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

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

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)