| The basis systems are loop transmission systems deployed in the network that provide the basis for establishing spectral compatibility criteria for within a loop cable. Selection of the basis systems list required industry consensus in order to be adopted in the spectrum management standard. The basis systems list per the first issue of the spectrum management standard T1.417 are as follows : -
Voice grade services, which include analog voice service, network signaling, data (e.g., via voice- band modem) and tones that utilize the frequency from 0 to approximately 4 kHz. -
Digital data service (DDS) based on signals defined in T1.410-1992. These signals use alternate mark inversion at a maximum line rate of 72 kb/s. The services supported are 64 kb/s and sub-rates of 2.4, 4.8, 9.6, 19.2, 38.8, and 56 kb/s. -
Basic rate ISDN (BRI) based on T1.601-1999. The basic rate ISDN signal uses 2B1Q line and operates a line rate of 160 kb/s. -
High-bit-rate digital subscriber line (HDSL) based on G.991.1 Annex A. HDSL was designed to transport a DS1 payload across two wire pairs, each carrying one-half of the DS1 payload plus overhead, at a distance within carrier serving area (CSA) range. Recall that CSA has a radius of 12 kft on 24-gauge wire and 9 kft of 26-gauge wire. -
Asymmetrical digital subscriber line (ADSL) based on T1.413-1998 with nonoverlapped (reduced near-end crosstalk) upstream/downstream mode in G.992.1 Annex A. ADSL uses DMT modulation to transmit upstream and downstream signal in non- overlapped frequency bands. -
Rate-adaptive asymmetric digital subscriber line (RADSL) based on Committee T1 TR.059-2000. RADSL uses the same frequency bands as ADSL but it uses single carrier modulation, namely carrierless AM/PM (CAP) or quadrature amplitude modulation (QAM). -
Splitterless ADSL based on T1.419-2000. Splitterless ADSL uses the same DMT modulation as conventional ADSL, but the downstream channel is limited to approximately one-half the bandwidth of ADSL. -
HDSL2 single pair HDSL transporting a DS1(1.544 Mb/s) payload based on T1.418-2000. -
2B1Q-based single pair symmetric DSL (SDSL) at bit rates of 400 kb/s, 1040 kb/s, and 1568 kb/s. -
Enhanced business services (P-Phone) based on interface specification NIS S102-2, Issue 02.02 October 1998. This service utilizes the spectrum from 0 to 10 kHz providing for providing voice grade services, but the signaling is provided on a 8 kHz carrier. The most notable criterion for systems on the above basis systems list is that each of the systems have been widely deployed and they are expected to be widely deployed, in the foreseeable future. There are some exceptions that need to be considered . Some technologies are old and are being replaced by newer technologies, where the newer technologies would have better spectral characteristics for the same quality of service than the older technologies. An example is 2B1Q SDSL will be replaced on the basis systems list with G.shdsl at an appropriate point in time. Another exception has to do with the T1 AMI signal, which is known to be a strong disturber into ADSL. Although many T1 AMI systems have been deployed in the network, it is expected that future deployments of such system would be very limited in an unbundled loop environment. Equivalent services offered with T1 AMI would be provisioned with HDSL or G.shdsl technologies, which are less disturbing to ADSL than T1 AMI. The basis systems list above uses frequencies up to approximately 1.1 MHz, which is determined by the downstream ADSL signal spectrum. There are future DSL systems being developed that will utilize frequencies above 1.1 MHz, such as very-high bit-rate digital subscriber line (VDSL). It is expected that VDSL will become a basis system once the standard is complete and large-scale deployments occur. For any new technology to be added to the basis systems list, the spectrum management standard T1.417 specifies the following factors in its consideration (taken from Section 4.3.1 of T1.417): -
It is highly preferred that the system be standardized by the ITU or an ANSI accredited standards organization or that a draft standard is expected to be approved by the time the forthcoming issue of the Spectrum Management standard is expected to be published. If an effort has been made to standardize the system and there is a clear reason why the system cannot be standardized, then a physical-layer specification shall be publicly available. -
The specification for a non-standard system shall be stable, widely accepted by most of the industry, and shall specify all aspects necessary to determine spectral compatibility (e.g., transmitted signal PSD, modulation method, coding, bit-rate, start-up process, and margin to be achieved for certain reference loops and reference noise). -
Preferably, a new basis system should not require changes to the existing spectrum management classes (including PSD definition, total average power, transverse balance, longitudinal output voltage and deployment guidelines) to maintain spectral compatibility with the new basis system. -
Preferably, a new basis system should not be adequately addressed by the existing systems on the basis system list. -
New basis systems should demonstrate possible scenarios where the new system could be disturbed while other basis systems are not. In the second issue of T1.417, G.shdsl has been added to the basis systems list, replacing 2B1Q SDSL. It is expected that VDSL will be added to the basis systems list once standardization is complete and systems are deployed. Note that spectral compatibility is not an explicit criterion for basis systems. Alhough it is not necessary for a basis system to be spectrally compatible, in practice the basis systems generally are spectrally compatible with each other. |
