xDSL in Brief
To fully appreciate the significance of ADSL technology, it is important to understand how the xDSL’s differ. Let us briefly looked at each member of the xDSL family.
High-bit-rate Digital Subscriber Line (HDSL)
HDSL offers symmetric service, where the upstream bit rate is equal to the downstream bit rate. However, the use of HDSL is limited to telephone networking areas satisfying the carrier serving area (CSA) specifications. The CSA limits the length of the subscriber line according to the diameter of its copper wire, and also defines constraints on the length of bridge taps. HDSL will operate over CSA’s without intermediate repeaters, which implies a distance limited to 3600m. This is the most mature of all DSL technologies, with two standard rates of 1.544Mbps and 2.048Mbps already in use. Due to speed, some telephone companies have deployed HDSL as an alternative to T1/E1. HDSL requires two pairs of copper wires to operate, and because the up/downstream are symmetric, its maximum speed is not as great as for ADSL. Its application include high speed LAN access and Internet server access.
Single-line Digital Subscriber Line (SDSL)
SDSL is a variation on the HDSL technology, utilizing a single wire pair instead of two. It has the same speed as HDSL but can only operate over a shorter distance. The advantage of SDSL over HDSL is that individual subscriber premises are often equipped with only one telephone line. Transmitting symmetrically, it can complement ADSL in applications that require symmetric access.
Asymmetric Digital Subscriber Line (ADSL)
ADSL is an extension of HDSL. ADSL deals with asymmetric traffic in which the downstream rate is much greater than the upstream rate. This can provide a cost-effective way to offer multimedia services to the end user via the existing telephone networks. Unlike HDSL, ADSL technology is not limited to CSA specifications, and thus proves to be of huge potential in commercial markets. This technology allocates most of its available bandwidth to the downstream traffic and less bandwidth to upstream. Bit rates range from 1.5-8Mbps downstream and 1.544Mbps upstream. This makes ADSL suitable for applications where the speed of the downstream channel is crucial, and restricts the upstream channel to give simple commands/requests or other less speed critical data transfers. Applications include Internet access, LAN access, Video-on-Demand (VoD), video broadcasting, and interactive multimedia. ADSL based services will be "always connected" just like a cable modem, and its operation will not affect the analog telephone. Unlike cable modems, ADSL modems do not suffer from reduced bandwidth due to line sharing. Each ADSL customer has its own dedicated copper line!
Rate adaptive Digital Subscriber Line (R-ADSL)
This type of DSL technology operates at a bit rate that is dynamically adjusted according to the length and quality of the twisted pair lines. It is also possible for the end user to set the data rate. Bit rates range from 1.5-8Mbps downstream and 1.544Mbps upstream.
Very-high-bit-rate Digital Subscriber Line (VDSL)
VDSL is an extension of ADSL and is the fastest xDSL technology. It may operate either symmetrically or asymmetrically over a single POTS subscriber line or a basic ISDN access line. The concept of VDSL is based on the fact that the potential capacity of copper loops exceeds 10 Mbps at relatively short distances. Hence, with the development of optical fibres into the distribution plants, the remaining copper can be used by VDSL. Bit rates range from 13-52Mbps downstream and 1.5-2.3Mbps upstream, or 34Mbps if symmetric. An advantage of VDSL over other DSL technologies is that it is able to support High Definition TV (HDTV), which requires a rate of 24Mbps.
The following table lists all the xDSL technologies as well as 56kbps analog modems, ISDN, and cable modems for comparison. Note the compromise between the distance limitation and the speed!
Technology and Application Comparison
Technology
Speed
Distance Limitation
(24-gauge wire)Applications
56 Kbps analog modems
56 Kbps downstream
None
28.8 or 33.6 Kbps upstreamE-mail, remote LAN access, Internet/intranet access
ISDN
Up to 128 Kbps (uncompressed) Full duplex
18,000 feet (additional equipment can extend the distance)
Video conferencing, disaster recovery, leased line backup, transaction processing, call center services, Internet/intranet access
Cable modem
10–30 Mbps downstream 128 Kbps–10 Mbps upstream (shared, not dedicated, bandwidth)
30 miles over coaxial (additional equipment can extend the distance to 200 miles)
Internet access
ADSL Lite
Up to 1 Mbps downstream
Up to 512 Kbps upstream18,000 feet
Internet/intranet access, Web browsing, IP telephony, video telephony
ADSL/R-ADSL
1.5–8 Mbps downstream Up to 1.544 Mbps upstream
18,000 feet (12,000 feet for fastest speeds)
Internet/intranet access, video-on-demand, remote LAN access, VPNs, VoIP
IDSL
Up to 144 Kbps full duplex
18,000 feet (additional equipment can extend the distance)
Internet/intranet access, Web browsing, IP telephony, video telephony
HDSL
1.544 Mbps full duplex (T1)
2.048 Mbps full duplex (E1)
(uses 2–3 wire pairs)12,000–15,000 feet
Local, repeatered T1/E1 trunk replacement, PBX interconnection, Frame Relay traffic aggregator, LAN interconnect
SDSL
1.544 Mbps full duplex (T1)
2.048 Mbps full duplex (E1)
(uses 1 wire pair)10,000 feet
Local, repeatered T1/E1 trunk replacement, collaborative computing, LAN interconnect
VDSL
13–52 Mbps downstream
1.5–2.3 Mbps upstream
(up to 34 Mbps if symmetric)1,000–4,500 feet
(depending on speed)Multimedia Internet access, high-definition television program delivery
Table 2. Technology comparision Source: [1]
