The invention relates to a method of identifying a protocol data unit in a
network layer of an air interface in a cellular radio network, to a
cellular radio network and to a protocol data unit. A network layer
usually includes connection management, mobility management and radio
resources management sublayers. In the invention, the radio resources
management sublayer is replaced by a radio network sublayer. A protocol
data unit header includes a protocol discriminator. Some of the protocol
discriminator values are allocated to the identification of protocol data
units in the connection management sublayer and the mobility management
sublayer. According tot the invention, at least one unreserved value of
the protocol discriminator is used for identifying protocol data units in
the radio network sublayer.
