Transmission Convergence (Tc) Layers - THOMSON SHDSL Manual

2| SHDSL Overview
2.2

Transmission Convergence (TC) Layers

Introduction
Thomson SHDSL devices support two Transmission Convergence (TC) layers: the traditional Asynchronous
Transfer Mode (ATM) TC and the newer Ethernet in the First Mile (EFM) TC.
A single platform such as the SHNT-F is capable of operating in any of the following configurations:
ATM TC on top of a single SHDSL link.
ATM TC on top of multiple SHDSL links configured in a single SHDSL 4-wire group or a scalable SHDSL
M-pair group.
EFM TC on top of a single SHDSL link.
EFM TC configured in a PAF group consisting of at least one and possibly multiple SHDSL links.
The remainder of this section briefly explains each combination and enumerates the main advantages and
disadvantages.
ATM TC - Single pair
In case the ATM TC-layer is enabled on a single SHDSL link, a stream of 53 byte ATM cells is converted into an
octet stream which is fed into the SHDSL transceiver.
Advantages:
ATM is a well proven technology.
This solution is multiprotocol, e.g. PPPoE over ATM, PPP over ATM (PPPoA) or Ethernet over ATM
(ETHoA).
Disadvantages:
Fixed 5 byte cell overhead + variable AAL5/fixed RFC 1483 overhead.
ATM TC - M-pair
If multiple SHDSL links are assigned to a DSL interface, then it can operate in 4-wire mode or in the scalable
M-pair mode. In this case, the cell stream is byte-interleaved over the two pairs (4-wire mode) or over the M
pairs (M-pair mode). This aggregation method is sometimes referred to as SHDSL bonding. This technique
requires that the SHDSL links within the bonding group are all in data mode and synchronized to the same
rate.
Advantages:
Deployed and proven track record.
No extra overhead due to M-pair operation.
Disadvantages:
The links must synchronize to the same rate.
All links must be simultaneously up.
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