Ooh. This is complicated.
There are 3 separate things you might mean:
Openreach have descriptions of their GEA-FTTC service, but use the term "Downstream Prioritisation Rate". According to SIN 498, this can be set to a value of either 15Mbps or 30Mbps, depending on package chosen. This is obviously reduced to the line rate if that is lower.
BT Wholesale use the term "assured rate" for something completely different, in their WBC portfolio. I'm assuming you aren't really referring to this.
BT Wholesale set a "minimum best-efforts speed" for their FTTC packages:
- Standard 40Mbps packages get 20Mbps min
- Elevated 40Mbps packages get 30Mbps min
- Standard 80Mbps packages get 40Mbps min
- Elevated 80Mbps packages get 60Mbps min
(The handbook predates the introduction of 55Mbps, so I have no idea there)
For example if a cabinet is some 80m from the cabinet and syncs at 78 ... what would that mean for the end user in the context of the assurance rate?
As others have mentioned, the Openreach "prioritisation rate" applies between cabinet and exchange.
This "prioritisation rate" comes into play when too many subscribers (on the one cabinet) are trying to download simultaneously, and congestion is felt in the fibre(s) running to the cabinet.
In these circumstances, the OLT (in the exchange) attempts to balance usage between lines on the cabinet, subject to the differing "prioritisation rates" that the lines have.
If the OLT has to drop some of your traffic as part of this balancing act, it uses some QoS markings to help decide what should go.
In your example, your "up to 80Mbps" line running at 78Mbps would get a prioritisation rate of 30Mbps.
Congestion in the backhaul, be it TTB, BTW or whoever, is down to the backhaul provider and (likely) the amount ordered by the ISP. It seems that BTW set their limits to something higher than Openreach.
Edited by WWWombat (Sat 14-Jan-17 03:40:35)