Symmetric DSL requirements
ADSL (Asymmetric Digital Subscriber Line) assigns different downstream and upstream rates. Leased-line services and enterprise links often require the same rate in both directions for inter-site data transfer, voice transport, or network access.
HDSL (High bit rate Digital Subscriber Line) and SDSL (Symmetric Digital Subscriber Line) provide bidirectional symmetric transmission. Their standards differ in line code, pair arrangement, payload rate, and treatment of analogue telephone service.
HDSL line options
HDSL (High bit rate Digital Subscriber Line) is specified by ITU-T G.991.1 for bidirectional symmetric transmission at 1544 or 2048 kbit/s over copper twisted pairs. It was designed for access sections carrying T1 or E1 services over metallic loops. use the corresponding regional digital hierarchy rate.
HDSL line characteristics
- Symmetric rates: G.991.1 defines 1544 kbit/s and 2048 kbit/s application rates. The selected rate is present in both transmission directions.
- Pair arrangement and line code: For 2048 kbit/s, 2B1Q supports duplex operation on one pair or parallel transmission on two or three pairs. The 1544 kbit/s 2B1Q application uses two pairs, while CAP is defined for one or two pairs at 2048 kbit/s.
- POTS interaction: The HDSL line arrangement does not define an analogue splitter for sharing the same pair with POTS. ETSI TS 101 135 separately specifies a digital mapping that can transport analogue telephone connections inside an HDSL core frame.
- Applications: Typical uses include T1 or E1 leased services, access digital sections, and business links that require a fixed symmetric rate.
SDSL and SHDSL on one pair
SDSL (Symmetric Digital Subscriber Line) is the name used by ETSI TS 101 524 for bidirectional symmetric transmission on one metallic pair in its basic mode. The specification also defines an optional four-wire mode with two active pairs. The transceivers use echo cancellation and are intended for existing unshielded access pairs.
SHDSL (Single-pair High-speed Digital Subscriber Line) is specified by ITU-T G.991.2. It is a related ITU-T standard with selected symmetric rates, optional multi-pair operation, and spectral-compatibility requirements.
Defined SDSL and SHDSL options
- Pair operation: ETSI TS 101 524 defines SDSL on one metallic pair and specifies an optional four-wire mode with two active pairs. G.991.2 permits optional multi-pair operation up to four pairs and describes SHDSL primarily for duplex operation on a mixed-gauge two-wire twisted pair.
- Standardized rates: G.991.2 defines selected single-pair payload rates from 192 kbit/s to 2312 kbit/s in 8 kbit/s steps. Annex F extends the range to 5696 kbit/s. In two-wire mode, Table 7.1 of ETSI TS 101 524 covers user payload rates from 192 kbit/s to 2312 kbit/s. The frame adds 8 kbit/s of overhead, so the total line rate is 200 kbit/s to 2320 kbit/s. The PSD mask description, including Figure 9.11, also lists selected profiles of 256, 512, 768, 1536, 2048, and 2304 kbit/s, so 256 kbit/s is not the lower limit of the complete ETSI range.
- Multi-pair option: G.991.2 permits optional operation on up to four pairs to increase the available transmission capacity. In ETSI TS 101 524, four-wire mode uses two pairs and interleaves successive frame blocks between pair 1 and pair 2. The reference configuration supports aggregate user payload rates from 384 kbit/s to 4624 kbit/s in 16 kbit/s steps in this mode.
- Line conditions: Reach depends on conductor gauge, attenuation, crosstalk, and noise. G.991.2 defines test conditions and optional regenerators instead of one universal distance.
Summary Comparison and Application
The choice between ADSL, HDSL, and SDSL or SHDSL depends on the required direction symmetry, payload rate, pair availability, and service architecture.
| Feature | ADSL/ADSL2+ | HDSL | SDSL / SHDSL |
|---|---|---|---|
| Symmetry | Asymmetric (High Download, Low Upload) | Symmetric | Symmetric |
| Copper Pairs | 1 pair | 1 pair or 2/3 pairs, depending on rate and line code | 1 pair in SDSL, optional two-pair ETSI four-wire mode, and up to 4 pairs in G.991.2 |
| POTS Coexistence | Yes (with splitter) | No analogue splitter | No analogue splitter |
| Primary Application | Residential Internet | Leased T1/E1 and access sections | Symmetric business access and digital transport |
HDSL provides fixed symmetric 1544 or 2048 kbit/s services with pair and line-code options defined by G.991.1. ETSI SDSL defines a basic single-pair mode and an optional two-pair four-wire mode, while ITU-T SHDSL describes single-pair symmetric transmission with selected rates. G.991.2 adds optional operation on up to four pairs and regenerators, while POTS transport requires an application-specific digital function or a separate voice path.