Framing Modes

Framing modes are also referred to as framing structure and framing overhead. Overhead refers to signaling and synchronization between two modems. There are two main standards-based framing modes: full overhead and reduced overhead. Each of these two main modes has two subsets. The resulting four modes are usually referred to as modes 0, 1, 2, and 3. In basic terms, the four modes refer to how data is allocated to an Embedded Operations Channel (EOC), to an ADSL Overhead Control (AOC) channel, and to cyclic redundancy check (CRC) functions.

Data can be transmitted in dual-latency or single-latency mode. Dual-latency implies that data is transmitted in both the interleaved and non-interleaved (fast) buffers. In single-latency mode, data is allocated either to the fast (non-interleaved) buffer or to the interleaved buffer. In framing mode 3, if data is transmitted only in the fast buffer, only a fast byte carries overhead (EOC, AOC, or CRC) information. If data is transmitted only in the interleaved buffer, only a sync byte carries overhead information. The resulting reduction in administrative overhead allows more bandwidth to be allocated to user traffic.

The terms fast byte and sync byte are generally accepted jargon for fast synchronization byte and interleaved synchronization byte, further referring to the non-interleaved and interleaved channels. The synchronization symbol permits recovery of the frame boundary after micro-interruptions that might otherwise force retraining.

These channels are known as the bearer channels. ADSL uses two types of bearer channels. AS bearer channels are simplex and used for downstream traffic. LS bearer channels are full duplex, and thus used for both upstream and downstream. Up to four (AS0-AS3) downstream simplex channels and up to three (LS0-LS2) duplex channels are synchronized to the 4-kHz ADSL data frame rate, then multiplexed into two separate data buffers (fast and interleaved). These bearer channels are not to be confused with the subchannels or tones in DMT; these are used for framing, not Layer-1 transmission.

Framing mode 0 is sometimes called full async (full overhead with asynchronous timing). This mode has an administrative overhead of 128 kbps, and is usually the required mode when training up to CPE that is only ANSI issue 1 compliant. Async refers to a mode where the timing of the bearer channels is not synchronized to the ADSL modem timing base. In asynchronous mode, additional synchronization functions are performed to synchronize the bearer channels with the modem's timing base.

Framing mode 1 is sometimes called full sync (full overhead with synchronous timing). This mode also has an administrative overhead of 128 kbps. Sync refers to a mode where the timing of the bearer channels is synchronized with the ADSL modem timing base. In sync mode, additional synchronization functions are not necessary and are not performed.

Framing mode 2 is sometimes called reduced separate (reduced overhead with separate fast and sync bytes in the fast and interleaved buffers). This mode incurs 64 kbps of overhead, with 32 kbps allocated to the fast byte and 32 kbps added to the sync byte.

Framing mode 3 is sometimes called reduced merged (reduced overhead with merged fast and sync bytes in either the fast or interleaved buffer). This mode incurs only 32 kbps of overhead, and is now the most commonly used by systems that are DMT issue 2 compliant.

Cisco ATU-C and ATU-R modems support all framing modes, with mode 3 preferred. If in doubt about which framing mode to select in a DSLAM, select reduced merged (mode 3).

Reduced overhead modes, especially mode 3, can significantly increase a user's data throughput by using less of the available bandwidth for administrative functions. The difference between 128 kbps of overhead and 32 kbps of overhead is much more apparent at slower DSL data rates than at higher DSL data rates.

For a complete description of the ADSL DMT framing modes and processes, refer to ANSI specification T1.413.

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