Title:        *DSL Reference
By:           James Bensley
Created:      07/12/2012
Last Updated: 05/01/2016
Corrections:    jwbensley <at> gmail <dot> com
    

xDSL Comparison
ADSL1 Reference
ADSL1 Topology Models
ADSL1 Protocol Stack
ADSL2+ Reference
SHDSL Reference

References:

• Frequencies: ITU G.992.5 (2009). SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS. Asymmetric digital subscriber line transceivers 2 (ADSL2) – Extended bandwidth (ADSL2plus). Available: https://www.itu.int/rec/T-REC-G.992.5/en, Last accessed 25/12/2014.
• Frequencies: ITU G.992.3 (2009). SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS. Asymmetric digital subscriber line transceivers 2 (ADSL2). Available: https://www.itu.int/rec/T-REC-G.992.3/en, Last accessed 25/12/2014.
• PMD, PMS-TC & TPS-TC: ITU G.992.3 (2009). SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS. Asymmetric digital subscriber line transceivers 2 (ADSL2). Available: https://www.itu.int/rec/T-REC-G.992.3/en, Last accessed 25/12/2014.

Physical Media-Dependent (PMD) Function
TPS-TC is the Transport Protocol-Specific Transmission Convergence (TPS-TC) function. The TPS-TC is application specific and consists largely of adaptation of the customer interface data and control signals to the (a)synchronous data interface of the TPS-TC. Additionally, the TPC-TC layer may also generate or receive control messages via the overhead channel of the PMS-TC layer.

PMS-TC is the Physical Media-Specific Transmission Convergence (PMS-TC) function. The primary purpose of the ATU PMS-TC function is to provide for the multiplexing and transport of several channels of information. The PMS-TC layer contains the framing and frame synchronization functions, as well as forward error correction, error detection, scrambler and descrambler functions. Additionally, the PMS-TC layer provides an overhead channel that is used to transport control messages generated in the TPS-TC, PMS-TC or PMD layers as well as messages generated at the management interface.

PMD is the Physical Media-Dependent function and it provides procedures for transporting a bitstream over the physical medium (i.e., over the copper pairs) in both the upstream and downstream directions. The transmit PMD function accepts data from the transmit PMS-TC function and the receive PMD function and delivers data to the receive PMS-TC function.

The principal functions of the PMD layer may include symbol timing generation and recovery, encoding and decoding, modulation and demodulation, echo cancellation (if implemented) and line equalization, link startup and physical layer overhead (superframing). Additionally, the PMD layer may generate or receive control messages via the overhead channel of the PMS-TC layer. In addition to transport functionality, the PMD transmit function also provides procedures for: tone ordering, constellation encoder, synchronization and L2 exit symbols, modulation, transmitter dynamic range, transmitter spectral masks (including spectrum shaping), conversion to analogue signal for transmission over the DSL, on-line adaptation and reconfiguration.

The transmit PMD function shall transmit 4000 data symbols per second. For each data symbol, the transmit PMD function requests and receives a constellation encoder input data frame (containing L data bits) from the transmit PMS-TC function. The data frame shall then be constellation encoded. After constellation encoding, the output data frame shall be modulated into a data symbols to produce an analogue signal for transmission across the digital subscriber line. The one-way payload transfer delay introduced by the PMD sublayer shall be less than or equal to 3.75 ms. NOTE – The one-way payload transfer delay is shared between the ATU-C and the ATU-R. The transmit PMD function shall use the superframe structure. Each superframe shall be composed of 68 data frames, numbered from 0 to 67, which are encoded and modulated into 68 data symbols, followed by a synchronization symbol, which carries no data frame and is inserted by the modulator to establish superframe boundaries. From the PMS-TC perspective, the data symbol rate shall be 4000 per second (symbol period = 250 μs) but, in order to allow for the insertion of the synchronization symbol, the transmitted data symbol rate is 69/68 × 4000 per second. The superframe duration shall therefore be 17 ms.

ITU G.992.5 Annex A Frequncies - ADSL2+ over PSTN with POTS
ITU G.992.3 Annex A "ADSL2" (G.dmt.bis) supports a maximum downstream rate of 12Mbps using a maximum PSTN frequency of 1104kHz (the same as ADSL) using imrpoved the Reed-Solomon encoding.
ITU G.992.5 Annex A "ADSL2+" (G.dmt.bis+) supports a maximum downstream rate of 24Mbps by doubling the maximum downstream frequency range of ADSL/ADSL2 up to 2208kHz, which doubles the total number of bins from 256 to 512. 26 bins are for upstream (6-32, although 16 and 32 aren't used), 478 bins are for downstream (33-511, although 511 & 64 aren't used).

• PSTN Bandwidth: 25.875kHz - 2208kHz
• Modulation: Discrete Multi-Tone (DMT)
• Max sub-carriers bands "bins": 512 (4.3125KHz wide, *512=2208Khz)
• Downstream:
  • Downstream sub-carriers: 478 (running alongside POTS)
  • Max usable sub-carriers downstream: 476 (sub-carrier 511 [the downstream Nyquist frequency] and 64 [the downstream pilot frequency] aren't used for data)
  • Max sub-carrier modulation per baud: 0 - 15 bits
  • Sub-carrier baud rate: 4000 symbols per second
  • Using all downstream bins with POTS support with 15 bits per bin gives the theoretical maximum wire-rate of 476*15*4000=14640000bps (14.6Mbps)
  • The Reed-Solomon cyclic error-correcting codes limits each transmitted ADSL superframe to 255 bytes,
    using 16 bytes of parity with a symbol size of 8 bits, RS(255,240). This means that only 8 bits per bin are used.
  • Using all downstream bins with POTS support with 8 bits per bin gives the theoretical maximum transfer rate of 476*8*4000=7136000bps (7.13Mbps)
  • The bins and spare sync space above the user data bins ending at 2208kHz are the OOB and FEC channels.
• Upstream:
  • Upstream sub-carriers: 26 (running alongside POTS)
  • Max usable sub-carriers upstream: 24 (sub-carrier 32 [the upstream Nyquist frequency] and 16 [the upstream pilot frequency] aren't used for data)
  • Max sub-carrier modulation per baud: 0 - 15 bits
  • Sub-carrier baud rate: 4000 symbols per second
  • Using all upstream bins with POTS support with 15 bits per bin gives the theoretical maximum wire-rate of 24*15*4000=1440000bps (1.44Mbps)
  • The Reed-Solomon cyclic error-correcting codes limits each transmitted ADSL superframe to 255 bytes,
    using 16 bytes of parity with a symbol size of 8 bits, RS(255,240). This means that only 8 bits per bin are used.
  • Using all downstream bins with POTS support with 8 bits per bin gives the theoretical maximum transfer rate of 24*8*4000=768000bps (768Kbps)

Frequency Overview (with POTS):
Each sub-carrier is centered on the frequency N*4.3125Khz

Sub-Carrier
     0 POTS      End of POTS    Gaurd Band Start    Gaurd Band End     Upstream Start    Upstream Pilot    Upstream End    Downstream Start    Downstream Pilot     Downstream End    Nyquist Frequency
     |----------------1-----------------2------------------5------------------6----------------16----------------32----------------33-----------------64------------------511-----------------512---|  
     |--------------4.3125------------8.625-------------21.5625------------25.875--------------69---------------138-------------142.3125--------------276---------------2203.6875------------2208---|  
     0
   kHz