UPDATE BT Enable Physical Retransmission G.INP on FTTC Broadband Lines

A new report appears to suggest that BTOpenreach has deployed or started to deploy Physical Retransmission (G.INP / ITU G.998.4) technology to improve the performance of their superfast ‘up to’ 40-80Mbps capable Fibre-to-the-Cabinet (FTTC / VDSL) broadband lines. Previously G.INP has only formed part of a trial on Openreach’s platform.

Openreach has already conducted a “limited trial” of G.INP with a small number of FTTC/VDSL lines during 2013 (here) and they expanded this last year as part of their Phase 2 FTTC Vectoring trials (here). The error correction technology is designed to tackle spikes / burst of electromagnetic interference (impulse noise) and can thus make some problematic lines more stable and less prone to errors (Sky Broadband already uses G.INP on their LLU network), which may also return a small performance benefit.

According to Thinkbroadband, a number of people have now reported that G.INP is suddenly showing up as being enabled on their FTTC broadband lines and this appears to be happening outside of the known trial locations for the Vectoring and G.INP technologies. We have recently been requesting more information on this and other matters from Openreach, although they have yet to clarify whether the change is part of an expanded trial or full deployment.

Last week Alcatel-Lucent described G.INP, especially when combined with Vectoring technology to iron out crosstalk interference, as “the secret to stable 100Mbps over copper“.

Alcatel-Lucent’s Jan Verlinden said:

“Any appliance with an electric motor, power switch or power adapter is capable of generating impulse noise. Telephone wiring in subscribers’ homes picks up the noise from such appliances which, in turn, impairs DSL transmission. The severity depends on how close the source of the impairment is to the telephone wiring and the quality of the wiring itself. Service providers tell us, however, that many of their subscribers’ homes have lower quality wiring, so are susceptible to impairment.

Impulse noise comes in two main flavors – intermittent and repetitive. Noise that occurs as sporadic, unpredictable events is called SHINE (single high impulse noise event). SHINE often originates from turning an appliance on or off. Impulse noise that is consistent is known as REIN (repetitive electrical impulse noise). Household dimmers and faulty power adaptors are a common source of REIN.

The G.inp standard specifies the use of physical layer retransmission to enhance INP. The approach is similar to the retransmission method used in TCP/IP. Instead of IP packets, however, data transfer units (DTU) are sent between transmitter and receiver. When packets get corrupted during transmission, the transmitting peer is informed and the DTU is resent.

There are several benefits to the G.inp approach. Compared to I-FEC, the method only consumes transmission capacity when retransmission is required, traditionally consuming less than 1% (more on that later). And unlike TCP/IP, all the traffic is protected, including TCP, IP, UDP, SMTP, HTTP and more.

In addition, the round-trip time — i.e., the (minimum) time required for retransmission — is very short in case of G.inp because the DTU error detection and retransmission occurs at the physical layer.. TCP/IP retransmission often takes up to 50 milliseconds, while G.inp only takes a couple of milliseconds (4 milliseconds is typical). The result is also that G.inp achieves enhanced INP with good efficiency at shorter delays compared to I-FEC.”

We hope to have an update soon.

UPDATE 6:08pm

A spokesperson for Openreach has confirmed that G.INP is indeed being deployed and the roll-out should take several months. A commercial deployment of Vectoring technology now seems likely to follow.

An Openreach Spokeswoman told ISPreview.co.uk:

“Openreach is in the early stages of introducing G.INP correction, also known as Retransmission, for FTTC lines that we think can benefit from it. Retransmission supports our Dynamic Line Management process and will benefit customers by providing a slight improvement in speed on FTTC lines where it has been used. It will also improve the volume of FTTC lines running error free.

FTTC lines will automatically accept Retransmission policies. Those currently with non-Retransmission compliant modem firmware will be identified by management systems and updated, after which Retransmission will become available.

An upgrade process has started which is expected to take several months to complete.”