Reference models - Model of the ADSL (Asymmetric Digital Subscriber Line)

Content:

• Block diagram
• Legend of the block diagram
• Reference points:
• ADSL U interface, ADSL transmitter/receiver
  • Line (ADSL) layer
    • Modulation
    • Spectrum of the ADSL line layer
    • Basic types of ADSL transceivers
    • Block diagram of the transmitter unit of ADSL DSLAM/modem
    • Schematic review of the ADSL frame
    • Main transmission parameters of line layer
    • Initialization
    • Power Management
    • Testing
  • Data path layer
  • Channel layer
    • VC, LLC multiplexing, PPPoA

The following figure shows the theoretical build-up of the ADSL system doing the connection of the subscriber LANs to the broadband backbones:

Its practical implementations can be found in the xDSL subnetwork of the telecommunication test network.

Legend:

• Broadband network - Broadband packet-switched network (Internet). On the border of this network and the ADSL system is located the router of the provider.
• Narrowband network - This is the name of the classical PSTN/ISDN network here. Here can be found the exchange of the provider.
• _{POTS - Plain Old Telephone Service - Name of the classical telephone service used in the ADSL.}
• DSLAM - DSL Access Multiplexer - Name of the network adapter device of the ADSL system. Parts of the equipment:
  • Broadband Interface - Broadband packet-switched adapter of the ADSL system.
    
    • Classical ADSL network access: STM-1 (optical) interface, payload isATM, IP packets are carried by ATM cells.
      
    • Network access of the improved ADSL (ADSL2, ADSL2+) DSLAMs is Ethernet (10/100/1000BASE-T).

  • The ADSL V interface
    Physical interface:
    • 10/100 Ethernet (IEEE 802.3)
    • ATM

  • Switch - Its task is the direction of the IP traffic towards the subscribers.
    
    • In case of the classic ADSL network connection the switch is an ATM switch
      With the help of VPI/VCI route identifiers located in the header of the ATM cells IP packets can be directed towards the subscribers simply, that is why the solution is so prevalent.
    • The switch function of the improved ADSL (ADSL2, ADSL2+) DSLAMs is made by an LAN switch. The direction of IP packets happens based on the VLAN IDs in tagged MAC frame headerben.
    Alignment of VPI/VCIs, VLAN IDs and user IP addresses happens in the router of the provider.

  • ATU-C - ADSL Transceiver Unit Central Office Side - ADSL transciever (modem) set, central signal transformer
    (Physically 16-32 transcievers are located in one unit (on a card)).
    Basic version of ATU-C was primarily developed for the fast Internet access. However, later appeared the improved versions of the basic version.
    • ADSL2 direction of the development was to support the forwarding of VoIP and a streaming information (video) here. Moreover, an ADSL2 ATU has numerous extra services compared to an ADSL ATU.
    • ADSL2+ knows all, that ADSL2 knows with twofold data transmission rate.

  • S - splitter - electronic filter.
    _{The signals of two services - the ADSL and the PSTN/ISDN - locate besides aech other in the frequency domain and can be separated with these filters.}

  • Management Network - Managing system of the ADSL network
    Cards of a DSLAM do not contain any handling gadget (DIP switch, jumper, potentiometer etc.) this device can be remote controlled entirely.
    (Managing system can simply be a terminal, but it can also be a computer network.) Managing methods:
    • Managing system keeps the connection through SNMP (Simple Network Management Protocol) with the DSLAM.
    • Furthermore, DSLAM can be managed through the WEB page.
    • Management of the ADSL modem can be realized through the U interface on the service channel embedded to ADSL frames (EOC - Embedded Operational Channel).
    • Furthermore, modem vendors equip modems with management port, too.

• CPN - Equipments on the subscribers' side:
  • ADSL modem. Its parts are the following:
    • ATU-R - ADSL Transceiver Unit Remote terminal end - ADSL transmitter/receiver, remote subscriber adapter (ADSL modem)
    • S - splitter - filetr for separation of the signal of the telephone and the ADSL
    • Modem Interface S-reference point - Adapter of the subscriber network.
      Physical interfaces:
      • 10/100 Mbit/s multiport Ethernet (router or bridge function)
      • 25,6 Mbit/s ATM
      • USB
      • WLAN

  • CPN - Customer Premise Network - Network at the user's site
    _{ADSL realize a LAN Bridge between CPN and Broadband Network .}

  • Narrowband user - PSTN/ISDN subscriber terminal
    _{(eg. telephone) or network (eg. ISDN subscriber network).}

  Reference points:

  Individual elements of the network connect to each other through standard, so called reference points (interface specifications are given to this points). Marks of these reference points are similar to marks applied on the ISDN subscriber networks:

  Q reference point	Reference point of the managing network. (Do not confuse it with the Q reference point of the ISDN subscriber network!)
  S reference point	An interface connecting subscriber devices with each other.
  T reference point	Boundary point of the subscriber network and the provider network .
  U reference point	An interface connecting the subscriber network with the provider.
  V reference point	Access point of the centre of provider.

  ADSL U interface, ADSL transceivers

  Application:

  ADSL	ADSL2, ADSL2+
  fast internet access	fast internet access speech transmission (VoIP) video transmission (streaming)

  Data transmission rate:

  Within the ADSL system the long distance digital signal transmission can a be realized on the U interface. It is called "asymmetric" because the data transmission rates are different at downstream (towards the subscriber) and upstream (towards the network) directions:

  -	ADSL	ADSL2	ADSL2+
  At downstream (network->subscriber) direction max:	8 Mbit/s	12 Mbit/s	25 Mbit/s
  At upstream (subscriber->network) direction max:	1024 kbit/s	1200 kbit/s	1200 kbit/s

  Two-direction duplex signal transmission:

  U interface generates a duplex subscriber digital line section with the ATU-R (NT1) subscriber network adapter, and with the ATU-C central adapter. Signal of the ADSL goes toward together with the telephone line signal of the common PSTN/ISDN on the subscriber copper pairs. At a duplex signal transmission - the frequency division technique is applied for the transmission of signals of the two directions at the same time, and for the separation of the signal of the PSTN/ISDN from the signal of the ADSL.

  Sublayers of U interface:

  Physical layer of the ADSL U interface according to OSI model consists of the following sublayers:

  line (ADSL)	line (ADSL) layer (bottom layer)	(DMT, ADSL frame)
  channel	channel layer	(fast, interleaved)
  data path	data path layer (upper layer)	(STM, ATM, (ADSL2, ADSL2+:) Packet)

  There are several notations for the identification of the two sides of the interface:

  
    C - Central office side            R - Remote end
        (Network side)                (Subscriber side)
  
         +-------+  subscriber coppe pair +-------+
         | ATU-C | ---------------------- | ATU-R |
         +-------+                        +-------+
                      downstream -->
                      <-- upstream
  
         +-------+                        +-------+
         | LTU   | ---------------------- | NTU   |
         +-------+                        +-------+
  
         +-------+                        +-------+
         | DSLAM | ---------------------- | MODEM |
         +-------+                        +-------+
  
         +-------+                        +-------+
         | COE   | ---------------------- | CPE   |
         +-------+                        +-------+
  
  

  • ATU-C - ADSL Transceiver Unit Central office side - ADSL transceiver, central side adapter
  • ATU-R - ADSL Transceiver Unit Central Remote terminal end - ADSL transceiver, remote subscriber side adapter
  • LTU - Line Terminating Unit - (DSLAM)
  • NTU - Network Terminating Unit - (ADSL modem)
  • DSLAM - Digital Subscriber Line Access Multiplexer (LTU)
  • COE - Central Office End - (DSLAM)
  • CPE - Customer Premise Equipment - Equipments at customer's site.

  ADSL-U Line (ADSL) layer

  • Modulation
  • Spectrum of the ADSL line layer signal
  • Basic types of ADSL transceivers
  • Block diagram of the ADSL DSLAM/modem transmission unit
  • Schematic review of the ADSL frame
  • Main transmission parameteres of the line layer
  • Initialization
  • Testing

  Modulation

  In the ADSL system DMT (Discret MultiTone) modulation is in use.

  frekvency raster:	4.3125 kHz
  number of DMT channel bearer frequencies: (ADSL, ADSL2)	256 #0...255
  number of DMT channel bearer frequencies: (ADSL2+)	512 #0...511
  Baud rate (data symbol rate) in the individual DMT channels:	4 kBaud (250 us)
  number of forwarded bits in the individual DMT channels:	2...15 Actual value in the individual channels depends on the value of signal-to-noise ratio. The concrete value is determined during the initialization.
  estimated channel capacity (net data rate):	ADSL Down: 256 * 8 * 4000 = 8192 kbit/s (we can forward 8 bits in every channel) ADSL2 Down: min 8 Mbit/s, Up: 800 kbit/s ADSL2+ Down: min 16 Mbit/s, Up: 800 kbit/s
  modulációs technika:	IDFT (Inverse Discrete Fourier Transform) It determines samples of the time function from the complex amplitude and phase values of signals to be forwarded in the individual channels.

  Spectrum of the ADSL line signal:

  as it can be found in the literature:

  

  and as it can be seen on the screen of the managing system.

  Basic types of ADSL transceivers:

  • ISDN/POTS - Under frequency band of the ADSL, ISDN or PSTN telephone services can operate. Separation of these two services can be made with splitters in the frequency domain.
  • EC/FDD - Echo cancelled (overlapped)/Frequency Division Distibution - Separation of upstream and downstream signals can be made with echo canceller/ electronis filter.

  Data of line spectrum for the individual basic types:

  ADSL type	direction	lower freq. [kHz]	upper freq. [kHz]	number of DMT bearers
  ADSL, ADSL2 EC POTS	downstream	25,875	1104	249 #6...255
  ADSL2+ EC POTS	downstream	25,875	2208	505 #6...511
  ADSL, ADSL2 FDD POTS	downstream	138	1104	223 #32...255
  ADSL2+ FDD POTS	downstream	138	2208	479 #32...511
  ADSL, ADSL2, ADSL2+ FDD/EC POTS	upstream	25,875	138	26 #6...32
  ADSL, ADSL2, EC ISDN	downstream	120	1104	227 #28...255
  ADSL2+ EC ISDN	downstream	120	2208	483 #28...511
  ADSL, ADSL2, FDD ISDN	downstream	254	1104	197 #59...255
  ADSL2+ FDD ISDN	downstream	254	2208	453 #59...511
  ADSL, ADSL2, ADSL2+ FDD/EC ISDN	upstream	120	276	36 #28...64

  Notes:
  1) There is a version for the ADSL2, ADSL2+ POTS modems worked out with an enhanced range (Reach Extended ADSL). Spectrum of the downstream signals of these modems differ from the spectrum of the signals of basic modems.
  2) There is a version for ADSL2, ADSL2+ POTS modems worked out with an extended upstream channel capacity (Extended Upstream Bandwidth). Spectrum of the upstream signals of these modems differ from the spectrum of the signals of basic modems.
  3) All digital mode - The whole frequency domain is used for an ADSL transmission, there is neither POTS, nor ISDN service.

  Data rate definitions:

  • net data rate: frame bearer data rates over all latency path in one direction
  • aggregate data rate: net data rates + ADSL overhead data rate
  • total data rate: aggegate data rate + RS FEC (error correction codes)
  • line rate: total data rate + trellis overhead

  Block diagram of the transmitter unit of ADSL DSLAM/modem (ATU-C/ATU-R) is shown on the folowing figure:

  
      V-C/                Fast path                   bits from init                        U-interface
      T-R                                                  |
       |  +----+ +-----+ +---+ +-+ +---+ +-+            +--|--+ +--------+ +----+ +---+ +---+
   ASx-|->|STM |-| Mux/|-|CRC|-|D|-|FEC|-|D|--------->--|Tone | |Constel-|-|    |-|   | |   |
   LSx-|->|or  |-|Sync | +---+ |A| +---+ |A|            |orde-| |lation  |-|IDFT|-|P/S| |DAC|   analog
   or -|  |ATM |-|Contr|       |T|       |T|            |ring | |encoder |-|    |-|   | |   |   output
       |  |    | |     |       |A|       |A|            |     |-|  and   |-|    |-|   |-|   |--->
   ATM-|->|if. |-|     | +---+ |B| +---+ |B|   +------+ |     | |Gain    |-|    |-|   | |   |
       |+-|    | |     |-|CRC|-|U|-|FEC|-|U|   |Inter-| |     | |scaling |-|    |-|   | |   |
        | |    | |     | +---+ |F| +---+ |F|->-|leaver|-|     | |        |-|    |-|   | |   |
        | +----+ +-----+       +-+       +-+   +------+ +-----+ +--------+|+----+|+---+ +---+
       EOC                Interleaved path                                Zi     Xi
  

  The user data flow to be forwarded with nx32 kbit/s of rate will arrive through ASx, LSx, ATM ports in a serial form to the input of ATU-C (STM or ATM interface). Mux/Sync Contr. circuit connects the bearer channels to the chosen transmission path (fast / interleaved path). Next step is the serial/parallel transformation, we store a 250-us part (1/4000 Baud) from the user data flow to a data buffer (DATABUF), so that we create a control sum (CRC). We compile the user data flow part to be forwarded in a framed form (ADSL frame) in the DATABUF with max size of 256 bytes.

  _{in case of ADSL2, ADSL2+ modems the Fast/Interleaved paths are multiplied.}

  Schematic presentation of the ADSL frame:

  • frame type: there are several types of frames developed for the classical and for the improved ADSL connections. We can choose from them (g.dmt (ADSL), ADSL2, ADSL2+) in the menu of modems, but the autoconfig service is developed here, too.
  • frame time: 250 us (it is equal to the baud rates of the individual DMT channels).
  • frame size: max. 256 bytes. Actual frame size is determined by the attainable bit rate parameter.
  • multiframe: (CRC, EOC forwarding) 68 ADSL frames, 17 ms
  • the frame is flexible
  • Frame can be divided into two main parts:

           +----------------------+
           | fast  data buffer    |  - frame part assigned to the fast transmission path
           +----------------------+
           | interleaved data buf.|  - frame part assigned to the interleaved transmission path
           +----------------------+
         

    Size of the buffer belonging to the unused path is 0.
  • Build-up of a fast data buffer

           +----------------------+
           | fast byte            | - forwarding of CRC, EOC
           +----------------------+
           | fast data bytes      | - Bearer channels, frame synch
           +----------------------+
           | FEC bytes            | - (FEC - Forward Error Coding)
           +----------------------+   
         

  • o In the interleaved data buffer the error coding is made by interleaving to several data frames.

  After compiling the content of the data buffer the next step is to order data buffer bits to the DMT channels (Tone ordering). If we can forward many data bits (8...15) on a channel, then we can improve with the performance of the transmission system an optional Trellis coding. Number of the data bits to be forwarded is determined by the signal-to noise ratio measured in the channel during the initialization (bits from init). Next step is, that the transmission unit orders a complex number to each DMT channels, which describes the amplitude and phase of the line symbols ordered to the bits to be forwarded (Constellation encoder). Executing this action to every DMT channel the spectrum of the line signal aggregates to the duration of a DMT symbol (250 us) at the output of the Constellation encoder. We can obtain the samples of the line signal with an inverse Fourier-transformation (IDFT). Number of patterns is 512 in case of ATU-C, and it is 64 in case of ATU-R. We can obtain the continuous line signal in time from the computed patterns after ining up (P/S) and D/A conversion (DAC).

  Main transmission parameters of the line layer

  These parameters are measuring results of the ADSL modems and they are available through the managing system:

  Bit rate, Stream Rate	attainable (planned) bit rate (specification (max. min. value))
  Attainable bit rate	the attainable bit rate determined according to the signal-to-noise ratio measured in DMT channels
  SNR (Signal to Noise Ratio) Margin, Stream Noise Margin	the measured signal-to-noise ratio in decibels related to the requirement. ADSL systems are calibrated to 10e-7 error rate, and the noise level belonging to this error rate is the relative basis of SNR Margin (relative signal-to-noise ratio) (0 dB). (Note: in PCM systems this error rate value was 10e-6.) We can give a requirement for the signal-to-noise ratio in the managing system (Desired Margin, Target SNR). Typical value is 6 dB. It means, that the required signal-to-noise ratio is with 6 dB bigger, than the value belonging to the 10e-7 error rate. It also means, that the noise performance can grow quadruple in the connection without disturbing it. Noise is mostly a crosstalk noise, and its fluctuation in time may caused by subscribers of changing number using the ADSL in one same cable in the individual points of time.
  Line attenuation, Stream Attenuation	Attenuation of subscriber copper pairs can be determined from the transmitted and received signal performances.
  Output power, Stream Output Power	Output power of the ADSL transmitter
  Service mode	Type of the ADSL frame
  Trellis Encoding	Usage of Trellis line coder
  Interleave delay	Delay on the interleave transmission path.
  Bits/channel	Distribution of bits by DMT channels. It gives, how many bits can be transmitted by DMT-channels.

  Illustration:

  

Initialization

• Handshake - set up a connection between transmitters/receivers, sending basic data (e.g. STM/ATM transmission)
• Training - Signal level measurements, setting of line equalizers, echo cancellers
• Channel analysis - measurement of the signal-to-noise ratio in DMT channels, and determination of tone ordering on the basis of that.
• Exchange - Changing of measuring results between transmitter/receivers

Power Management

ADSL-U Channel layer

There are two paths at our disposal for forwarding the bearer channels in the U-interface:

• fast path - low delay, weak error protection. It is used for transmission of an information, which cannot tolerate delay - e.g. VoIP.
• interleaved path - high delay, strong error protection. It is used e.g. for Internet transmission
  Strong error protection is based on the issue, that the error correction codes are generated by interleaving them to several ADSL frames. Measure of interleaving can be programmed on the handling surface of the DSLAM (Interleave Delay).
  In ADSL2, ADSL2+ modems there are four interleaved path at our disposal.

ADSL-U Data path layer

• STM - unframed, synchronous transmission mode (data signals arrive continuously) (Do not confuse it with the SDH STM-1 frame!)
• ATM - framed, synchronous transmission mode (data frames (PDU) can be read from a store), then we forward data signals packed to ATM cells to the input of the ADSL layer. We map the information to be forwarded on the user side of the ATM network to AAL5 frames. Draft of this action:

                        Tx PDU1 .. PDUn
                            |       |
                           +|-------|+
                           | MUX, ENC| --- VC, LLC encapsulation (multiplexing)
                           +----|----+
                                |------- AAL5 frame
                           +----|----+
                           | ATM AAL |
                           +----|----+
                                |------- 48 bytes segments
        another ATM        +----|----+
         interface         | ATM ATM |
               |           +----|----+
               +--------------+ |------- 53 bytes (ATM) cells (VPI/VCI)
                           AS1| |AS0
                           +--|-|----+
                           | PHY ADSL|
                           +----|----+
                               DMT
  

  Rate of the data flow can be programmed in 32 kbit/s steps in the managing system of DSLAM.

• PACKET - (only in case of ADSL2, ADSL2+) (G.993.1 Annex H)
  

                          HDLC
                +--------------|-----------------+
                | Address                        |      0xFFFF (Broadcast)
                +--------------------------------+
                | Control                        |      0x0003 (Unnumbered information)
                +--------------------------------+
                | Data                           |
                |                                |
                |                                |
                +-----------|--------------------+
                           FCS
  
          This form is used mainly for forwarding streams (VoIP, video).
  
  
  

VC, LLC multiplexing, PPPoA

Beyond the physical layer the data connection (ATM) layer also gives a possibility for data flow multiplexing to users in form of ATM VC-k (virtual circuits). There are several ATM virtual circuits can be generated in an ADSL connection.

There two processes worked out for multiplexing data flows:

• VC-multiplexing: Usage of separated virtual channels (VPI,VCI) by packet types (e.g.. IP, IPX, DECnet, Appletalk).
• LLC-multiplexing: We encapsulate the different packets, and forward them to a common virtual circuit. Header of the frame is the header of the LLC sublayer of DLC. Name of the process can be derived from this.
  Header is also necessary, because the AAL5 frame does not contain a payload type identifier, so then we will not know, how to handle the received PDU on the reception side.
  

_{Build-up of AAL5 frames after multiplexing:
Here we pay an emphasized attention to multiplexing of the PPP protocol used in these systems for setting up connection into the AAL5 frame. (PPPoA, RFC 2364)}

• VC-multiplexing: the payload gets to the AAL5 frame in an unchanged format. Examples:

  
         Bridged Ethernet                 Routed Protocols            PPPoA
            (RFC 1483)                       (RFC 1483)
  
          +----AAL5---+                    +----AAL5---+          +----AAL5---+
          |           |                    |           |          |           |
          |  Ethernet |                    |  Payload  |          |   PPP     |
          |  frame    |                    |           |          |  Payload  |
          |           |                    |           |          |           |
          +-----------+                    +-----------+          +-----------+
          |  AAL5     |                    |  AAL5     |          |  AAL5     |
          |  trailer  |                    |  trailer  |          |  trailer  |
          +-----------+                    +-----------+          +-----------+
  
      

• LLC-multiplexing: LLC header comes before the payload in the AAL5 frame. Examples:

  
         Bridged Ethernet                 Routed Protocols             PPPoA
            (RFC 1483)                       (RFC 1483)
  
          +----AAL5---+                    +----AAL5---+            +----AAL5---+
          | LLC header|                    | LLC header|            | LLC header|
          +-----------+                    +-----------+            +-----------+
          |           |                    |           |            |           |
          |  Ethernet |                    |  IPv4     |            |   PPP     |
          |  frame    |                    |  frame    |            |  payload  |
          |           |                    |           |            |           |
          +-----------+                    +-----------+            +-----------+
          |  AAL5     |                    |  AAL5     |            |  AAL5     |
          |  trailer  |                    |  trailer  |            |  trailer  |
          +-----------+                    +-----------+            +-----------+
       

• Content of the LLC-headers: (it is not necessary to learn!)

  	Bridged Ethernet	Routed Protocols	PPPoA
  DSAP	0xAA	0xAA	0xFE
  SSAP	0xAA	0xAA	0xFE
  CONTROL	03 (UI PDU)	03 (UI PDU)	03
  OUI	00-80-C2	00-00-00	NLPID=PPP 0cCF
  Type	00-01 (802.3) 00-02 (802.4)	----------

Notes:
Multiplexing, framing processes are described by IETF RFC 2684 (an earlier version is the IETF RFC 1483, it is frequently used as the idenfitier of processes):
PPPoE frames are bridged by the ADSL modems, as it follows from the above.