A method performed at a first device (600, 500) in a wireless network, wherein the first device is one of a network node and a mobile terminal, the method comprising: determining whether or not to use an extended header for a protocol layer in wirelessly communicating with a second device (500, 600) in the wireless network, wherein the second device is the other one of a network node and a mobile terminal, the protocol layer being a layer above a physical layer; and using the extended header for the protocol layer in transmitting and/or receiving a protocol data unit, PDU, of the protocol layer when it is determined that the extended header is to be used, wherein the extended header comprises a legacy indicator, L-I, field of a predetermined number of indicator bits for use in indicating a value of a characteristic of the PDU, wherein there is a legacy indicator threshold value associated with the characteristic, the legacy indicator threshold value being a maximum value of the characteristic that can be indicated by the legacy indicator field L-I, wherein the extended header comprises an extended indicator field, E-I, logically concatenated with the legacy indicator field L-I such that the E-I/L-I concatenation comprises sufficient number of bits to specify a value of the characteristic of the PDU beyond the legacy indicator threshold value, characterized in that the step of using the extended header for the protocol layer in transmitting and/or receiving a PDU of the protocol layer comprises any one or more of: using an extended sequence number field, PDCP ESN, to be logically concatenated with a legacy sequence number field, PDCP SN, in an extended PDCP header of a PDCP data unit; using an extended first missing PDCP SN field, EFMS, to be logically concatenated with a legacy first missing PDCP SN field, FMS, in the extended PDCP header of the PDCP data unit; and using an extended sequence number field, RLC ESN, to be logically concatenated with a legacy sequence number field, RLC SN, in the extended RLC header of the RLC data unit. determining whether or not to use an extended header for a protocol layer in wirelessly communicating with a second device (500, 600) in the wireless network, wherein the second device is the other one of a network node and a mobile terminal, the protocol layer being a layer above a physical layer; and using the extended header for the protocol layer in transmitting and/or receiving a protocol data unit, PDU, of the protocol layer when it is determined that the extended header is to be used, wherein the extended header comprises a legacy indicator, L-I, field of a predetermined number of indicator bits for use in indicating a value of a characteristic of the PDU, wherein there is a legacy indicator threshold value associated with the characteristic, the legacy indicator threshold value being a maximum value of the characteristic that can be indicated by the legacy indicator field L-I, wherein the extended header comprises an extended indicator field, E-I, logically concatenated with the legacy indicator field L-I such that the E-I/L-I concatenation comprises sufficient number of bits to specify a value of the characteristic of the PDU beyond the legacy indicator threshold value, characterized in that the step of using the extended header for the protocol layer in transmitting and/or receiving a PDU of the protocol layer comprises any one or more of: using an extended sequence number field, PDCP ESN, to be logically concatenated with a legacy sequence number field, PDCP SN, in an extended PDCP header of a PDCP data unit; using an extended first missing PDCP SN field, EFMS, to be logically concatenated with a legacy first missing PDCP SN field, FMS, in the extended PDCP header of the PDCP data unit; and using an extended sequence number field, RLC ESN, to be logically concatenated with a legacy sequence number field, RLC SN, in the extended RLC header of the RLC data unit. using an extended sequence number field, PDCP ESN, to be logically concatenated with a legacy sequence number field, PDCP SN, in an extended PDCP header of a PDCP data unit; using an extended first missing PDCP SN field, EFMS, to be logically concatenated with a legacy first missing PDCP SN field, FMS, in the extended PDCP header of the PDCP data unit; and using an extended sequence number field, RLC ESN, to be logically concatenated with a legacy sequence number field, RLC SN, in the extended RLC header of the RLC data unit.