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2.2 ADSL TRANSPORT MODES: STM OR ATM?

The original ADSL standard was designed to carry compressed digital video (i.e., MPEG2), nx64kbit/s and DS1 dedicated circuits. This class of information transfer is known as synchronous transport mode (STM). With the redirection of ADSL to transport IP packets, there was a movement to support variable-length frames (e.g., HDLC or Ethernet MAC) as part of STM. Since 1997 ATM, or cell-based transport, has been favored over STM (in order to support IP packets as well as compressed video and other real-time or QOS-based applications), and G.992.2 ( G.lite ) supports only ATM transport. Since the majority of telco networks now have ATM backbones, the extension of ATM over the subscriber enables the telco to take advantage of economies of scale. It also dispenses with protocol conversion at the access-network-to-core-network-interface. Finally, an ATM network can more easily scale up to accommodate more subscribers and/or higher access speeds. This would make it easy for a carrier to accommodate growth in both the numbers and downstream bit rates of ADSL lines and to build the infrastructure for VDSL (see the discussion on network architecture in Section 2.3).

With ATM over ADSL, users are connected to a network service provider (NSP) via virtual circuits1. Currently, both a PPP over ATM stack (for Internet and secure corporate server access) and a native-mode ATM protocol stack (for real-time and multimedia applications) are used in conjunction with PVCs. In the future ATM SVC signaling (a.k.a. ATM Forum UNI or ITU Q.2931signaling) and ATM network management (ATM Forum ILMI) messages will be supported in the access node and the ATM over ADSL CPE. For ATM over ADSL as defined in T1.413 or G.992.2, user data is segmented into cells, which are then transmitted and received over the subscriber loop by the pair of ADSL modems (the NT on the customer premises and the access node in the network-typically on a line card within a DSLAM or ATM edge switch).

The ATM network supports various traffic classes to realize the desired user service. These are specified on a virtual circuit basis, along with subordinate traffic class/QOS parameters. From highest to lowest priority, these traffic classes are:

1. Constant bit rate (CBR)

2. Real-time and non-real-time variable bit rate (VBR)

3. Available bit rate (ABR)

4. Unspecified bit rate (UBR)

specified Today these are private virtual circuits (PVCs), but carriers plan to offer switched virtual circuits (SVCs) in the future. In the meantime two techniques-soft PVCs, which are effectively PVCs that have been set up but never taken down, and auto-configuration extensions to the ILMI MIB-can be used for more flexible provisioning.


 * ATM END-TO-END NETWORK ARCHITECTURES AND PROTOCOL STACKS

Today, most ADSL networks use only UBR, but those supporting high-quality video or audio also use CBR. Those ADSL modems that support both these traffic classes must implement multiclass queuing and traffic scheduling, so as always to give priority to CBR traffic.

All three ADSL-DMT standards specify the same cell TC for mapping ATM cells into the user data field of an ADSL physical layer frame. There are separate cell TCs for the  interleave and fast paths: corresponding to the ADSL channels (AS0 and AS1 downstream and LS1 and LS2 upstream) in use. Only one channel, in each direction of transmission, exists for G.992.2, but up to two upstream and downstream channels are optional in T1.413-II and G.992.12.Hence for dual latency in a given direction of transmission, the cell TC appears as two physical layers to the ATM layer. An example of this would be the concurrent use of video retrieval on the interleave path and Internet access or digital telephony (e.g., VToA) on the fast path.

In addition to cell delineation, the cell TC performs other functions:

1. It inserts and removes idle cells from the ADSL physical layer user data.

2. It scrambles/descrambles the cell payload.

3. It checks for HEC violations on each received cell and discards cells with HEC errors.

4. It performs sublayer bit timing ordering.

5. It reports both the inability of the receiver to acquire cell delineation (no cell delineation) and the loss of cell delineation after it had been acquired. These anomalies are reported in indicator bits within the ADSL superframe. The ATU-R is required to maintain three cell TC counters to monitor cell TC performance.

Sublayer interfaces for the cell TC are defined in a T1.413-II Annex for the ATM layer above (nominally, the UTOPIA or UTOPIA 2 interface from the ATM Forum) and the sync/control multiplexing PHY sublayer below. Again, one cell TC is required for each latency path/ADSL channel