Use of multiplexing shall be negotiated between the HNB and HNB-GW.
Before each multiplexed RTP/codec payload PDU inserted into the UDP/IP packet a Multiplex Header, which identifies the multiplexed packet, shall be inserted.
The Multiplex Header includes :
The field has two possible values. Value 0 shall be used for an uncompressed RTP header, as decribed in the present sub-clause. Value 1 is FFS.
Mux ID, 15 bits.
For identification of different user plane connections. The value shall be the UDP destination port of the corresponding non-multiplexed RTP PDU packet divided by two (only even numbered ports are used for RTP sessions).
Length Indicator (LI), 8 bits, unsigned integer.
Gives the length of the multiplexed RTP PDU packet (RTP header + RTP) in bytes (the last byte of the RTP PDU is padded to the next byte boundary if necessary). Maximum length is 255 bytes. This LI allows to calculate where the next Multiplex Header for the next multiplexed RTP PDU packet starts.
Reserved for future use. Shall be set to 0 by the sending entity and be ignored by the receiving entity.
Source ID, 15 bits.
For identification of the different connections. The value shall be the source UDP port of the corresponding non-multiplexed RTP/codec PDU packet divided by two (only even numbered ports are used for RTP sessions).
The multiplexed RTP PDU shall be inserted in the IP/UDP packet directly after the corresponding Multiplex Header. The multiplexed RTP packet PDU shall follow the rules defined in IETF RFC 1889  and consists of the full RTP header and the RTP payload. If the multiplexed RTP packet PDU does not end at a byte boundary, then the remaining bits of its last byte shall be padded with zeros.
The multiplexing method does not limit the number of packets being multiplexed and it is thus the data link layer protocol that defines the maximum frame size. In order to avoid additional delay in the network the packets should not be delayed more than 1 ms to 2 ms, which also effectively limits the number of multiplexed packets and makes the multiplexing-jitter low.