Interface Configuration Guide 7705 SAR Interfaces
Edition: 01 3HE 11011 AAAC TQZZA 155
PTM bonding and EFM bonding are sometimes used interchangeably, with EFM
bonding generally used in conjunction with SHDSL. The SHDSL interface on a 6-port
DSL Combination module complies with both PTM specified in ITU-T G.998.2, and
EFM specified in IEEE 802.3ah-2004.
PTM bonding applies to DSL links with or without identical transmission speeds.
Since PTM implies the use of variable-size PDUs, it makes the use of IMA
techniques impossible. PTM bonding combines EFM-based transmission links with
limited, or reach-dependent, bandwidth, specifically VDSL2, SHDSL, or ADSL2+ on
the 7705 SAR-M and ADSL2+ or VDSL2 on the 7705 SAR-Wx. PTM bonding adds
sequence information to transmitted frames or frame fragments, allowing
resequencing by adding delay variation to account for speed and PDU size
differences across multiple physical links in one bonding group.
In PTM channel-bonding schemes, the end-user packets are split into small
fragments of up to 512 bytes. The PTM bonding implementation on xDSL ports uses
a fixed fragment size of 204 bytes, which is not user-configurable. The PTM fragment
size on the SHDSL ports is set to 256 bytes. When PTM bonding is active on a DSL
bundle, fragments are distributed over individual DSL lines. At the receiving end, the
fragments are realigned to recover from differential delays in the transmission path,
then reassembled into packets.
In order to realign correctly, each fragment is prefixed with a header containing a
sequence identifier (SID), a start of packet (SOP) indicator, and an end of packet
(EOP) indicator. The receiver side uses the SID to rearrange the incoming cells in
the correct order, while the SOP and EOP indications are used to reassemble the
stream of cells into complete data packets. If transmitted Ethernet packets are
smaller than the PTM fragment size, they are transmitted inside a single fragment.
3.2.18.4 Pairs Within a Bonded Group
Pairs within a bonded group must start with pair 1 and are then sequentially added
into each module or port. For example, if a 4-pair bonded group is desired on an
interface capable of supporting 4 or more pairs, pairs 1 through 4 should be
connected to the DSLAM and configured by the DSLAM into the bonded group. For
pinout diagrams, refer to the 7705 SAR DSL Module Installation Guide for RJ-11
pinouts and the 7705 SAR-Wx Chassis Installation Guide for RJ-45 pinouts.
On the ISAM, bonding makes use of either chipset or LT-level (Line Termination
card-level) bonding. For LT-level bonding, the interworking function on the LT board
allows non-contiguous ports on the same LT to be bonded. LT-level bonding is used
to configure 8-pair bonding on the ISAM, which is also handled by dedicated
hardware that incorporates both bonding and xDSL interworking functions. SHDSL
bonding functions are provided on the ISAM via chipset level bonding.
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