2 Digital subscriber line VDSL2
2.8 Profiles of VDSL2 connections

It was mentioned that increasing of transmission speeds is realized due to an extension of utilized frequency band. But extended bandwidth reduces the maximum length of the local loop, because there is an increase copper transmission path attenuation at higher frequencies. Therefore, with regard to disparities in access networks each region (North America, Europe, Japan), the need to implement data transmission services and various other conditions, connections with VDSL2 set different width utilized frequency bands in VDSL2 profiles. The profiles are differentiated by upper cutoff frequency, width of subchannel and a maximum total power of the transmitted signal.

Common parameters of VDSL2 connections profiles used in Europe.
Profile | Max. power in downstream [dBm] | Max. power in upstream [dBm] | Width of subchannel [kHz] | MBDC [Mbit/s]
8a | +17,5 | +14,5 | 4,3125 | 50
8b | +20,5 | +14,5 | 4,3125 | 50
8c | +11,5 | +14,5 | 4,3125 | 50
8d | +14,5 | +14,5 | 4,3125 | 50
12a | +14,5 | +14,5 | 4,3125 | 68
12b | +14,5 | +14,5 | 4,3125 | 68
17a | +14,5 | +14,5 | 4,3125 | 100
30a | +14,5 | +14,5 | 8,625 | 200

Frequency band cannot be used as a single unit. Subscriber wants through its single metallic lines to send and receive data. Therefore it is necessary to split the frequency band into at least two sub bands for two directions of transmission. To separate directions of traffic will use two basic methods. The first method is echo cancellation, the second method is the frequency division.

VDSL2 connection can only use the method of FDD with dedicated zones for both directions of transmission and the crossover between bands. This method reduces the effects of NEXT (Near End Cross Talk). Method FDD is therefore for this property preferred in VDSL2 connection.

Parameter MBDC (Minimum Bidirectional Net Data Rate) represents the minimum value of the transmission rate, which is defined as the sum of transmission speeds in both directions. This value MBDC terminal must achieve in order to be classified for use in a given frequency profile. Speed NDR (Net Data Rate) is offered to the subscriber, and does not include requirements for the transmission of business information and header bits between VDSL2 modems. Useful speed will be even lower than the value of NDR (part is used e.g. for header of Ethernet frames or IP packets).