PDF of this tutorialThere are three major categories of twisted-pair solutions that are being used for Internet access (see Figure 4):
- voiceband data (VBD) modems
- ISDN digital subscriber line (DSL)
- other DSL approaches (xDSL)
Figure 4. VBD and ISDN
VBD modems are well known and understood by residential and small-business users. They operate by using the voice-frequency band of the twisted-pair facility to transmit data, using frequency shift keying (FSK) or quadrature amplitude modulation (QAM) transmission techniques. Symmetric rates exist up to 33.6 kbps, with the majority running at 14.4 and 28.8 kbps. Emerging is an asymmetric capability with a nominal server rate of 56 kbps and return-path rate operating up to 33.6 kbps.
Integrated services digital network (ISDN) is a digital baseband technology that operates with a 144–kbps bidirectional payload rate using 2B1Q encoding scheme. The 144–kbps rate is divided into two 64–kbps (B) channels and one 16–kbps (D) channel. The B channels can be used for two separate voice calls, two 64–kbps data calls, a separate voice and data call, or a combined 128–kbps data call. The wire limit for ISDN is 18,000 feet on standard twisted pair.
xDSL Technologies
A variety of xDSL rates and technologies have been standardized, or are in the process of standardization, by American National Standards Institute (ANSI) and the Asymmetric Digital Subscriber Line (ADSL) Forum. As Figure 5 shows, the higher rates are for customers that are a short distance away from the network provider's xDSL modem. This modem may be located either in a central office (CO) or at a remote terminal site closer to many end users.
Figure 5. xDSL Downstream Rates
ADSL
ADSL is one of several types of xDSL technologies. ADSL has two main standards: ADSL–1 specifies a downstream rate of 1.5 or 2 Mbps and an upstream rate of 16 to 64 kbps; ADSL–3 specifies a downstream rate of up to 6.144 Mbps and a bidirectional channel of up to 640 kbps.
Good twisted-pair lines with no bridged taps can support ADSL–1 rates up to 18,000 feet (24-gauge wire), and ADSL–3 up to 12,000 feet.
ANSI and the ADSL Forum have endorsed discrete multitone (DMT) technology. However, carrierless amplitude and phase (CAP) technology has the most market-share thus far, with 30 times as many ADSL lines using CAP. DMT and CAP modems are incompatible, but the issue is not nearly as great as with VBD modems. VBD modems must be compatible end-to-end, from end user to end user. But ADSL modems only operate over the end user's twisted pair, from end user to network provider.
VDSL
Very-high-speed DSL (VDSL) promises even higher speeds than ADSL, although over much shorter distances. Standardization is underway in four different standards bodies: ANSI, the ADSL Forum, the ATM Forum, and the Digital Audio-Visual Council (DAVIC). There are four different technologies proposed (CAP, DMT, DWMT, and subscriber line charge [SLC]), aiming at a goal of lower power and less cost than ADSL.
RADSL
As the name implies, rate-adaptive DSL (RADSL) modems adjust the data rate to match the quality of the twisted-pair connection. Emerging software should make this an automated process with little human intervention.
HDSL and SDSL
High-data-rate DSL (HDSL) modems transmit 1.5 Mbps in each direction. Two twisted pairs of wires are used, with half of the traffic on each pair. A 2.0–Mbps transmission rate is also available, using three pairs of wires (one-third of the traffic on each pair). The wire limit is 12,000 feet (24 ga.) or 9000 feet (26 ga.)
Symmetrical digital subscriber line (SDSL) is similar to HDSL but requires only one pair of wires. Transmission speed ranges from n x 64 kbps to 2.0 Mbps in both directions.
HDSL and SDSL are intended as lower-cost replacements for dedicated T1 and fractional–T1 lines, rather than for residential access.
