D1 network
The D1 network is a digital mobile communications network operated by T-Mobil and based on GSM. The D1 network and D2 network belong to the D networks and operate in the 900 MHzfrequency range.
The D1 network went into operation in 1992 and supported only telephony in the initial phase. In the meantime, the range of services has been expanded to include some value-added services such as call barring, call forwarding, mobile mailbox, fax and data services. The option of call barring is a service feature with different variants. For example, call barring can be used to bar all outgoing international calls or all national calls, as well as all national and international incoming calls.
The D1 network, like the other GSM networks, uses digital encryption and therefore offers the greatest possible security against eavesdropping. The secret key for encoding and decoding is located on the SIM card.
Like the D2 network, the D1 network includes the option of individual traffic guidance services, which are offered as value-added services via cell phone by the service providers. D1 network numbers can be identified by the network codes 0160, 0170, 0171 and 0175.