Is a trademarked version of DSL with speeds that are a little slower than typical ADSL speeds?

II.E Digital Subscriber Line

Digital subscriber line (DSL) is a transmission technology that achieves high speeds over existing twisted pair lines. To provide DSL service, a provider installs a digital subscriber line access multiplexer (DSLAM) at the company central office (CO). This is used to aggregate multiple customer DSL lines. At the customer location, a DSL modem is installed to terminate the DSL connection.

DSL comes in a variety of flavors and transmission speeds. To date, asymmetric digital subscriber line (ADSL) has been deployed the most. It offers speeds of up to 7 Mbps downstream but significantly lower upstream speeds. It is marketed primarily to the residential market for Internet activity. High-speed digital subscriber line (HDSL) is also available. It is a T1-like service having comparable bandwidth. Symmetric digital subscriber line (SDSL) allows for symmetrical data rates of up to 2.2 Mbps, which is adequate for small and medium-sized businesses.

The fact that DSL uses existing copper wire makes it particularly attractive to ILECs. There are, however, some limitations. Higher frequency signals attenuate faster over metallic loops. DSL service is therefore limited in distance to approximately 12,000 feet. It is adversely affected by network elements commonly found on voice copper lines, such as taps and loading coils. For these reasons, DSL service has not been deployed as rapidly and extensively as the ubiquity of twisted-pair infrastructure would suggest. Companies like Rhythms, Covad, and Northpoint have positioned themselves to sell ADSL, SDSL, and HDSL to businesses around the United States. However, they often find themselves at the mercy of the ILECs who own the existing copper loops. Analysts see vast growth in residential use of ADSL. While current penetration is approximately 1% of households, Goldman Sachs, among others, predicts that DSL will be used in nearly 44.8 million homes by 2008.

Other DSL standards are emerging. These include Very High Data Rate Digital Subscriber Line (VDSL), designed to offer capacity suitable for video, and Rate-Adaptive Digital Subscriber Line (RADSL), which is able to adjust the delivery rate based on line conditions.

DSL

Digital Subscriber Line (DSL) has a “last mile” solution similar to ISDN: use existing copper pairs to provide digital service to homes and small offices. DSL has found more widespread use due to higher speeds compared with ISDN, reaching speeds of 10 megabits and more.

Common types of DSL are Symmetric Digital Subscriber Line (SDSL, with matching upload and download speeds), Asymmetric Digital Subscriber Line (ADSL, featuring faster download speeds than upload), and Very High Rate Digital Subscriber Line (VDSL, featuring much faster asymmetric speeds). Another option is HDSL (High-data-rate DSL), which matches SDSL speeds using two pairs of copper; HDSL is used to provide inexpensive T1 service.

Symmetric DSL is also called Single-Line DSL. An advantage of ADSL is that it allows the simultaneous use of a POTS line, often filtered from the DSL traffic. As a general rule, the closer a site is to the Central Office (CO), the faster the available service.

Table 5.9 summarizes the speeds and modes of DSL.

Table 5.9. DSL Speed and Distances [10]

Digital Subscriber Line

DSL is commonly denoted as xDSL, where the x specifies what type of DSL is in use. DSL is commonly used to access the Internet from both residential and business locations to provide high-speed access to the Internet. DSL became very popular as dial-up technologies become increasingly unable to meet the demand for fast access to the Internet.

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DSL and other high-speed technologies are slowly displacing dial-up service to the Internet. DSL is one of the most highly used because it can use preexisting phone lines in your home, so installation is a bit cheaper and less intrusive.

DSL is not a shared medium, unlike cable networks, which use shared access. Shared access means that when there is heavy usage of the system, less bandwidth is available to individual users. DSL has dedicated bandwidth, so the only one using that bandwidth is you.

DID YOU KNOW?

One drawback of DSL, however, is that the QoS is dependent on the user's distance from the central office (CO). The CO is where the network endpoint is located and is generally run by your Internet service provider (ISP). The farther you are from the CO, the slower the service is. There are many forms of DSL. The most common forms of DSL are asymmetric DSL (ADSL) and symmetric DSL (SDSL).

VII.B.4. xDSL Technology

Digital subscriber line (DSL) standards include a range of variations including ADSL (asymmetric digital subscriber line), HDSL (high-bit-rate digital subscriber line) and SDSL (single pair digital subscriber line). These standards support the transmission of high-speed digital data over voice-grade lines, often aiming to deliver T1 or E1 speeds more efficiently. This technology has become available in many areas and offers considerable benefits over the use of a modem for transmission of data. Benefits include the ability to support the simultaneous use of a single line for phone calls while transmitting data (web browsing, etc.). The technology also supports much higher data transmission rates. ADSL, for example, offers up to 640 Kbps upstream (e.g., for sending e-mail) and up to 9 Mbps downstream (e.g., for downloading from the Internet).

The technology, as always, is not without its challenges however. For most implementations of DSL technology, the user must be located reasonably close to the telco switching exchange office. The further the user is from the switch, the slower the transmission speeds. If the user is too far from the exchange she cannot use DSL at all.

DSL

Digital subscriber line (DSL) has a “last mile” solution that uses existing copper pairs to provide digital service to homes and small offices.

Common types of DSL are symmetric digital subscriber line (SDSL, with matching upload and download speeds); asymmetric digital subscriber line (ADSL), featuring faster download speeds than upload speeds; and very high-rate digital subscriber line (VDSL, featuring much faster asymmetric speeds). Another option is high-data-rate DSL (HDSL), which matches SDSL speeds using two copper pairs. HDSL provides inexpensive T1 service. As a general rule, the closer a site is to the Central Office (CO), the faster the available service will be.

Table 4.4 summarizes the speeds and modes of DSL.

Table 4.4. DSL Speed and Distances1

7.2.7.6 Standardizing IPTV Delivery

IPTV stressed the existing broadband connectivity model. First, PPP is not well suited to deliver a single stream to multiple subscribers (“multicast”). Second, Ethernet is emerging as the networking technology of choice for the WAN, especially for high-bandwidth applications such as IPTV.

DSL Forum TR-101 defines the technologies for delivering IPTV service as well as for using Ethernet as the underlying network technology instead of ATM. Unlike the previous DSL Forum implementation standards, TR-101 provides numerous alternatives rather than dictating a single approach. Some of the key alternatives include:

Ethernet delivery using the N:1 (or services VLAN or S-VLAN) or 1:1 (customer VLAN or C-VLAN) model. Service VLANs deliver each service (voice, video, and data) to a dedicated client such as a PC or STB and allocate a fixed amount of bandwidth to each service. Customer VLANs deliver all services to each subscriber via a single logical connection (Ethernet VLAN), allowing all bandwidth to be shared among all services.

In addition to PPP, the operator could use the simpler (but less functional) IP over Ethernet (IPoE) model. The DHCP proponents argue that the “PPP login” is no longer required, and PPP does not support multicast IPTV well. However, this simplistic argument downplays many of PPP’s benefits. PPPoE and DHCP can be used concurrently to deliver different services, even to the same subscriber.

Use of a single B-RAS to support all traffic (single edge) or the ability to use a separate edge router to support IPTV traffic (multi-edge).

DSL Pluses and Minuses

Like cable access, DSL is a viable choice for Internet access for a small or home business. The positive aspects of DSL service include:

Good performance: DSL is a dedicated line over the local loop to the CO. It therefore is not subject to the slowdowns that can occur with cable service when traffic to and from the fiber optic node is very heavy. (To be fair, most users won't see cable service slowdowns from heavy traffic; this only occurs in very high-density, heavy-usage areas.)

Reasonable cost: Depending on the speed and type of service you purchase, DSL will cost anywhere from $15 to $150 a month.

High reliability: DSL has very high uptime, with relatively few service outages.

High speed: DSL is very fast compared to dial-up access.

Note: The DSL-versus-cable access choice can be a tough one. Both cost about the same, provide good performance, don't require a dedicated phone line, and are very reliable. Often the choice is based simply on which service is available in a given area. If you have both available, then you can look at package content and prices.

DSL does have several disadvantages:

DSL is generally limited to a distance of 15,000 feet from the CO. This measurement relates to cable length rather than physical distance.

Even within the 15,000 foot limit, the farther you get from the CO, the slower the transmission speed.

DSL requires a telephone landline. If you are a “cell phone only” user, then this could be a major stumbling block.

DSL's availability is more limited than that of cable access.

Note: Which do I use? DSL. Why? Because when bundled with my telephone service, DSL is $15 a month cheaper than the cable service. (I have satellite TV rather than cable.) In addition, my premises are only 1,250 feet from a CO. The service might be rated at up to 3 Mbps, but I've had much faster downloads.

Digital Subscriber Line and Cable Modems

Cable modems and digital subscriber line (DSL) modems access technology that provides connection speeds in the megabit per second (Mbps) range. Characteristics of cable modems are as follows:

They use the broadband technology of cable television lines.

They transmit data into a coaxial-based technology, which is used to split Internet access from television signals.

The transmission speeds are typically around 1.544 Mbps, but broadband Internet can provide greater speeds allowing a download path of up to 27 Mbps.

Cable modems communicate with a cable modem termination system (CMTS) and provide a constant connection to the cable service provider that also acts in the role of an Internet service provider (ISP).

Characteristics of DSL modems are as follows:

They allow simultaneous voice and data communications.

They transmit and receive data digitally across the phone line's twisted-pair cable providing Internet access using existing telephone wiring.

They provide transmission speeds of 1.544 Mbps and can go up to data transfer rates of 6.1 Mbps.

The speed of DSL decreases the further you are from a telephone company's offices or a repeater that regenerates the signal. The closer you are to the telephone company's offices, the faster your DSL connection will be.

There are several different variations of DSL available (shown in Table 3.2), which offer different data transfer rates and distance limitations.

Table 3.2. Types of DSL

Fast Facts

An Integrated Services Digital Network (ISDN) is a system of digital telephone connections that enables data to be transmitted simultaneously end to end. It consists of multiple components:

ISDN Channels An ISDN transmission circuit consists of a logical grouping of data channels which carry voice and data. Each ISDN connection consists of two channels, a B channel and a D channel, each with their own function and bandwidth constraints. The bearer channels (B channels) transfer data and offer a bandwidth of 64 Kbps per each channel, and the data channel (D channel) handles signaling at 16 or 64 Kbps so that the B channel doesn't have to do it. This includes the session setup and teardown using a communications language known as Digital Subscriber Signalling System No. 1 (DSS1). The bandwidth available for the D channel is dependent upon the type of service – basic rate interfaces (BRIs) usually require 16 Kbps and primary rate interfaces (PRIs) use 64 Kbps. Typically, ISDN service contains two B channels and a single D channel. H channels are used to specify a number of B channels. The following list shows the implementations:

H0 384 Kbps (six B channels)

H10 1472 Kbps (23 B channels)

H11 1536 Kbps (24 B channels)

H12 1920 Kbps (30 B channels) – Europe

ISDN Interfaces There are two basic types of ISDN service:

BRI consists of two 64 Kbps B channels and one 16 Kbps D channel for a total of 144 Kbps. Only 128 Kbps is used for user data transfers. BRIs were designed to enable customers to use their existing wiring. This provided a low-cost solution for customers and is why it is the most basic type of service today intended for small business or home use. To use BRI services, you must subscribe to ISDN services through a local telephone company or provider. By default, you must be within 18,000 ft. (about 3.4 miles) of the telephone company's central office for BRI services.

PRI requires T1 carriers to facilitate communications. Normally, the channel structure contains 23 B channels plus one 64 Kbps D channel for a total of 1536 Kbps. This standard is used only in North America and Japan. European countries support a different kind of ISDN standard for PRI. It consists of 30 B channels and one 64 Kbps D channel for a total of 1984 Kbps. A technology known as Non-Facility Associated Signaling (NFAS) is available to enable you to support multiple PRI lines with one 64 Kbps D channel.

ISDN devices The standard refers to the devices that are required to connect the end node to the network.

ISDN reference points They are used to define logical interfaces. They are, in effect, a type of protocol used in communications. The following list contains the reference points:

R defines reference point between a TE2 device and a TA device.

S defines reference point between TE1 devices and NT1 or NT2 devices.

T defines reference point between NT1 and NT2 devices.

U defines reference point between NT1 devices and line termination equipment. This is usually the central switch.

ISDN identifiers They use five separate identifiers when making a connection. The provider assigns two of these when the connection is first set up: the service profile identifier (SPID) and the directory number (DN). These are the most common numbers used because the other three are dynamically set up each time a connection is made. The three dynamic identifiers are terminal endpoint identifier (TEI), bearer code (BC), and service access point identifier (SAPI).