ11 type standard (called the BRI-U).

The maximum distance for a BRI-S/T line is approximately 3,000 feet and the maximum distance for the BRI-U is 18,000 feet. Beyond these distances, the service provider may install repeaters to provide service. However, repeaters are expensive to install and setup.

The ISDN BRI allows the user to change the use of the B channels whenever desired. For example, an ISDN user may be sending data using the two B channels at 128 Kbps. If a voice call comes in or is initiated, the data transmission is not interrupted; but is automatically reduced to one B channel at 64 Kbps. When the voice call ends, the data transmission returns to 128 Kbps on the two B channels.

Figure 1 provides the different interfaces that are available in the integrated services digital network (ISDN). The two interfaces shown are BRI and PRI. These are all digital interfaces from the PSTN to the end customers network termination. 1 (NT1) equipment. devices that are ISDN compatible can directly connect to the NT1 connection. Devices that require other standards (such as POTS or data modems) require a terminal adapter (TA).


Figure 1: Integrated Digital Services Network (ISDN)

Technologies : Advanced Intelligent Networks (AIN)

Advanced intelligent networks (AIN’s) are telecommunications networks that are capable of providing advanced services through the use of distributed databases that provide additional information to call processing and routing requests.

In the mid 1980’s, Bellcore (now Telcordia) developed a set of software development tools to allow companies to develop advanced services for the telephone network[4]. The advanced intelligent network (AIN) is a combination of the SS7 signaling network, interactive database nodes, and development tools that allow for the processing of signaling messages to provided for advanced telecommunications services.

The AIN system uses a service creation environment (SCE) to created advanced applications. The SCE is a development tool kit that allows the creation of services for an AIN that is used as part of the SS7 network. A service management system (SMS) is the interface between applications and the SS7 telephone network. The SMS is a computer system that administers service between service developers and signal control point databases in the SS7 network. The SMS system supports the development of intelligent database services. The system contains routing instructions and other call processing information.

To enable SCPs to become more interactive, intelligent peripherals (IPs) may be connected to them. IPs are a type of hardware device that can be programmed to perform a intelligent network processing for the SCP database. IPs perform processing services such as interactive voice response (IVR), selected digit capture, feature selection, and account management for prepaid services.

To help reduce the processing requirements of SCP databases in the SS7 network, adjunct processors (APs) may be used. APs provide some of the database processing services to local switching systems (SSPs).

Figure 1 shows the basic structure of the AIN. Companies that want to enable information services use the SMS to interface to SCP databases within the SS7 network. This diagram shows how a prepaid calling card company manages a portion of a SCP node using the SCE tool kit. The SCP is connected to an IP that contains an IVR unit that prompts callers to enter the personal identification number (PIN). The IP then reviews the account and determines available credit remains and informs the SCP of the destination number for call routing.


Figure 1: Advanced Intelligent Network (AIN)