1.2.1 VDSL2 rates

The upstream and downstream VDSL2 data rate is reported to the CP BRAS/Radius upon PPP discovery (a similar mechanism exists for DHCP � see Section 2.1.7). The VDSL2 rate states the upper rate at which Ethernet traffic can be transmitted on the link. This traffic comprises of:
� A 4 byte per frame overhead added by Openreach for internal routing,
� A degree of overhead introduced by DSL (Packet Transfer Mode layer),
� The EU traffic sent from the CP, or from the EU.
As a result of these overheads, the actual achievable throughput in bits per second is dependent on the reported VDSL2 rate and frame size of the data being transmitted. Openreach advise CPs to consider carefully how they interpret the reported VDSL2 rate in relation to the services they sell, the specifics on an individual EU's use of the service, and any impacts of their own network.
For example, if the reported downstream VDSL2 data rate is 40,000 kbit/s and the IP packet size is 1500 bytes (i.e. Ethernet frame size at End User LAN is 1514 bytes) the maximum throughput achievable is actually 39,178 kbit/s (when measured at the EU LAN i.e. no VLAN header, but including Ethernet header). Alternatively, if the IP packet size is 64 bytes the maximum throughput achievable is further reduced to 35,721 kbit/s. This overhead is particularly important to consider in respect to the downstream shaper setting on the CP�s BRAS. CPs are advised to understand and account for the method that their BRAS uses to implement traffic shaping.