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Core network aspects 1

9.2 Traffic adjustment mechanism

In a multi-path transmission, the MPT-enhanced MUE and MUE/UE share the network resources. In contrast
to single-path transmission, services with large bandwidth requirements can be separated for parallel
transmission. It must be considered that the separated service is transmitted through distinct paths and
should be coordinated based on a path scheduler algorithm, which may refer to [b-IETF RFC 6356] or [b-IETF
RFC 4960].

In addition, there are several retransmission paths available rather than (just) the original one, i.e., single-
path is less efficient, especially when the original path is under high load. According to the performance of
each path available, retransmission traffic can be enhanced to transmit through a different path depending
on the QoS requirements of the retransmission traffic. In order to avoid unnecessary retransmission, RTO of
a path should be carefully calculated as described in [b-IETF RFC 5682].
Moreover, packet scheduling takes both bandwidth aggregation and end-to-end delay into account to obtain
the optimal transmission throughput, and network coding can recover original packets from receiving packets
even if partial information is lost in the packets. The application of packet scheduling and network coding can
also improve the network throughput, reduce energy consumption and reduce transmission delay.

Traffic adjustment can be initiated by the MPT-enhanced MUE or the network (MPT-enhanced SCF as a multi-
path transmission). The high level information flow is described separately as follows.

9.2.1 Traffic adjustment mechanism initiated by MPT-enhanced MUE
Suppose that MUE_A and MUE_B are both MPT-enhanced MUEs with two interfaces simultaneously
accessing WLAN (IF (1)) and 3G (IF (2)) access networks. Consider the example scenario where MUE_A
initiates a large file transfer service to MUE_B. To achieve a larger bandwidth for time savings, MUE_A splits
the file transfer to go through both the WLAN path and the 3G path simultaneously.

After a period of time, the 3G path becomes congested due to the increased traffic load. MUE_A would then
shift some of the traffic from the 3G path to the WLAN path, which is less congested based on the paths'
performance parameters such as Cwnd and RTT. The traffic adjustment of each path is required to take path
conditions into consideration in a fully coupled, coupled or semi-coupled way. At the same time, the packets
transmitted by each path are required to be adjusted according to the available bandwidth and end-to-end
delay of each path for in-order delivery. Thereafter, when detecting that data packets in WLAN path
retransmit frequently, MUE_A could decide to adjust part of the retransmitted traffic to go through the 3G
path for better retransmission efficiency.
The detailed information flow of this example scenario is shown in Figure 9-3.

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